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Posts Tagged ‘Higgs’

Theoretically, the Higgs boson solves a lot of problems. Theoretically, this Higgs boson is a problem.

Greetings from the good ol’ U.S. of A.

Now that Fall is here, classes are going, holidays are wrapping up, and research programs are in full steam. Unfortunately, all is not well in the Wonderful World of Physics. To refresh, back on 4th of July, the LHC experiments announced the outstanding and historical discovery of a new particle with properties consistent with the Standard Model Higgs boson. No doubt, this is a fantastic feat by the experiments, a triumph and culmination of a decades-long endeavor. However, there is deep concern about the existence of a 125 GeV Higgs boson. Being roughly 130 times the proton’s mass, this Higgs boson is too light. A full and formal calculation of the Higgs boson’s mass, according to the theory that predicts it, places the Higgs mass pretty close to infinity. Obviously, the Higgs boson’s mass is less than infinite. So let’s talk mass and why this is still a very good thing for particle physics.

For an introduction to the Higgs boson, click here, here, or here (This last one is pretty good).

The Standard Higgs

The Standard Model of Particle Physics (SM) is the theory that describes, well, everything with the exception of gravity (Yes, this is admittedly a pretty big exception).  It may sound pompous and arrogant, but the SM really does a good job at explaining how things work: things like the lights in your kitchen, or smoke detectors, or the sun.

Though if this “theory of almost-everything” can do all this, then when written out explicitly, it must be pretty big, right? Yes. The answer is yes. Undeniably, yes. When written out fully and explicitly, the “Lagrangian of the Standard Model” looks like this (click to enlarge):

Figure 1: The Standard Model Lagrangian in the Feynman Gauge. Credit: T.D. Gutierrez

This rather infamous and impressive piece of work is by Prof. Thomas Gutierrez of Cal Poly SLO. Today, however, we only care about two terms (look for the red circles):

Figure 2: The Standard Model Lagrangian in the Feynman Gauge with the Higgs boson tree-level mass and 4-Higgs vertex interactions terms circles. Original Credit: T.D. Gutierrez

The first term is pretty straightforward. It expresses the fact that the Higgs boson has a mass, and this can represented by the Feynman diagram in Fig 3. (below). As simple and uneventful as this line may appear, its existence has a profound impact on the properties of the Higgs boson. For example, because of its mass, the Higgs boson can never travel at the speed of light; this is the complete opposite for the massless photon, which can only travel at the speed of light. The existence of the diagram if Fig. 3 also tells us exactly how a Higgs boson (denoted by h) travels from one place in the Universe, let’s call is x, to another place in the Universe, let’s call it y. Armed with this information, and a few other details, we can calculate the probability that a Higgs boson will travel from point x to point y, or if it will decay at some point in between.

Figure 3: The tree-level Feynman diagram the represents a SM Higgs boson (h) propagating from a point x in the Universe to a point y somewhere else in the Universe. Credit: Mine

The second term is an interesting little fella. It expresses the way the Higgs boson can interact with other Higgs bosons, or even itself. The Feynman diagram associated with this second term is in Fig. 4. It implies that there is a probability a Higgs boson (at position w) and a second Higgs boson (at position x) can collide into each other at some point in the Universe, annihilate, and then produce two Higgs bosons (at point z and y). To recap: two Higgses go in, two Higgses go out.

Figure 4: The tree-level Feynman diagram the represents two SM Higgs bosons (h) at points w and x in the Universe annihilating and producing two new SM Higgs bosons at points z and y somewhere else in the Universe. Credit: Mine

This next step may seem a little out-of-the-blue and unmotivated, but let’s suppose that one of the incoming Higgs bosons was also one of the outgoing Higgs bosons. This is equivalent to supposing that w was equal to z. The Feynman diagram would look like Fig. 5 (below).

