The Physics with Linearly-Polarized Photons in Hall B of JLab

Dr. Philip Cole
Idaho State University
Department of Physics

Constituent quark models, such as the Koniuk-Isgur model, successfully describe the low-mass baryon spectrum, particularly those resonances that decay through the pion-nucleon channel.  Furthermore, they predict the existence of a fairly large number of resonances that couple to the 2pi-N channel.  Although some of these resonances have been identified, far more are predicted than have yet been observed.  It is expected that several of these "missing baryons" will decay via the rho-N, omega-N, K-Lambda, and/or Delta-pi modes, especially those with masses above cms  energies of 1.7 GeV, which lie within the energy regime where the  gamma + N -->  pi + pi + N  channel dominates.  On the other hand, diquark models, which restrict the internal degrees of freedom of the quarks by requiring that two quarks be bound in a diquark pair, do not predict any additional resonances than have already been seen.  This pairwise binding of quarks reduces the internal degrees of freedom of the quarks, which thereby lowers the level density of baryon resonances over that predicted by symmetric quark models.

With the use of the photon tagger and the CLAS detector in Hall B of Jefferson Lab, we can accurately measure the the azimuthal and polar angular distributions of the decay pions as functions of the center-of-mass energy.  Employing a linearly-polarized beam of photons provides additional information on the spin and angular momentum degrees of freedom of the underlying quarks, such as the spin-parity of the state; this serves to disentangle the broadly overlapping resonances from one another.  Recently analyzed data from our run in the summer of 2001 will be shown.  This recent photoproduction experiment in concert with our future experiments (summer 2005) using a linearly-polarized  photon beam are expected to help settle the issue of the whether the  quarks are bound pairwise, or not, and thereby provide additional insight  into the nature of the strong force.