OU Strangeness Physics Program

Professor Hicks is spokesperson on several Jefferson Laboratory experiments using the CLAS spectrometer in Hall B that explore the production of strange quarks in reactions on the proton in order to better understand the structure of the nucleon and fundmental aspects of the strong nuclear force. These experiments are:

E93-030 - Spokespersons: K.H. Hicks (OU) and M.D. Mestayer (JLab) - Measurement of the Structure Functions for Kaon Electroproduction
E03-113 - Spokespersons: K.H. Hicks (OU) and S. Stepanyan (JLab) - Investigation of Exotic Baryon States in Photoproduction Reactions with CLAS
E12-06-117 - Spokespersons: W. Brooks (JLab), K.H. Hicks (OU) and others - Quark Propagation and Hadron Formation
K* CAA- Spokesperson: K.H. Hicks (OU) - K* Electroproduction from the Proton (CLAS Approved Analysis)
Z+ CAA- Spokesperson: K.H. Hicks (OU) - K0 Photoproduction of the S=+1, Z+(1520) Resonance using CLAS (CLAS Approved Analysis)
K0 CAA- Spokesperson: K.H. Hicks (OU) - K0 Hadronization from Nuclei (CLAS Approved Analysis)

Each of these experiments is described in more detail in the following sections.

E93-030: K+ Electroproduction at CLAS

Professor Hicks is a spokesman of experiment E93-030 at Jefferson Lab (JLab) which uses the CLAS spectrometer in Hall B. This experiment was designed to measure the four unpolarized structure functions, T, L, TT, and LT, which describe the dynamics of kaon electroproduction, over the range of Q2 from 1.0 to 2.5 GeV2 and W from threshold up to 2.2 GeV. The unique capabilities of the CLAS detector allow for detection of the kaon over practically the entire solid angle, making it possible to measure all four structure functions. The main goal of this experiment is to search for ``missing" nucleon resonances (N*s) that couple strongly to decay channels with strange particle final states. Several N*s and *s, such as the N*(1650), N*(1710), and *(1900), are known to decay into both strange and non-strange channels. Presently quark-model calculations predict more N* states than have been found experimentally in studies of reactions with pions, etas, and other non-strange meson final states. Quark models predict that some N*s are predicted to have large decay branching ratios to strange particle channels. Until recently, kaon photo- and electroproduction data were scarce or non-existent. Furthermore, kaon scattering data are nowhere near the quality of pion scattering data, and even then the complications of the strong interaction have prevented conclusive evidence for any missing N*s from the scant database that often has uncertainties, both statistical and systematic, of 15% or more.

The experimental analysis was carried out in parallel at OU and at Carnegie Mellon University (CMU). This work led to the Ph.D. of CMU graduate student Rob Feuerbach. The first paper from this experiment has been written and is undergoing internal review by the CLAS collaboration and should be submitted soon for publication. These data have statistical uncertainties of less than a few percent over a broad range of kinematics, about an order of magnitude better than the limited data presently available.

Comparison of theoretical models to the new E93-030 data is unsatisfactory. One of the most advanced models is that by Mart and Bennhold (MB). The MB model uses a hadrodynamic framework, where the coupling constants between baryons and mesons are fit to photoproduction data from SAPHIR (and earlier data from the 1970s). Even though the calculations fit the photoproduction data quite well, the electroproduction data are fit poorly, suggesting that either the electromagnetic form factors of the N* resonances (and/or those of hyperons and mesons with strangeness) are not correctly parameterized, or perhaps that the hadrodynamic model is not the proper framework for such calculations. The latter is unlikely since there is considerable freedom in the parameterization of the form factors. Other models, using either hadrodynamic or Regge-trajectory frameworks, also give poor predictions of the data. (top of page)

K* Vector Meson Electroproduction at CLAS

Professor Hicks is the spokesman of the CLAS Approved Analysis focussing on K* electroproduction. The K* meson plays a central role in hadrodynamic models of kaon photoproduction. In particular, the data cannot be fit without the assumption of the K* as a virtual meson in t-channel processes. However, the cross section for K* electroproduction has never been measured. At CLAS, we have measured the reaction: e + p --> e' + K*0(890) + +(1189) at a beam energy of 4.1 GeV. Preliminary cross sections were reported in the M.S. thesis of OU graduate student Adam Weisberg. This work was extended by the PhD thesis of OU graduate student Ishaq Hleiqawi to measure photoproduction. This analysis is designed to provide complementary information to the ongoing K+ experiments.

In principle, the coupling constants for K* production should be constrained by K+ photoproduction, since the K* is an important part of the t-channel. However, direct measurement of the K* can test these predictions, in addition to providing constraints on models of the vector meson reaction mechanism. Our results show an enhancement near threshold, with cross sections larger than expected from theoretical calculations. The larger mass of the K*, at about 890 MeV, allows one to investigate N* resonances at higher masses (W > 2.1 GeV) that decay to a simple two-body final state. These N* resonances may not be easily extracted from pion channels, due to the light mass of the pion (and the heavy mass of the N*). (top of page)

Last modified: October 21, 2007
Kenneth Hicks