PYSICS NEWS UPDATE A digest of physics news items by Phillip F. Schewe, American Institute

PYSICS NEWS UPDATE
A digest of physics news items by Phillip F. Schewe,
American Institute of Physics
Number 176  April 26, 1994

FERMILAB SEES SOME EVIDENCE FOR THE TOP QUARK at a mass of 174 GeV.  The
standard model of particle physics holds that all matter is made from a
small alphabet of elementary particles consisting of six quarks and six
leptons.  The heaviest of these, the top quark, is unstable and only exists
when it is created artificially in the high-energy proton-antiproton
collisions carried out at the Fermilab Tevatron.  Finding the top quark, a
major goal of Fermilab scientists for more than a decade, is important since
it is the only quark not yet observed.  Failure to find the top quark would
distrupt the current theory of basic particle physics.  Produced in
conjunction with an anti-top, the top quickly decays into a variety of
daughter particles. The best way to search for the top, theorists say, is to
look for its decay into a W boson and the next lightest quark, the b quark.
One of the chief problems, in this regard, is the fact that energetic b's
and W's are also unstable and quickly decay into the kind of particle jets
that emanate from less-interesting background collisions. For the purpose of
identifying a top quark, the cleanest event (a di-lepton event) is one in
which both W's decay leptonically, that is, into an electron or muon, plus a
neutrino.  But the W's can also decay into hadrons, the collective name for
protons, neutrons, and mesons.  In general the lepton decay events are less
liable to misinterpretation than are the hadron-decay events.  A third
category of event (a semi-leptonic event) is one in which one W decays
leptonically and the other hadronically.  A claim for the top quark would be
the observation of a number of events well above the number of events one
would expect from background processes. Fermilab's claim, which it carefully
cites as "evidence for" and not a "discovery of" the top quark, is of this
type.  Speaking at a seminar today at Fermilab, Melvyn Shochet of the
University of Chicago, chief spokesman of the 440-person CDF collaboration,
and William Carrithers of LBL, reported that an analysis of a trillion
collisions had netted a dozen candidate top events in an energy range around
170 GeV: 2 di-lepton events (with an estimated background of 0.13 events), 7
semi-leptonic events (compared to an expected 3 background events), and 6
non-leptonic events (2 background events); 3 events were counted twice.
Fermilab's way of assessing the "signal-to-noise ratio" for this data sample
is to say that there is only a 1-in-400 chance that the 12 events, compared
with the expected background, do not represent a top-quark signal.  The CDF
scientists have calculated the top mass to be 174 GeV with an uncertainty of
about 10% and the cross section (the inherent likelihood) for producing top
pairs at that mass to be about 14 pico-barns (1 barn equals 10**-24 cm**2).
In addressing the question of whether there is enough statistical
significance for an announcement at this time, Carrithers said that CDF was
on a "middle ground" where they could not claim a discovery nor yet dismiss
the presence of what looks like an excess of events at an interesting energy
range.  Meanwhile, Hugh Montgomery, a spokesman for the D0 collaboration,
CDF's rival at the Tevatron, reported at the same seminar that his team had
recorded a few interesting events but that these were comparable in number
to the expected background.  The data for both groups were recorded during a
run which stretched from August 1992 to June 1993.  They expect to take
several times more data in a run currently underway.