Quantum Diaries

Aidan Randle-Conde | USLHC | USA

The biggest news at CIPANP 2012 for particle physicists seems to be coming from the “low” energy frontier, at energies in the ballpark of 10GeV and lower. This may come as a surprise to some people, after all we’ve had experiments working at these energies for a few decades now, and there’s a tendency to think that higher energies mean more potential for discovery. The lower energy experiments have a great advantage over the giants at LHC and Tevatron, and this is richer collection of analyses.

There’s a big difference between discovering a new phenomenon and discovering new physics, which is something that most people (including physicists!) don’t appreciate enough. Whenever a claim of new physics is made we need to look at the wider implications of the idea. For example, let’s say that we see the decay of a $\tau$ lepton to an proton and a $\pi^0$ meson. The Feynman diagram would look something like this:

tau lepton decay to a proton and a neutral pion, mediated by a leptoquark

The “X” particle is a leptoquark, and it turns leptons into quarks and vice versa. Now for this decay to happen at an observable rate we need something like this leptoquark to exist. There is no Standard Model process for $\tau\to p\pi^0$ since it violates baryon number (a process which is only allowed under very special conditions). So suppose someone claims to see this decay, does this mean that they’ve discovered new physics? The answer is a resounding “No”, because if they make a claim of new physics they need to look elsewhere for similar effects. For example, if the leptoquark existed the proton could decay with this process:

proton decay to an electron and neutral pion, mediated by a leptoquark

We have very stringent tests on the lifetime of the proton, and the lower limits are currently about 20 orders of magnitude longer than the age the universe. Just take a second to appreciate the size of that limit on the lifetime. The proton lasts for at least 20 orders of magnitude longer than the age of the universe itself. So if someone is going to claim that they have proven the leptoquark exists we need to check that what they have seen is consistent with the proton lifetime measurements. A claim of new physics is stronger than a claim of a new phenomena, because it must be consistent with all the current data, not just the part we’re working.

How does all this relate to CIPANP 2012 and the low energy experiments? Well it turns out that there are a handful of large disagreements in this regime that all tend to involve the same particles. The $B$ meson can decay to several lighter particles and the BaBar experiment has seen the decays to the $\tau$ lepton are higher than they should be. The disagreement is more than $3\sigma$ disagreement with the Standard Model predictions for $B\to D^{(*)}\tau\nu$, which is interesting because it involves the heaviest quarks in bound states, and the heaviest lepton. It suggests that if there is a new particle or process, that it favors coupling to heavy particles.

Standard model decays of the B mesons to τν, Dτν, and D*τν final states

In another area of $B$ physics we find that the branching fraction $\mathcal{B}(B\to\tau\nu)$ is about twice as large as we expect from the Standard Model. You can see the disagreement in the following plot, which compares two measurements ($\mathcal{B}(B\to\tau\nu)$ and $\sin 2\beta$) to what we expect given everything else. The distance between the data point and the most favored region (center of the colored region) is very large, about $3\sigma$ in total!

The disagreement between B→τν, sin2β and the rest of the unitary triangle measurements (CKMFitter)

Theorists love to combine these measurements using colorful diagrams, and the best known example is the unitary triangle. If the CKM mechanism describes all the quark mixing processes then all of the measurements should agree, and they should converge on a single apex of the triangle (at the angle labeled $\alpha$). Each colored band corresponds to a different kind of process, and if you look closely you can see some small disagreements between the various measurements:

The unitary triangle after Moriond 2012 (CKMFitter)

The blue $\sin 2\beta$ measurement is pulling the apex down slightly, and green $|V_{ub}|$ measurement is pulling it in the other direction. This tension shows some interesting properties when we try to investigate it further. If we remove the $\sin 2\beta$ measurement and then work out what we expect based on the other measurements, we find that the new “derived” value of $\sin 2\beta$ is far off what is actually measured. The channel used for analysis of $\sin 2\beta$ is often called the golden channel, and it has been the main focus of both BaBar and Belle experiments since their creation. The results for $\sin2\beta$ are some of the best in the world and they have been checked and rechecked, so maybe the problem is not associated with $\sin 2\beta$.

Moving our attention to $|V_{ub}|$ the theorists at CKMFitter decided to split up the contributions based on the semileptonic inclusive and exclusive decays, and from $\mathcal{B}(B\to\tau\nu)$. When this happens we find that the biggest disagreement comes from $\mathcal{B}(B\to\tau\nu)$ compared to the rest. The uncertainties get smaller when $\mathcal{B}(B\to\tau\nu)$ is combined with the $B$ mixing parameter, $\Delta m_d$, which is well understood in terms of top quark interactions, but these results still disagree with everything else!:

Disagreement between B→τν, Δmd and the rest of the unitary triangle measurments (CKMFitter)

What this is seeming to tell us is that there could be a new process that affects $B$ meson interactions, enhancing decays with $\tau$ leptons in the final state. If this is the case then we need to look at other processes that could be affected by these kinds of processes. The most obvious signal to look for at the LHC is something like production of $b$ quarks and $\tau$ leptons. Third generation leptoquarks would be a good candidate, as long as they cannot mediate proton decay in any way. Searching for a new particle of a new effect is the job of the experimentalist, but creating a model that accommodates the discoveries we make is the job of a theorist.

That, in a nutshell is the difference between discovering a new phenomenon and discovering new physics. Anyone can find a bump in a spectrum, or even discover a new particle, but forming a consistent model of new physics takes a long time and a lot of input from all different kinds of experiments. The latest news from BaBar, Belle, CLEO and LHCb are giving us hints that there is something new lurking in the data. I can’t wait to see wait to see what our theorist colleagues do with these measurements. If they can create a model which explains anomalously high branching fractions $\mathcal{B}(B\to\tau\nu)$, $\mathcal{B}(B\to D\tau\nu)$, and $\mathcal{B}(B\to D^*\tau\nu)$, which tells us where else to look then we’re in for an exciting year at LHC. We could see something more exciting than the Higgs in our data!

