Tuesday, August 1, 2017

CMS Observes Higgs Boson Decays To Tau Leptons That Are Exactly As Predicted

Every new month of discoveries makes it that much more clear that the Higgs boson observed at the LHC is consistent with a theoretically predicted Higgs boson of the observed mass in every way.  
A measurement of the coupling strength of the Higgs boson to tau leptons is performed using events recorded in proton-proton collisions by the CMS experiment at the LHC in 2016 at a center-of-mass energy of 13 TeV. The data set corresponds to an integrated luminosity of 35.9 inverse femtobarns. The H to tau tau signal is established with a significance of 4.9 standard deviations, to be compared to an expected significance of 4.7 standard deviations. The best fit of the product of the observed H to tau tau signal production cross section and branching fraction is 1.09+0.27-0.26 times the standard model expectation. The combination with the corresponding measurement performed with data collected by the CMS experiment at center-of-mass energies of 7 and 8 TeV leads to an observed significance of 5.9 standard deviations, equal to the expected significance. This is the first observation of Higgs boson decays to tau leptons by a single experiment.
CMS Collaboration "Observation of the Higgs boson decay to a pair of tau leptons" (August 1, 2017).

The LHC has also ruled out hypothetical two Higgs doublet additional Higgs bosons predicted by supersymmetry over increasingly large mass ranges as a recent experimental result from the ATLAS experiment illustrates.

Experiment after experiment is ruling out beyond the Standard Model physics. And, the power of these observations is greatly underestimated, because in the Standard Model or any plausible extensions of it, everything is related to everything else at a high enough number of loops. You can't have just around the corner superpartners and extra Higgs bosons, without those particles wrecking havoc on the theoretical predictions for slightly lower energy scales and we're just not seeing that at all.

While individual experimental results consistent with the Standard Model don't by themselves rule out particular phenomena conclusively. The failure of experiments to detect strong experimental BSM signals anywhere over a robust panoply of different kinds of experimental methods magnifies the power of the individual results tremendously.

Realistically, we have reached a "new physics" desert that extends for many order of magnitude in energy scale from the electroweak scale where the Standard Model is situated.

At a minimum, there are no new fundamental particles between the top quark mass and masses dozens of times as large or more. Almost surely, there is not a fourth generation of Standard Model fermions. Higher order fundamental bosons are likewise very unlikely up to the many TeV scale.

Not every last mystery of high energy physics has been unraveled yet. Some of the finer points of hadron physics and neutrino physics still need to be worked out.

But, the Standard Model is now UV complete all of the way up to the GUT scale. There are really no HEP phenomena that require new particles or forces to explain. None of the lingering experimental anomalies are so large that they can't be resolved with better data and a minor theoretical insight or two.

Prospects for a GUT or a TOE look pretty bleak for the foreseeable future, but we are about three physical constants short of a complete Standard Model and not unthinkably far from a theory of quantum gravity that can replace dark matter and dark energy, giving us a complete, if ugly, collection of the laws of Nature. It may take more precision measurement to have a whole that is capable of being definitively tied to one particular theory. But, we're getting there.

While HEP experimentation seems to be at something of a dead end, new astronomy data continues to poor in, which makes it possible to distinguish between competing particle dark matter and modified gravity theories and dark energy theories based upon hard data rather than aesthetic concerns.

No comments: