Both ATLAS and CMS presented many analyses of the 2015 dataset today. The energy of the LHC proton collisions increased from 8 to 13 TeV. All the production rates of known particles changed, so many studies measured those. In addition, the higher energy allows to search for even heavier particles than before. The number of collisions was lower than in 2012 (“run 1” together with smaller datasets from 2010-2011), but for potential heavy particles the increased energy is more important. Therefore, analyses mainly looked for particles heavier than 1-2 TeV. Nothing significant was found there. Somewhere else, however…:
The diphoton spectrum
Among many other analyses, both experiments studied the production of two photons (“diphotons”) and their invariant mass spectrum. Particles that decay to two photons can lead to a clear peak in this spectrum. The ##\pi^0## particle was seen in that way in 2009 (it was known for decades, but measuring it helped to calibrate the detectors), and it was one of the two main discovery modes for the Higgs boson in 2012. Experimentally, it is a very clean measurement: if you do something wrong, you reduce the experimental precision, but it is hard to get a wrong result.
The diphoton results
The two experiments independently analyzed their data, and found more events than expected at a mass of around 750-760 GeV. There is no known particle at this mass, and no other known process that could lead to such an effect, apart from statistical fluctuations. The overall significance of the peak is hard to evaluate and depends on the model used (see below) - this was easier with the Higgs discovery, as the Higgs properties had clear predictions. If you consider just the ATLAS result or just the CMS result, a statistical fluctuation is certainly possible (and expected somewhere given the large number of analyses). Two statistical fluctuations of that size at the same place in two independent experiments? Still possible, but it is getting interesting.
Don’t look here, look elsewhere
In those searches, the look-elsewhere-effect is important: a fluctuation at a given mass might be unlikely, but there are many mass points where a fluctuation can happen. Therefore, experiments usually give two significance numbers: a local significance ("what is the probability that we see so many events at this specific point?”) and a global one ("what is the probability that we see such an excess at some place in the tested range?”). CMS gives 2.6 local and <1.2 global significance, ATLAS quotes 3.6 sigma local and 1.9 sigma global significance for a narrow signal and 3.9 sigma local / 2.3 sigma global for a broader signal.
2.6 sigma correspond to a probability of 0.47% for a random fluctuation, 3.6 sigma corresponds to 0.016%, 3.9 sigma to 0.005%.
Comparison to run 1
A possible particle at 750-760 GeV should have been produced at the lower energy in 2012 as well. With the much larger number of collisions, it gives an important cross-check. Both CMS and ATLAS re-investigated the old studies to check the compatibility, and the results are compatible. Both ATLAS and CMS had a bit more events than expected, but the deviation was not significant enough to get more attention.
CMS made a full combination of run 1 and run 2 data, giving 3 sigma local and <1.7 sigma global significance.
If the excess comes from a new particle, it would be a small hint that it is probably produced by gluon-gluon fusion, as this gives a better compatibility between run 1 and new data for ATLAS. No full combination from ATLAS (yet?).
Possible interpretations
To summarize: ?
The peaks are unexpected. Diphoton spectra are mainly investigated for Higgs- and Graviton-like particles. A Higgs at that mass should have a broader peak (on the other hand, the SM-like Higgs is at 125 GeV so new particles can behave differently), and Gravitons at that mass should be produced much more frequently.
I’m sure some theoreticians are writing explanations right now what this could be...
Edit:
Jester found some toy model, a heavy scalar that couples to vector-like quarks.
What comes next?
I guess the experiments will refine their analyses where possible, make even more cross-checks and investigate the run 1 dataset in more detail. We might get new results at the
Moriond conferences in March. The numbers can change a bit, but I don’t expect the message to change significantly. I guess the diphoton spectrum will be one of the key analyses with the 2016 dataset. Collisions are expected to start end of April, by July the dataset size could be sufficient to have an impact, at the end of the year it should be much larger than the dataset of this year. It will either show that the excess seen here was not new physics, or conclusively prove the existence of something new.
What else happened?
Various limits were set, many of them better than in run 1. The excesses seen in run 1 were re-investigated with run 2 data, and while more data is needed to fully rule them out, no excess was clearly seen again. ATLAS sees an excess in Z plus missing transverse momentum in run 1 and 2, but CMS does not see it in either run, and the significance is not very high (3 and 2 sigma local significance, respectively, with many places to look).
CMS had a very high-energetic electron/positron event earlier this year, which was surprising. It stayed the most high-energetic event of this type for the whole year, and the probability to find one event of at least this energy is about 3-4%, so one event is not too surprising.
