Lots of particle physics news from the Tevatron the past two weeks, including:
- the five-sigma observation of single top quark production in both CDF and D0,
- a new measurement of the W boson mass from the D0 collaboration, and
- a new combined result from CDF and D0 on the search for the Higgs boson.
At some level the first two have a bearing on the last one, which has started to get some attention in the media, including a nice interview with Prof. Heidi Schellman from Northwestern today on Science Friday, and a piece in Scientific American to which I contributed quotes.
The observation of the production of single top quarks (rather than the easier-to-see production of top-antitop quark pairs) has been a goal of the Tevatron experiments for years. The success of this analysis demonstrates that extremely complex dissections of the data like this can be undertaken, and reveal faint signals like that of single top. The search for the Higgs boson is more difficult still.
It all hinges on the mass of the Higgs boson. In different mass ranges it decays to different final states, changing the experimental approach and changing the sensitivity of the Tevatron and LHC experiments. The new measurement of the W mass by D0 adds a bit more knowledge about what the Higgs boson mass might be, since it depends on the mass of the W boson (the carrier of the weak force, which governs nuclear beta decay) and the mass of the top quark directly.
The plot at left demonstrates where we are right now; click on it for an annotated version. I call it the “billion dollar plot” but it probably cost a lot more than that to produce, because it shows the results of decades of experimentation at the Tevatron at Fermilab, LEP at CERN, the SLC at SLAC, and other measurements. As you can see (I hope) we are in a very interesting situation: the world’s data seem to indicate that the best Higgs boson mass is deep into the territory already excluded by LEP 2 in 2000! They set a 95% confidence bound on the Higgs mass at about 114 GeV. Their limit does not extend very far beyond that mass at all; it was limited by the energy of the LEP accelerator. The mass range above 114 GeV is open experimentally, but if you take the W mass/top mass constraint seriously (in the context of the Standard Model you must) then it would certainly appear very likely that the Higgs must lie in the range 114-185 GeV, with a strong preference for the lower end.
The new Tevatron result takes a new bite out of the upper end of the range, excluding from 160-170 GeV the “sweet spot” where the Higgs can decay to two W bosons. This is in some sense “first blood” for the Tevatron: at last the two experiments can exclude a Standard Model Higgs boson somewhere it hasn’t already been excluded!
But, to my mind, the interesting end of the range is at the low end. The data favor it, and theory favors it in the sense that if nature is more complicated, and supersymmetry is manifest, then one would expect that the light Higgs boson in supersymmetry exists in the range 120-130 GeV or so. In this picture there would be heavier Higgs bosons lying in wait for either the LHC or the Tevatron, though the LHC has the edge here with higher energy.
Assuming that the improvements in the analyses continue to outpace the data, as shown in the plot below, it is not impossible that the Tevatron could start to extend the region excluded by LEP, by this summer. But discovery? A three sigma result is possible with a good deal more data, but a five-sigma discovery looks very hard, and always has.
For a gold-plated, five-sigma-significance discovery, my money is on the LHC, I have to say. But the LHC will initially see the Higgs boson decaying to two photons, and we really need to see it decaying to two quarks or two leptons to really know its nature. I think the Tevatron could do that before the LHC, measuring the decay of the Higgs boson to two b quarks, and that alone is reason enough to keep the machine running until it does, to my mind, provided there are sufficient personnel to run the detectors and analyze the data. That decision, though, is above my pay grade…
The LHC will likely be able to see the Higgs boson decaying to two tau leptons before the Tevatron can see it decaying to two b quarks. Is that a race? I view the two observations as complementary, and they both add to the scientific picture. Without the Tevatron, it will just take longer.
One last word…the Higgs boson is damned hard to see, and when the LHC turns on, a ton of other new physics may pour out first. It is a very interesting year, that’s for sure!