Eta and two-pion electroproduction with CLAS

James A. Muellera for the CLAS collaboration

a Department of Physics and Astronomy, University of Pittsburgh

In the mass region above 1.5 GeV many overlapping baryon states are present and some
of them are not well known. There are also many baryon resonances predicted to be in
this mass range which have never been observed. To fully understand the fundamental
properties of baryon structure, the nucleon excitation spectrum must be measured in
a variety of reaction channels. In this paper, we present results on the reactions ep →
epπ+π− and ep → epη obtained with the CLAS detector at the Thomas Jefferson National
Laboratory. In the two-pion channel, the data show resonant structures not visible in
previous experiments. Phenomenological predictions based on available information on
known N∗ and ∆ states fail to match our results. In the η channel, results from CLAS
have already been published[1]. Now we present results from the an order of magnitude
more statistics, yielding a precise measurement of the proton-S11(1535) photocoupling
over a wider range in Q2 than any previous experiment. We also observe structure in
the differential cross section that is not predicted by current phenomenological models.
In both of these channels the observed discrepancies either indicate the presence of new
baryon resonances[2] or a significant revision of the properties of known states in the
models.

References
[1] R. Thompson, et al., (The CLAS Collaboration), Phys. Rev. Lett. 86 (2001) 1702.
[2] B. Saghai and Z.P. Li, Eur. Phys. J. A11 (2001) 217, S. Capstick, et al., Phys. Rev.
C59 (1999) 3002.