Figure 5: By making an incoming Higgs boson (h) the same as an outgoing Higgs boson in the 4-Higgs interaction term, we can transform the tree-level 4-Higgs interaction term into the 1-loop level correction to the Fig. 1, the diagram the represents the propagation of a Higgs boson in the Universe. Credit: Mine

In words, this “new” diagram states that as a Higgs boson (h) at position x travels to position y, it will emit and absorb a second Higgs boson somewhere in between x and y. Yes, the Higgs boson can and will emit and absorb a second Higgs boson.

If you look carefully, this new diagram has the same starting point and ending point at our first diagram in Fig. 3, the one that described the a Higgs boson traveling from position x to position y. According to the well-tested rules of quantum mechanics, if two diagrams have the same starting and ending conditions, then both diagrams contribute to all the same processes and both must be included in any calculation that has the same stating and ending points. In terms of Feynman diagrams, if we want to talk about a Higgs boson traveling from point x to point y, then we need to look no further than Fig. 6.


Figure 6: The tree-level (L) and 1-loop level (R) contributions to a Higgs boson (h) traveling from point x to point y. Credit: Mine

What Does This All Mean?

Now that I am done building things up, let me quickly get to the point. The second diagram can be considered a “correction” to the first diagram. The first diagram is present because the Higgs boson is allowed to have mass (mH). In a very real sense, the second diagram is a correction to the Higgs boson’s mass. In a single equation, the two diagrams in Fig. 6 imply

Equation 1: The theoretical prediction for the SM Higgs boson's observed mass, which includes the "tree-level" contribution ("free parameter"), and 1-loop level contribution ("cutoff"). Credit: Mine

In Eq. (1), term on the far left is the Higgs boson’s mass that has been experimentally measured, i.e., 125 GeV. Hence the label, “what we measure.” The term just right of that (the “free parameter”) is the mass of the Higgs boson associated with the first term in the SM Lagrangian (Fig. 2 and 3). When physicists talk about the Standard Model not predicting the mass of the Higgs boson, it is this term (the free parameter) that we talk about. The SM makes no mention as to what it should be. We have to get down, dirty, and actually conduct an experiment get the thing. The term on the far right can be ignored. The term “Λ” (the “cutoff scale“), on the other hand, terrifies and mystifies particle physicists.

Λ is called the “cutoff scale” of the SM. Physically, it represents the energy at which the SM stops working. I mean it: we stop calculating things when we get to energies equal to Λ. Experimentally, Λ is at least a few hundred times the mass of the proton. If Λ is very LARGE, like several times larger than the LHC’s energy range, then the observed Higgs mass gets an equally LARGE bump. For example, if the SM were 100% correct for all energies, then Λ would be infinity. If this were true, then

(the Higgs boson’s mass) = (something not infinity) + (something infinity) ,

which comes out inevitably to be infinity. In other words, if the Standard Model of Physics were 100% correct, then the Higgs boson’s mass is predicted to be infinity. The Higgs boson is not infinity, obviously, and therefore the Standard Model is not 100%. Therefore, the existence of the Higgs boson is proof that there must be new physics somewhere. “Where and at what energy?,” is a whole different question and rightfully deserves its own post.


Happy Colliding

– Richard (@bravelittlemuon)


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)


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.


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
“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!


A snapshot of the recent media coverage on the recently discovered Higgs-like particle is now online as part of the Fermilab Today archives. View television and newspaper coverage of the Tevatron results, opinion pieces on CERN’s particle discovery and photos of groups around the world who watched the CERN seminar broadcast live. See these and other Higgs media highlights.


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.


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.


Dark matter: No model, just guesses

Wednesday, July 11th, 2012

On the last day of the International Conference on High Energy Physics dark matter took a central seat.

As many of you know, ourselves, the earth, all stars and galaxies are made of atoms. These atoms emit light when they are excited and that is how astronomers can explore the vast universe. But this matter only accounts for 4% of the content of the universe while dark matter makes up 24% of it. An unknown type of energy dubbed “dark energy” makes up the remaining 76%.

Dark matter was discovered in 1933 by Swiss physicist Fritz Zwicky. But to this day, scientists still don’t know what it is made of. This matter emits no light, which is why it was called “dark matter”.

