The study of the structure of nuclei containing strange
baryons
is one
of the frontier areas of nuclear
research. While there has been a
marked increase in the data available from (K-,p) and (p+,K+)
reactions, relatively little progress has been made in providing
unambiguous answers to questions relating to modification of
interactions, the symmetries and simple degrees of freedom that arise
when, in addition to neutrons
and protons, a third kind
of quasistable
particle
is introduced into the nuclear medium. Examples of such
questions are the relationships between 2-body and 3-body interactions
in complex hypernuclei and the existence and widths of bound
sigma-hypernuclear states. The proposal focuses upon hypernuclear
interactions in the lightest nuclei inasmuch as study of two-body and
few-body systems limits the complexity of possible interactions and
provides information in systems that are amenable to detailed
theoretical calculations.
The experiment involves coincident detection of scattered electrons
and kaons, i.e., (e,e'K+)
reactions, on targets of liquid or cooled,
pressurized D, 3He, and 4He. The HMS
and SOS
spectrometer systems in
Hall C will be used to detect
the emergent electrons and K+,
respectively. Since the reaction involves three-body (or more) final
states, each detector covers a broad range of energies for a given
final state or fixed missing mass. The focal plane detector packages
will be instrumented to provide optimum detection capability for
detection of kaons in the presence of high fluxes of protons and
pions.
A number of issues are addressed with each of the targets. The D
target will provide information about K-Nucleon-Y coupling constants,
where Y is either Lambda or Sigma (the masses of the Lambda and Sigma
differ by <80 MeV), and a unique method for studying the poorly
understood Y-neutron interaction at low relative energy. Of particular
interest is a cusp, expected to be seen near the threshold for Sigma
production, whose shape and magnitude depend upon the unknown phase
between the elementary production amplitudes for Lambda and
Sigma. More speculative are the possibilities for observing strange
di-baryons with masses predicted to be near the Sigma threshold. Bound
Lambda-hypernuclear states will be produced by the first studies
(e,e'K+) reactions on 3He and 4He targets. Investigation of the bound
state angular distributions will provide tests of the wave functions
used to describe these states, while the mass spectra will be examined
carefully to see if there is evidence for low-lying unbound
states. Other features of this study include possible threshold cusps
in mass three and four and searches for bound, or nearly bound,
Sigma-hypernuclei. Several purely hadronic studies have observed
unexpectedly narrow peaks at masses corresponding to Sigma states and
some theoretical models predict bound Sigma states in light
nuclei. Observation of such states via electroproduction of strange
particles will, therefore, have significant implications for future
experiments. This initial study will provide a foundation for further
investigation of the structure of complex hypernuclei. |