Back in December 2015, there was some excitement when the experiments ATLAS and CMS at the Large Hadron Collider [LHC] — especially ATLAS — reported signs of an unexpectedly large number of proton-proton collisions in which
- two highly energetic photons [particles of light] were produced, and
- the two photons could possibly have been produced in a decay of an unknown particle, whose mass would be about six times the mass of the Higgs particle (which ATLAS and CMS discovered in 2012.)
This suggested the possibility of an unknown particle of some type with rest mass of 750 GeV/c². However, the excess could just be a statistical fluke, of no scientific importance and destined to vanish with more data.
The outlook for that bump on a plot at 750 GeV has gotten a tad brighter… because not only do we have ATLAS’s plot, we now have increasing evidence for a similar bump on CMS’s plot. This is thanks largely to some hard work on the part of the CMS experimenters. Some significant improvements at CMS,
- improved understanding of their photon energy measurements in their 2015 data,
- ability to use 2015 collisions taken when their giant magnet wasn’t working — fortunately, the one type of particle whose identity and energy can be measured without a magnet is… a photon!
- combination of the 2015 data with their 2012 data,
have increased the significance of their observed excess by a moderate amount. Here’s the scorecard.*
- CMS 2015 data (Dec.): excess is 2.6σ local, < 1σ global
- CMS 2015 data (improved, Mar.) 2.9σ local, < 1σ global
- CMS 2015+2012 data: 3.4σ local, 1.6σ global
- ATLAS 2015 data (Dec. and Mar.): 3.6σ local, 2.0σ global to get a narrow bump [and 3.9σ local , 2.3σ global to get a somewhat wider bump, but notice this difference is quite insignificant, so narrow and wider are pretty much equally ok.]
- ATLAS 2015+2012 data: not reported, but clearly goes up a bit more, by perhaps half a sigma?
You can read a few more details at Resonaances.
*Significance is measured in σ (“standard deviations”) and for confidence in potentially revolutionary results we typically want to see local significance approaching 5σ and global approaching 3σ in both experiments. (The “local” significance tells you how unlikely it is to see a random bump of a certain size at a particular location in the plot, while the “global” significance tells you how unlikely it is to see such a bump anywhere in the plot … obviously smaller because of the look-elsewhere effect.)
This is good news, but it doesn’t really reflect a qualitative change in the situation. It leaves us slightly more optimistic (which is much better than the alternative!) but, as noted in December, we still won’t actually know anything until we have either (a) more data to firm up the evidence for these bumps, or (b) a discovery of a completely independent clue, perhaps in existing data. Efforts for (b) are underway, and of course (a) will get going when the LHC starts again… soon! Next news on this probably not til June at the earliest… unless we’re very lucky!