The latest in particle physics soon revealed!

I just arrived at the 25th edition of the Lepton-Photon conference in Mumbai, India. This is one of the largest particle physics conferences in the world where all the very latest results in the world of particle physics are presented and discussed. This is going to be quite a unique experience! Right now, the eyes of the particle physics community are turned onto us, the LHC experiments, as we are rapidly extending the range of our sensitivity to the Higgs boson.

At the last big conference, the International Europhysics Conference on High Energy Physics (casually abbreviated as EPS), the ATLAS experiment announced “tantalizing hints” to the Higgs boson, while CMS, our direct competitor sitting on the opposite side of the LHC, didn’t see anything yet. To be fair, ATLAS may have been over-stating what it was seeing.

ATLAS EPS result in the WW Higgs decay channel. Wherever you see the "observed" curve above the "expected", there is an excess. If the "observed" curve is outside the yellow band, then the excess is categorized as "tantalizing hint" by ATLAS, but as less than "evidence".

Like I said in a previous post, particle physics experiment are statistical in nature. We need to count how many events satisfy a bunch of criteria that determine if the events is Higgs-like or not, and then we need to estimate how much this selection will be contaminated by events that are not from actual Higgs bosons. It is only by measuring an excess over this expected contamination that we can start gaining certainty that we see the Higgs. However, you will agree that if you estimate that 1 event will be from contamination, and that you observe 2 events passing your selection, it is a bit early to claim discovery.

On the other hand, if you have 1000 times more data, and that you first correct your background estimate to be 953 events, and that you observe 1234 events, you can be a lot more confident that the excess you are seeing is real. You can imagine a spectrum of degrees of certainty between these two cases. Right now, the degree of certainty with which ATLAS sees an excess in the EPS result is just not high enough to declare victory, but high enough to warrant some excitement but most importantly, further investigation.

This is the cool thing about this Lepton Photon conference. In the time between EPS and Lepton-Photon, the LHC has been churning out a lot of data, which is now being incorporated into the Higgs searches. The degree of certainty of what we see will go up. If the excess that ATLAS saw was real, we could be able to tell by now. And the result is going to be shown at Lepton Photon. I will soon know, in a few hours, what are the latest official results from CMS and ATLAS on the Higgs searches. Also, I heard that the first combination of ATLAS and CMS Higgs searches is going to be shown.

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