(CKMFitter images kindly provided by the CKMfitter Group (J. Charles et al.), Eur. Phys. J. C41, 1-131 (2005) [hep-ph/0406184], updated results and plots available at: http://ckmfitter.in2p3.fr)

Tags: B-Physics, CKM, LHC, new physics, physics
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Higgs update (CIPANP 2012)

Friday, June 1st, 2012

Now that we’re in the conference season we’re treated to the latest results from the LHC and Tevatron. For now we focus on squeezing as much as we can from the 2011 data, so it’s a great time to look at the status of the Higgs searches. We’ll see some of the 2012 results at ICHEP in July (as summer abruptly turns into winter, with ICHEP being held in Australia.) Until then we must be content with what we can see with the data up to the end of 2011.

Both CMS and ATLAS are still searching for the Higgs boson, and that means that if it exists, it must exist in the difficult low mass region. This is something that Standard Model advocates have “known” all along, since the global fit to electroweak data all point to a Higgs mass around 95GeV. The further away the mass of the Higgs is from 95GeV the more we need to explain why it has the mass that it does. The diagram below shows the electroweak fit and the right hand axis shows how many sigmas away the point is from what we expect. (I explained about sigmas in a previous post. About one third of all results are more than $1\sigma$ away from expectation. For 2, 3, 4 and 5$sigma$ these numbers are about 5% , 0.25%, 1 in 15,000, and 1 in 1.7 million respectively.) As we can see, moving up to about 160GeV the probability for discovering the Higgs is already as low as a few percent.

The electroweak fit ( arXiv:1107.0975v1 hep-ph)

It gets very tricky to reconcile a very high mass Higgs boson with existing constraints, so a high mass Higgs suggests physics beyond the Standard Model. The high mass region is cleaner, it’s easier to study, and it’s more exciting if there is a discovery. By contrast the lower mass region is takes much longer to see any evidence, the final states are more complicated and take more time to analyze. If we discover the Higgs bosons and only the Higgs boson then all that happens is we confirm that the Standard Model is an accurate description of reality. It looks like nature is teasing us with a low mass scenario.

Taking a look into the low mass regime (less than about 150GeV) we can see why there is such a challenge. The dominant decays of the Higgs boson are $b\bar{b}$ quarks, $\tau^+\tau^-$ pairs, and other quark and gluon processes. There are rarer decays too, and the most important is the $\gamma\gamma$ final state. The branching fractions are shown in the plot below. A branching fraction is the fraction of Higgs bosons which will decay into each final state:

$The Standard Model Higgs boson branching fractions (arXiv:1101.0593v3 hep-ph)$

The Standard Model Higgs boson branching fractions (arXiv:1101.0593v3 hep-ph)

The analyses from ATLAS and CMS are closing in on the Standard Model Higgs boson now. The limits are a few times the Standard Model, and once the yellow and green bands (“Brazil band plots”, as one speaker called them) pass below the line $1\times$Standard Model we can exclude the Higgs boson. If the Higgs boson exists then one point will stay far above the $1\times$Standard Model line, and that’s the location of the Higgs boson. If you want a primer on how to read these plots see my previous post on the topic.

There are three main ways to produce a Higgs boson:

• from gluon gluon fusion, which is the dominant process. In this case we get a Higgs boson, some jets from QCD and not much else. It’s a higher statistics sample, but there is nothing remarkable about the events.
• with associated production, which is about a factor of ten smaller. Higgs bosons love to couple of massive vector bosons, so whenever we have a massive vector boson there’s a small but significant chance we’ll also see a Higgs boson. We can use the massive vector boson to “tag” these extraordinary events, making the search with lower statistics, but cleaner.
• from vector boson fusion, a weird process that has a similar rate to associated production. In this mode the quarks from the protons exchange some massive bosons, which create a Higgs, and then the protons scatter off each other, leaving two jets at shallow angles. These events can be hard to reconstruct, but they are cool to look at.

The size of the background for $b\bar{b}$ quarks is about 50 million times larger than the Higgs processes, so any analysis using a $b\bar{b}$ final state must be very crafty. Generally we require that the Higgs is produced in association with a massive vector boson. When this happens the two bosons usually move back to back in the lab frame, so we can look for a high momentum Higgs boson. This makes things easier for the $b\bar{b}$ final state because the two b-jets should be on the same side of the detector, and look like a “fat” jet. Even so, there are still large backgrounds from QCD processes. Since December 2011 physicists have been busy working to get as much discrimination between the Higgs and the background processes as possible, so its no surprise that we see more use of multivariate analyses in these searches. With a more dedicated study we can split up our searches based on the final states and tailor each final state accordingly. This “divide and conquer” method has lead to improved limits. The current exclusion for $H\to b\bar{b}$ is already a few times the Standard Model:

ATLAS limits for Higgs decaying to b quarks (B LaForge, CIPANP2012)

CMS limits for Higgs decaying to b quarks (C Palmer, CIPANP2012)

For the next dominant mode, the $\tau^+\tau^-$ final state, we have a different set of challenges. $tau$ leptons produce neutrinos, which carry away some of the momentum, making it harder for us to reconstruct the event. To make things worse, the $\tau$ can decay to leptons or to hadrons, so we need to split up our analyses and treat each case separately. And if that wasn’t enough, we also have a large background from decays of the Z boson, which have exactly the same final state. Given all this it’s a wonder we can use this channel at all. Unfazed by the challenges, both ATLAS and CMS have shown great improvements in this channel:

ATLAS limits for Higgs decaying to tau leptons (B LaForge, CIPANP2012)

CMS limits for Higgs decaying to tau leptons (C Palmer, CIPANP2012)

