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Posts Tagged ‘ATLAS’

Higgs update, HCP 2012

Thursday, November 22nd, 2012

Last week, Seth and I met up to discuss the latest results from the Hadron Collider Physics (HCP) Symposium and what they mean for the Higgs searches. We have moved past discovery and now we are starting to perform precision measurements. Is this the Standard Model Higgs boson, or some other Higgs boson? Should we look forward to a whole new set of discoveries around the corner, or is the Higgs boson the final word for new physics that the LHC has to offer? We’ll find out more in the coming months!

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The art of data mining is about searching for the extraordinary within a vast ocean of regularity. This can be a painful process in any field, but especially in particle physics, where the amount of data can be enormous, and ‘extraordinary’ means a new understanding about the fundamental underpinnings of our universe. Now, a tool first conceived in 2005 to manage data from the world’s largest particle accelerator may soon push the boundaries of other disciplines. When repurposed, it could bring the immense power of data mining to a variety of fields, effectively cracking open the possibility for more discoveries to be pulled up from ever-increasing mountains of scientific data.

Advanced data management tools offer scientists a way to cut through the noise by analyzing information across a vast network. The result is a searchable pool that software can sift through and use for a specific purpose. One such hunt was for the Higgs boson, the last remaining elementary particle of the Standard Model that, in theory, endows other particles with mass.

With the help of a system called PanDA, or Production and Distributed Analysis, researchers at CERN’s Large Hadron Collider (LHC) in Geneva, Switzerland discovered such a particle by slamming protons together at relativistic speeds hundreds of millions of times per second. The data produced from those trillions of collisions—roughly 13 million gigabytes worth of raw information—was processed by the PanDA system across a worldwide network and made available to thousands of scientists around the globe. From there, they were able to pinpoint an unknown boson containing a mass between 125–127 GeV, a characteristic consistent with the long-sought Higgs.

An ATLAS event with two muons and two electrons - a candidate for a Higgs-like decay. The two muons are picked out as long blue tracks, the two electrons as short blue tracks matching green clusters of energy in the calorimeters. ATLAS Experiment © 2012 CERN.

The sheer amount of data arises from the fact that each particle collision carries unique signatures that compete for attention with the millions of other collisions happening nanoseconds later. These must be recorded, processed, and analyzed as distinct events in a steady stream of information. (more…)

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It’s been over a month since CERN hosted a seminar on the updated searches for the Higgs boson. Since then ATLAS and CMS and submitted papers showing what they found, and recently I got news that the ATLAS paper was accepted by Physics Letters B, a prestigious journal of good repute. For those keeping score, that means it took over five weeks to go from the announcement to publication, and believe it not, that’s actually quite fast.

Crowds watch the historic seminar from Melbourne, Australia (CERN)

Crowds watch the seminar from Melbourne, Australia (CERN)

However, all this was last month’s news. Within a week of finding this new particle physicists started on the precision spin measurement, to see if it really is the Higgs boson or not. Let’s take a more detailed look at the papers. You can see both papers as they were submitted on the arXiv here: ATLAS / CMS.

The Higgs backstory

In order to fully appreciate the impact of these papers we need to know a little history, and a little bit about the Higgs boson itself. We also need to know some of the fundamentals of scientific thinking and methodology. The “Higgs” mechanism was postulated almost 50 years ago by several different theorists: Brout, Englert, Guralnik, Hagen, Higgs, and Kibble. For some reason Peter Higgs seems to have his name attached to this boson, maybe because his name sounds “friendliest” when you put it next to the word “boson”. The “Brout boson” sounds harsh, and saying “Guralnik boson” a dozen times in a presentation is just awkward. Personally I prefer the “Kibble boson”, because as anyone who owns a dog will know, kibble gets everywhere when you spill it. You can tidy it up all you like and you’ll still be finding bits of kibble months later. You may not find bits often, but they’re everywhere, much like the Higgs field itself. Anyway, this is all an aside, let’s get back to physics.

It helps to know some of history behind quantum mechanics. The field of quantum mechanics started around the beginning of the 20th century, but it wasn’t until 1927 that the various ideas started to get resolved into a consistent picture of the universe. Some of the greatest physicists from around the world met at the 1927 Solvay Conference to discuss the different ideas and it turned out that the two main approaches to quantum mechanics, although they looked different, were actually the same. It was just a matter of making everything fit into a consistent mathematical framework. At that time the understanding of nature was that fields had to be invariant with respect to gauge transformation and Lorentz transformations.

The Solvay Conference 1927, where some of the greatest physicists of the 20th century met and formulated the foundations of modern quantum mechanics. (Wikipedia)

The Solvay Conference 1927, where some of the greatest physicists of the 20th century met and formulated the foundations of modern quantum mechanics. (Wikipedia)

A gauge transformation is the result of the kind of mathematics we need to represent particle fields, and these fields must not introduce new physics when they get transformed. To take an analogy, imagine you have the blueprints for a building and you want to make some measurements of various distances and angles. If someone makes a copy of the blueprints, but changes the direction of North (so that the building faces another direction) then this must not change any of the distances or angles. In that sense the distances and angles in blueprint are rotation-invariant. They are rotation-invariant because we need to use Euclidean space to represent the building, and a consequence of using Euclidean space is that any distances and angles described in the space must be invariant with respect to rotation. In quantum mechanics we use complex numbers to represent the field, and a gauge transformation is just a rotation of a complex number.

The Lorentz transformation is a bit simpler to understand, because it’s special relativity, which says that if you have a series of events, observers moving at different speeds and in different directions will agree on the causality of those events. The rest of special relativity is just a matter of details, and those details are a lot of fun to look at.

By the time all of quantum mechanics was coming together there were excellent theories that took these symmetries into account. Things seemed to be falling into place, and running the arguments backwards lead to some very powerful predictions. Instead of observing a force and then requiring it to be gauge and Lorentz invariant, physicists found they could start with a gauge and Lorentz invariant model and use that to predict what forces can exist. Using plain old Euclidean space and making it Lorentz invariant gives us Minkowski space, which is the perfect for making sure that our theories work well with special relativity. (To get general relativity we start with a space which is not Euclidean.) Then we can write the most general description of a field we can think of in this space as long as it is gauge invariant and that’s a valid physical field. The only problem was that there were some interactions that seemed to involve a massive photon-like boson. Looking at the interactions gave us a good idea of the mass of this particle, the \(W\) boson. In the next few decades new particles were discovered and the Standard Model was proposed to describe all these phenomena. There are three forces in the Standard Model, the electromagnetic force, the weak force, and the strong force, and each one has its own field.

Inserting the Higgs field

The Higgs field is important because it unifies two of the three fundamental fields in particle physics, electromagnetism and the weak fields. It does this by mixing all the fields up (and in doing so, it mixes the bosons up.) Flip Tanedo has tried to explain the process from a theorist’s point of view to me privately on more than one occasion, but I must admit I just ended up a little confused by some of the finer points. The system starts with three fields which are pretty much all the same as each other, the \(W_1\), \(W_2\), and the \(W_3\). These fields don’t produce any particles themselves because they don’t obey the relevant physical laws (it’s a bit more subtle in reality, but that’s a blog post in itself.) If they did produce their own fields then they would generate massless particles known as Goldstone bosons, and we haven’t seen these, so we know there is something else going on. Instead of making massless bosons they mix amongst themselves to create new fields, giving us massive bosons, and the Goldstone bosons get converted into extra degrees of freedom. Along comes the Higgs field and suddenly these fields separate and mix, giving us four new fields.

The Higgs field, about to break the symmetry and give mass (Flip Tanedo)

The Higgs field, about to break the symmetry and give mass (Flip Tanedo)

The \(W_1\) and \(W_2\) mix to give us the \(W^+\) and \(W^-\) bosons, and then the \(W_3\) field meets the \(B\) field to give us the \(Z\) boson and the photon. What makes this interesting is that the photon behaves well on its own. It has no mass and this means that its field is automatically gauge invariant. Nature could have decided to create just the electromagnetic field and everything would work out fine. Instead we have the photon and three massive bosons, and the fields of these massive bosons cannot be gauge invariant by themselves, they need something else to make it all balance out. By now you’ve probably guessed what this mystery object is, it’s the Higgs field and with it, the Higgs boson! This field fixes it all up so that the fields mix, we get massive bosons and all the relevant laws (gauge invariance and Lorentz invariance) are obeyed.

