Constraining the Higgs mass

In the standard model of particle physics, the predicted and much-sought Higgs boson (H) remains the principal missing link. The theory attributes the nonzero masses of the quarks, leptons, and weak vector bosons to their interaction with the H’s quantum field. Searches at CERN’s Large Electron–Positron collider have put a lower limit of 114 GeV (about 120 proton masses) on the H mass, and theoretical analysis of a variety of well-measured particle-physics parameters suggests an upper mass limit of about 185 GeV. Now a significant bite has been taken out of the interval 114–185 GeV by a new analysis of Higgs-search data accumulated in nine years of running at Fermilab’s 2-TeV Tevatron proton–antiproton collider. The analysis, a combined undertaking of the large CDF and D0 detector collaborations at the collider, concluded with a confidence limit of 95% that the H mass does not lie between 160 and 170 GeV—presuming that its production and decay properties are those predicted by the standard model (see the figure). The combined data set comes from 10¹⁵ proton–antiproton collisions, but the teams had to limit their searches to events that produced a W or Z weak vector boson. Only a few percent of H-producing collisions are expected also to produce a W or Z. But the decays of those very heavy particles are spectacular enough to provide a discernable signal under the haystack of routine events that would otherwise hopelessly obscure the tiny fraction of events that create an H. The two collaborations expect to accumulate a lot more data before CERN’s new 14-TeV Large Hadron Collider joins the Higgs search early next year. (CDF and D0 collaborations, http://arxiv.org/abs/0903.4001vl .)