First, you may recall a tempest in a teapot that erupted in late 2012, when ATLAS’s two measurements of the Higgs particle’s mass disagreed with each other by more than one would normally expect. This generated some discussion over coffee breaks, and some silly articles in on-line science magazines, even in Scientific American. But many reasonable scientists presumed that this was likely a combination of a statistical fluke and a small measurement problem of some kind at ATLAS. The new results support this interpretation. ATLAS, through some hard work that will be described in papers that will appear within the next couple of days, has greatly improved their measurements, with the effect that now the discrepancy between the two measurements, now dominated by statistical uncertainties, has gone down from nearly 3 standard deviations to 2 standard deviations, which certainly isn’t anything to get excited about. Experts will be very impressed at the reduction in the ATLAS systematic uncertainties, arrived at through significantly improved energy calibrations for electrons, photons and muons.
Experts: More specifically, the measured mass of the Higgs in its decay to two photons decreased by 0.8 GeV/c², and the systematic uncertainty on the measurement dropped from 0.7 GeV/c2 to 0.28 GeV/c2. And by the way, the rate for this process is now only 1 standard deviation higher than predicted for the simplest possible type of Higgs (a “Standard Model Higgs“); it was once 2 standard deviations high, which got a lot of attention, but was apparently just a fluke.
Meanwhile, for the decays to two lepton/anti-lepton pairs, the systematic error has dropped by a factor of ten — truly remarkable — from 0.5 GeV/c2 to 0.05 GeV/c2. The Higgs mass measurement itself has increased by 0.2 GeV/c2.
Second, as reported by the CMS experiment a couple of months ago, the lifetime of the Higgs particle has been constrained, using a clever method developed in papers by Kauer and Passarino, by Campbell, Ellis and Williams, and by Caola and Melnikov. For a “Standard Model Higgs” (the simplest possible type of Higgs particle) that has a mass around 125 GeV/c², the lifetime of the Higgs is predicted to be about 150 trillionths of a trillionth of a second. According to CMS, the lifetime has now been measured to be at
most 6 times that large [oops! at least 1/6th as long a lifetime as predicted — sorry] (though there’s still some debate about how precise that measurement really is.) [No, we don’t measure this lifetime with a stopwatch. The clever method involves noting that a Higgs particle with an unexpectedly short lifetime can lead, via a quantum uncertainty principle, to a big increase in the rate for the production of pairs of real Z particles. (Recall the Higgs particle itself can only decay to one real and one virtual Z particle.)]
Third, both experiments are trying to make measurements of the Higgs particle decaying to tau lepton/anti-lepton pairs and to bottom quark/anti-quark pairs. The measurements aren’t very precise yet, but there’s now strong evidence for the tau decays. And a rough measurement of the Higgs particle’s mass in its tau decays is within a few GeV/c² of the measurements made via the more precise methods mentioned above… so all seems consistent.