Before we go any further it’s worth pointing a few things out. The mass of the \(W\) boson is so large in comparison to other particles that it slows down the interactions of a lot of particles, and this is one of the reasons that the sun burns so “slowly”. If the \(W\) boson was massless then it could be produced in huge numbers and the rate of fusion in the sun would be much faster. The reason we have had a sun for billions of years, allowing the evolution of life on Earth (and maybe elsewhere) is because the Higgs field gives such a large mass to the \(W\) boson. Just let that thought sink in for a few seconds and you’ll see the cosmic significance of the Higgs field. Before we get ahead ourselves we should note that the Higgs field leads to unification of the electromagnetic and weak forces, but it says nothing about the strong force. Somehow the Higgs field has missed out one of the three fundamental forces of the Standard Model. We may one day unite the three fields, but don’t expect it to happen any time soon.

“Observation” vs “discovery”, “Higgs” vs “Higgs-like”

There’s one more thing that needs to be discussed before looking at the papers and that’s a rigorous discussion of what we mean by “discovery” and if we can claim discover of the Standard Model Higgs boson yet. “Discovery” has come to mean a five sigma observation of a new resonance, or in other words that probability that the Standard Model background in the absence of a new particle would bunch up like this is less than one part in several million. If we see five sigma we can claim a discovery, but we still need to be a little careful. Suppose we had a million mass points, what is the probability that there is one five sigma fluctuation in there? It’s about \(20\%\), so looking at just the local probability is not enough, we need to look at the probability that takes all the data points into account. Otherwise we can increase the chance of seeing a fluctuation just by changing the way we look at the data. Both ATLAS and CMS have been conscious of this effect, known as the “Look Elsewhere Effect”, so every time they provide results they also provide the global significance, and that is what we should be looking at when we talk about the discovery.

Regular readers might remember Flip’s comic about me getting worked up over the use of the word “discovery” a few weeks back. I got worked up because the word “discovery” had been misused. Whether an observation is \(4.9\) or \(5.1\) sigma doesn’t matter that much really, and I think everyone agrees about that. What bothered me was that some people decided to change what was meant by a discovery after seeing the data, and once you do that you stop being a scientist. We can set whatever standards we like, but we must stick to them. Burton, on the other hand, was annoyed by a choice of font. Luckily our results are font-invariant, and someone said “If you see five sigma you can present in whatever durn font you like.”

Getting angry over the change of goalposts.  Someone has to say these things.

Getting angry over the change of goalposts. Someone has to say these things.

In addition to knowing what we mean by “discovery” we also need to take hypothesis testing into account. Anyone who claims that we have discovered the Higgs boson is as best misinformed, and at worst willingly untruthful. We have discovered a new particle, there’s no doubt about that, but now we need to eliminate things are not the Higgs until we’re confident that the only thing left is the Higgs boson. We have seen this new particle decay to two photons, and this tells us that it can only only have spin 0 or spin 2. That’s eliminated spin 1, spin 3, spin 4… etc for us, all with a single measurement. What we are doing now trying to exclude both the spin 0 and spin 2 possibilities. Only one of these will be excluded, and then will know for sure what the spin is. And then we know it’s the Standard Model Higgs boson, right? Not quite! Even if we know it’s a spin 0 particle we would still need to measure its branching fractions to confirm that it is what we have been looking for all along. Bear this in mind when thinking about the paper- all we have seen so far is a new particle. Just because we’re searching for the Higgs and we’ve found something new it does not mean that it’s a the Higgs boson.

The papers

Finally we get to the papers. From the titles we can see that both ATLAS and CMS have been suitably agnostic about the particle’s nature. Neither claim it’s the Higgs boson and neither even claim anything more than an “observation”. The abstracts tell us a few useful bits of information (note that the masses quoted agree to within one sigma, which is reassuring) but we have to tease out the most interesting parts by looking at the details. Before the main text begins each experiment dedicates their paper to the memories of those who have passed away before the papers were published. This is no short list of people, which is not surprising given that people have been working on these experiments for more than 20 years. Not only is this a moving start to the papers, it also underlines the impact of the work.

Both papers were dedicated to the memories of colleagues who did not see the observation. (CMS)

Both papers were dedicated to the memories of colleagues who did not see the observation. (CMS)

Both papers waste no time getting into the heart of the matter, which is nature of the Standard Model and how it’s been tested for several decades. The only undiscovered particle predicted by the Standard Model is the Higgs boson, we’ve seen everything else we expected to see. Apart from a handful of gauge couplings, just about every prediction of the Standard Model has been vindicated. In spite of that, the search for the Higgs boson has taken an unusually long time. Searches took place at LEP and Tevatron long before the LHC collided beams, and the good news is that the LEP limit excluded the region that is very difficult for the LHC to rule out (less than \(114GeV\)). CDF and D0 both saw an excess in the favored region, but the significance was quite low, and personally I’m skeptical since we’ve already seen that CDF’s dijet mass scale might have some problems associated with it. Even so we shouldn’t spend too long trying to interpret (or misinterpret) results, we should take them at face value, at least at first. Next the experiments tell us which final states they look for, and this is where things will get interesting later on. Before describing the detectors, each experiment pauses to remind us that the conditions of 2012 are more difficult than those of 2011. The average number of interactions per beam crossing increased by a factor of two, making all analyses more difficult to work with (but ultimately all our searches a little more sensitive.)

At this point both papers summarize their detectors, but CMS goes out of their way to show off how the design of their detector was optimized for general Higgs searches. Having a detector which can reconstruct high momentum leptons, low momentum photons and taus, and also tag b-jets is not as easy task. Both experiments do well to be able to search for the Higgs bosons in the channels they look at. Even if we limit ourselves to where ATLAS looked the detectors would still have trigger on leptons and photons, and be able to reconstruct not only those particles, but also the missing transverse energy. That’s no easy task at a hadron collider with many interactions per beam crossing.

The two experiments have different overall strategies to the Higgs searches. ATLAS focused their attention on just two final states in 2012: \(\gamma\gamma\), and \(ZZ^*\), whereas CMS consider five final sates: \(\gamma\gamma\), \(ZZ^*\), \(WW^*\), \(\tau\tau\), and \(b\bar{b}\). ATLAS focus mostly on the most sensitive modes, the so-called “golden channel”, \(ZZ^*\), and the fine mass resolution channel, \(\gamma\gamma\). With a concerted effort, a paper that shows only these modes can be competitive with a paper that shows many more, and labor is limited on both experiments. CMS spread their effort across several channels, covering all the final states with expected sensitivities comparable to the Standard Model.

\(H\to ZZ^*\)

The golden channel analysis has been presented many times before because it is sensitive across a very wide mass range. In fact it spans the range \(110-600GeV\), which is the entire width of the Higgs search program at ATLAS and CMS. (Constraints from other areas of physics tell us to look as high as \(1000GeV\), but at high masses the Higgs boson would have a very large width, making it extremely hard to observe. Indirect results favor the low mass region, which is less than around \(150GeV\).) Given the experience physicists have had with this channel it’s no surprise that the backgrounds are very well understood at this point. The dominant “irreducible” background comes from Standard Model production of \(Z/\gamma*\) bosons, where there is one real \(Z\) boson, and one “off-shell”, or virtual boson. This is called irreducible because the source of background is the same final state as the signal, so we can’t remove further background without also removing some signal. This off-shell boson can be an off-shell \(Z\) boson or an off-shell photon, it doesn’t really matter which since these are the same for the background. In the lower mass range there are also backgrounds from \(t\bar{t}\), but fortunately these are well understood with good control regions in the data. Using all this knowledge, the selection criteria for \(8TeV\) were revisited to increase sensitivity as much as possible.