The next dominant processes are $c\bar{c}$ and $gg$, which are of no use to us at all. Backgrounds from QCD processes are just too high for these modes to be useful. So that leaves the $\gamma\gamma$ final state, and this is the cleanest mode for the lower mass scenarios. To decay $\gamma\gamma$ the Higgs boson must go through some intermediate particles in a loop. The challenges presented by the $\gamma\gamma$ final states are mostly associated with the detectors. How do we know when we see a photon in the detector, and not a jet? What control samples can we use to calibrate our energy scale? These are tough questions to answer, and since the backgrounds for this channel are so high we need to have confidence in our abilities to recognize and reconstruct photons. (I’m actually a bit skeptical that we have seen hints of a Higgs based on these kinds of questions. Our most sensitive channel is the one with some of the biggest questions.) Even so, the limits are looking encouraging:

ATLAS limits for Higgs decaying to photons (B LaForge, CIPANP2012)

CMS limits for Higgs decaying to photons (C Palmer, CIPANP2012)

I’ve skipped the massive vector boson final states ($ZZ^*$ and $WW^*$), although these are sensitive to some of the range too. As we look to lower and lower mass ranges the contributions from these final states diminish rapidly, and the kinematic constraints get worse and worse. (At high mass the Higgs boson would produce real $WW$ and $ZZ$ pairs, giving us fantastically clean mass peaks. At lower masses one of the bosons must be virtual, and we lose one of our most useful constraints.)

Combining the results gives better exclusions. As we can see there is not much space left for the Higgs boson!

ATLAS limits for combined Higgs channels (B LaForge, CIPANP2012)

CMS limits for combined Higgs channels (C Palmer, CIPANP2012)

Most people’s money is on the region 124-126GeV. All we need to do now is collect the 2012 data and see if it shows the same bump. The waiting is the hardest part.

Tags: ATLAS, CMS, conference, Higgs
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It’s conference season again!

Thursday, May 31st, 2012

Greetings from Florida! The summer conference season has just started, and on both sides of the Atlantic, in Florida and France, physicists are meeting to share the latest news from the LHC and the Tevatron. I’m at the Eleventh Conference on the Intersections of Particle and Nuclear Physics (CIPANP 2012), and with 70 parallel sessions, 10 plenary sessions, and 64 posters there’s a lot to explore here! While the Higgs boson is a hot topic, it’s not the main focus of the conference, topics include neutrino physics, cosmology, nuclear physics, dark matter and hadronic structure. Physicists are chatting over coffee, catching up on gossip and rumors, and trying to find the time to fit in the most interesting talks.

I delivered my talk yesterday (a whirlwind tour of Higgs bosons decaying to final states with tau leptons) so I can now relax and enjoy the rest of the conference. Given the diverse nature of CIPANP this is a great opportunity to find out about the other areas of physics. In the very low mass region there are extremely stringent tests of the Standard Model which keep getting better. It’s easy to forget that the most precise tests are not found at the high energy frontier, so hearing from colleagues who work with muons and neutrinos is vital.

Presenting my talk

So far I’ve mostly limited myself to the Higgs sessions and the plenary talks. We’ve seen ATLAS, CMS, CDF, and D0 squeeze as much as they can out of their datasets, looking in much more detail at the decay channels, splitting analyses into ever finer categories in order to improve the techniques. Even so, we’re going to have to wait for ICHEP in July to see some substantially improved exclusion limits.

Perhaps the best part of traveling to conferences is the change of scenery and break from the usual habits. I don’t want to give the impression that it’s like a vacation- nearly everyone is still working very hard while they’re here. Instead the travel breathes new life into our approach to physics, giving us a chance to think a bit differently about what we do.

A popular plenary session.

As I sit in talks I find my mind wandering to the public understanding of physics, because I struggle to understand a lot of the presentations from theorists. We tend to skip over a lot of information when we present our work, so it would be useful to be able to take things more slowly when explaining the more important areas. Unfortunately we need to get permission to present plots using data, so for now we are stuck with the plots that have been approved. They are often busy, pragmatic, and try to condense as much information as possible in as little space as possible. Putting in a few more steps could make the ideas much more accessible to the wider public, so if I get time in the next few months I want to explore making it easier to get more suitable plots approved for the public.

A physicist takes a break between sessions

I’ll focus more on the physics results in a different blog post. For now I just want to say that it’s great to be back in the USA again and (tedious border control aside) it’s been a very pleasant experience to be on this side of the Atlantic for a week. At these conferences there are always social events and receptions, so imagine how happy I was to see that there was a dolphin watching cruise on the schedule!

Dolphins!

Tags: CIPANP2012, conference
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Happy birthday, Richard Feynman!

Friday, May 11th, 2012

Richard Feynman was one of the most influential physicists of the twentieth century. Not only did he revolutionize quantum theory with his development of quantum electrodynamics, but he also revolutionized the way we think about physics and physicists. He spoke to people from all kinds of backgrounds about physics, from lecturing students destined to change the field themselves, to appearing on television to discuss physics and the philosophy of science, to meeting with the greatest minds of the time.

Feynman in the middle of a lecture. (www.richard-feynman.net)

For me, Feyman’s great contribution was the way he thought about physics. His Lectures on Physics are world famous, and rightly so. (In fact, one of the first things I did after landing in San Francisco to work at SLAC was to buy a copy of his lectures from the Stanford bookstore. Shortly afterwards by bank froze my card, suspecting fraud. It was worth the inconvenience!)

As a jaded undergraduate they were a source of inspiration to me. A faint glimmer of hope turned into a roaring inferno after reading his lectures on electromagnetism, and I’ve never looked back since. Finally, here was someone who wanted to discuss the beauty of the subject, as well as the truth. He had no time for obscuring the underlying symmetry of a concept, nor for lying to students in order to make things easier. Inevitably having to unlearn and relearn ideas leaves people confused, disillusioned and unable to trust their tutors. In that spirit, this is how he started his course on electromagnetism:

“We begin now our detailed study of the theory of electromagnetism. All of electromagnetism is contained in the Maxwell equations.

