Well, 2011 was certainly an interesting and exciting year for particle physics. And 2012 promises to be even better.
LHC and the Higgs search
At the Large Hadron Collider [LHC], the accelerator physics team did a fantastic job of assuring the collider worked effectively, and provided significantly more proton-proton collisions than were originally expected. Meanwhile the experimental teams found clever ways to dig more information out of the collision data than was initially anticipated. And thanks to this, the search for the Higgs particle (or Higgs particles, or whatever replaces the Higgs particle) is most of the way through Phase 1 — the search for the simplest possible form of Higgs particle, known as the “Standard Model Higgs”. We started 2011 knowing that the mass of the Standard Model Higgs particle might lie almost anywhere between 115 GeV/c2 and 800 GeV/c2 (where GeV, a measure of energy, is described here, and c is the speed of light, as in E = m c2.) The exception was a narrow gap around 160-170, excluded by the Tevatron experiments. We ended the year, thanks to the great work at the ATLAS and CMS experiments, with the Standard Model Higgs excluded everywhere except above 600 GeV/c2 (where it is disfavored for other reasons) and in a window between 115 and about 128 GeV/c2. Even more exciting, there is a serious hint of a Higgs particle signal at around 124–126 GeV/c2.
The hint emerges from a number of different preliminary measurements, and for this reason it is, in my view, still quite unstable. Toward the end of this month we may expect the final versions of these results to appear, and it will be interesting to see if the hint changes its character, becoming more or less solid. We will also see some gaps in the results filled in over time, perhaps by mid-March, and possibly an official combination of the ATLAS and CMS results. [Note Added: I’ve been reminded (thanks Ian Sample) that the Tevatron experiments CDF and DZero will also present final results combining their data, presumably in March. A Standard Model Higgs particle at 125 GeV would give something like a 2 to 3 sigma excess, though not in a narrow peak; the result might support or detract from the case but certainly won’t clinch it.] And finally, in late March or April, 2012 data taking at the LHC will begin again, quite likely with proton-proton collisions at 8 TeV, rather than the 7 TeV we saw in 2010 and 2011. (The decision about whether to increase the collision energy will be made in February; there are various pros and cons.) We will not see significantly new results on the Higgs search before summer, and only gradually will it become clear whether there is indeed a Higgs particle in nature at around 125 GeV/c2. So we have a many-months-long wait ahead of us, but we’ve waited a long time to get to this point… and it’s better to be right than hasty.
Meanwhile, the LHC experiments are actively analyzing their fall 2011 data looking for signs of new and unexpected phenomena having nothing to do with the Higgs per se. There will be a trickle of new results appearing over the next month, and this will gradually become a flood as March approaches. Since not only is there a lot of new data but also many new analysis techniques are being brought to bear, the opportunity for discoveries — or more likely, for hints of possible discoveries — is substantial. At worst, the predictions of many variants of an array of speculative theories will be tested, and if nothing unexpected is seen, falsified.
These analysis efforts will of course redouble when the 2012 data comes in. With luck, the amount of data collected in 2012 will be 3 to 4 times the amount collected in 2011, and (as just mentioned) possible at slightly higher energy. And then the LHC will most likely shut down for 2013 (and perhaps 2014) for upgrades and repairs, so that it can operate at 13-14 TeV per collision. However, there will be many more things to do with the 2011-2012 data set than can be completed by the end of 2012, so important scientific results will continue to emerge during the shutdown period.
2011 also brought us the faster-than-light neutrino claim of the OPERA experiment. Many attempts, both by members of OPERA and by outside researchers, to find a mistake in this claim have so far failed. OPERA itself made an important cross-check — using short neutrino pulses rather than the long ones that were used originally — that confirmed, at least, that there were no mistakes in a controversial aspect of the analysis technique. 2012 will apparently bring us a lot of news here too. Short neutrino pulses from CERN, I am told, will be used again in the coming months, and not only OPERA but also its neighbors in the Gran Sasso laboratory, ICARUS and BOREXINO, will try to duplicate OPERA’s measurement. They will do the timing and distance measurements on their own, starting from scratch and not relying on anything the OPERA team did. We will therefore have three experiments independently measuring the neutrinos from CERN. Furthermore, as I hoped, the experiments will collect enough data to allow the energy-dependence of any apparent speed-excess to be observed. If the faster-than-light effect refuses to go away, the energy-dependence of the effect will give us crucial insights into what is going on (either a real breakdown of special relativity, or some other subtlety with the time or distance measurement that everyone has somehow missed up to now.) And even before this happens, I understand we will likely hear something from the MINOS experiment, which measures neutrinos traveling from Fermilab (near Chicago) to the SOUDAN mine in Minnesota, and which can improve on their older neutrino-speed measurement simply by calibrating some of their electronics more carefully.
Of course, that’s not even close to a summary of all that is going on in particle physics and related areas in 2012. I haven’t mentioned the LHCb experiment at the LHC, which does indeed have a hint of a new phenomenon, and will have more to say about it in 2012, along with many other measurements. There are various experiments looking for dark matter to keep an eye on, and other searches for rare processes or hard-to-measure particles that might exist in nature are ongoing too. And the skies always may offer us the unexpected.
So keep tuned to this and related channels in 2012. I think we’re in for a wild ride this year — perhaps fun, perhaps instructive, or if we’re lucky, both.