The invariant mass spectrum for ATLAS's H→ZZ* search (ATLAS)

The invariant mass spectrum for ATLAS's H→ZZ* search (ATLAS)

Since this mode has a real \(Z\) boson, we can look for two high momentum leptons in the final state, which mames things especially easy. The backgrounds are small, and the events are easy to identify, so the trigger is especially simple. Events are stored to disk if there is at least one very high momentum lepton, or two medium momentum leptons which means that we don’t have to throw any events away. Some triggers fire so rapidly that we can only store some of the events from them, and we call this prescaling. When we keep \(1\) in \(n\) events then we have a prescale of \(n\). For a Higgs search we want to have a high efficiency as possible so we usually require a prescale of \(1\). Things are not quite so nice for the \(\gamma\gamma\) mode, as we’ll see later.

The invariant mass spectrum for CMS's H→ZZ* search (CMS)

The invariant mass spectrum for CMS's H→ZZ* search (CMS)

After applying a plethora of selections on the leptons and reconstructing the \(Z\) and Higgs boson candidates the efficiency for the final states vary from \(15\%-37\%\), which is actually quite high. No detector can cover the whole of the solid angle, and efficiencies vary with the detector geometry. The efficiency needs to be very high because the fraction of Higgs bosons that would decay to these final states is so small. At a mass of \(125GeV\) the branching fraction to the \(ZZ^*\) state is about \(2\%\), and then branching fraction of \(Z\) to two leptons is about \(6\%\). Putting that all together means that only \(1\) in \(10,000\) Higgs bosons would decay to this final state. At a mass of \(125GeV\) the LHC would produce about \(15,000\) Higgs bosons per \(fb^{-1}\). So for \(10fb^{-1}\) we could expect to have about \(11\) Higgs bosons decaying to this final state, and we could expect to see about \(3\) of those events reconstructed. This is a clean mode, but it’s an extremely challenging one.

The selection criteria are applied, the background is estimated, and the results are shown. As you can see there is a small but clear excess over background in the region around \(125GeV\) and this is evidence supporting the Higgs boson hypothesis!

CMS see slightly fewer events than expected, but still see a clear excess (CMS)

CMS see slightly fewer events than expected, but still see a clear excess (CMS)

\(H\to\gamma\gamma\)

Out of the \(H\to ZZ^*\) and \(H\to\gamma\gamma\) modes the \(\gamma\gamma\) final state is the more difficult one to reconstruct. The triggers are inherently “noisy” because they must fire on something that looks like a high energy photon, and there are many sources of background for this. As well as the Standard Model real photons (where the rate of photon production is not small) there are jets faking photons, and electrons faking photons. This makes the mode dominated by backgrounds. In principle the mode should be easy: just reconstruct Higgs candidates from pairs of photons and wait. The peak will reveal itself in time. However ATLAS and CMS are in the middle of a neck and neck race to find the Higgs boson, so both collaborations exploit any advantage they can, and suddenly these analyses become some of the most difficult to understand.

A typical H→γγ candidate event with a striking signature (CMS)

A typical H→γγ candidate event with a striking signature (CMS)

To get a handle on the background ATLAS and CMS each choose to split the mode into several categories, depending on the properties of the photons or the final state, and each one with its own sensitivity. This allows the backgrounds to be controlled with different strategies in each category, leading to increased overall sensitivity. Each category has its own mass resolution and signal-to-background ratio, each is mutually independent of the others, and each has its own dedicated studies. For ATLAS the categories are defined by the presence of two jets, whether or not the photon converts (produces an \(e^-e^+\) pair) in the detector, the pseudorapidity of the photons, and a kinematic quantity called \(p_{T_T}\), with similar categories for CMS.

When modelling the background both experiments wisely chose to use the data. The background for the \(gamma\gamma\) final state is notoriously hard to predict accurately, because there are so many contributions from different backgrounds, from real and fake photon candidates, and many kinematic or detector effects to take into account. The choice of background model even varies on a category by category basis, and choices of model vary from simple polynomial fits to the data, to exponential and skewed Gaussian backgrounds. What makes these background models particularly troublesome is that the background has to be estimated using the signal region, so small deviations that are caused by signal events could be interpreted by the fitting algorithm as a weird background shape. The fitting mechanism must be robust enough to fit the background shapes without being fooled into thinking that a real excess of events is just a slightly different shape.

ATLAS's H→γγ search, where events are shown weighted (top) and unweighted (bottom) (ATLAS)

ATLAS's H→γγ search, where events are shown weighted (top) and unweighted (bottom) (ATLAS)

To try to squeeze even more sensitivity out of the data CMS use a boosted decision tree to aid signal separation. A boosted decision tree is a sophisticated statistical analysis method that uses signal and background samples to decide what looks like signal, and then uses several variables to return just one output variable. A selection can be made on the output variable that removes much of the background while keeping a lot of the signal. Using boosted decision trees (or any multivariate analysis technique) requires many cross checks to make sure the method is not biased or “overtrained”.

CMS's H→γγ search, where events are shown weighted (main plot) and unweighted (inset) (CMS)

CMS's H→γγ search, where events are shown weighted (main plot) and unweighted (inset) (CMS)

After analyzing all the data the spectra show a small bump. The results can seem a little disappointing at first, after all the peak is barely discernable, and so much work has gone into the analyses. Both experiments show the spectra after weighting the events to take the uncertainties into account and this makes the plots a little more convincing. Even so, what matters is the statistical significance of these results, and this cannot be judged by eye. The final results show a clear preference for a boson with a mass of \(125GeV\), consistent with the Higgs boson. CMS see a hint at around \(135GeV\), but this is probably just a fluctuation, given that ATLAS do not see something similar.

ATLAS local significance for H→γγ (ATLAS)

ATLAS local significance for H→γγ (ATLAS)

(If you’ve been reading the blog for a while you may remember a leaked document from ATLAS that hinted at a peak around \(115GeV\) in this invariant mass spectrum. That document used biased and non peer-reviewed techniques, but the fact remains that even without these biases there appear to be a small excess in the ATLAS data around \(115GeV\). The significance of this bump has decreased as we have gathered more data, so it was probably just a fluctuation. However, you can still see a slight bump at \(115GeV\) in the significance plot. Looking further up the spectrum, both ATLAS and CMS see very faint hints of something at \(140GeV\) which appears in both the \(ZZ^*\) and \(\gamma\gamma\) final states. This region has already been excluded for a Standard Model Higgs, but there may be something else lurking out there. The evidence is feeble at the moment, but that’s what we’d expect for a particle with a low production cross section.)

\(H\to WW^*\)

One of the most interesting modes for a wide range of the mass spectrum is the \(WW(*)\) final state. In fact, this is the first mode to be sensitive to the Standard Model Higgs boson searches, and exclusions were seen at ATLAS, CMS, and the Tevatron experiments at around \(160GeV\) (the mass of two on-shell \(W\) bosons) before any other mass region. The problem with this mode is that it has two neutrinos in the final state. It would be nice to have an inclusive sample of \(W\) bosons, including the hadronic final states, but the problems here are the lack of a good choice of trigger, and the irreducible and very large background. That mean that we must select events with two leptons and two neutrinos in them. As the favored region excludes more and more of the high mass region this mode gets more challenging, because at first we lose the mass constraint on the second \(W\) boson (as it must decay off-shell), and secondly because we must be sensitive in the low missing transverse energy region, which starts to approach our resolution for this variable.

While we approach our resolution from above, the limit on the resolution increases from below, because the number of interactions per beam crossing increases, increasing the overall noise in the detector. To make progress in this mode takes a lot of hard work for fairly little gain. Both papers mention explicitly how difficult the search is in a high pileup scenario, with CMS stating

“The analysis of the \(7TeV\) data is described in [referenced paper] and remains unchanged, while the \(8TeV\) analysis was modified to cope with more difficult conditions induced by the higher pileup of the 2012 data taking.”

and ATLAS saying

“The analysis of the \(8TeV\) data presented here is focused on the mass range \(110<m_H<200GeV\) It follows the procedure used for the \(7TeV\) data described in [referenced paper], except that more stringent criteria are applied to reduce the \(W\)+jets background and some selections have been modified to mitigate the impact of the high instantaneous luminosity at the LHC in 2012.”

