From Moriond, Higgs Evidence Piles Up

A busy week and a computer crash has delayed my report on a number of new results on the Higgs particle from the current Moriond conference on particle physics, but the quiet not only on my blog but on some others should be a clue: the new results shown do not significantly change what we have previously known, and to the extent they do, they do not point to anything unexpected.

As a summary before I mention a few details, let me say that all in all, I think it is pretty safe now to award the Nobel prize to the theoretical physicists behind this story; last year was too early, but this year is not. Confidence is steadily growing that this “Higgs-like” particle really is a type of Higgs (Brout-Englert) (Guralnik-Hagen-Kibble) boson [what’s a boson?], and most alternatives are now significantly disfavored.  Whether it is the one and only type of Higgs particle in nature, and whether it is exactly of Standard Model type (the simplest possible type of Higgs particle), we cannot yet be sure, but its properties are more or less in line with what Higgs and friends proposed, enough to give them credit for having correctly imagined (to greater and lesser degrees) how nature might provide mass to force-carrier particles like the W and Z particles, and how we might test this notion experimentally. We should also remember some theorists who came before them and some who came after, but that’s a story for another day.

The fact that most of the new results are pretty similar to what we saw in November and December isn’t too surprising. This autumn’s results were mostly based on analysis of data from 2011 and from the first two-thirds of 2012. Since the additional data from the remainder of 2012 represents less than a doubling of the size of the data set, the new measurements, mostly using the full data set, are not drastically different from the old ones, except for

• those that were not updated in the autumn,
• those that used a new measurement technique in the current update, and
• those that were never previously performed.

Still, it should be noted that the most important result we are waiting for — the measurement by the CMS experiment (one of the two general purpose experiments at the Large Hadron Collider [LHC]) of the rate for Higgs particles to be produced and then decay to two photons [particles of light], apparently isn’t ready for presentation yet. Given the extreme importance of this measurement (see below), there’s a strong argument that it is better to get it right then to get it soon, so it is not surprising that the CMS experimental collaboration is taking its time. There are a number of conferences between now and the summer during which this result might appear.

Meanwhile, the results we did see this week are quite consistent with the hypothesis that the new particle is a Higgs particle, and most are reasonably consistent with the hypothesis that it is the simplest type of Higgs particle (a “Standard Model Higgs”).  You can expect a more detailed summary from me in the next week or two of these results, in which I’ll try to discuss the implications of what we’re learning. But here are the highlights expressed in qualitative form.

First, every attempt to check the properties of the new particle indicates it is a Higgs particle of some type.

• its decays to pairs of lepton/anti-lepton pairs and lepton/anti-neutrino pairs indicates it interacts with W and Z particles with strengths that are proportional to the W and Z particle masses, and with a form of interaction that would be expected for a particle that gives the W and Z particles their masses;
• it is produced in proton-proton collisions, and it decays to photons and to tau particles, with roughly the expected probabilities for a Higgs particle;
• it has the right intrinsic properties (specifically, the right “spin” [its intrinsic angular momentum] and the right “parity” [behavior of its properties when one imagines the world is reflected in a mirror]) to be a Higgs particle.

Second, there are no strong indications that the new particle isn’t a Higgs particle of the simplest possible type (a “Standard Model Higgs particle”.)

• The only measurements of its production and decay properties which are notably out of alignment with predictions for a Standard Model Higgs particle are in the process in which a Higgs is produced and then decays to two photons; this occurs about 1.65 times as often as expected, according to the ATLAS experiment’s new measurement, with uncertainties that put this result about 2.3 standard deviations above the Standard Model prediction. That’s not nearly enough to be anything more than intriguing [we’d want at least 3 standard deviations to get excited and something like 5 to be convinced], and the excess is slightly smaller (and of similar statistical significance) to their last measurement, which suggests the excess is gradually going away as more data is collected. CMS also found an excess in this decay process in July, but (apparently for reasons described here) have not updated their results since then… and as I said above, we are waiting anxiously for their result, because we can only start to take this violation of the expectation for a Standard Model Higgs seriously if both ATLAS and CMS find a statistically significant excess. With ATLAS’s result drifting downward over time, a widespread unofficial understanding that CMS’s measurement jumped downward when they looked at it in November (see here for related discussion), and a 15% theoretical uncertainty in the prediction in the overall rate for Higgs particle production in the Standard Model, optimism that this is a real effect is decreasing.
• I should mention that the new ATLAS measurement of Higgs decaying to two lepton/anti-lepton pairs is also somewhat more common than expected. But CMS’s new measurement of the same process is not. So there’s no sign of anything remarkable there.
• In December there was a little (and rather unjustified) flurry of interest in the fact that ATLAS’s measurement of the new particle’s mass in its decays to two photons was about 126.6 GeV/c² while the mass measurement in the particle’s decay to two lepton/anti-lepton pairs was 123.5 GeV/c² — which, when uncertainties were accounted for, represented a nearly 3 standard deviation discrepancy. The discrepancy has now decreased a bit; the latter measurement has crept up by nearly a full GeV/c², while the former shifted up by 0.2 GeV/c². But CMS’s mass measurements lie between these two, suggesting ATLAS’s two measurements are simply a bit high and a bit low, respectively, compared to the true value. (Again, CMS hasn’t updated its mass measurement from photons since July, so this will be something to watch when they finally release their photon-based result.) All indications are that there is nothing interesting going on here and that we’re just seeing some statistical effects combined most likely with some small problems (of 0.5-1.0% in size) with setting the overall energy scale in some of the measurements of particle energies. This will get sorted out over time, though perhaps not before we get more data in 2015 or so.
• Finally, measurements of processes that should be too rare to observe, if the Higgs is of Standard Model type, but that might be observable if the Higgs is of a more complex type, have found nothing: these include decays to a Z particle and a photon, to a muon/anti-muon pair [which should be rare since the muon has a small mass], to particles which would go undetected at ATLAS and CMS [such as neutrinos, dark matter particles, or something else unknown], to certain types of previously unknown long-lived particles that decay to known particles while crossing the detector, and to clusters of electron/positron [i.e. anti-electron] pairs.

In the near future we’ll be looking for CMS’s photon measurement, and we’ll be seeing more measurements looking for unexpected decays of the Higgs particle, and for unexpected ways that it might be produced, over the coming year.