| One of the greatest theoretical triumphs in the history of physics has
been the unification of the electromagnetic and weak forces. This
theory successfully predicted the masses of the W and Z bosons which
were later measured at CERN, and involves a mechanism that provides
all particles with mass. This mechanism also predicts the existence
of another observable particle, known as the Higgs boson. Experiments
at the LEP collider have placed a lower bound on its mass of 114 Gev/c2,
but direct measurement of the Higgs has thus far eluded all efforts.
Currently, the CDF and D0 experiments at Fermilab are pushing to probe
the mass regions not excluded by LEP with a number of analyses optimized
for masses extending from 100 to 200 GeV/c2. Near the LEP boundary where
the Higgs is expected to decay primarily to a pair of bottom quarks, the
most promising channels involve Higgs produced in association with a W
or Z boson. In particular, the ZH modes have very interesting properties
which can be taken advantage of at the analysis level. The mode in
which the Z decays to electrons or muons is extremely "clean", as leptons
from vector boson decay are typically well measured and all final state
particles are directly reconstructed. Conversely, the mode in which the Z
decays to neutrinos is extremely challenging, as the presence of the Z can
only be inferred from momentum imbalance provided by recoil with the Higgs.
Utilizing new analysis techniques developed to isolate a Higgs signal
amongst its seemingly overwhelming backgrounds, no significant excess of
signal has currently been observed. However, limits have been set on the
production cross section of a Higgs boson. Currently, limits of 16 times
the standard model expectation has been set in the ZH->llbb mode, and 8
times the standard model expectation in the ZH->vvbb mode for a Higgs
mass of 115 GeV/c2. Combining these results with all low mass analyses
at CDF and D0, the Tevatron has placed a limit of 3.7 times the standard
model expectation directly above LEP's lower mass limit. |