Today the ATLAS experiment at CERN announced the strongest evidence so far that the Higgs gives mass to leptons

At a seminar at CERN this morning, Aliaksandr Pranko announced an interesting new result on the Higgs boson from the ATLAS experiment.

In brief - we know the Higgs boson is there, and is involved in giving mass to some fundamental particles. Today we saw strong evidence that it gives mass to all of them.

In more detail...

In the Standard Model of physics, the fundamental building blocks of nature are quarks (which live inside hadrons) and leptons (such as the electron, and its heavier siblings, the muon and the tau). These building blocks interact with each other via fundamental forces carried by bosons - the photon carries electromagnetism, the W and Z bosons carry the weak nuclear force, and the gluon carries the strong force.

All those particles (except the photon and the gluon, which are massless) acquire their mass by interacting with the Higgs boson, the discovery of which was announced last year on the fourth of July.

When produced, a Higgs boson decays very quickly, and there are several options open to it in terms of what particles it can produce when it decays. If a particle gets its mass from the Higgs boson, then the Higgs boson will sometimes decay to that particle.

The Higgs discovery was made by observing an excess of events in which pairs of photons were seen, and in which four leptons were seen, probably coming from the Higgs decaying to pairs of Z bosons. You can see the "bumps" (animated!) which betray the existence of a new boson here. We also saw strong evidence for Higgs bosons decaying to W bosons.

All of this was compelling evidence that the new boson is responsible for the masses of the W and Z bosons. That was enough to get Englert and Higgs the Nobel prize.

However, in the Standard Model, the Higgs has to give mass to the matter particles as well as the force carrying particles. And while we have indirect evidence that it gives mass to quarks, there was, until today, precious little to show that it gave mass to leptons.

What ATLAS announced today was very strong evidence for the Higgs boson decaying to tau leptons. Tau leptons themselves decay pretty quickly, and produce at least one invisible neutrino when they do so, so spotting them is hard and reconstructing a bump in a distribution is hard. The evidence is the result of a complex analysis which makes use of the fact the we now know the mass of the Higgs. I think the best distribution to visualise it is this one:

Mass distribution of tau pairs reconstructed in the ATLAS detector. The fact that the red line (which includes a Higgs decaying to taus) agrees better with the data (black spot) than the other lines is evidence that the Higgs does, indeed, give mass to leptons

... although if you are au fait with boosted decision trees there a more compelling one in the presentation linked from the talk page above (slide 39). The evidence is 4.1 sigma, which is more than the 3 sigma threshold for taking it seriously as evidence, but below the 5 sigma we conventionally require for a discovery. And it is still preliminary - the paper backing it up has not yet been released. But still, it is out there, public and in my opinion pretty solid, so I thought you might like to know.