Maxwell’s equations:

\[
\nabla \cdot \vec{E} = \frac{\rho}{\varepsilon_0}
\]
\[
\nabla \times \vec{E} = - \frac{\partial \vec{B}}{\partial t}
\]
\[
c^2\nabla \times \vec{B} = \frac{\partial \vec{E}}{\partial t} + \frac{\vec{j}}{\varepsilon_0}
\]
\[
\nabla \cdot \vec{B} = 0
\]

Don’t worry about trying to understand these equations. The important thing here is that Feynman has given the students the complete truth about electromagnetism. With these four equations he can solve any problem about the shape and nature of electromagnetic fields for any configuration of charges and currents. The equations he provides are not some approximation of the theory, or some equations that only work some of the time, these are the equations that all physicists and engineers use and they are, as far as we know, complete and state of the art. Feynman has shown a level of honesty and respect for his students/readers that was not present when I sat through lectures. My lecturers taught me backwards, Feynman taught me forwards.

(Experts might notice that the Lorentz force law is missing here, but Feynman already mentioned it a few pages before Maxwell’s equations. With the Lorentz force law physicists can relate the electromagnetic fields to the forces on charged particles.)

Feynman continues:

The situations that are described by these equations can be very complicated. We will consider first relatively simple situations, and learn how to handle them before we take up more complicated. The easiest circumstance to treat is one in which nothing depends on time- called the static case. All charges are permanently fixed in space, or if they do move, they move as a steady flow in a circuit (so $\rho$ and $\vec{j}$ are constant in time). In these circumstances, all of the terms in the Maxwell equations which are time derivatives of the field are zero. In this case Maxwell’s equations become:

Electrostatics:
\[
\nabla \cdot \vec{E} = \frac{\rho}{\varepsilon_0}
\]
\[
\nabla \times \vec{E} = \vec{0}
\]

magnetostatics:
\[
c^2\nabla \times \vec{B} = \frac{\vec{j}}{\varepsilon_0}
\]
\[
\nabla \cdot \vec{B} = 0
\]

You will notice an interesting thing about this set of four equations. It can be separated into two pairs. The electric field $\vec{E}$ appears only in the first two, and the magnetic field $\vec{B}$ appears only in the second two. The two fields are not interconnected. This means that electricity and magnetism are distinct phenomena so long as charges and currents are static.

And he goes on. Immediately at the start of the course he’s pointed out one of the most important and beautiful symmetries in electromagnetism. He also lets us know how the course is going to proceed, with static cases first and the full treatment later. This leaves the student with a wonderful surprise later in the course, when the two fields finally get united again. When this happens Feynman goes on to show us how electromagnetism comes about as a result of special relativity, and if done properly that is one of the most breathtaking moments in physics! This is the way physics should be taught, and I wish I could have been in that lecture hall to see it happen!

The rest of the lectures are a fascinating journey, full of neat little asides, teasers, paradoxes, and it’s all handled with refreshing clarity. He even pokes fun at physics itself from time to time, showing how our mathematical notation is just a trick to make complicated things look simple and how different problems appear to have similar solutions only because we choose to use the same kinds of methods to solve them. Towards the end of his electromagnetism course he even goes out of his way to show how electromagnetism fails in an epic way. The problem of the infinite energy of the field, and the intractable problem of the mass of the electron are two major failings of the classical theory, and he dedicates a lecture to showing us just many questions were left unanswered by the subject.

Feynman with bongos, because some physicists are cool (www.richard-feynman.net)

Feynman gave us a lot to digest, from Nobel prize worthy discoveries, to a view of scientists that was anything but a crusty old professor, and for me what I value most is the lectures he gave, packed with inspiration and clarity. If you have a chance, go read some of the lectures and find out what made this man get out of bed in the morning. You won’t be disappointed. His other books are also excellent (Six Easy Pieces, Six Not So Easy Pieces, QED and Surely You’re Joking, Mr Feynman!) and well worth a read. Put them on your Christmas wish list!

Feynman’s birthday should be a national day of celebration, not just for physics, but for getting people hooked on physics! (I’m just sorry I’m a bit late to the party here, have a great weekend.)

If you want to find out a bit more about Richard Feynman check out this lecture about Feynman from Lawrence Krauss, one of today’s most eloquent speakers and best advocates for physics.

(Quotes taken from “The Feyman Lectures on Physics, The Definitive Edition Volume II”, Feynman Leighton and Sands, ISBN 0-8053-9047-2)

Tags: feynman, physics
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A sigma here, a sigma there…

Wednesday, May 9th, 2012

Whenever we come across a new result one of the first things we ask is “How many sigma is it?!” It’s a strange question, and one that deserves a good answer. What is a sigma? How do sigmas get (mis)used? How many sigmas is enough?

The name “sigma” refers to the symbol for the standard deviation, σ. When someone says “It’s a one sigma result!” what they really mean is “If you drew a graph and measured a curve that was one standard deviation away from the underling model then this result would sit on that curve.” Or to use a simple analogy, the height distribution for male adults in the USA is 178cm with a standard deviation of 8cm. If a man measured 170cm tall he would be a one sigma deviation from the norm and we could say that he’s a one sigma effect. As you can probably guess, saying something is a one sigma effect is not very impressive. We need to know a bit more about sigmas before we can say anything meaningful.

The term sigma is usually used for the Gaussian (or normal) distribution, and the normal distribution looks like this:

The normal distribution

The area under the curve tells us the population in that region. We can color in the region that is more than one sigma away from the mean on the high side like this:

The normal distribution with the one sigma high tail shaded

This accounts for about one sixth of the total, so the probability of getting a one sigma fluctuation up is about 16%. If we include the downward fluctuations (on the low side of the peak) as well then this becomes about 33%.

If we color in a few more sigmas, we can see that the probability of getting two, three, four and five sigma effect above the underlying distribution is 2%, 0.1%, 0.003%, and 0.00003%, respectively. To say that we have a five sigma result is much more than five times as impressive as a one sigma result!

