After a long-anticipated data run, LHC proton-proton running concludes in early November. A mere six weeks later, on a mid-December afternoon, the ATLAS and CMS collaborations present their first results from the full dataset to a packed CERN auditorium, with people all over the world watching the live webcast. Both collaborations see slight excesses in events with photon pairs; the CMS excess is quite modest, but the ATLAS data show something that could be interpreted as a peak. If it holds up with additional data, it would herald a major discovery. While the experimenters caution that the results do not have much statistical significance, news outlets around the world run breathless stories about the possible discovery of a new particle.
December 15, 2015? No — December 13, 2011, four years ago. That seminar presented what we now know were the first hints of the Higgs boson in the LHC data. At the time, everyone was hedging their bets, and saying that the effects we were seeing could easily go away with more data. Yet now we look back and know that it was the beginning of the end for the Higgs search. And even at the time, everyone was feeling pretty optimistic. Yes, we had seen effects of that size go away before, but at this time four years ago, a lot of people were guessing that this one wouldn’t (while still giving all of the caveats).
But while both experiments are reporting an effect at 750 GeV — and some people are getting very excited about it — it seems to me that caution is needed here, more so than we did with the emerging evidence for the Higgs boson. What’s different about what we’re seeing now compared to what we saw in 2011?
I found it instructive to look back at the presentations of four years ago. Then, ATLAS had an effect in diphotons around an invariant mass of 125 GeV that had a 2.8 standard deviation local significance, which was reduced to 1.5 standard deviations when the “look elsewhere effect” (LEE) was taken into account. (The LEE exists because if there is a random fluctuation in the data, it might appear anywhere, not just the place you happen to be looking, and the statistical significance needs to be de-weighted for that.) In CMS, the local significance was 2.1 standard deviations. Let’s compare that to this year, when both experiments see an effect in diphotons around an invariant mass of 750 GeV. At ATLAS, it’s a 3.6 standard deviation local effect which reduced to 2.0 standard deviations after the LEE. For CMS the respective values are 2.6 and 1.2 standard deviations. So it sounds like the 2015 signals are even stronger than the 2011 ones, although, on their own, still quite weak, when we consider that five standard deviations is the usual standard to claim a discovery because we are sure that a fluctuation of that size would be very unlikely.
But the 2011 signals had some other things going for them. The first were experimental. There were simultaneous excesses in other channels that were consistent with what you’d expect from a Higgs decay. This included in particular the ZZ channel, which had a low expected rate, but also very low backgrounds and excellent mass resolution. In 2011, both experiments had the beginning of signals in ZZ too (although at a slightly different putative Higgs mass value) and some early hints in other decay channels. There were multiple results supporting the diphotons, whereas in 2015 there are no apparent excesses in other channels indicating anything at 750 GeV.
And on top of that, there was something else going for the Higgs in December 2011: there was good reason to believe it was on the way. From a myriad of other experiments we had indirect evidence that a Higgs boson ought to exist, and in a mass range where the LHC effects were showing up. This indirect evidence came through the interpretation of the “standard model” theory that had done an excellent job of describing all other data in particle physics and thus gave us confidence that it could make predictions about the Higgs too. And for years, both the Tevatron and the LHC had been slowly but surely directly excluding other possible masses for the Higgs. If a Higgs were going to show up, it made perfect sense for it to happen right where the early effects were being observed, at just that level of significance with so little data.
Do we have any of that with the 750 GeV effect in 2015? No. There are no particular reasons to expect this decay with this rate at this mass (although in the wake of last week’s presentations, there have been many conjectures as to what kind of new physics could make this happen). Thus, one can’t help but to think that this is some kind of fluctuation. If you look at enough possible new-physics effects, you have a decent chance of seeing some number of fluctuations at this level, and that seems to be the most reasonable hypothesis right now.
But really there is no need to speculate. In 2016, the LHC should deliver ten times as much data as it did this year. That’s even better than what happened in 2012, when the LHC exceeded its 2011 performance by a mere factor of five. We can anticipate another set of presentations in December 2016, and by then we will know for sure if 2015 gave us a fluctuation or the first hint of a new physical theory that will set the research agenda of particle physics for years to come. And if it is the latter, I will be the first to admit that I got it wrong.