That's why I am quite happy today to report that CMS has again taken the lead in this LHC-internal competition. One which is less idle than it looks: the two detectors were designed, over ten years ago, with the explicit purpose of discovering and measuring the Higgs boson properties as precisely as possible. The hardware choices made by the two experiments were quite different, and so it is quite interesting to see who ended up having the upper hand in the precision of the Higgs boson mass measurement.
Measuring the mass of a particle with high precision entails not only collecting as many of its decays as possible -something which requires a high-acceptance, redundant detection system. It of course also requires that the subdetector components devoted to measuring the energies and momenta of the decay products are as precise as possible, and that they enable a successful intercalibration.
For the Higgs boson, which decays a few times in a thousand into a pair of energetic photons, determining the energy of those photons as precisely as possible became an obsession during the design and construction of the CMS detector. The electromagnetic calorimeter which does that job is a technological marvel, composed of thousands of crystals of lead tungstate, an optimal material due to its transparency, high atomic number, and response.
Similarly, the reconstruction of electrons and muons from Z boson decays became the other crucial focus of the detector builders, as the Higgs may decay to two Z bosons and those two Z bosons will sometimes yield a signature of four energetic electrons or muons. To measure those particles with high precision, CMS chose an all-silicon inner tracker, and a very redundant muon detection system made up of three different types of muon detectors. For electrons, the same electromagnetic calorimeter optimized for photons was of course the right tool.
So you can see how obtaining the best precision on the measurement of the mass of the Higgs boson can be seen as a proof of the winning strategy for detector design. And I am glad to see that once both LHC experiments have spelled their last word on the Higgs mass, the one coming out on top is indeed CMS.
(Above: the CMS spectrum of the invariant mass of photon pairs passing the Higgs candidates selection (top), and the background-subtracted signal at 125 GeV (bottom). The events entering these graphs have been weighted by the expected signal over signal plus background fraction in the various categories from which the data comes. So, e.g., events in the vector-boson-fusion category have a larger weight, as there is less background there. This method allows one to see more clearly the Higgs signal, in a way not dissimilar to how the likelihood "sees" the data.)
The measurement released today is MH=125.03 +- 0.30 GeV, a combination of the determinations (fully compatible, in the case of CMS) of the gamma-gamma and ZZ decay modes. If you compare it to the corresponding ATLAS number, MH=125.36+-0.41 GeV, you might at first sight say "what's the big deal?". Indeed, the two numbers are consistent with one another, and the uncertainties are both in the few tenths of a GeV region. However... 0.41 GeV is a lot larger than 0.30 GeV!
The ATLAS Higgs mass uncertainty is 37% larger than the CMS one. This would occur for two systematics-free, equally precise experiments if the latter had collected 85% more data than the former. In other words, the better CMS measurement (a result allegedly due to a better detector and/or better software algorithms for energy and momentum measurements) can be "translated" into saying that for every Higgs event collected by ATLAS, it is like CMS collected 1.85 !
The above is of course just a view on a small part of the whole set of measurements of Higgs boson properties that the two experiments have performed: cross section in the various production processes, branching ratios, couplings, spin, etcetera. In this post I do not have the time nor the energy to cover all of that, but I would not like to leave you with the impression that I am just picking the one bit where CMS is on top. In fact, the other measurements by the two experiments show a very good agreement -among each other and with Standard Model predictions- and in some cases ATLAS has a better precision; for instance, we know that ATLAS has since 2012 had a higher significance in its Higgs signals. Whether that was a statistical fluctuation, it is matter for another post.
For more information on the CMS results on the Higgs boson, released for the ICHEP conference going on in Valencia (Spain), please see the CMS press release here, which contains a link to the preprint. I will devote a different article to a more careful examination of those results.