Quantum Diaries

The EPS conference is now over, and, for the benefit of our readers, here is an attempt at a roundup of what LHC physics was presented there. Before we get started, please note that I didn’t attend the conference myself! I didn’t hear the talks, or actually talk to anyone who attended; I just read the slides that were posted online and the comments of others. (In fact, those of you who know me personally can attest that I’m actually on vacation this week. I will be attending the DPF conference in two weeks, and will try to let you know what develops there.) I welcome comments from others who were on the scene and would like to give a different take on things.

Higgs searches: This is of course one of the headline measurements from the LHC, and also the Tevatron which is still going strong. Here are some plots summarizing the results of the searches from the Tevatron (combined D0 and CDF), CMS, and ATLAS:

The bottom line for reading these plots is that if the solid black line falls below 1 on the vertical axis, it means that a Higgs of the mass given on the horizontal axis is excluded at 95% confidence level. (The dashed line indicates how well the experiments would expect to do if there is no Higgs boson at that mass.) All of the experiments exclude a fairly wide range around 165 GeV, and the LHC experiments are able to exclude a Higgs at much higher masses, a range that the Tevatron cannot access. These are the strongest direct limits on the Higgs masses to date. It seems to indicate that if there is actually a Higgs boson, it must be relatively light, with a mass between about 115 and 150 GeV. As it happens, this is one of the most challenging mass ranges for an observation at the LHC, although you can be sure that all the experiments will pull out all the stops to explore that region, while the Tevatron experiments will make the most of the data that will be recorded before the accelerator shuts down in a mere two months.

However, all of the experiments independently see that the limits are weaker than expected in that lower mass range, as indicated by the fact that the solid lines are higher than the dashed lines. This might (might!) indicate that in fact there is something to be observed there. But there is a lot more to learn yet; in particular, ATLAS and CMS will work on a combination of their results to try to make a joint statement soon, and of course recording and analyzing more data will help clarify things. Excesses like this have come and gone in the past, but of course we have to remain hopeful.

Searches for physics beyond the standard model: For all the hype about the Higgs, it’s “merely” a standard-model particle, in that it is a critical element of the theory that has served as an excellent model of particle physics for decades now. What would be truly exciting is if we were to find something that’s not predicted by that model. (On top of that, we know that the standard model is inherently problematic, and we believe some new physics must come into play to remedy that.) All of the LHC experiments are looking for a wide variety of new phenomena. And, so far, no one has found anything; instead, increasingly stringent limits have been set on the properties of hypothetical new particles, suggesting that if they exist, they must have very large masses. It’s a disappointment; certainly we might have hoped that we’d see something very new soon after turning on the LHC. Instead, while perhaps we are close to finding the last particle of the standard model, it looks like we might be some distance away from observing something that will give us clues about a new model for particle interactions. That won’t keep us from trying, and the data that the LHC will record during this rest of this year will make a difference in these searches.

B_s: In a previous post we discussed the CDF search for the decay of the B_s meson to a pair of muons, and their modest excess of signal over background. At EPS, both CMS and LHCb presented their own searches. So far, the CDF excess is not confirmed, and neither experiment observes the decay. The two experiments have already combined their results to show that their limit on the branching fraction is a factor of 3.4 times the expected value from the standard model, and about a factor of two below the implied CDF branching fraction. All three experiments will need to record more data to try to resolve this discrepancy.

At this conference, all of the experiments laid their best cards on the table. Over the next few weeks, everyone will be trying to interpret these results, and we can expect a thorough consideration of them, including new syntheses, at the upcoming DPF and Lepton-Photon conferences. And, more importantly, we have seen what the LHC experiments can do with what is still relatively little data. We’ll perhaps double the LHC dataset by the end of the year (or so I hope); the experimental conditions will be increasingly challenging, but the detectors appear to be ready to handle them, and we know that the scientists can turn the data around quickly. We’ll have an even better understanding of what’s going on at this energy scale over the next few months. Stay tuned!

Here we are, just eight days away from the start of the EPS conference, the first big meeting of the summer conference season. You can bet that all of the LHC experiments are racing to come up with results that will make as big a splash as possible there. (Or, failing that, at DPF two and a half weeks later, or Lepton-Photon two weeks after that.)

However, even before then, we have the first interesting result of the season in a paper submitted by CDF just yesterday. The topic is a search for a B_s meson that decays to a μ⁺μ^- pair. The B_s is made of a bottom antiquark and strange quark, and a student in an introductory particle physics course would (or should) tell you immediately that this decay can’t happen, as the standard model doesn’t allow for any “flavor changing neutral currents”, and there is no means for these two quarks of different flavors to annihilate each other. But a student in the next course would (or should) point out that no FCNC only applies to leading-order processes, and that there are higher-order, more rare interactions that would allow or this. Still, the rate is expected to be extremely low, with a branching ratio of about 3.2 x 10^-9. Various models of new physics, including supersymmetry, predict that there should be additional interactions that would lead to a higher rate for this decay. So far, the decay has never been observed; the best measured upper limit is 4.2 x 10^-8, more than an order of magnitude above the SM prediction.

