Tuesday, August 14, 2012

The Higgs, From The Tevatron With Love

So we had the preprint of the ATLAS and CMS paper on the possible discovery of the Higgs. Not to be left out in all of this, the grand old lady of high energy collider, the Tevatron, reaches out from the dead, and with her last gasp, reveals the guilty party and announces her own discovery of the Higgs. (Well, OK, so I've been watching too many Mystery shows on PBS).

Both the CDF and D0 detectors at the Tevatron combined their effort to publish this result in this week's PRL (you can get the paper free of charge from that link). To be sure, the evidence isn't as strong at that produced at the LHC a few weeks ago. But what is neat here is that they are looking at a different decay channels than both ATLAS and CMS. They could have easily thrown a wrench into the discovery if they had, in fact, come up with a result that is inconsistent with the LHC results. But they didn't.

In fact, experimentalists don’t directly detect the Higgs boson. Instead, they look at all the different sequences of particles—or “channels”—that the unstable Higgs boson decays into. ATLAS and CMS were able to detect the Higgs boson by looking for its decay into two photons and two Z bosons, and, albeit with somewhat weaker significance, two W bosons.

All of these channels involve the Higgs boson decaying into bosons, but if the Higgs particle explains the masses of quarks and leptons (the electron, muon, and tau), it should be possible to see it decay into these particles, which are fermions, too. (This is another way of saying that the leptons and quarks couple to the Higgs field.) Although CMS has analyzed their data to look for evidence that the new particle decays into fermions, namely into two tau leptons or two bottom quarks (b quarks), they haven’t observed a clear signal in this channel yet.
The new paper [1] combines the results of the CDF and the D0 experiments in one particular search channel, namely, the one where the Higgs boson decays into two bottom quarks (b quarks). In the combined dataset, the groups see an excess over what is expected from the background-only hypothesis, but is this excess caused by the same particle observed in the LHC experiments, and if so, does that mean that this Higgs particle indeed decays into fermions?
In any case, with the LHC continuing to run, and the planned increase to 14 TeV, I suspect that we will be hearing quite a bit more on the Higgs search during the next couple of years. This completely mystery isn't solved yet.


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