Search for the Charged Higgs Decays of the Top Quark Using Hadronic
Decays of the Tau Lepton

The CDF experiment at Fermilab's Tevatron, the world's highest-energy
proton-antiproton collider, has completed a new study searching
for evidence of a new particle, the charged Higgs boson, in decays
of the top quark.  These results, submitted for publication in the
journal Physical Review Letters, set tight new minimum mass limits
on charged Higgs bosons over a range of theoretical assumptions.

In 1995 the CDF and D0 collaborations at Fermilab completed the
Standard Model picture of the matter particles called quarks with
the discovery of the top quark.  Only one matter particle predicted
by the Standard Model, the tau neutrino, has yet to be observed directly.

The top quark's surprisingly large mass, about 175 GeV/c^2, or 185
times the proton mass, underscored some of the main remaining outstanding
questions in the field of high-energy physics: what is the origin of
the quark and lepton masses?  Why are there three "generations" of
these fundamental particles, with very large differences in their
masses?

In the simplest version of the Standard Model, there is an as-yet-
undiscovered electrically neutral particle, the Higgs boson, which gives
masses to the matter fermions.  Quarks and leptons, having half-integer
spin, are the fundamental fermions, or "matter particles."  Particles
with integer spin, such as the photon, gluon, W, Z and Higgs, are the
fundamental bosons, or "force particles" mediating the interactions of
the matter particles.  So far, experiments seeking evidence for the Higgs
have determined that its mass must exceed about 70 GeV/c^2, roughly 75
times the mass of a proton.

But the Standard Model can not tell the whole story, since it provides no
account of its many free parameters, such as the masses and interaction
strengths of the fundamental fermions.  To help explain these, nature
could have a more complicated Higgs "sector".  One favored possibility has
five Higgs particles, three of which are electrically neutral, and two
of which have the same mass, and opposite charge (the same magnitude
as the electron charge).  Many theorists speculating on what might lie
beyond the Standard Model believe that nature respects a symmetry
between bosons (integer-spin particles such as photons) and fermions
(half-integer-spin particles).  These "supersymmetry" theories require
a Higgs sector with these five physical Higgs bosons.

The discovery of the top quark opens a new window for the search for
the charged Higgses, since if the charged Higgs exists and has a mass
less than that of the top quark, the top quark can decay to a charged
Higgs and a b quark.  This would compete with the Standard Model decay of
the top to a W boson and a b quark, first observed in 1995.

The charged Higgs, in turn, would decay almost exclusively to a tau lepton,
unlike the W, which can decay to quarks and the other leptons.  CDF has
looked for "top-quark-like" events in which there is evidence for a tau
lepton decay.  The researchers report seven observed events, with roughly
the same number expected from Standard Model background processes.  Had a
charged Higgs existed, the experimenters would have observed more than
twice this number.

These results rule out the possibility of a charged Higgs with mass less
than the top quark minus the b quark mass, if the "coupling" of the charged
Higgs to the top quark is large.  It is possible, though, in the theory,
that this coupling may be small and thus it will take more data from
future running at the Tevatron, and later at the LHC at CERN, to
definitively rule out the existence of this particle.

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