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Kevin Black | USLHC | USA

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Startup 2012

Sunday, March 25th, 2012

The LHC will start colliding beams again in a few weeks after the traditional winter shutdown. 2012 could be THE year. This is not just idle speculation. Hints of the elusive Higgs boson may have been seen in both multipurpose experiments at the LHC (ATLAS and CMS) as well as the Tevatron full luminosity analysis at D0 and CDF. To be clear from the start these hints are – as of this writing – not significant enough to make a claim to have seen anything yet. The level of statistical significance has been described as ‘interesting and tantalizing’ though not conclusive. What makes people so excited about this result is that unlike many other episodes of temporary excitement there is a reasonable level of agreement BETWEEN experiments.

Let me explain. For a so called “low mass” Higgs boson (110-130 GeV) the most prominent decay channel is to the heaviest available fermions which the boson can decay into and conserve energy and momentum- a b and anti-bquark pair. Unfortunately, at both the Tevatron and the LHC the backgrounds to this channel are tremendously large. So its not just the decay branching ratios that are important but also how large the backgrounds are, the difficulty of reconstructing the final state, and the experimental resolution on the observed objects which matter (among other things).

As it turns out the channels with the largest expected statistical power at the LHC in this range are decays into two photons, two W bosons, or two Z bosons. In fact in December of 2011 both the ATLAS and CMS collaborations saw small bumps on an otherwise fastly falling decay spectrum in the diphoton resonant search. In that channel one looks for pairs of photons and reconstructs the invariant mass. If the pair of photons came from the production of a heavy particle you would expect to see a clustering of events around a particular mass. An excess of events was also seen in both experiments in the gauge boson decays (pairs of W and Z particles) around the same mass region.

All of this was known since last December when both collaborations made their results public with the full 2011 datasets for the first time. Then at the annual Moriond conference in 2012 the Tevatron experiments released their results with their full datasets. Because of the different center of mass energy the cross-sections for both signal and background processes are different. It turns out that the they too see an excess of events in the region consistent with the LHC experiments but in the so called “associated Higgs production” where the Higgs is produced along with a W or Z boson. This allows them to make use of the large branching ratio into b and anti-bquark pairs while reducing the background due to the presence of the gauge boson.

So what does this all mean? Well – for now I am optimistically cautious. The exciting thing for me as a part of the ATLAS collaboration is that we will start taking data in a few weeks at an even higher center of mass energy – 8 TeV. This year we could get even more data than we did in 2011 which will give us the opportunity to possibly confirm our preliminary results and make a discovery or exclude the existence of the particle which may have even more significant impact to our field.

One of the roles I currently play in the ATLAS experiment is the coordination of the muon high level trigger. Since we collide protons at the LHC at a rate that is much much higher than we can record to tape for analysis we have to choose which events we can keep. We have to have enormous rejection power on the order of accepting 1 event in many million in order to accomplish this. But on the other hand we have to be very careful which events we choose to save! We must maintain very high efficiency for triggering on the events we care about the most. This is becoming an ever more exciting and difficult game as the event rate gets larger and larger and the stakes get higher and higher.

Looking forward to an exciting year!

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