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Physics
The Fermilab Fixed Target program has long been concerned with understanding
the physics of charm hadron production and decays. The aim of E-781 was
to complement previous or contemporaneous work in hadroproduction and
photoproduction by emphasizing physics at large Feynman-x, where the
charm hadron carries off a large fraction of the incident beam momentum.
Most charm hadroproduction experiments have used only pion beams and
worked near xF = 0, where production of all types of secondary
particles
is maximal. Charm mesons are by far the dominant charm species in these
experiments. Empirical observations of the strange hyperons indicate that
the baryon/meson ratio increases at large xF. E-781 is unique
in its
ability to see whether this feature of hadroproduction also holds true for
heavy quark systems like charm. There are also important features of
charm hadroproduction that may depend on the incident beam particle.
E-781, using different beam hadrons from the Fermilab Hyperon beam,
is the only experiment that can address these issues.
E-781 employed a novel impact-parameter software trigger to select charm
candidates for writing to tape. Charm particles have a short but finite
decay length. A high-resolution vertex detector close to the production
point can select charm candidates based on the miss-distance of the
decay tracks evaluated at the primary production vertex. E-781 built a
50,000 strip silicon vertex detector system to reconstruct on-line all
high-momentum ( >15 GeV/c) tracks from each interactions with 6 micron
resolution. Events were recorded on tape only when the reconstruction
indicated that these tracks did NOT come from a single primary vertex
The goal was to take a large data set with a loose hardware trigger but
to avoid huge software overheads in extracting physics. The
full spectrometer, shown in
this Figure,
includes a two-stage
magnetic spectrometer and excellent particle identification information from
the downstream Ring-Imaging Cerenkov Counter. This is especially important
for identifying charm baryon decays in the large xF region.
Physics questions for charm studies have to do both with production and
decay mechanisms. In charm baryon decays the charm quark may decay or
interact through exchange mechanisms with the light quarks. Unlike meson
decays, there is no helicity suppression for exchanges, and a rich spectrum
of quasi-two-body decay modes may occur. Do they? There is little
experimental information on the question. Such a study requires
both good charged particle identification and good photon detection.
Comparison of non-leptonic and semi-leptonics decays are also important.
E-781 has good photon coverage, electron tagging and fast charged-particle
identification. They expect to make new studies of the higher-order
corrections to the charm decay mechanisms explored by combining Heavy Quark
Effective Theory and perturbative QCD.
Strong interaction physics can be studied in the production of charm hadrons.
There are open questions about possible direct charm content of non-charmed
mesons and nucleons, as well as color-drag effects in production at
large xF. Such studies demand comparisons between different beam hadrons
and also good acceptance at large xF. E-781 is designed to make these
studies.
The physics potential of the experiment touches many little-known areas
of heavy quark physics. The focus on charm baryons is especially
appropriate for a hadron machine. The experiment recorded
events from 15 x 109 inelastic collisions during the 1996-97 Fixed Target
period. The group developed
a run-time Data Summary Tape (DST) strategy for the first-level processing
pass, akin to the skimming pass of the Tevatron Collider experiments. They
identified interesting events during intial track reconstruction and
wrote out condensed records having only physics information and identifiers
for those events. Sample charm mass plots from this condensed output file
can be seen below. This has worked well. Initial physics results have been
presented at international conferences and have been submitted to journals.
Topics range from total cross section
measurements to precision charm hadron lifetimes to new features of
charm hadroproduction and decay.
The group is now preparing the second analysis
pass over all data to improve selection of the charm-strange baryons that
are an important objective of this charm baryon experiment. That pass will
be done by summer, 2001. Analysis should continue for another year
after that.
MP