The normal distribution with each sigma band shown in a different color. Within one sigma is green, two sigma is yellow, three sigma is... well can you see past the second sigma?

When confronted with a result that is (for example) three sigma above what we expect we have to accept one of two conclusions:

the distribution shows a fluctuation that has a one in 500 chance of happening
there is some effect that is not accounted for in the model (eg a new particle exists, perhaps a massive scalar boson!)

Unfortunately it’s not as simple as that, since we have to ask ourselves “What is the probability of getting a one sigma effect somewhere in the distribution?” rather than “What is the probability of getting a one sigma effect for a single data point?”. Let’s say we have a spectrum with 100 data points. The probability that every single one of those data points will be within the one sigma band (upward and downward fluctuations) is 68% to the power 100, or $2\times 10^{-17}$, a tiny number! In fact, we should be expecting one sigma effects in every plot we see! By comparison, the probability that every point falls within the three sigma band is 76%, and for five sigma it’s so close to 100% it’s not even worth writing out.

A typical distribution with a one sigma band drawn on it looks like the plot below. There are plenty of one and two sigma deviations. So whenever you hear someone says “It’s an X sigma effect!” ask them how many data points there are. Ask them what the probability of seeing an X sigma effect is. Three sigma is unlikely for 100 data points. Five sigma is pretty much unheard of for that many data points!

A typical distribution of simulated data with a one sigma band drawn.

So far we’ve only looked at statistical effects, and found the probability of getting an X sigma deviation due to fluctuations. Let’s consider what happens with systematic uncertainties. Suppose we have a spectrum that looks like this:

A sample distribution with a suspicious peak.

It seems like we have a two-to-three sigma effect at the fourth data point. But if we look more closely we can see that the fifth data point looks a little low. We can draw three conclusions here:

the distribution shows a fluctuation that has a one in 50 chance of happening (when we take all the data points into account)
there is some effect that is not accounted for in the model
the model is correct, but something is causing events from one data point to “migrate” to another data point

In many cases the third conclusion will be correct. There are all kinds of non-trivial effects which can change the shape of the data points, push events around from one data point to another and create false peaks where really, there is nothing to discover. In fact I generated the distribution randomly and then manually moved 20 events from the 5th data point to the 4th data point. The correct distribution looks like this:

The sample distribution, corrected.

So when we throw around sigmas in conversation we should also ask people what the shape of the data points looks like. If there is a suspicious downward fluctuation in the vicinity of an upward fluctuation be careful! Similarly, if someone points to an upward fluctuation while ignoring a similarly sized downward fluctuation, be careful! Fluctuations happen all the time, because of statistical effects and systematic effects. Take X sigma with a pinch of salt. Ask for more details and look at the whole spectrum available. Ask for a probability that the effect is due to the underlying model.

Most of the time it’s a matter of “A sigma here, a sigma there, it all balances out in the end.” It’s only when the sigma continue to pile up as we add more data that we should start to take things seriously. Right now I’d say we’re at the point where a potential Higgs discovery could go either way. There’s a good chance that there is a Higgs at 125GeV, but there’s also a reasonable chance that it’s just a fluctuation. We’ve seen so many bumps and false alarms over the years that another one would not be a big surprise. Keep watching those sigmas! The magic number is five.

Tags: data analysis, sigma, Statistics
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Shifting expectations

Saturday, April 14th, 2012

It’s 2012. We have stable beams. We’re at 8TeV. We’re taking data and I’m sitting in the ATLAS Control Room again. Fans of my blog will remember my previous on-shift posts and, yes, today I had an awesome breakfast of roasted duck (a special treat from a visiting professor).

So ATLAS Control Room, we meet again...

The last time I took shifts was about 6 months ago, and since we’ve had a shutdown. Both the LHC and ATLAS have used this break as an opportunity to make substantial improvements and move things around a bit. The change to 8TeV came at the same time as a change in the luminosity calibration. For some reason it looks like CMS are getting about 10% more collisions than ATLAS is. That’s a little unnerving.

The writing's on the wall, literally. CMS have more collisions than we do.

As the beam conditions changed, so has the Trigger Shifter’s desk. Performing the checks used to take me about 20 minutes, but with the new layout it took me one hour. Hopefully as I get used to the new system it will be quicker! Since I’m supposed to perform these checks about once an hour I could spend my whole shift staring at one set of histograms! That’s the kind of environment that leads to simple mistakes which could cost data.

Just when things were going well I heard a sound over the intercom and all my trigger rates dropped to 0Hz. There were no error messages, nothing seemed to be wrong with the detector and every system seemed to be working fine. After discussing the situation with colleagues in the Control Room I realized that it was a scheduled beam dump. A scheduled beam dump. We don’t get those often, and the training doesn’t include an MP3 file of the “scheduled beam dump” sound. But then again it’s 1:00am and it’s been 6 months since I was last on shift, so I think I can be forgiven for forgetting what a scheduled beam dump sounds like.

Discussing the beam dump with the other shifters.

I’ll be on shift for the tonight and the next two night, racking up credit for SMU and keeping the trigger alive. If all goes well it’s a good chance to catch up on work, write a few blog posts and get some time to ponder the bigger challenges in my analyses. For a few days I’m essentially free from all meetings and distractions, giving me the time and space to sort out all the little problems that have built up in the past few weeks. The broken code, the old E-mails, the unasked questions. Shifts are great.

If you liked this post you might also like:
On shift
The best and worst moment on shift

Tags: ATLAS, control room, LHC, shift
Posted in Latest Posts | 3 Comments »

Physicists discover large body orbiting Earth!

Sunday, April 1st, 2012

After gathering a huge amount of data the physicists at the Ice Cube experiment in Antarctica have come to an inescapable and startling conclusion. There is a massive body orbiting the Earth, and the scientists can see its “shadow” in their data. They can even trace its path across the sky.