It’s not all bad news though, because the final branching fraction to this state is much higher than that of the \(ZZ^*\) final state. The branching fraction for the Standard Model Higgs boson to \(WW^*\) is about \(10\) times higher than that for \(ZZ^*\), and the branching fraction of the \(W\) boson to leptons is also about \(3\) times higher than the \(Z\) boson to leptons, which gives another order of magnitude advantage. Unfortunately all these events must be smeared out across a large spectrum. There is one more trick we have up our sleeves though, and it comes from the spin of the parent. Since the Standard Model Higgs boson has zero spin the \(W\) bosons tend to align their spins in opposite directions to make it all balance out. This then favors one decay direction over another for the leptons. The \(W^+\) boson decays with a neutrino in the final state, and because of special relativity the neutrino must align its spin against its direction of motion. The \(W-\) boson decays with an anti-neutrino, which takes its spin with its direction of motion. This forces the two leptons to travel in the same direction with respect to the decay axis of the Higgs boson. The high momenta of the leptons smears things out a bit, but generally we should expect to see one high momentum lepton, and a second lower momentum lepton n roughly the same region of the detector.

The transverse mass for ATLAS's H→WW* search (ATLAS)

The transverse mass for ATLAS's H→WW* search (ATLAS)

ATLAS did not actually present results for the \(WW^*\) final state on July 4th, but they did show it in the subsequent paper. CMS showed the \(WW^*\) final state on July 4th, although it did somewhat reduce their overall significance. Both ATLAS and CMS spend some of the papers discussing the background estimates for the \(WW^*\) mode, but ATLAS seem to go to more significant lengths to describe the cross checks they used in data. In fact this may help to explain why ATLAS did not quite have the result ready for July 4th, whereas CMS did. There’s a trade off between getting the results out quickly and spending some extra time to understand the background. This might have paid off for ATLAS, since they seem to be more sensitive in this mode than CMS.

The invariant mass for CMS's H→WW* search (CMS)

The invariant mass for CMS's H→WW* search (CMS)

After looking at the data we can see that both ATLAS and CMS are right at the limits of their sensitivity in this mode. They are not limited by statistics, they are limited by uncertainties, and the mass point \(125GeV\) sits uncomfortably close some very large uncertainties. The fact that this mode is sensitive at all is a tribute to the hard work of dozens of physicists who went the extra mile to make it work.

CMS's observed and expected limits for H→WW*, showing the dramatic degradation in sensitivity as the mass decreases (CMS)

CMS's observed and expected limits for H→WW*, showing the dramatic degradation in sensitivity as the mass decreases (CMS)

\(H\to b\bar{b}\)

At a mass of \(125GeV\) by far the largest branching fraction of the Standard Model Higgs boson is to \(b\bar{b}\). CDF and D0 have both seen a broad excess in this channel (although personally I have some doubts about the energy scale of jets at CDF, given the dijet anomaly they see that D0 does not see) hinting at a Higgs boson of \(120-135GeV\). The problem with this mode is that the background is many orders of magnitude larger than the signal, so some special tricks must be used to remove the background. What is done at all four experiments is to search for a Higgs boson that is produced in associated with a \(W\) or \(Z\) boson, and this greatly reduces the background. ATLAS did not present an updated search in the \(b\bar{b}\) channel, and taking a look at the CMS limits we can probably see why, the contribution is not as significant as in other modes. The way CMS proceed with the analysis is to use several boosted decision trees (one for each mass point) and to select candidates based on the output of the boosted decision tree. The result is less than \(1\) sigma of significance, about half of what is expected, but if this new boson is the Higgs boson then this significance will increase as we gather more data.

A powerful H→bb search requires a boosted decision tree, making the output somewhat harder to interpret (CMS)

A powerful H→bb search requires a boosted decision tree, making the output somewhat harder to interpret (CMS)

It’s interesting to note that the \(b\bar{b}\) final state is sensitive to both a spin 0 and a spin 2 boson (as I explained in a previous post) and it may have different signal strength parameters for different spin states. The signal strength parameter tells us how many events we see compared to how many events we do see, and it is denoted with the symbol \(\mu\). A there is no signal then \(\mu=0\), if the signal is exactly as large as we expect then \(\mu=1\), and any other value indicates new physics. It’s possible to have a negative value for \(\mu\) and this would indicate quantum mechanical interference of two or more states that cancel out. Such an interference term is visible in the invariant mass of two leptons, as the virtual photon and virtual \(Z\) boson wavefunctions interfere with each other.

\(H\to\tau\tau\)

Finally, the \(\tau\tau\) mode is perhaps the most enlightening and the most exciting right now. CMS showed updated results, but ATLAS didn’t. CMS’s results were expected to approach the Standard Model sensitivity, but for some reason their results didn’t reach that far, and that is crucially important. CMS split their final states by the decay mode of the \(\tau\), where the final states include \(e\mu 4\nu\), \(\mu\mu 4\nu\), \(\tau_h\mu 3\mu\), and \(\tau_h e3\nu\), where \(\tau_h\) is a hadronically decaying \(\tau\) candidate. This mode has at least three neutrinos in the final state, so like the \(WW^*\) mode the events get smeared across a mass spectrum. There are irreducible backgrounds from \(Z\) bosons decaying to \(\tau\tau\) and from Drell-Yan \(\tau\tau\) production, so the analysis must search for an excess of events over these backgrounds. In addition to the irreducible backgrounds there are penalties in efficiency associated with the reconstruction of \(\tau\) leptons, which make this a challenging mode to work this. There are dedicated algorithms for reconstructing hadronically decaying \(\tau\) jets, and these have to balance out the signal efficiency for real \(tau\) leptons and background rejection.

CMS's H→τtau; search, showing no signal (CMS)

CMS's H→τtau; search, showing no signal (CMS)

After looking at the data CMS expect to see an excess of \(1.4\) sigma, but they actually see \(0\) sigma, indicating that there may be no Standard Model Higgs boson after all. Before we jump to conclusions it’s important to note a few things. First of all statistical fluctuations happen, and they can go down just as easily as they can go up, so this could just be a fluke. It’s a \(1.5\) sigma difference, so the probability of this being due a fluctuation if the Standard Model Higgs boson is about \(8\%\). On its own that could be quite low, but we have \(8\) channels to study, so the chance of this happening in any one of the channels is roughly \(50\%\), so it’s looking more likely that this is just a fluctuation. ATLAS also have a \(\tau\tau\) analysis, so we should expect to see some results from them in the coming weeks or months. If they also don’t see a signal then it’s time to start worrying.

CMS's limit of H→ττ actually shows a deficit at 125GeV.  A warning sign for possible trouble for the Higgs search! (CMS)

CMS's limit of H→ττ actually shows a deficit at 125GeV. A warning sign for possible trouble for the Higgs search! (CMS)

Combining results

Both experiments combine their results and this is perhaps the most complicated part of the whole process. There are searches with correlated and uncorrelated uncertainties, there are two datasets at different energies to consider, and there are different signal-to-background ratios to work with. ATLAS and CMS combine their 2011 and 2012 searches, so they both show all five main modes (although only CMS show the \(b\bar{b}\) and \(\tau\tau\) modes in 2012.)

When combining the results we can check to see if the signal strength is “on target” or not, and there is some minor disagreement between the modes. For the \(ZZ^*\) and \(WW^*\) modes, the signal strengths are about right, but for the \(\gamma\gamma\) mode it’s a little high for both experiments, so there is tension between these modes. Since these are the most sensitive modes, and we have more data on the way then this tension should either resolve itself, or get worse before the end of data taking. The \(b\bar{b}\) and \(\tau\tau\) modes are lower than expected for both experiments (although for ATLAS the error bars are so large it doesn’t really matter), suggesting that this new particle may a non-Standard Model Higgs boson, or it could be something else altogether.

Evidence of tension between the γγ and fermionic final states (CMS)

Evidence of tension between the γγ and fermionic final states (CMS)

While the signal strengths seem to disagree a little, the masses all seem to agree, both within experiments and between them. The mass of \(125GeV\) is consistent with other predictions (eg the Electroweak Fit) and it sheds light on what to look for beyond the Standard Model. Many theories favor a lower mass Higgs as part of a multiplet of other Higgs bosons, so we may see some other bosons. In particular, the search for the charged Higgs boson at ATLAS has started to exclude regions on the \(\tan\beta\) vs \(m_{H^+}\) plane, and the search might cover the whole plane in the low mass region by the end of 2012 data taking. Although a mass of \(125GeV\) is consistent with the Electroweak Fit, it is a bit higher than the most favored region (around \(90GeV\)) so there’s certainly space for new physics, given the observed exclusions.

