H. Avakian

Measurement of Beam-Spin Asymmetries for Deep Inelastic pi{sup +} Electroproduction

We report the first evidence for a non-zero beam-spin azimuthal asymmetry in the electroproduction of positive pions in the deep-inelastic region. Data have been obtained using a polarized electron beam of 4.3 GeV and with the CLAS detector at the Thomas Jefferson National Accelerator Facility (JLab). The amplitude of the sin phi modulation increases with the momentum of the pion relative to the virtual photon, z, with an average amplitude of 0.038+/-0.005+/-0.003 for 0.5

Electroexcitation of the Roper resonance for 1.7<Q{sup 2}<4.5 GeV{sup 2} in e-vectorp{yields}en{pi}{sup +}

The helicity amplitudes of the electroexcitation of the Roper resonance are extracted for 1.7 < Q 2 < 4.5 GeV 2 from recent high precision JLab-CLAS cross section and longitudinally polarized beam asymmetry data for π + electroproduction on protons at W = 1.15 - 1.69 GeV. The analysis is made using two approaches, dispersion relations and a unitary isobar model, which give consistent Q 2 behavior of the helicity amplitudes for the γ* p → N(1440)P 11 transition. It is found that the transverse helicity amplitude A 1/2 , which is large and negative at Q 2 = 0, becomes large and positive at Q 2 ≃ 2GeV 2 , and then drops slowly with Q 2 . The longitudinal helicity amplitude S 1/2 , which was previously found from CLAS ep → epπ 0 , enπ + data to be large and positive at Q 2 = 0.4, 0.65 GeV 2 , drops with Q 2 . Available model predictions for γ*p → N(1440)P 11 allow us to conclude that these results provide strong evidence in favor of N(1440)P 11 as a first radial excitation of the 3q ground state. The results of the present paper also confirm the conclusion of our previous analysis for Q 2 < 1 GeV 2 that the presentation of N(1440)P 11 as a q 3 G hybrid state is ruled out.

Cascade production in the reactions {gamma}p{yields}K{sup +}K{sup +}(X) and {gamma}p{yields}K{sup +}K{sup +}{pi}{sup -}(X)

Photoproduction of the cascade resonances has been investigated in the reactions {gamma}p{yields}K{sup +}K{sup +}(X) and {gamma}p{yields}K{sup +}K{sup +}{pi}{sup -}(X). The mass splitting of the ground state ({xi}{sup -},{xi}{sup 0}) doublet is measured to be 5.4{+-}1.8 MeV/c{sup 2}, consistent with existing measurements. The differential (total) cross sections for the {xi}{sup -} have been determined for photon beam energies from 2.75 to 3.85 (4.75) GeV and are consistent with a production mechanism of Y*{yields}K{sup +}{xi}{sup -} through a t-channel process. The reaction {gamma}p{yields}K{sup +}K{sup +}{pi}{sup -}[{xi}{sup 0}] has also been investigated to search of excited cascade resonances. No significant signal of excited cascade states other than the {xi}{sup -}(1530) is observed. The cross-section results of the {xi}{sup -}(1530) have also been obtained for photon beam energies from 3.35 to 4.75 GeV.

Search for {theta}{sup +}(1540) Pentaquark in High-Statistics Measurement of {gamma}p{yields}K{sup 0}K{sup +}n at CLAS

The exclusive reaction gammap-->K0K+n was studied in the photon energy range between 1.6 and 3.8 GeV searching for evidence of the exotic baryon Theta+ (1540)-->nK+. The decay to nK+requires the assignment of strangeness S=+1 to any observed resonance. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility corresponding to an integrated luminosity of 70 pb-1. No evidence for the Theta+ pentaquark was found. Upper limits were set on the production cross section as function of center-of-mass angle and nK+ mass. The 95% C.L. upper limit on the total cross section for a narrow resonance at 1540 MeV was found to be 0.8 nb.

Single spin asymmetries in electroproduction at CLAS

We present measurements of spin asymmetries in semi-inclusive processes in hard scattering kinematics using a 5.7 GeV electron beam and the CEBAF Large Acceptance Spectrometer (CLAS) at JLab. Scattering of longitudinally polarized electrons of an unpolarized liquid-hydrogen and off a polarized NH{sub 3} targets was studied over a wide range of kinematics. Non-zero single-beam and single-target spin asymmetries have been observed in semi-inclusive pion production in hard-scattering kinematics (Q{sup 2} > 1.2 GeV{sup 2}, W{sup 4} > 4 GeV{sup 2}). Systematic studies of factorization of x and z dependences have been done for different spin-dependent and spin-independent observables. No significant x/z dependence has been observed within statistical uncertainties, which is consistent with factorization of hard scattering and fragmentation processes.