The new CDF result, however, provides a tantalizing hint of a signal. Their latest analysis uses more data than their previous (2008) analysis, is sensitive to 20% more of the signal decays, and has improved the signal to background by a factor of two, to give a factor of 3.3 increased sensitivity overall. There is an excess of signal events over the predicted background, and if interpreted as a B_s signal gives a branching fraction of (1.8 +- 1.0) x 10^-8. This is about two standard deviations away from zero — not significant enough to be called a discovery, by any means (CDF predicts that there is about a 2% chance that this is a background fluctuation), but interesting. If it does hold up, then the branching ratio would be about five times bigger than that expected in the SM. (Insert your own speculations here.)

So the question we must ask here is: what does the LHC have to say about this? The b-hadron production rate of the LHC is huge, perhaps an order of magnitude above that of the Tevatron, and so the experiments can have similar sensitivity with less integrated luminosity. This measurement is one of the goals of the LHCb experiment; they have a result based on the 2010 data sample alone setting an upper limit of 5.6 x 10^-8, which is not in conflict with the CDF result. With the LHC having delivered about 30 times as much data in 2011 compared to 2010, LHCb will surely have increased sensitivity. ATLAS and CMS also have active b-physics programs, and will be able to look for this decay. Of course we are all very eager to find out if these experiments will see results consistent with CDF or not. If they do — and this point we can merely speculate — it’s going to be very interesting news. If not, well, sometimes life gives you statistical fluctuations.

Coming soon? We can hope.

Here I am on another trip to CERN, but this one is different. I’m in the midst of a five-week stay here, in the company of my family. Usually my trips are much shorter, and they have always been solo. But we wanted to give this a shot — we figured that it would be a good experience for the family, everyone would get to see a lot of new things, and perhaps we will get a sense of what it would be like to spend a sabbatical-length stay here sometime in the future.

Even though I have been to CERN and Geneva many times before, I am getting to experience it all anew because it is new to my family. The sort of things that I had noted on my own early visits and then filed away have been pulled back into prominence. We brought a college student with us to help take care of the kids, so that my wife and I can both work. She has been accompanying me to CERN most days and getting her own experience of lab life. She is enjoying the international flavor of CERN — hearing so many different languages being spoken and seeing the cultural interaction. And she also gets a sense of my harried days here! (Then again, that is compensated for by leisurely lunches on the Restaurant 1 terrace.) For our children, ages two and four, the new experiences have been much more quotidian, but when you are that small even the quotidian stands out. There are different foods to eat here (nutella! for breakfast!), and in the town where we are staying there is a fresh milk vending machine out on the street. People here tend to hang their laundry to dry instead of using a dryer, so the drying rack is a source of fascination. And so forth.

For myself, I’m really feeling the impact of being in the different time zone. Back at home, seven hours behind CERN, my day is very front-loaded, with all of the CMS meetings happening early in my day. Here, everything is at the end of the day, with the action not really getting going until 3 PM. This interferes with family life quite a bit; I can’t really stay for a late meeting if I’m going to have dinner from the kids. Worse still, I find myself trying to work both time zones while I’m here, as there is plenty of US business happening during the CERN evening hours. When I’m here on my own, I have more time to manage this, but while tending to my family also, I am constantly scrambling to keep up! (Note: blog post being written at 11:30 CERN time….)

But, despite all of that, we’re enjoying our stay here. We’re getting out to see Geneva and environs, and trying to enjoy ourselves. In particle physics, we’re very lucky to be able to visit places all around the world. If we’re going to travel so much, we might as well take advantage of it!

There are lots of physical phenomena that arise from changes in temperature. High-temperature environments are high-energy environments. That energy goes into the kinetic energy of particles. Perhaps the most common manifestation of this is evaporation — when you set a liquid out on a hot day, the molecules gain thermal energy, and some of them gain enough energy to overcome the attractive forces of the other molecules in the liquid; those molecules then float away into the air.

You can see similar phenomena at the atomic level and below. There, binding energies of particles tend to be bigger, and thus it takes more thermal energy to separate the bound states. For instance, at a temperature of about 158,000 degrees above absolute zero (could someone check my math on that?), electrons in hydrogen atoms will gain enough energy for them to separate from the their nuclei. Under such conditions, atoms don’t really exist anymore; you just have a “plasma” of electrons and protons. And we imagine that the early universe, shortly after the Big Bang, was so hot that protons didn’t exist; the quarks and gluons had enough thermal energy to keep from being bound together into hadrons.

A new result from CMS shows just this kind of phenomenon. The upsilon particle is a bound state of a bottom and anti-bottom quark, much like a hydrogen atom is a bound state of an electron and proton. In the ground state of the upsilon, the two particles are pretty tightly bound and require a lot of energy to separate. But the upsilon, like hydrogen, has a number of higher-energy bound states, in which the quarks have greater kinetic energy, and thus are easier to separate. A bit more thermal energy, a few hundred MeV, and these excited upsilon states should just fall apart.

This is what CMS observes. In proton-proton collisions, the excited upsilon states are clearly visible. But in lead-lead collisions, when there is a lot more ambient energy due to all of the colliding nuclei, the excited states begin to disappear. Actually, all of the upsilon states are suppressed, but the excited states are even more so, by about a factor of three, which indicates that the more energetic states are more sensitive to the increased temperature. It’s a pretty neat trick, and the first time that it’s been observed in bound states of bottom quarks.