The masses seem to agree, although the poor resolution of the WW* mode is evident when compared to the ZZ* and γγ modes (ATLAS)

The masses seem to agree, although the poor resolution of the WW* mode is evident when compared to the ZZ* and γγ modes (ATLAS)

To summarize the results, ATLAS sees a \(5.9\) sigma local excess, which is \(5.1\) sigma global excess, and technically this is a discovery. CMS sees a \(5.0\) sigma local excess, which is \(4.6\) sigma global excess, falling a little short of a discovery. The differences in results are probably due to good luck on the part of ATLAS and bad luck on the part of CMS, but we’ll need to wait for more data to see if this is the case. The results should “even out” if the differences are just due to fluctuations up for ATLAS and down for CMS.

ATLAS proudly show their disovery (ATLAS)

ATLAS proudly show their disovery (ATLAS)

Looking ahead

If you’ve read this far then you’ve probably picked up on the main message, we haven’t discovered the Standard Model Higgs boson yet! We still have a long road ahead of us and already we have moved on to the next stage. We need to measure the spin of this new boson and if we exclude the spin 0 case then we know it is not a Higgs boson. If exclude the spin 2 case then we still need to go a little further to show it’s the Standard Model Higgs boson. The spin analysis is rather complicated, because we need to measure the angles between the decay products and look for correlations. We need to take the detector effects into account, then subtract the background spectra. What is left after that are the signal spectra, and we’re going to be statistically limited in what we see. It’s a tough analysis, there’s no doubt about it.

We need to see the five main modes to confirm that this is what we have been looking for for so long. If we get the boson modes (\(ZZ^*\), \(WW^*\), \(\gamma\gamma\)) spot on relative to each other, then we may have a fermiophobic Higgs boson, which is an interesting scenario. (A “normal” fermiophobic Higgs boson has already been excluded, so any fermiophobic Higgs boson we may see must be very unusual.)

There are also many beyond the Standard Model scenarios that must be studied. As more regions of parameter space are excluded, theorists tweak their models, and give us updated hints on where to search. ATLAS and CMS have groups dedicated to searching for beyond the Standard Model physics, including additional Higgs bosons, supersymmetry and general exotica. It will be interesting to see how their analyses change in light of the favored mass region in the Higgs search.

A favored Higgs mass has implications for physics beyond the Standard Model.  Combined with the limits on new particles (shown in plot) many scenarios can be excluded (ATLAS)

A favored Higgs mass has implications for physics beyond the Standard Model. Combined with the limits on new particles (shown in plot) many scenarios can be excluded (ATLAS)

2012 has been a wonderful year for physics, and it looks like it’s only going to get better. There are still a few unanswered questions and tensions to resolve, and that’s what we must expect from the scientific process. We need to wait a little longer to get to the end of the story, but the anticipation is all part of the adventure. We’ll know is really happening by the end of Moriond 2013, in March. Only then can we say with certainty “We have proven/disproven the existence of the Standard Model Higgs boson”!

I like to say “We do not do these things because they are easy. We do them because they are difficult”, but I think Winston Churchill said it better:

This is not the end. It is not even the beginning of the end, but it is perhaps the end of the beginning.” W. Churchill

References etc

Plots and photos taken from:
“Webcast of seminar with ATLAS and CMS latest results from ICHEP”, ATLAS Experiment, CERN, ATLAS-PHO-COLLAB-2012-014
Wikipedia
“Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC”, ATLAS Collaboration, arXiv:1207.7214v1 [hep-ex]
“Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC”, CMS Collaboration, arXiv:1207.7235v1 [hep-ex]
Flip Tanedo

It’s been a while since I last posted. Apologies. I hope this post makes up for it!

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BOOST!

Sunday, August 5th, 2012

A couple weeks ago, about 80 theorists and experimentalists descended on Valencia, Spain in order to attend the fourth annual BOOST conference (tag-line: “Giving physics a boost!”). On top of the fact that the organizers did a spectacular job of setting up the venue and program (and it didn’t hurt that there was much paella and sangria to be had) overall I’d have to say this was one of the best conferences I’ve attended.

so....much.....sangria......

Differing from larger events such as ICHEP where the physics program is so broad that speakers only have time to give a cursory overview of their topics, the BOOST conferences have more of a workshop feel and are centered specifically around the emerging sub-field of HEP called “boosted physics”. I’ll try to explain what that means and why it’s important below (and in a few subsequent posts).

Intro to top quark decay

In order to discuss boosted physics, something already nicely introduced in Flip’s post here, I’m going to use the decay of the top quark as an example.

Obligatory Particle Zoo plushie portraying the top quark in a happy state

The most massive of all known fundamental particles by far, weighing in at around 173 GeV/c2, the top quark has an extremely short lifetime….much shorter than the time scale of the strong interaction. Thus the top quark doesn’t have time to “hadronize” and form a jet…instead, it will almost always decay into a W boson and a b quark (more than 99% of the time), making it a particularly interesting particle to study. The W boson then decays into either a lepton and a neutrino or two lighter quarks, and the full top decay chain is colloquially called either “leptonic” or “hadronic”, respectively.

From the experimental point of view, top quarks will look like three jets (one from the b and two from the light quarks) about 70% of the time, due to the branching fraction of the W boson to decay hadronically. Only 20% of tops will decay in the leptonic channel with a jet, a muon or electron, and missing energy. (I’m ignoring the tau lepton for the moment which has it’s own peculiar decay modes)

In colliders, top quarks are mostly produced in top/anti-top (or “t-tbar”) pairs….in fact, the top-pair production cross section at the LHC is about 177 pb (running at sqrt(s)=7 TeV), roughly 25 times more than at the Tevatron!! Certainly plenty of tops to study here. Doing some combinatorics and still ignoring decay modes with a tau lepton, the whole system will look:

  1. “Fully hadronic”: two hadronically-decaying tops (about 44% of the time)
  2. “Semi-leptonic”: one leptonically-decaying and one hadronically-decaying top (about 30% of the time)
  3. “Fully leptonic”: two leptonically-decaying tops (only about 4% of the time)

Branching fractions of different decay modes in t-tbar events (from Nature)

 

The point: if a t-tbar event is produced in the detector, it’s fairly likely that at least one (if not both) of the tops will decay into jets! Unfortunately compared to the leptonic mode, it turns out this is a pretty tough channel to deal with experimentally, where at the LHC we’re dominated by a huge multi-jets background.

What does “boost” mean?

If a t-tbar pair was produced with just enough energy needed to create the two top masses, there wouldn’t be energy left over and the tops would be produced almost at rest. This was fairly typical at the Tevatron. With the energies at the LHC, however, the tops are given a “boost” in momentum when produced. This means that in the lab frame (ie: our point of view) we see the decay products with momentum in the same direction as the momentum of the top.

This would be especially conspicuous if, for example, we were able to produce some kind of new physics interaction with a really heavy mediator, such as a Z’ (a beyond-the-Standard-Model heavy equivalent of the Z boson), the mass of which would have to be converted into energy somewhere.

Generally we reconstruct the energy and mass of a hadronically-decaying top by combining the three jets it decays into. But what if the top was so boosted that the three jets merged to a point where you couldn’t distinguish them, and it just looked like one big jet? This makes detecting it even more difficult, and a fully-hadronic t-tbar event is almost impossible to see.

At what point does this happen?

It turns out that this happens quite often already, where at ATLAS we’ve been producing events with jets having a transverse momentum (pT) of almost 2 TeV!

A typical jet used in analyses in ATLAS has a cone-radius of roughly R=0.4. (ok ok, the experts will say that technically it’s not a “cone,” let alone something defined by a “radius,” as R is a “distance parameter used by the jet reconstruction algorithm,” but it gives a general idea.) With enough boost on the top quark, we won’t be able to discern the edge of one of the three jets from the next in the detector. Looking at the decay products’ separation as a function of the top momentum, you can see that above 500 GeV or so, the W boson and the b quark are almost always within R < 0.8. At that momentum, individual R=0.4 jets are hard to tell apart already.

