"The mu tau and e tau Decays of Top Quark Pairs Produced in p-pbar 
 Collisions at root s = 1.8 TeV"

In a paper submitted to Physical Review Letters, the CDF collaboration
presents four events as candidates for decays of the heaviest quark, called 
the top quark, into a bigger brother of the electron, known as the tau lepton.

The fundamental building blocks of matter, six different types of quarks and 
six different types of leptons, are found to come in three groups, called
"generations" by particle physicists. The most important difference between the
generations is the masses of the particles: the first generation comprises the
lightest particles, while the third generation includes the heaviest. For
example, the well-known electron is a lepton of the first generation. The
second generation contains the "muon" lepton, which is about 200 times heavier
than the electron. The "tau" lepton in the third  generation is about 3500
times as heavy as the electron. Similarly, the heaviest quark of all, called
the top quark, belongs to the third generation. It is more than 10,000 times
heavier than the quarks in the first generation. The origin of this hierarchy
of masses is little understood and is considered one of the most fundamental
problems in particle physics today.

Top quarks are unstable particles which decay quickly into bottom quarks plus
pairs of light quarks or pairs of leptons. The highly successful Standard Model
of Particle Physics predicts that top quark decays produce leptons from the
three generations (electrons, muons, taus) equally often. On the other hand,
certain extensions of the Standard Model, which attempt to address the problem
of particle masses, predict more decays into tau leptons than into electrons or
muons. This makes top quark decays that involve tau leptons an interesting
place to look for problems with the Standard Model, which could then provide
further insight to help physicists solve the mass problem.

When the top quark was discovered at Fermilab in 1995, only decays involving
electrons and muons were used because identifying tau leptons in the detectors
is more difficult. CDF reported seven events with either two electrons, two
muons, or an electron and a muon from the simultaneous decay of a top and an
antitop quark.

While electrons and muons leave quite specific signatures in the detector, tau
leptons themselves actually decay into electrons, muons, or hadrons (composite
particles made of quarks and/or antiquarks) and must be identified using the
properties of their decay products. CDF has now developed such an
identification technique for tau leptons that decay into hadrons. 

The Standard Model predicts that CDF should have observed about one event from
top quark decays into tau leptons. Because of the difficulty with the
identification of tau leptons it is possible that events that are not actually
from the decays of top quarks into tau leptons can be mistaken for top quark 
decays. These events are called background events and CDF expects to have
observed about two such background events. In their data CDF find four events
with an electron or muon plus a tau lepton candidate, consistent with
Standard Model expectations.

An additional handle for reducing the background is the identification of 
bottom quarks that are always produced in top quark decays but usually not in
background events. If the identification of at least one bottom quark is
required only 0.3 background events are expected in the data. In three of the
four candidate events a bottom quark has been identified. This supports the
hypothesis that some of the observed four events are caused by top quarks 
decaying into tau leptons.

Marcus Hohlmann [hohlmann@uccdf.uchicago.edu]

Michele Gallinaro [michgall@fnald.fnal.gov]

Henry Frisch [frisch@uccdf.uchicago.edu]