Dark matter seems to react only to gravitational force and this is how it was discovered. Zwicky realized there was more matter in the universe than what was visible from the light emitted by stars and galaxies. This matter creates a much stronger gravitational field than what can be accounted for if you only rely on visible matter.

Neal Weiner, a theorist from New York University, started his lecture saying that contrary to the Higgs boson, for dark matter “we have no model, only guesses”. There is nothing within the Standard Model of particle physics to account for dark matter. This is one key reason we physicists are all convinced there is a bigger theory hiding behind the current known one.

So theorists and experimentalists are in the dark… As Neal stressed, there are many manifestations of dark matter. Different experiments observe strange signals where dark matter could be the explanation. But formulating an explanation is far from being trivial.

For example, several experiments have reported seeing more positrons than electrons coming from outer space. Positrons are the antimatter for electrons. Recently, the Pamela and the Fermi experiments both saw an excess of positrons, particularly at high energy. Given that the universe is made of matter, one needs to explain where these anti-electrons come from.

Some astronomers think it could be produced by pulsars but the jury is still out on this. Others argue that dark matter could annihilate into a pair of electron and positron, creating more positrons than expected. But it is not easy to cook up a theory that would do that. Hopefully, new data will come in 2013 from the Planck satellite to resolve this issue.

The DAMA/Libra experiment has been reporting a loud and clear signal (8.7 sigma) from dark matter for years. Unfortunately, nobody else can detect this signal as Lauren Hsu from Fermilab explained in her review of dark matter experiments. One possibility is that their detector, which is made of iodine, is sensitive to dark matter particles but other chemical elements used by the other experiments were not. Two new experiments were built using iodine, COUPP and KIMS, and should soon have enough data to get the final word on this long-standing anomaly.

Dark matter might interact with the Higgs boson. If that’s the case, now that we have a mass for it, we can test specific hypotheses. The XENON100 experiment is just at the limit of sensitivity for this and new results will come soon.

This is a huge, open question in particle physics. Let’s hope the new (Higgs?) boson discovery will soon be followed by some clues on the nature of dark matter. Exciting times ahead.

Pauline Gagnon

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Lors de la dernière journée de la Conférence Internationale sur la Physique des Hautes Energies, on a fait le point sur la matière noire. Comme plusieurs d’entre vous le savent, nous sommes tous: nous-mêmes, la terre, les étoiles et les galaxies faits d’atomes. Ces atomes émettent de la lumière lorsqu’ils sont excités, ce qui permet aux astronomes d’étudier l’univers. Mais toute cette matière ne compte que pour 4% du contenu total de l’univers alors que la matière noire en fait 24%. Les 76% restant viennent d’énergie d’un type inconnu surnommée « énergie noire. »

La matière noire fut découverte en 1933 par le physicien suisse Fritz Zwicky. Mais on ignore toujours de quoi il s’agit. Cette matière n’émet aucune lumière, d’où son nom.

La matière noire semble jusqu’à maintenant ne réagir qu’à la force gravitationnelle et c’est ce qui a permis de la déceler. Zwicky constata qu’il y avait plus de matière dans l’univers que ce qu’il voyait basé sur la lumière émise par les étoiles et les galaxies. Cette matière crée un champ gravitationnel bien plus fort que ce que la matière visible peut engendrer.

Neal Weiner, un théoricien de l’université de New York, a ouvert sa présentation en disant que, contrairement au boson de Higgs, pour la matière noire « on n’a aucun modèle, que des hypothèses ». Il n’y a rien dans le Modèle Standard de la physique des particules qui la décrive. C’est d’ailleurs un point clé indiquant clairement que le modèle standard a ses limites, et qu’une autre théorie plus globale devra le remplacer.

Les théoriciens et les expérimentatrices sont donc tous dans le noir… Come Neal l’a souligné, il y a déjà plusieurs manifestations de cette matière noire. Plusieurs expériences observent d’étranges signaux qui pourraient s’expliquer en termes de particules de matière noire. Mais formuler la bonne explication s’avère compliqué.

