The Higgs Search at Fermilab

The standard model of particle physics says that in the early moments of the universe many of the particles we know, such as the electron and the quark, were endowed with mass in a process called the Higgs mechanism (named for physicist Peter Higgs). Indeed the process is implicitly still at work, behind the scenes, and an associated particle for this purpose, the Higgs boson, should be lurking in the vacuum. By adding a lot of energy to a small volume of space, one should be able to make the Higgs show itself, and this effort is given by the highest priority by particle physicists.

Electron-positron collisions at CERN in Geneva might have accomplished this, but the CERN machine had to be shut down before enough evidence could definitely settle the issue (Update 502). The rudimentary data from CERN indicated a possible Higgs mass at around 115 GeV. Now, across the Atlantic, Fermilab's Tevatron machine will resume its operations in March 2001 and will run for five years, and it too will search for the Higgs. Fermilab can, in principle, search for Higgs's as massive as 180 GeV, but CERN's efforts will have helped Fermilab, at least at first, in sifting the incoming returns for signs of the Higgs. With its new higher luminosity (essentially the intensity of the beams), the Tevatron should be able to produce about 10¹⁵ proton-antiproton collisions.

In the small fraction of these in which a quark and antiquark meet nearly head on, a Higgs can be assembled mainly in three ways: (1) via the fusion of two gluons; (2) in the company of a W boson; and (3) accompanied by a top and an anti-top quark. According to current theoretical estimates these three modes should respectively produce about this many 115-GeV Higgs particles: 15,000, 4500, and 120.

Ironically it is mode 3 which is of greatest interest to a group of Fermilab scientists. Stephen Parke (630-840-4517, parke@fnal.gov) says that mode 1 is much better suited to producing heavier Higgs. Mode 3, although rarer than mode 2, has a more distinctive signature. In mode 3 each of the top quarks decays into a bottom (b) quark plus a W boson, while the Higgs itself decays quickly into a pair of b quarks (actually a b and anti-b). In turn the W's decay into either a lepton (such as electron or muon) plus neutrino or into two quark jets. The final ensemble of daughter particles would therefore be four b quarks, a lepton, two other quark jets, and a neutrino which, although it can't be detected, would leave a telltale shortfall of momentum in a particular direction. Because of this striking signature, and because the Tevatron detectors have greatly improved their sensitivity to b quarks, Parke feels that mode 3 will be "the Cinderella discovery mode for Higgs production, long overlooked but eventually the queen." The expected number of Higgs produced in this way, 120, is not a large number, and Fermilab will be under pressure to produce a higher-than-planned number of collisions. (Goldstein et al., Physical Review Letters, 26 February 2001; text at Physics News Select)