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

E12-06-117: Quark Propagation and Hadron Formation

Professor Hicks is a co-spokesman of experiment E12-06-117 at Jefferson Lab to use the CLAS12 detector, which will be built as part of the 12 GeV upgrade that will start in 2009 and finish after 2011. Although this experiment is far in the future, it has already been approved the the international Proposal Advisory Committee at Jefferson Lab. The goals of the experiment are to measure the production of hadrons for exclusive deep inelastic scattering experiments using an electron beam incident on a variety of nuclear targets. Both transverse momentum broadening and attenuation due to interactions with the nuclear medium will provide the world's best data on effects such as gluon radiation. Hicks is particularly interested in the production of hadrons with strange quarks. The strange quark is likely to be produced due to "string-breaking" of the color flux tube, as the result of the struck quark propagating away from the parent nucleon.

In order to take data with CLAS12, Hicks and his students have taken on the task of helping to build the enhanced electromagnetic calorimeter, called the PCAL. Hicks has taken on a leadership role as PI (Principle Investigator) of a MRI (Major Research Instrumentation) proposal to the NSF (National Science Foundation). The proposal will also include colleagues from James Madison University, Norfolk State University, Yerevan Institute (Armenia) and the work will be done in coordination with staff at Jefferson Lab. (top of page)

E03-113: Search for Exotic Baryon States

Professor Hicks is a spokesman of experiment E03-113 at Jefferson Lab which searched for a purported narrow pentaquark resonance called the Theta+ which was first reported by the LEPS Collaboration (Japan) in 2003. The Theta+ was predicted to have a narrow width by a theoretical chiral soliton model in 1997 by Diakonov, Petrov and Polyakov. The structure of the Theta+ has 4 light quarks (two each of up and down flavors) and one strange antiquark. The photoproduction reaction on a neutron target with a final state of a kaon and an antikaon was investigated in this experiment to search for a narrow peak in the invariant mass of the neutron-kaon system. The result was that no peak was seen, which overturned an earlier, low-statistics measurment of the same reaction.

The results of this experiment received much attention by the international scientific community. The data was the PhD thesis of graduate student Brian McKinnon from Glasgow University. The results have now been published in the Journal Physical Review Letters, Volume 96 (2006). (top of page)

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 published in Physical Review C, Vol. 75 (2007). 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.

Theoretical models cannot explain the new E93-030 within experimental uncertainties. One model is by authors 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. Other models, using either hadrodynamic or Regge-trajectory frameworks, also give poor predictions of the E93-030 data. This suggests that we still have a lot to learn about resonant states of baryons. (top of page)

K0 Hadronization from Nuclei

Professor Hicks is the spokesman of this CLAS Approved Analysis focussing on the production of K0 mesons from the onset of deep inelastic scattering of 5-6 GeV electrons from nuclear targets. The K0 is produced is a state of definite strangeness, but mixes with both K0-short and K0-long CP eigenstates. Only the K0-short decay mode is used for these measurements, so both K0 and K0-bar photoproduction contribute. The goal of this study is to measure the attenuation of K0, as it propagates through the nuclear medium, as a function of the variable z (the ratio of the K0 total energy and the energy transfer from the scattered electron). From these data, theorists can deduce the interaction probability of the struck quark with the other quarks and gluons bound in the nucleus. A second observable of interest is the transverse momentum distribution, which is predicted to broaden as a result of gluon radiation, with a quadratic dependence on the nuclear radius. Preliminary results have been presented to the CLAS Collaboration, and the results are being analyzed by Hicks, with help by his students and colleagues in the CLAS Collaboration. After an internal review, the data are expected to be submitted for publication sometime in 2008. (top of page)

Coherent Phi-meson Photoproduction

Postdoctoral Researcher Tsutomu Mibe, working with Professor Hicks and other collaborators at CLAS, took the leading role on the analysis of Phi-meson photoproduction from a deuteron target. The Phi is unique among mesons in being a nearly pure state made from one s-quark and one s-antiquark. As a result, its dominant interaction with nucleons is via the exchange of gluons, through a process known as Pomeron exchange.

At high energies, the Pomeron exchange is thought to dominate in all meson-baryon interactions, but for the Phi meson, it also dominates near threshold energies. Theoretical calculations by Rogers, Frankfurt and Strickman show that photoproduction of the phi off the deuteron (where the deuteron remains bound) can be used to deduce the phi-nucleon scattering cross section under the assumption of vector meson dominance. The results have been published in Physical Review C, Volume 76 (2007), and suggest that the assumption of vector meson dominance is not the best fit to the data, but more precise measurements are needed before this conclusion can be reached. (top of page)

The S=+1 Z⁺(1520) Resonance

Professor Hicks is the spokesman of the CLAS Approved Analysis focussing on photoproduction of the pentaquark resonance once called the Z+ and later renamed as the Theta+. This analysis used the existing "g2" data at CLAS, where few-GeV photons were incident on a deuterium target, and the exclusive final state with a kaon, an antikaon, a proton and a neutron was isolated. Other final states were also investigated, but required additional assumptions on the Fermi momentum of the target neutron, which was not considered as reliable. In the exclusive channel, five parallel analyses were carried out, each with their own independent data selection criteria, and the results were reviewed by the CLAS Collaboration before being submitted for publication. The result had low-statistics, but indicated a narrow peak at a mass of 1540 MeV in the missing mass of the proton-antikaon system.

The analysis of the exclusive final state was first presented by Stepan Stepanyan (Jefferson Lab) who is the first author on the publication in Physical Review Letters, Volume 91 (2003). Later, this result was overturned by a higher-statistics experiment using a follow-up experiment with the same exclusive reaction measured at CLAS (see E03-113 above). (top of page)

K* Vector Meson Photoproduction at CLAS

Professor Hicks is the spokesman of the CLAS Approved Analysis focussing on K*0 photoproduction. 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. At CLAS, we have measured the reaction of few-GeV photons incident on a proton target and isolated the final state of: K*⁰(890) + ⁺(1189). These are the world's first cross section for K*0 photoproduction having reasonable statistics near threshold. The results are from the PhD thesis of OU graduate student Ishaq Hleiqawi and were published in Physical Review C, Volume 75 (2007). The cross sections were later corrected (due to a missing isospin factor of 3/2) in an Erratum in this journal of the same year.

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)