Par exemple, plusieurs expériences mesurent un excès de positons par rapport au nombre d’électrons observés venant du cosmos. Les positons sont l’antimatière des électrons. Récemment, les satellites Pamela et Fermi ont mesuré que cet excès est plus prononcé à haute énergie. Mais comme l’univers est fait de matière, d’où viennent ces positons?

Certains astronomes pensent qu’ils pourraient provenir de pulsars mais cela reste à prouver, ce qui est difficile. D’autres proposent plutôt qu’ils émanent de l’annihilation de particules de matière noire en une paire d’électron et positon.

Mais encore là, ce n’est pas facile à justifier théoriquement. Espérons que les nouvelles données attendues en 2013 par le satellite Planck aidera à résoudre ce problème.

Et puis il y a l’expérience DAMA/Libra qui clame depuis des années avoir obtenu un signal très fort (8.7 sigma). Le seul hic est que personne d’autre ne le capte comme l’a expliqué Lauren Hsu de Fermilab dans sa revue des résultats expérimentaux. Il est possible que les autres détecteurs n’y soient pas sensibles puisque seul DAMA/Libra utilisait un détecteur à l’iode. Deux nouvelles expériences COUPP et KIMS sont maintenant en cours ayant elles aussi de l’iode comme détecteur. Elles devraient avoir bientôt suffisamment de données pour trancher la question.

Autre possibilité: la matière noire interagit peut-être avec le boson de Higgs. Maintenant qu’on en connaît la masse, il se pourrait que l’expérience XENON100 puisse bientôt atteindre la sensibilité nécessaire pour tester cette hypothèse.

C’est donc une énorme question encore ouverte en physique des particules.

Peut-être que le nouveau boson (de Higgs?) apportera quelques indices qui nous permettront d’en apprendre plus.  Ça promet.

Pauline Gagnon

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Everything must fit nicely together

Tuesday, July 10th, 2012

Yesterday, at the International Conference on High Energy Physics in Melbourne we heard three presentations from ATLAS, CMS and the Tevatron experiments, (D0 and CDF) on the Higgs boson searches. It was great to see how all these results are consistent with each other: between the four experiments, the two accelerators operating at different energies and the six independent decay channels. All give the same picture: we have really found a new boson.

Everybody’s attention is now turned towards establishing the exact identity of this boson. Is it the one predicted by the Standard Model or one of the five Higgs bosons associated with supersymmetry, another theory that attempts to fix the few remaining problems of the Standard Model.

Although the theory was unable to predict the exact mass of the Higgs boson, it provided strong constraints on where it could be found. Many quantities are interconnected by the equations of the Standard Model. This is why we keep improving the uncertainty margin on these quantities. Putting all this information together allows us to check the consistency of the model.

This has been the highlight of many conferences for a decade or two. Each new update showed how much progress had been accomplished when all the measurements were combined in a complex algorithm designed to test the so-called “electroweak” part of the Standard Model all in one go. This is very similar to checking the stability of a very elaborate mobile after modifying each of its components slightly.

Yesterday, for the first time, we saw what the newly measured mass of what is most-likely a Higgs boson adds to this global picture.

The vertical axis shows the measured mass of the W boson and the horizontal axis, the mass of the heaviest quark, the top quark. The blue ellipse is centered on the measured values of these two masses. The ellipse gives the error margin. There is a narrow blue band below the large green band. This represents the actual measured mass of the Higgs boson announced on July 4th, the width being its uncertainty. So as it stands, given the overlap, there is agreement, at least within errors.

The black ellipse is a projection of what this picture will look like once the LHC experiments reduce the uncertainty on the W mass from the current 15 MeV to only 5 MeV. If all is consistent within the Standard Model, the black ellipse will have to overlap with the narrow blue band indicating the Higgs boson mass. If the central value of the W mass does not change, then there will be some inconsistency with the Standard Model (the very narrow blue strip). On the other hand, if supersymmetry or SUSY is the real, more global theory of nature, the green area gives the mass values allowed for the W and top quark. MSSM stands for Minimal Supersymmetric Model and is just one specific model within the vast SUSY space.