This body is called “Luno” by some scientists and it seems to be cross the sky once every 29.5 days. The mass of Luno is estimated to be quite staggering- about 1% of the mass of the Earth! Despite its large size there seems to be little danger posed by this body, It seems to be orbiting happily, showing no sign deviating from its course. Taking a look at the data once the movement of Luno is taken into account gives a striking pattern, confirming that its orbit is indeed stable over long periods of time:

The position of neutrinos in the sky respect to Luno (Ice Cube) (Link to pdf)

The Ice Cube experiment is a neutrino observatory that searches for high energy neutrinos from outer space. These are thought to be given off by gamma ray bursts, neutron stars and alien TV broadcasts. (Some controversial theories also state that we can expect high energy neutrinos from malfunctioning microwave ovens and vacuum cleaners. But it would have to be some extreme form of malfunction.) As the ~~neutrinos~~ cosmic rays hit Luno they interact and the associated neutrinos don’t make it to Ice Cube. This is how Ice Cube see the “shadow” of Luno:

Schematic of the shadow of Luno (Ice Cube) (Link to pdf)

Other observations of Luno

This is not the first time that a particle physics experiment has speculated about a massive extra terrestrial body. The experiments at LEP postulated the existence of a massive body outside the Earth that changed their centre of mass energies. The assumptions went as far as to say that Luno was responsible for huge tidal forces that changed the shape of the Earth subtly around LEP. Then again, the LEP experiments were also sensitive to the TGV train timetables and meetings of the CERN Yoga Club.

Scientists at NASA have been studying Luno and they have come to some interesting conclusions. The most striking prediction is that Luno should be visible to the naked eye. Luno should reflect electromagnetic radiation from the sun, making it particularly visible at night. It is also thought that Luno is largely responsible for the tides we see in the seas and oceans across the world, a phenomenon which had been a mystery for centuries. Luno could even block the line of sight between the sun and the Earth, causing nightfall for a brief period of time. This could cause panic for people from scientifically illiterate cultures, nocturnal animals and biochemists. After much study there have been a number of artist’s impressions to help with identification of Luno:

Artist's impression of Luno to aid identification (NASA)

Ancient prophecy

Although Ice Cube has only discovered Luno recently, there are several examples of prophecy of Luno in various forms. Several ancient civilizations drew pictograms that represented Luno in some way with some examples, such as the Tarot deck, surviving to the present day. Some cultures even had a Luno deity, such as Khonsu of the ancient Egyptians. His pictogram includes a large figure, which carries Luno. Given the size of Luno, we should be able to see the large figure as well, but all searches have been fruitless. Some people think that this figure may be even harder to find than SUSY, or even extra dimensions (outside of the Terry Pratchett universe.)

Khonsu (discovered portions shown in gray) (Wikipedia)

Whatever Luno is, it should be heralded as one of the greatest discoveries of 2012, and I wouldn’t be surprised if it won the Nobel Prize!

Tags: check the date, Ice Cube, Luno, neutrinos
Posted in Latest Posts | 15 Comments »

Ramping up

Tuesday, March 27th, 2012

At the moment the LHC is making the transition from no beams to stable beams. It’s a complicated process that needs many crosschecks and calibrations so it takes a long time (they have already been working on the transition since mid February.) The energy is increasing from 7TeV to 8TeV, and the beams are being squeezed tighter, and this means more luminosity, more data, and better performance. As the LHC prepares for stable beams, so do the experiments. I can only see what is happening within ATLAS, but the story will be the same for CMS and LHCb.

As the LHC moves through its checks and changes its beam parameters the experiments have an opportunity to request special beam setup. We can ask that the LHC “splashes” the detector with beam in order to calibrate our hardware. This is similar to the famous first beam plots that we saw in 2008. In addition to splashes we can also request very low pileup runs to test our simulation. “Pileup” refers to the average number of events we expect to get every time the beams collide in the detectors, and by increasing the pileup we cram as many events as we can into the limited periods of time available to us. For 2011 our pileup was about 15, and this is going to increase in 2012 to about 20-30. This meant I was surprised to find out that we can use pileup of 0.01 for some of our simulation calibrations!

First ATLAS splash from 2008 (ATLAS Collaboration)

The timetable for the ramping up the LHC is announced as far in advance as possible, but it’s subject to small changes and delays as new problems arise. In general, the LHC outperforms its expectations, delivering higher luminosities than promised and stable beams for longer than expected, so when we factor in unexpected problems and unexpected higher performance we have to take the timetable with a pinch of salt. We expect to get stable beams around Easter weekend. You can see the timetable in the pdf document provided by the LHC team.

In the meantime the ATLAS hardware has been checked and maintenance performed to get it in good working order for the data taking. The thresholds are fine tuned to suit the new beam conditions and the trigger menu is updated to make the best use of the data available. There are plenty of decisions that need to be made and discussions that need to take place to make sure that the hardware is ready for stable beams. Today I got a glimpse at the checks that are performed for the electromagnetic calorimetry system, the trigger system and some changes to the muon systems. It’s easy to lose sight of how much work goes into maintaining the machine!

The LHC team preparing for beams.

As the hardware improves, so does the software. Software is often a cause of frustration for analysts, because they develop their own software as a collaboration and the software is sometimes “bleeding edge”. As we learn more about the data and the differences between data and simulation we can improve our software, and that means that we constantly get new recommendations, especially as the conferences approach. There is a detailed version tracking system in place to manage these changes, and it can be difficult to keep up to date with it all. Unfortunately, updated software usually means analyzing the data or simulation again, which is time consuming and headache-inducing in itself. That is how things worked in 2011. This year it looks like we’ve already learned a lot about how the data look, so we can start with much better simulation and we can start with an improved release for all the software. This should make progress much easier for analyses and simpler for everyone (which is a very important consideration, given that we have a large range of experience with software, and a large range of knowledge of physics processes.)

The banks of super computers are ready and waiting...

