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Plot Of The Day – Higgs @ ATLAS

March 13, 2011

Today’s plot (with two more as a bonus!) comes from the ATLAS collaboration. They analyzed their 2010 dataset in search for the clean but rare decay of a Higgs boson into a pair of energetic photons. The data is not nearly sufficient to grant an observation of the signal, even if the Higgs was there and it had a mass in the most favourable range for the diphoton final state -say from the LEP II limit to 130-135 GeV; nonetheless, the analysis is quite interesting, since an update of the same will hopefully show a signal in a not too distant future.

I have not yet had a chance to read the details of the ATLAS search, but the general characteristics of the events are simple to describe: these are proton-proton collisions where the two most energetic objects do not leave charged tracks, and are only detected as localized energy deposits in electromagnetic calorimeter cells.

ATLAS finds 99 diphoton candidates in the data. Their invariant mass distribution is shown below: the black points show the data, the yellow band shows the background prediction. Too bad the background is above the data in most of the mass distribution: if it was below, one could see a nice bump at about 115 GeV… But I leave these scandalistic considerations to less serious bloggers.


From the agreement of data with background prediction ATLAS extracts a upper limit on the Higgs production rate as a function of its mass. The rate is computed in units of the SM prediction, such that a limit below 1.0 would signify that the Higgs boson cannot exist, at that particular mass value. The result is shown below.

Note that the “brazil band” which has become customary in many particle searches as of late has no lower yellow region. This is because ATLAS these days is using a “power-constrained” method for setting upper limits. This amounts to drawing a line at minus one sigma: if the data fluctuates more than one sigma below the background expectation, it is ignored and the -1sigma limit is taken instead. This method is rather uncustomary and it has some drawbacks, but I will not discuss those here. However, I will not abstain from showing what a more common method would produce: the CLs result is also provided by ATLAS, mainly because this eases comparisons and combinations with other experiments. See below:

As you can see, the ATLAS 16% PCL method is quite less conservative than CLs. The limit in the 125-130 GeV region is at 8xSM with the ATLAS method, while it is at 18xSM with good-old CLs… Not a small difference!




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