R. Nasseripour

Absorption of the ω and ϕ mesons in nuclei.

Because of their long lifetimes, the ω and ϕ mesons are the ideal candidates for the study of possible modifications of the in-medium meson-nucleon interaction through their absorption inside the nucleus. During the E01-112 experiment at the Thomas Jefferson National Accelerator Facility, the mesons were photoproduced from 2H, C, Ti, Fe, and Pb targets. This Letter reports the first measurement of the ratio of nuclear transparencies for the e+e- channel. The ratios indicate larger in-medium widths compared with what have been reported in other reaction channels. The absorption of the ω meson is stronger than that reported by the CBELSA-TAPS experiment and cannot be explained by recent theoretical models.

Proton G_E/G_M from beam-target asymmetry

The ratio of the protons electric to magnetic form factor, G{sub E}/G{sub M}, can be extracted in elastic electron-proton scattering by measuring cross sections, beam-target asymmetry, or recoil polarization. Separate determinations of G{sub E}/G{sub M} by cross sections and recoil polarization observables disagree for Q{sup 2}>1 (GeV/c){sup 2}. Measurement by a third technique might uncover an unknown systematic error in either of the previous measurements. The beam-target asymmetry has been measured for elastic electron-proton scattering at Q{sup 2} = 1.51 (GeV/c){sup 2} for target spin orientation aligned perpendicular to the beam momentum direction. This is the largest Q{sup 2} at which G{sub E}/G{sub M} has been determined by a beam-target asymmetry experiment. The result, {mu}G{sub E}/G{sub M}=0.884{+-}0.027{+-}0.029, is compared to previous world data.

Probing the high momentum component of the deuteron at high Q2.

W.U. Boeglin, L. Coman, P. Ambrozewicz, K. Aniol, J. Arrington, G. Batigne, P. Bosted, A. Camsonne, G. Chang, J.P. Chen, S. Choi, A. Deur, M. Epstein, J.M. Finn, ∗ S. Frullani, C. Furget, F. Garibaldi, O. Gayou, 5 R. Gilman, 5 O. Hansen, D. Hayes, D.W. Higinbotham, W. Hinton, C. Hyde, H. Ibrahim, 11 C.W. de Jager, X. Jiang, M. K. Jones, L.J. Kaufman, † A. Klein, S. Kox, L. Kramer, G. Kumbartzki, J.M. Laget, J. LeRose, R. Lindgren, D.J. Margaziotis, P. Markowitz, K. McCormick, Z. Meziani, R. Michaels, B. Milbrath, J. Mitchell, ‡ P. Monaghan, M. Moteabbed, P. Moussiegt, R. Nasseripour, K. Paschke, C. Perdrisat, E. Piasetzky, V. Punjabi, I.A. Qattan, 3 G. Quéméner, R.D. Ransome, B. Raue, J.S. Réal, J. Reinhold, B. Reitz, R. Roché, M. Roedelbronn, A. Saha, ∗ K. Slifer, P. Solvignon, V. Sulkosky, § P.E. Ulmer, ‡ E. Voutier, L.B. Weinstein, B. Wojtsekhowski, and M. Zeier

Proton Spin Structure in the Resonance Region

We have examined the spin structure of the proton in the region of the nucleon resonances (1.085 GeV<W<1.910 GeV) at an average four momentum transfer of Q2=1.3 GeV2. Using the Jefferson Lab polarized electron beam, a spectrometer, and a polarized solid target, we measured the asymmetries A|| and A(perpendicular) to high precision, and extracted the asymmetries A1 and A2, and the spin structure functions g1 and g2. We found a notably nonzero A(perpendicular), significant contributions from higher-twist effects, and only weak support for polarized quark-hadron duality.

Probing Quark-Gluon Interactions with Transverse Polarized Scattering

We have extracted QCD matrix elements from our data on doubly polarized inelastic scattering of electrons on nuclei. We find the higher twist matrix element d˜2, which arises strictly from quark-gluon interactions, to be unambiguously nonzero. The data also reveal an isospin dependence of higher twist effects if we assume that the Burkhardt-Cottingham sum rule is valid. The fundamental Bjorken sum rule obtained from the a0 matrix element is satisfied at our low momentum transfer.

Medium modifications of light vector mesons in photoproduction reactions at JLab

Theoretical calculations predict the modification of properties of vector mesons, such as a shift in their masses and/or broadening of their widths in dense nuclear matter. These effects can be related to partial restoration of chiral symmetry at high density or temperature. Photoproduction of vector mesons off nuclei were performed at Jefferson Lab using the CEBAF Large Acceptance Spectrometer. The data were taken with a beam of tagged photons with energies up to 4 GeV on various nuclear targets. The properties of the ρ vector mesons were investigated via their rare leptonic decay to e+e−. This decay channel is preferred over hadronic modes in order to eliminate final state interactions in the nuclear matter. The combinatorial background in the mass spectrum was removed by a self-normalizing mixed-event technique. The ρ meson mass distributions were extracted for each of the targets. Statistically significant results regarding medium modification of the ρ meson in the nuclear medium rule out large medium effects. Transparency studies of the ω and vector mesons allow a determination of their widths in the medium.

Photodisintegration of 4 He into p + t

The two-body photodisintegration of

VECTOR MESON MODIFICATION IN NUCLEAR MATTER AT CLAS

^4

Search for medium effects on light vector mesons

He into a proton and a triton has been studied using the CEBAF Large-Acceptance Spectrometer (CLAS) at Jefferson Laboratory. Real photons produced with the Hall-B bremsstrahlung-tagging system in the energy range from 0.35 to 1.55 GeV were incident on a liquid

In-medium properties of light vector mesons

^4