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….
16 December 2015 at 17:50
So, what did they say? Any big surprises or news?
17 December 2015 at 15:56
There is a 3-sigma (my hunch of a global significance, accounting for both experiments and the look-elsewhere effect) excess of events with diphotons,
with a mass of 750 GeV. The effect is interesting but my take is that it is just
one fluctuation. However dozens of papers by theorists are already pouring in
on the arxiv as we speak…