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Hyperon Particle Physics at the Joint Project
R. E. Mischke, Los Alamos National Laboratory
This paper presents a summary of information available on hyperon decays and
some of the relevant physics issues for these decays. It is an updated and expanded
version of a paper that was prepared for the International Workshop on JHF Science in
March, 1998 [1], and is intended to provide background material fora discussion of a
program in hyperon decay physics at the 50 GeV PS of the Joint Project. The outline
of a possible experiment to measure the asymmetry parameter in the decay A -+ ny is
included as an example of the potential to be offered by the Joint Project.
The study of hyperon decays has a history of over 40 years of experimental results.
The static properties of hyperons (masses, magnetic moments) are in relatively good
shape, but much work remains to be done on the decays of hyperons and hyperon
resonances, as has been documented in reviews and conferences [2]. Figure 1 plots the
date of references from the Particle Data Group compilation [3] for the stable hyperons.
While new results appear at a steady rate, many of the earliest results are still the best
available.
An understanding of the spectroscopy of the hyperons will make a major contribu-
tion to understanding the spectrum of baryons. The hadrons are the testing ground
for descriptions of non-perturbative QCD. Much progress is being made in the meson
sector with convincing evidence emerging for non qq states (glueballs, hybrids). To
make similar progress in the baryon sector will probably entail obtaining essentially a
complete set of all observables, which can then be subjected to a global analysis.
It has long been recognized that introducing a strange quark into hadronic matter
is a valuable tool to remove degeneracies and probe hadronic structure. Thus the study
of hyperons and hypernuclei has been fruitful, but limited by flux and resolution. The
50-GeV PS at the Joint Project will provide much improvement in beams of K mesons
over present facilities, both in flux and beam quality. This new capability may be
utilized to make a significant contribution to the understanding of hadrons.
The decays fall into four categories, three of which are weak transitions (semi-
leptonic, non-leptonic, and radiative) and one that is electromagnetic (radiative). A
comprehensive study of all types of hyperon decays is essential to understand the un-
derlying structure and the interrelated contributions from strong, electromagnetic, and
weak interactions. In particular, hyperon decays provide a superb laboratory to probe
the weak interactions of the quarks. This has always been a major thrust area for nu-
clear physics and hyperon 0 decays offer distinct advantages over neutrons and nuclei.
Semi-leptonic Weak Decays
Hyperon # decay can be viewed as a generalization of neutron 0 decays to systems
containing one or more strange quarks. In addition to the AS = 0 process encountered
exclusively in the neutron 0 decays, hyperon 0 decay can involve AS = 0 as well as
AS = 1. A large number of hyperon 0 decays are allowed kinematically, although only
two of them have been studied with reasonable statistics. Unlike the case for neutron /
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Mischke, R.E. Hyperon Particle Physics at the Joint Project, article, March 17, 1999; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc721034/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.