On the occasion of summer conferences the LHC experiments dumped a large number of new Higgs results. Most of them have already been advertised on blogs, see e.g. here or here or here. In case you missed anything, here I summarize the most interesting updates of the last few weeks.
1. Mass measurements.
ATLAS and CMS recently presented improved measurements of the Higgs boson mass in the diphoton and 4-lepton final states. The errors shrink to 400 MeV in ATLAS and 300 MeV in CMS. The news is that Higgs has lost some weight (the boson, not Peter). A naive combination of the ATLAS and CMS results yields the central value 125.15 GeV. The profound consequence is that, for another year at least, we will call it the 125 GeV particle, rather than the 125.5 GeV particle as before ;)
While the central values of the Higgs mass combinations quoted by ATLAS and CMS are very close, 125.36 vs 125.03 GeV, the individual inputs are still a bit apart from each other. Although the consistency of the ATLAS measurements in the diphoton and 4-lepton channels has improved, these two independent mass determinations differ by 1.5 GeV, which corresponds to a 2 sigma tension. Furthermore, the central values of the Higgs mass quoted by ATLAS and CMS differ by 1.3 GeV in the diphoton channel and by 1.1 in the 4-lepton channel, which also amount to 2 sigmish discrepancies. This could be just bad luck, or maybe the systematic errors are slightly larger than the experimentalists think.
2. Diphoton rate update.
released a new value of the Higgs signal strength in the diphoton channel. This CMS measurement was a bit of a roller-coaster: initially they measured an excess, then with the full dataset they reported a small deficit. After more work and more calibration they settled to the value 1.14+0.26-0.23 relative to the standard model prediction, in perfect agreement with the standard model. Meanwhile ATLAS is also revising the signal strength in this channel towards the standard model value. The number 1.29±0.30 quoted on the occasion of the mass measurement is not yet the final one; there will soon be a dedicated signal strength measurement with, most likely, a slightly smaller error. Nevertheless, we can safely announce that the celebrated Higgs diphoton excess is no more.
3. Off-shell Higgs.
Most of the LHC searches are concerned with an on-shell Higgs, that is when its 4-momentum squared is very close to its mass. This is where Higgs is most easily recognizable, since it can show as a bump in invariant mass distributions. However Higgs, like any quantum particle, can also appear as a virtual particle off-mass-shell and influence, in a subtler way, the cross section or differential distributions of various processes. One place where an off-shell Higgs may visible contribute is the pair production of on-shell Z bosons. In this case, the interference between gluon-gluon → Higgs → Z Z process and the non-Higgs one-loop Standard Model contribution to gluon-gluon → Z Z process can influence the cross section in a non-negligible way. At the beginning, these off-shell measurements were advertised as a model-independent Higgs width measurement, although now it is recognized the "model-independent" claim does not stand. Nevertheless, measuring the ratio of the off-shell and on-shell Higgs production provides qualitatively new information about the Higgs couplings and, under some specific assumptions, can be interpreted an indirect constraint on the Higgs width. Now both ATLAS and CMS quote the constraints on the Higgs width at the level of 5 times the Standard Model value. Currently, these results are not very useful in practice. Indeed, it would require a tremendous conspiracy to reconcile the current data with the Higgs width larger than 1.3 the standard model one. But a new front has been opened, and one hopes for much more interesting results in the future.
4. Tensor structure of Higgs couplings.
Another front that is being opened as we speak is constraining higher order Higgs couplings with a different tensor structure. So far, we have been given the so-called spin/parity measurements. That is to say, the LHC experiments imagine a 125 GeV particle with a different spin and/or parity than the Higgs, and the couplings to matter consistent with that hypothesis. Than they test whether this new particle or the standard model Higgs better describes the observed differential distributions of Higgs decay products. This has some appeal to general public and nobel committees but little practical meaning. That's because the current data, especially the Higgs signal strength measured in multiple channels, clearly show that the Higgs is, in the first approximation, the standard model one. New physics, if exists, may only be a small perturbation on top of the standard model couplings. The relevant question is how well we can constrain these perturbations. For example, possible couplings of the Higgs to the Z boson are
this note. What they do is not perfect yet, and the results are presented in an unnecessarily complicated fashion. In any case it's a step in the right direction: as the analysis improves and more statistics is accumulated in the next runs these measurements will become an important probe of new physics.
5. Flavor violating decays.
CMS turned to lepton flavor violation, searching for Higgs decays to τμ pairs. This decay cannot occur in the standard model, so the search is a clean null test. At the same time, the final state is relatively simple from the experimental point of view, thus this decay may be a sensitive probe of new physics. Amusingly, CMS sees a 2.5 sigma significant excess corresponding to the h→τμ branching fraction of order 1%. So we can entertain a possibility that Higgs holds the key to new physics and flavor hierarchies, at least until ATLAS comes out with its own measurement.