The opening angle between the W and b in top decays as a function of the top pT in simulated PYTHIA Z'->ttbar (m_Z' =1.6 TeV) events.

 

We’ll definitely want to develop tools to identify tops over the whole momentum range, not just stopping at 500 GeV. The same goes for other boosted decay channels, such as the imminently important Higgs boson decay to b-quark pairs channel, or boosted hadronically-decaying W and Z bosons. So how can we detect these merged jets over a giant background? That’s what the study of boosted physics is all about.

Next: Finding boosted objects using jet “mass” and looking for jet substructure

Next next: Pileup at the LHC….a jet measurement nightmare.

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On Tuesday, CMS and ATLAS submitted their papers on their observation of a new boson to the journal Physics Letters B. These are surely the most significant publications of the LHC experiments to date, and, without airing too much internal laundry, you can imagine that the content and the phrasing of the papers was very thoroughly discussed within the collaborations. Within CMS, the length of all the comments submitted during collaboration review was longer than the paper itself. You will also notice that CMS and ATLAS came up with slightly different titles; one says that a boson was observed, the other says that a particle (spin unspecified) was observed in a search for the Higgs boson. And for sure neither one says that what is observed is the Higgs boson; as has been discussed in many other posts, we’re very far away from being able to make any confident statements about that.

We can expect that these papers will soon be accepted for publication (in fact, sooner than you might think), and then go on to be fixtures of the scientific literature of particle physics, cited many times over in future papers. Which got me thinking — what are the most highly cited papers in particle physics, and where might the “Higgs” observation papers end up in that list? (Note how he takes pains to put “Higgs” in quotation marks!)

Now, you’ve heard me sing the praises of the Particle Data Group before, but now let me put in a word for the people at INSPIRE, which has recently succeeded SLAC’s SPIRES database as the repository of publication information in our field. I wouldn’t be able to put my CV together or brag about my crazy-big h-index without them. Not only do they track every paper by author, they also keep track of paper citations. How often a paper is cited is a measure of the impact of the paper on the field.

It’s not hard to generate a list of the most cited papers tracked by INSPIRE. And the results may surprise you! A few observations:

  1. The most cited papers are theory papers, not papers that describe measurements. The number one paper, with 8414 citations, is by Juan Maldacena, describing a major breakthrough in string theory. (Don’t ask me to explain it, though!) This paper is only 14 years old. Number two, at 7820, is Steven Weinberg’s paper that was among the first to lay out the electroweak theory. It’s from 1967, predating the Maldacena paper by more than thirty years. And number three, at 6784, is by Kobayashi and Maskawa, explaining how a third generation of quarks could straightforwardly accommodate the phenomenon of CP violation; it’s from 1973.
  2. That famous paper by Peter Higgs? Only #95, with 2043 citations.
  3. The first experimental paper that shows up, at #4, is actually an astrophysics paper, the first results from the WMAP satellite, which among other things really nailed down the age of the universe for the first time. There are in fact many highly-cited papers on experimental results on cosmology. This is of course partly a function of the kind of papers that INSPIRE tracks.
  4. The first experimental papers that show up are actually compendia of results, from the PDG. They release a new review every two years, so many of them are on the list.
  5. The most-cited paper on a single experimental measurement is at #27, with 3769 citations. It’s the Super-Kamiokande paper from 1998 that showed the first evidence of the oscillation of atmospheric neutrinos.

So while it’s true that these observation papers will be among the most highly cited from the LHC experiments, the evidence already suggests that they will be pikers compared to many other publications in the literature. (So was it worth all that effort on what the title should be?) It will be interesting to watch…if nothing else, it will surely be one of the most cited papers that I am an author on, and it is definitely an achievement that we can be proud of.

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from PhD to postdoc

Tuesday, July 31st, 2012

Hello!  I’ll probably write more technical posts later, but since I’m a new US LHC blogger, I thought I would spend this first post talking a little about my background and how I decided what kind of postdoc position to look for.  Lately, quite a few of my friends in their last year of graduate school have been asking about the latter. 😉

I’ve been with Columbia University for almost a year now, having defended my PhD thesis at the University of Massachusetts, Amherst on June 10, 2011.  I knew I wanted to stay in the field after graduating, if only for the fact that it would have been a shame to look at only 42 pb-1 of data, see no new physics, and miss out on being around for (what eventually became!) the biggest particle physics discovery in decades. Just to give you an idea of how much data I had for my thesis compared to what we have now, see this histogram showing the integrated luminosity recorded by ATLAS in 2010, 2011 and so far in 2012:

I was that green line.

Before applying for postdoc positions, I needed to decide what kind of research to do in the next stage of my career, and where I would want to do it. Almost all the work I had done as a graduate student was related to the muon spectrometer on ATLAS; from helping in the installation and commissioning of the muon precision chambers, to muon reconstruction performance studies, to measuring the first Z→μμ cross section at sqrt(s)=7 TeV and finally performing a search for new physics in the high-mass tail of the mu-mu invariant mass spectrum. Muons were my thing.

The advice I got from most of my colleagues at the time, including my adviser, was to switch experiments. The reasoning made sense. If you stay with the same experiment for your postdoc, you miss out on a free pass to do research on something completely new. It’s a rare opportunity to start from scratch while still having some allowance for time to catch up.

But that was the thing….most of the people I was seeking advice from had come from other experiments to the LHC, not the other way around. In fact, I was one of the first US students able to write a thesis on LHC data (the delay partly due to the incident in 2008….let’s not talk about that). So where could I have gone from here? If I wanted to stay in collider physics, I needed to stay at the LHC.

Knowing I wanted to come back to CERN, it also took some time to figure out exactly what kind of analysis I wanted to work on after the PhD. I talked to a lot of people that semester, asking who would be working on what and getting lots of advice. I certainly had many interesting options for research, but it wasn’t until I was sitting in a talk about the evidence for forward-backward asymmetry of the top quark when I thought now hey, top physics…

In the end, I decided to make as big of a switch as possible while still staying on ATLAS, moving from the muon spectrometer and dimuon analyses to work on top quark physics and jets at an institute responsible for the liquid argon calorimeter electronics. The move seemed to cover the best of all possible scenarios…I didn’t need to worry about the year-long wait to qualify for authorship or to figure out ATLAS software, but I did get the opportunity to learn something ultimately different when it came to hardware work and physics analysis. However, because of the size of the collaboration, where each subdetector community has roughly the same number of people as one Tevatron experiment, it took some time to get enough exposure to be recognized for the new work I was doing. That will be the case whenever you start a new job, no matter what.

Even more difficult was going from feeling like an expert in my thesis topic to suddenly being thrown in the deep end of a new topic amongst other experts. I found I wasn’t the only one who experienced that.  Before I began, a few senior postdoc friends of mine who wrote their PhDs at the Tevatron said that their first year at the LHC felt just like being a brand new graduate student all over again and that it was hard to feel like anything really substantial had gotten done during that time, just because there was the additional learning curve thrown in. When I looked a little sad, one of them said “well for you, since you’re staying on ATLAS…maybe only 6 months.”

My advice to anyone wrapping up their graduate studies and thinking about getting a postdoc would be to talk to as many people as possible and get as many opinions as possible. My experience is just one of many! I can say though that the more I knew going in, the easier the transition was, and now one year later everything is going really smoothly.

Anyway, have a look at my upcoming posts, where I’ll talk about jet substructure, new physics searches involving the top quark, and whatever other cool beyond-the-Higgs stuff is happening at CERN.

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We’ve all heard the big news from CERN by now (if not then you might want to catch up on the latest gossip!) Right now most of the focus at ATLAS and CMS is on measuring the properties of the new boson we’ve found. The numbers of events are small, so studies are very difficult. One of the most important properties that we need to study is the particle’s spin, and luckily we can say something about that right now!

A typical Higgs boson candidate in the "golden mode" (ATLAS Collaboration)

The big news: One of many Higgs boson candidates in the "golden mode" (ATLAS Collaboration)

There are two ways to study the spin of this boson, the hard way and the easy way. The hard way involved looking at angles between the final state particles and that’s tricky, but it can be done with the existing data. This method is hard because we have to model both signal and background to get it right. The easy way is to look at the decays of the boson and see which ones happen and which ones don’t. We need a little more data to do this, but we can perform this study by the end the data taking for the year. Richard has already discussed the “hard” method, so I’m going to show the “easy” method. It comes with nice pictures, but there are a few subtleties.

