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Ken Bloom | USLHC | USA

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EPS: Close, but far….

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:

Tevatron Higgs exclusion plot

CMS Higgs exclusion limits

ATLAS Higgs exclusion limits

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.

Bs: In a previous post we discussed the CDF search for the decay of the Bs 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!

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