The Standard Model in physics seems to have split personalities. It's very obviously incomplete since it has no mechanism to give neutrinos mass, and it has no particles that correspond to dark matter. But it handles the phenomena it does include with a precision that seems to frustrate some physicists, who are anxious for signs of a new physics.

This was clearly demonstrated by the discovery of the Higgs boson, which showed up pretty much exactly as predicted. There were a couple of potential discrepancies between prediction and reality, but the researchers behind the ATLAS detector have now slammed the door shut on one of those.

The Higgs is a massive, unstable particle that decays almost as soon as it pops into existence. But it can decay down a number of different pathways, referred to as "channels" by the physicists who were searching them. When its discovery was announced, researchers had spotted the Higgs in two channels: decay into two high-energy gamma rays and decay into pairs of W or Z bosons, which would then decay into a total of four leptons.

What seemed to be missing was a decay into pairs of particles called taus (sometimes tauons). These particles are members of a group that includes the familiar electron and its heavier relative, the muon. Muons are also negatively charged, weigh nearly as much as a proton, and decay in about 10-6 seconds. Taus weigh over 10 times more and decay in about 10-13 seconds. Typically, these heavier particles decay down to the next-lighter version, releasing some energy in the process.

Because taus are so heavy and unstable, they barely move before decaying, meaning their decay typically happens amid the general debris of the proton-proton collision. (Muons, in contrast, move quickly enough that they can typically exit the detector despite their relative instability.) In addition, other processes like the decay of Z bosons can also produce two taus—all of which makes detecting a Higgs decay in this channel a significant challenge.

Nevertheless, using the full data set collected over the entire run of the Large Hadron Collider, researchers have now identified a series of Higgs decays via the two-tau channel. The significance of these results are 4.1 sigma, which wouldn't reach the five sigma level required for discovery; fortunately, they don't need to, since we've already reached that level in the other channels.

Finding decays in this channel is rather important from a physics perspective. The Higgs couples with these particles to give them mass and therefore should be able to decay via this channel. If we hadn't been able to detect it doing so, then it would be a sign of new physics, perhaps an indication that there's a second Higgs particle that performs some of the functions not handled by the one we know about. The discovery of these decays now closes the door on this prospect, undoubtedly to the disappointment of some physicists.