Speaker
Prof.
Reinhard Schumacher
(Carnegie Mellon University)
Description
An overview of a decade of elementary strangeness production results
from CLAS at Jefferson Lab will be presented. Photoproduction off the
proton of the ground state $\Lambda$ and $\Sigma^0$ states and kaons
has been instrumental in identifying the $N^*$ resonance structure of
the nucleon around 2 GeV. Spin observables, aiming at ``complete''
determination of the photoproduction amplitudes, promise to further
constrain the excitation spectrum of nucleons. Electroproduction
measurements have extended to non-zero 4-momentum transfer $Q^2$ the
structure function information about strangeness production.
Photoproduction of the excited hyperons, the $\Sigma^{0}(1385)$,
$\Lambda(1405)$, and $\Lambda(1520)$ in the reactions $\gamma + p \to
K^+ + Y^{\ast} \to K^+ + \Sigma + \pi$, can be compared to the hyperon
ground state reactions for the first time. The cross sections have been
compared to current theoretical models based on the effective
Lagrangian approach, with varying success. The cross sections for the
$\Lambda(1405)$ region are strikingly different for the
$\Sigma^+\pi^-$, $\Sigma^0\pi^0$, and $\Sigma^-\pi^+$ decay channels,
indicating the effect of isospin interference, especially at $W$
values close to the threshold. We show how this behavior is reflected
in the differing mass distributions for the $\Lambda(1405)$ in the
different decay channels. Chiral unitary models of the
$\Lambda(1405)$ and related non-strange baryonic states suggest how
the $\Lambda(1405)$ is a structure of several interfering poles. We
highlight also the first measurement of the spin and parity of the
$\Lambda(1405)$. Finally, we outline the next experimental steps to
be taken in strangeness electromagnetic production in the Jefferson
Lab 12 GeV era.
Author
Prof.
Reinhard Schumacher
(Carnegie Mellon University)
