As most of you already know, today at 3PM two back-to-back talks by Jim Olsen (CMS) and Marumi Kado (ATLAS) at CERN will disclose the latest results of physics analyses performed on 13 TeV proton-proton collisions recorded this year by the two experiments. (To follow the talks see here).
Among the rumours that circulated before the event, one is persistent: it speaks of an excess of events with two photons, at an invariant mass of 750 GeV or so.
As ground-breaking as 13 TeV are, one must remember that only 4 inverse femtobarns of collisions have been collected this year, as opposed to the 20 inverse femtobarns of 8 TeV collisions thoroughly analyzed from the 2012 run of the LHC. At 13 TeV a 750 GeV particle is produced 2 to 4 times more frequently than at 8 TeV (it depends on what partons produce it). It follows that if anything is seen in the 13 TeV data, it must have been seen also in the earlier 8 TeV data, and in bigger amounts!
So let us see what the CMS and ATLAS experiments published on 8 TeV results with diphotons. Below you find the money plots.
Above is the CMS result. A bump at 750 GeV is indeed present, but only if you totally disregard the data in nearby regions... What appears is rather a deficit at 700 GeV.
And here we see the ATLAS results, again from 8 TeV data. Do you see a bump at 750 GeV ? I don't.
But wait, ATLAS did release some early 13 TeV data too, this July. Below you can see the diphoton spectrum they published then.
Aha! Again, no bump - but admittedly, this is based on 2% of the statistics they have now analyzed. So that lone event at 730 GeV in the graph above might now be a towering 50-event peak...
It only remains to sit and wait....