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.
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, email@example.com) 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)