This plot might reassure a few: there is still plenty of room for supersymmetry. This theory is far from being dead. But as someone commented: “The huge number of SUSY presentations at this conference was inversely proportional to the number of evidence for it!”

The bets are still open on what will come next. Is this Higgs boson the one predicted by the Standard Model, supersymmetry or some other version? Patience is in order but the answer will eventually come.

Pauline Gagnon

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Hier, lors de la Conférence Internationale de la Physique des Hautes Énergies à Melbourne, les expériences ATLAS, CMS et celles du Tevatron (D0 et CDF) ont fait le point sur le boson de Higgs. C’était super de constater comment tous les résultats sont consistants entre eux: ceux des quatre expériences, des deux accélérateurs opérant à des énergies différentes et des six canaux de désintégration indépendants. Tous donnent le même message: on a bel et bien trouver un nouveau boson.

Prochaine étape: établir s’il s’agit du boson de Higgs prédit par le modèle standard de la physiqe des particules ou d’un des cinq bosons de Higgs venant de la supersymmétrie, une autre théorie qui tente de remédier aux dernières lacunes du modèle standard.

Bien que la théorie soit incapable de prédire exactement la masse du boson de Higgs, elle impose des contraintes strictes sur sa valeur. Ses équations interconnectent plusieurs paramètres. On améliorant la précision des mesures de masses et autres paramètres du modèle et en combinant toute l’information, on peut voir si tout se tient.

Cette vérification a constitué un des points forts des grandes conférences durant les vingt dernières années. A chaque nouvelle mise à jour, on pouvait se rendre compte des progrès accomplis lorsque les mesures les plus récentes étaient incorporées dans un programme complexe conçu pour vérifier la théorie dite « électrofaible » d’un seul coup. Un peu comme si on vérifiait la stabilité d’un immense mobile après avoir modifié chacun de ses éléments.

Hier, pour la première fois, la masse du boson mesurée la semaine dernière était incorporée aux équations pour voir son impact global.

La verticale représente la masse mesurée expérimentalement pour le boson W et à l’horizontale, la valeur de la masse du quark top, le plus lourd de tous les quarks. L’ellipse en bleu est centrée sur la mesure de ces deux quantités. Sa hauteur et largeur correspondent à la marge d’erreur sur ces deux mesures. L’ étroite bande en bleue représente la masse obtenue la semaine dernière pour ce qui pourrait être le boson de Higgs. Le fait que l’ellipse et cette étroite bande se chevauche en partie indique que tout est cohérent avec le modèle standard, du moins à l’intérieur de la marge d’erreur actuelle.

Mais si les expériences du Grand Collisionneur de Hadrons ou LHC réussissent à réduire la marge d’erreur sur la masse du boson W de 15 MeV (valeur actuelle) à 5 MeV (ellipse en noir), et si la valeur centrale des masses du top et du W ne change pas, il y aura une certaine tension i.e. la bande bleue représentant la masse du Higgs ne coïncidera plus avec l’ellipse en noir.

Par contre, la bande verte indique les valeurs encore possible pour les masses du W et du top si le boson de Higgs correspond non pas à celui prédit par le modèle standard, mais plutôt à un des cinq bosons de Higgs postulés par un des modèles de supersymmétrie connu sous le nom de MSSM ou Minimum Supersymmetric Model.

La figure ci-dessus réconfortera quelques personnes: il y a encore beaucoup de place pour la supersymmétrie même si toutes les tentatives actuelles n’ont toujours pas révélées sa présence. Comme un conférencier l’a exprimé hier: « La quantité de présentations à cette conférence sur la supersymmétrie est inversement proportionnelle à son évidence ». Entre temps, le modèle standard demeure toujours valide.

Tous les paris sont ouverts sur ce que l’on va maintenant découvrir. Est-ce le boson de Higgs du modèle standard, de la supersymétrie ou d’une autre théorie? Il faudra encore un peu de patience avant d’en avoir le cœur net mais la réponse viendra éventuellement.

Pauline Gagnon

Pour être averti-e lors de la parution de nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou par e-mail en ajoutant votre nom à cette liste de distribution



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!