I want to consider four decays: a decay to two photons, a decay to two \(Z\) bosons (the same applies to two \(W\) bosons), a decay to two \(\tau\) leptons, and a decay to two \(b\) quarks. All of these decay modes should be seen by both experiments if what we have seen is the Standard Model Higgs boson.

We need to label our particles properly and describe them a little before we begin. We can never measure the spin of a particle exactly, and the best we can do is measure its total spin, and its projection along a certain axis. The spin along the other two axes remains a mystery, because as soon as we measure its spin along one axis, the other two components of spin become indeterminate. That’s quantum mechanics for you! A component of spin can be increased or decreased with “raising” and “lowering” operators, and the change is always in natural units of 1. (This is just a result of the universe having three spatial dimensions, so if the answer was any different then the universe would look very different!)

Let’s take the electron and work out what spin states it can have. The electron’s total spin has been measured to be 1/2, so we need to project this spin onto an axis and find out the allowed values. A little thought shows that there are only two states that can exist: spin +1/2 and spin -1/2 (which we call “spin up” and “spin down”.) The \(J/\psi\) meson has spin 1, so it’s allowed states are +1, 0, -1. When the \(J/\psi\) is in state spin 0 what really mean is that it has “hidden” its spin at 90 degrees to the axis, so it’s total spin is still 1 and its projection along our chosen axis is 0.

So let’s get on with the job of considering the spins of all these other particles. The photon is a massless boson with spin 1, and it can only arrange its spin transversely (for obscure reasons that Flip explains very well), so it can’t hide its spin when it projects along an axis. That means that it can only have spin of +1 and -1. (There’s one more particle we’re going to use in these arguments, and that’s the gluon. The gluon is the same as the photon, except it interacts with a different field, so like the photon it can only have spin states +1 and -1):

The spin projections of the photon

The spin projections of the photon

The \(Z\) and \(W\) bosons are similar, except they have mass, so they have the luxury of hiding their spin. This means that they can have spin -1, 0, and 1, just like the \(J/\psi\) did:

Spin projections of the massive boson

Spin projections of the massive boson

Both the \(b\) quark and \(\tau\) lepton are fermions, which means that they have spin 1/2. We already know what spin states are allowed for fermions, spin up and spin down:

Spin projections of fermions

Spin projections of fermions

Now that we know the spin states of all these particles we can just add them up and confirm or refute which spin our new boson has. Let’s see how we can get spin 0:

Possible decays of a spin 0 particle

Possible decays of a spin 0 particle

It looks like we can a spin 0 particle by combining any of our particles.

Let’s try spin 1:

Possible decays of a spin 1 particle

Possible decays of a spin 1 particle

Uh-oh, it looks like we can’t make a spin 1 particle from photons! To align the spins correctly the photons must be in an antisymmetric state, which is absolutely forbidden by Bose-Einstein statistics. (Incidentally the term “boson” comes from the name Bose.) That means that this new boson is definitely not spin 1, because we see it decay to two photons.

So that means we have to do things the hard way to measure the spin of this new particle. For those who are interested, one of the main challenges presented here comes from the “acceptance” of the detectors- the kinematics of the final states we observe are significantly biased by the geometry of the detector. Even for a spin-0 boson, which decays isotropically, the distributions of the final decay products in the detector will not be isotropic, because the detectors do not have completely hermetic coverage. Fortunately since this post was first written we’ve gathered more data, and detailed studies have been performed eliminating all but the spin 0 hypothesis with a positive parity, indicating that what we have seen is most likely the long sought Standard Model Higgs boson after all.

Errata: In the original post I incorrectly made an argument stating that the decay of a spin 2 boson to a pair of quarks would be significantly more probable than the decay to a pair of leprons. Following discussions with Frank Close and Bob Cousins it was pointed out that well established graviton models would give a tensor interaction that would decay to leptons roughly 2% of the time per lepton flavour, making these final states accessible to the LHC experiments, and likely before the dijet final states would be accessible. My thanks go out to Close and Cousins for their correction.

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Versão em Português abaixo…

Passed the crazy week of the Higgs finding, or, if we are to keep the complete scientific correctness, the weird different particle which is most likely the one that we have been searching for “just” 30 years, well, after that I feel like it is time to explain the different pieces that contributed to such historic finding. The whole thing depends, as is often said, in a number of different factors which we will never be able to put in a few pages of a blog or anything of the sort. Still, I’d like to urge you to hang on a bit and hopefully, you will find as I do, lots of interesting little details. I will try to make a little weekly series that should tell the history of the parts of the ATLAS detector which I happen to be closer to : The ATLAS Calorimeter and Trigger systems.

Let’s start right at the moment when the huge energy accumulated by a proton (this little system of 3 massive particles called quarks and nobody knows how many gluons) is concentrated in a very small volume of space. This energy, following the famous E=mc2, crystalizes in the form of different types of particles. Many of the collisions happen at the so-called parton (a quark or a gluon) level. That means that most likely a shower of particles called a jet will come out of the collision. Well, actually, two jets will usually be produced (things must always balance!). Very rarely, however, other processes take the role and make something completely different. For instance, sometimes, they will produce a Z or W boson. Both are, in the kingdom of particles that we call the Standard Model, what we could call the heavy weights, having lots of mass (one can say more than 80 GeV – Giga-electron-Volts). These guys, when formed, have a very short life (around 3×10-25s), but don’t waste your time thinking on how many zeros do you have to write, just keep in mind that no Z’s or W’s will ever leave the small pipes that bring the protons into collision. Much before that happens, a Z, for instance, will decay, producing a pair of particles that take the energy unfrozen (if you want) in their mass as speed. So, we talk about Z->ee. The Z particle has a mass around 91 GeV and the electrons will have, on average, half of that in “speed energy”. One interesting thing that is always present in physics (check my other colleagues in this blog) is that many properties must be conserved. For instance, the Z particle has no electric charge, but the electron has a negative charge. So, actually, what we get is not a pair of electrons, but, rather, an electron-positron pair, the positron being the positive charged version of the electron, or as we call the electron antiparticle. So, if I wanted to be more rigorous, I should have written Z->e+ e, meaning that a zero charged particle results in a positive and negative charges : the sum is zero again!

The particles (and the anti-particles!) will invade the detector coming from the center (the beam pipe) crossing layers of detectors in the way and will finish their journey in the calorimeters. These devices were developed during many years and now, only in one of the ATLAS calorimeters, we are around 300 people working together!!. For now I will stop here. In the next week, I will explain what happens when each of the electrons enter in the calorimeter and how we use this information to detect the electron and make physics out of it!

To give you a quick taste of what is to come, I call your attention to two videos available in youtube. In the first one, you see the chain of accelerators with increasing size and proton energy. When we get to the LHC, the image zooms inside the tunnel and you will see the equations of the Standard Model of particles in the walls (like we would do that..) The proton will cross the French/Switzerland border in a complete illegal form (no passports!!!) and you will see the colored quarks inside the proton until they meet inside the detector. In the second collision, you will see the Z->ee event. After the collision, the software marks the two blue tracks left be the electron-position pair in the tracking detector and “illuminates” a few of the calorimeter cells represented in green in the movie. We will discuss what happens and how we can see these cells in the next postings. And, later on, you will understand the relation between detecting a Z and detecting a Higgs…
First video : http://www.youtube.com/watch?v=NhXMXiXOWAA
Second video : http://www.youtube.com/watch?v=RdYvtm4CIAE

Portugaise version :

Como Funciona um Detetor de Partículas!!

Passada a semana louca da descoberta do Higgs, ou se quisermos manter a imparcialidade e a retidão científica, a estranha e diferente partícula que muito provavelmente é aquela que estivemos procurando por “apenas” 30 anos, penso que é hora de explicar um pouco todas as peças que contribuíram para essa descoberta histórica. A coisa toda depende numa multitude de fatores os quais nunca poderemos colocar em algumas páginas de um blog. Ainda assim, peço que vocês agüentem firme e, quem sabe, vocês encontraram o mesmo prazer que eu em compreender os pequenos detalhes que fazem o sucesso dessa incrível experiência. Tentarei manter um fluxo de episódios semanais explicando como funciona a parte do ATLAS que conheço mais de perto : O Calorímetro e o Sistema de seleção do ATLAS.

