As the LHC gets ready to start up again, two reananlyses of the data already taken look at last year’s intriguing hints...

I’m here at the “Rencontres de Moriond” conference, up in the mountains in Italy. The conference has run annually for the last 50 years and it started with a group of physics friends staying in a chalet, skiing and working together. By now, it’s grown into an important meeting where exciting new experimental results are sometimes announced for the first time.

The week long conference is tiring and great fun: we listen to presentations in the morning and evenings and ski in the afternoons. I’m reporting to you on three interesting talks we just heard: an exciting update from the experiments about the so-called “diphoton” excess seen at the high energy collisions at the Large Hadron Collider (the second run of the LHC).

If this signal, which shows too many collisions producing two particles of light at a certain energy (750 times the mass of the proton), becomes statistically significant, it means the discovery of a new particle – plus some others need to make the details of the collisions work.



Hint of new particle at CERN's Large Hadron Collider? Read more

Such a discovery would be extremely important: worth a Nobel prize or two for sure. The LHC has been under maintenance since the end of 2015, so there is no additional data taken since the initial announcement on December 18th last year. However, ATLAS has re-analysed its data taken at in the first run, and now sees a 1.9 sigma excess at 750 GeV (they hadn’t analysed this region in December). CMS has analysed the data it took while its magnet was off due to a malfunction, and the local significance of the resonance has increased from 2.6 sigma to 2.9 sigma. Both of these improvements, while not a massive deal on their own, do move in the direction of making the experimental result even more intriguing, and will likely cause another avalanche of theoretical papers explaining the collisions.



One of the papers in the initial avalanche was written by me and three collaborators. Our explanation is that the new particle comes from a mathematical theory called supersymmetry. It is a copy of the neutrino but is heavier and has no spin. It can decay into two photons through a loop involving other supersymmetric particles (smuons). This model (along with many other explanations) is still alive, and we are all waiting with baited breath to see what happens later this year in the new data.

Facebook Twitter Pinterest Diphoton update from the ATLAS experiment at the CERN LHC Illustration: ATLAS/CERN

This graph shows the number of collisions as the black dots according to the vertical axis, that were measured at an energy that is on the horizontal axis (for experts, this is really the invariant mass of the photon pair) in ATLAS data measured during the first run. Around 750 GeV, there are a few points above the red line (the red line shows the background estimate of ordinary uninteresting processes that produce two particles of light). This really looks like nothing much on its own, but it supports the previous, much more impressive, bump found in the second run at higher collision energies.



PS From Jon: the LHC is nearly back in action. We won’t have too long to wait for a definitive answer...