Putting all this together we can conclude the following: we will have higher energy beams giving us more data, we’ll have a better functioning detector based on previous experience, we’ll have improved simulation, and we’ll have more stable and simpler software. This is very exciting on the one hand, but a bit intimidating on the other, because it means that the weak link in the chain could be the physicists performing the analyses! There are plenty of analyses which are limited by statistics of the dataset, or by resolution of the detector, or stymied by last minute changes in the software or bugs in the simulation. If we hit the ground running for 2012 we could find ourselves with analyses limited by how often the physicists are willing to stay at work until 3am to get the job done.

I’ve already explained why 2012 is going to be exciting in terms of results in another blog post. Now it looks like it will bring a whole new set of challenges for us. Bring it on, 2012, bring it on.

Tags: ATLAS, LHC, physics, ramp up
Posted in Latest Posts | 5 Comments »

Cleaning the world’s biggest machine

Tuesday, March 6th, 2012

Today I spent much of my time crawling around on hands and knees, picking pieces of rubbish from the innards of the ATLAS detector. It’s just one of those things that comes with the job and gives you a different view of the experiment (literally.) Before we start taking data we need to make sure that the ATLAS cavern is clean and safe. I call this process “Grooming the Beast”.

We started our shift with a briefing in the ATLAS Control Room.

The ATLAS detector is housed in the ALTAS cavern, just behind the Globe at CERN. The journey down is long (more than 100 meters) and convoluted, with all kinds of doorways, locks, passages and elevators. Work has been taking place in the cavern during the winter shutdown to make improvements and sort out minor problems with the detector. Is a piece of the hardware getting damaged by interactions with matter? This is an excellent time to replace it!

Some of the team survey the work ahead of them.

Cleaning the cavern just as people start to leave it may seem like an unusual thing to do, but it serves a very important purpose. There has been a lot of work to improve the detector during the shutdown, and this leaves some debris. The engineers clear up as much as they can as they go along, but the odd screw or piece of wire goes missing, and over the months this builds up. The real danger to the machine is metal debris. The detector contains large magnets and these can interact with metallic objects lying around. They need to be removed before we turn on and take data!

The problem with photographing ATLAS is that it's just too big and the cavern is too small.

The cleaning also serves a milestone in the life of the experiment. It serves as a reminder that the shutdown is over, the repairs are complete and that we need to look forward to the new data that’s going to arrive. It’s no coincidence that at the same time as we clean the cavern, we present our work at the Moriond conferences. (These two weeks are going to be my favorite of this year! So many interesting results, and getting the chance to poke around inside a large detector.)

We got treated to a coffee break halfway through! (Note the security guard preventing use of the "Goods In" entrance, and the retinal scanners in the green "Pedestrians" entrance.)

As you would expect, health and safety are very important in this process. To get access to the cavern I had to pass 4 levels of safety training, get a dosimeter to monitor radioactive dose, a hardhat with a light, and hard boots. In addition we had to register our names and phone numbers in case anything happened while we were down there. There is an elaborate key system in place as well, which is mainly for safety. We each take a key as we enter, and the beams cannot pass through the cavern until every key is returned. We each get a pass (either on our CERN ID cards or a magnetic key fob) that is linked to our names and dosimeters, so that if a key is not returned we know who is still in the cavern and when they entered. Safety isn’t the only concern though, these systems have the added advantage of protecting the machinery. Everyone who goes down to the cavern has to have safety training and the correct permission, which significantly improves the quality of all the work down there. If we knew we could just pop along at any time to fix a minor problem there would be people down in the cavern all the time!

Armed with a proton pack (I mean a vacuum cleaner) I take on the dirt and debris of the ATLAS cavern!

Most of the actual work involved picking pieces up off the floor and cleaning the areas that others can’t reach. It seems simple, but the shape and size of the detector make it very difficult. Balancing on one foot on a low friction floor as you lean into a crevice to see if that wire is actually attached to anything, while wearing a hard hat that makes your head bigger than usual is exactly as difficult as it sounds! For bonus points you can do this in a dark space with a special tool for grabbing objects in places too small for your hands to reach. Some of the “treasure” I found included a drill bit, a box of screws and tubes (“It’s good, but it’s not the Higgs”), a guide to some important looking apparatus, and some rusted metal in a box of rusty water. When faced with those objects it’s not always obvious what to do. Is it trash? Is it safe to move? Is somebody missing it?

"How can we make the ATLAS cavern even cooler than it already is?" "Put a scorpion like crane in it, of course!"

This is the first time I’ve seen the ATLAS detector in person and it’s impressive. But on the other hand, I couldn’t relate to it very easily. I saw some piece of the toroid and some piece of the muon system, but from the outside it looked like an amorphous chunk of wires and pipes. My officemate, Julia, pointed out the muon systems, and showed me the sensors they used to calibrate their position. In addition to this, they also have geodetic instrumentation in the cavern so that they can work out the position of any part of the detector. That’s some neat hardware to have 100 meters underground! I’d hoped to be able to recognize a lot more of the detector, but it’s just not that kind of experiment. It’s far too huge to appreciate in a single day.

Is this ATLAS? Or a space age submarine? Or the best clubhouse in the world?

Towards the end of the day my friend Katie showed me the inside of the detector. At the bottom of the detector there’s a crawl space (mind your head on the muon system!) into a series of chambers and tunnels. These give access to some instruments and hardware so that we make some changes or repairs, and through an intricate set of ladders and tunnels you can actually get quite far into the outer parts of the detector. It’s warm down there, and you can see parts of the famous toroid, which is neat. It was around this time that Katie suggested that it would make the best clubhouse in the world. I can see myself enjoying somewhere like that as a 12 year old boy! (Actually, I can see my self enjoying it as an adult, but that’s just how amazing it looks when you’re inside.)

Right now I’m exhausted, and my throat is a little dry, but it’s been an awesome day and I’m glad to have the chance to share some photos of the ATLAS cavern with you!