Comecemos exatamente no momento em que a enorme energia acumulada por um próton (esse pequeno sistema de três partículas massivas e não sabemos quantos glúons) se concentra num pequeno volume de espaço. Seguindo o famoso E=mc2, essa energia “se cristaliza” na forma de diferentes tipos de partícula. A maior parte das colisões ocorre entre partons (quarks ou glúons), resultando numa cascata de diferentes partículas, à qual damos o nome de “jato”. Normalmente, como a experiência tem um certo balanço a respeitar, temos dois jatos sendo produzidos com energias bastante similares. Muito raramente, entretanto, outros processos acontecem e algo completamente diferente pode surgir. Por exemplo, algumas vezes, tais processos podem produzir um bóson Z ou W. Ambos são, no reinado das partículas que chamamos de Modelo Padrão, o que podemos chamar de Pesos Pesados (pode-se dizer falar de uma massa maior que 80 GeV – Giga-elétron-Volts). Tais partículas têm uma vida muito curta de 3×10-25s, mas nem perca tempo pensando em quantos zeros se deve colocar depois da virgula. Saiba apenas que um Z formado não chega jamais a tocar o tubo que traz os prótons até o ponto de colisão. Um Z decai, produzindo, um par de partículas que levam a energia contida na massa do Z. Assim, falamos de Z->ee. Como o Z tem uma massa próxima a 91 GeV, os elétrons vão carregar média metade desse valor em “energia do movimento”. Outra coisa interessante (pesquise um pouco os artigos de meus colegas nesse blog) e que é sempre importante em física é que muitas quantidades devem ser conservadas. Assim, como o Z não tem carga elétrica e o elétron tem uma carga negativa, um dos elétrons é, na verdade, um pósitron, a anti-partícula do elétron com carga positiva. Assim, o Z sem carga resulta em uma carga positiva e uma negativa : a soma é zero! Se eu quiser ser realmente rigoroso, tenho que escrever Z->e+ e.

As partículas (e as anti-partículas!) invadem o detetor vindo do centro (onde está o tubo com os feixes) atravessando camadas de detetores e terminando sua viagem nos calorímetros. Esses aparelhos foram desenvolvidos em muitos anos de estudo e, hoje em dia, apenas um dos calorímetros do ATLAS ainda precisa de 300 pessoas trabalhando continuamente!! Por agora, eu vou parar por aqui. Na próxima semana, vou explicar o que acontece quando cada um dos elétrons entra no calorímetro e como usamos essa informação para detectar o elétron e “fazer física”!

Para dar um gostinho do que está por vir, gostaria de chamar atenção de vocês pra dois vídeos disponíveis no youtube. No primeiro, vocês podem ver toda a seqüência de aceleradores com tamanho e energia cada vez maiores. Quando chegamos no LHC, a imagem entra no túnel, em cuja parede, podemos ver as equações do Modelo Padrão de partículas (como se fosse verdade!). O próton que seguimos atravessa “ilegalmente” (alguém já viu um próton com passaporte?!) a fronteira da França com a Suíça e vocês podem ver os quarks viajando dentro do próton até a colisão dentro do detetor. Na segunda colisão, vocês podem ver um evento Z->ee se formando. Depois da colisão, o programa identifica os traços deixados pelo par elétron-pósitron no detetor de traços com linhas azuis. O par também “ilumina” algumas células do calorímetro representadas em verde no filme. Vamos discutir na semana que vem o que acontece e como podemos ver essas células no próximo blog… E, mais tarde, vamos entender qual a diferença entre detectar um Z e um Higgs…

Primeiro vídeo : http://www.youtube.com/watch?v=NhXMXiXOWAA
Segundo vídeo : http://www.youtube.com/watch?v=RdYvtm4CIAE
Canal ATLAS/Brasil : http://webcast.web.cern.ch/webcast/play.php?type=permanent&event=12

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Suite aux récents résultats du LHC concernant le boson de Higgs , Jacques Martino, Directeur de l’Institut national de physique nucléaire et de physique des particules du CNRS, adresse ses félicitations aux personnels de l’Institut.

Depuis le CERN ou par webcast depuis les laboratoires, les personnels de l'Institut national de physique nucléaire et de physique des particules du CNRS ont été nombreux à suivre en direct le séminaire LHC du 4 juillet 2012. Image : CERN

« Nous avons vécu, mercredi dernier, avec l’annonce de la découverte d’un boson à très forte saveur Higgs, une “folle” journée où l’ensemble de l’Institut a été récompensé d’un effort de recherche qui s’est étalé sur une bonne vingtaine d’années, et qui je l’espère va continuer et nous apporter d’autres découvertes.
Je souhaite que tout l’Institut se sente félicité et honoré par cette découverte ; bien sûr tous ceux qui ont travaillé directement sur cette recherche, mais aussi tous ceux qui ont rendu possible la participation de l’IN2P3 à cette découverte : les chercheurs, mais aussi les ingénieurs, techniciens et administratifs impliqués sur ou autour d’Atlas, CMS, du LHC et ses accélérateurs, du centre de calcul… Mais aussi tous les agents, dans tous nos labos, qui ont contribué à rendre cet effort possible, et fructueux.
Je souhaite ici associer toutes les autres disciplines de l’Institut : nous sommes dans un même bateau, notre recherche est avant tout “subatomique”, et le résultat majeur obtenu aujourd’hui par la physique des particules doit et va tous nous “booster”.
Nous sommes tous heureux et fiers que l’IN2P3 y ait participé, et que notre organisation ait permis d’y apporter une contribution très significative. Faut-il ici rappeler que cette organisation en réseau est, entre autre, celle qui a permis de coordonner nos efforts ? C’est une plus-value significative sans laquelle ni le CNRS, ni les Universités n’auraient pu avoir une place si visible. C’est aussi notre organisation qui nous a permis une excellente coordination avec nos collègues de l’Irfu, que j’associe à ce message.

Les résultats dévoilés mercredi dernier constituent un moment historique de la physique des particules. L’IN2P3, grâce à ses chercheurs, ingénieurs, techniciens et administratifs, a su être présent dès le début et faire les choix pertinents lors de la conception, de la construction, des analyses tant dans Atlas que CMS, choix qui nous ont donné une position très visible et reconnue.
Nous sommes tous fiers que ces investissements humains et financiers aient permis à nos chercheurs de jouer un rôle leader dans la découverte du “Higgs”. Le LHC n’est clairement qu’au début de son histoire, et nous ne doutons pas que d’autres résultats de grande qualité sont encore à venir dans Atlas et CMS, mais aussi LHCb et Alice.
Je voudrais terminer en vous rapportant un mail de félicitations que j’ai reçu de la part d’un ami médecin : il nous remercie pour cette journée où l’IN2P3 lui a permis de rêver. Oui, en effet, le progrès des connaissances, dans chacune de nos disciplines, porte une part de rêve, d’enchantement qui sont aussi un fort soutien, voire un moteur, à nos actions. Et si en plus c’est partagé au-delà de notre discipline, je crois que notre raison d’être et de travailler en est confortée. Continuons à faire progresser les connaissances de notre domaine, et poursuivons les activités de recherches plus appliquées d’ores et déjà entreprises : ceci doit être notre double leitmotiv.
Je terminerai en vous rappelant que lors de notre conférence de presse, avec le CEA, à Paris, nous avons reçu un appel téléphonique de notre Ministre, Madame Fioraso, dont les mots de félicitations sont sur notre page Web IN2P3. S’il est aujourd’hui encore bien tôt pour en tirer quelques certitudes quant à nos budgets à venir, il va sans dire qu’un tel intérêt ne peut être lu que positivement.

Bravo encore et félicitations à tous. »

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Following the Higgs seminar on Wednesday July 4th (Higgsdependence Day), fellow bloggers Steve Sekula and Seth Zenz joined me to discuss the results. We discussed all sorts of topics from the measurements themselves, to the nature of the work, to the future of the study of the Higgs boson. Enjoy!

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