Tags: ATLAS, cleaning, detector, LHC
Posted in Latest Posts | 7 Comments »

But what if they are faster than light?

Friday, February 24th, 2012

This week the OPERA experiment released a statement about their famous “faster than light” neutrino measurement. In September scientists announced that they had measured the speed of neutrinos traveling from CERN to Gran Sasso and they found that they arrived slightly sooner than they should do according to special relativity. There was a plethora of scientific papers, all kinds of rumors and speculation, and most physicists simply refused to believe that anything had traveled faster than light. After months of diligent study, OPERA announced that they may have tracked down two sources of experimental error, and they are doing their best to investigate the situation.

But until we get the results of OPERA’s proposed studies we can’t say for sure that their measurement is right or wrong. Suppose that they reduce the lead time of the neutrinos from 60ns to 40ns. That would still be a problem for special relativity! So let’s investigate how we can get faster than light neutrinos in special relativity, before we no longer have the luxury of an exciting result to play with.

The OPERA detector (OPERA Collaboration)

Special relativity was developed over a hundred years ago to describe how electromagnetic objects act. The electromagnetic interaction is transferred with electromagnetic waves and these waves were known to travel extremely quickly, and they seemed to travel at the same speed with respect to all objects, no matter how those objects were moving. What Einstein did was to say that the constancy of the speed of light was a fundamental law of nature. Taking this to its logical conclusion meant that the fastest speed possible was the speed of light. We can call the fastest possible speed $s$ and the speed of light $c$. Einstein then says $c=s$. And that’s how things stood for over a century. But since 1905 we’ve discovered a whole range of new particles that could cast doubt on this conclusion.

When we introduce quantum mechanics to our model of the universe we have to take interference of different states into account. This means that if more than one interaction can explain a phenomenon then we need to sum the probabilities for all these interactions, and this means we can expect some strange effects. A famous example of this is the neutral kaon system. There two lightest neutral kaons are called $K^0$ and $\bar{K}^0$ and the quark contents of these mesons are $d\bar{s}$ and $s\bar{d}$ respectively. Now from the “outside” these mesons look the same as each other. They’ve got the same mass, they decay to the same particles and they’re made in equal numbers in high energy processes. Since they look identical they interfere with each other, and this gives us clues about why we have more matter than antimatter in the universe.

Since we see interference all over the place in the Standard Model it makes sense to ask if we see interference with a photon. It turns out that that we do! The shape of the Z mass peak is slightly asymmetric because of interference between virtual Z bosons and virtual photons. There are plenty of other particles that the photon can interfere with, including the $J/\psi$ meson, and the $\rho$ meson. In fact, any neutral vector meson with no net flavor will do. Einstein didn’t know about any of these particles, and even if he did he never really accepted the conclusions of quantum mechanics, so it’s no surprise that his theory would require that the speed of light is the fastest speed (that is, $c=s$.) But if the photon interferes with other particles then it’s possible that the speed of light is slightly lower than the fastest possible speed ($c<s$). Admittedly, the difference in speed would have to be very small!

In terms of quantum mechanics we would have something like this:
\[
|light>_{Einstein} = |\gamma>
\]
\[
|light>_{reality} = a_\gamma |\gamma> + a_{J/\psi} |J/\psi> + a_Z |Z> + \ldots
\]

As you can see there are a lot of terms in this second equation! The contributions would be tiny because of the large difference in mass between the massive particles and the photon. Even so, it could be enough to make sure that the speed of light is ever so slightly slower than the fastest possible speed.

At this point we need to make a few remarks about what this small change in speed would mean for experiments. It would not change our measurements of the speed of light, since the speed of light is still extremely fast and no experiment has ever showed a deviation from this extremely fast speed. Unless somebody comes up with an ingenious experiment to show that the difference between the speed of light and the fastest possible speed is non-zero we would probably never notice any variation in the speed of light. It’s a bit unfortunate that since 1983 it’s been technically impossible to measure the speed of light, since it is used in the definition of our unit of length.

Now we know that photons can interfere with other particles it makes sense to ask the same question about neutrinos. Do they interfere with anything? Yes, they can interfere, so of course they do! They mix with neutrinos of other flavors, but beyond that there are not many options. They can interfere with a W boson and a lepton, but there is a huge penalty to pay in the mass difference. The wavefunction looks something like this:
\[
|\nu_e>(t) = a(t)_{\nu_e}|\nu_e> + a(t)_{\nu_{\mu}}|\nu_\mu> + a(t)_{\nu_{\tau}}|\nu_\tau> + a(t)_{We}|We>
\]
(I’ve had to add a time dependence due to neutrino mixing, but it’s essentially no more complicated than what we had for the photon.)

That means that the photon could get slowed down slightly by the interference with other particles (including particles in the vacuum) and that neutrinos could get slowed down more slightly by their interference terms with other particles. And that way we could get neutrinos traveling faster than the speed of light and special relativity could remain intact. (In this description of the universe we can do what used to seem impossible, we can boost into the rest frame of a photon. What would it mean to do that? Well I suppose it would mean that in this frame the photon would have to be an off-shell massive particle at rest.)

The SN 1987 supernova, a rich source of slower than light electron neutrinos (Hubble, ESA/NASA)

Now I’ll sit back and see people smarter than I am pick holes in the argument. That’s okay, this isn’t intended to be a serious post, just a bit of fun! There are probably predictions of all kinds of weird effects such as shock waves and time travel that have never been observed. And there are plenty of bits I’ve missed out such as the muon neutrinos traveling faster than electron neutrinos. It’s not often we get an excuse to exercise our analytic muscles on ideas like this though, so I think we should make the most of it and enjoy playing about with relativity.

Tags: faster than light, neutrinos, OPERA, relativity
Posted in Latest Posts | 17 Comments »

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Aidan Randle-Conde | USLHC | USA

Other observations of Luno

Ancient prophecy

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