M. Lucas

First beam-target double-polarization measurements using polarized HD at legs

A new polarized target using HD in the solid phase has been developed for studies of the nucleon spin structure at Q(2) = 0 using pion photo-production. In combination with the high quality LEGS photon beam and a large solid angle spectrometer this target allows practically background-free measurements on the proton and on the neutron. The first beam-target double-polarization data taken with this target are reported here.

Recent Polarization Experiments and the GDH Sum Rule

Measurements of the spin‐difference cross sections entering the Gerasimov‐Drell‐Hearn (GDH) sum rule are reviewed. Results on the proton from Mainz and Bonn exceeded the GDH prediction by 22 μb, requiring as yet unmeasured canceling high‐energy components. Recent experiments with frozen‐spin HD at BNL reveal a different angular dependence for π0 production than what was assumed in Mainz analyses in lieu of direct measurements and integrate to a value that is 18 μb lower, suggesting a rapid convergence. Results for deuterium over limited energy ranges are consistent with large canceling contributions but differ from existing state of the art calculations.

Exclusive pion production from 16O using polarized photons

The (γ↘,π−p) reaction was measured for 16O at photon energies of 200 to 320 MeV, and angles ranging from about 20° to 160° for the proton and 32° to 135° for the pion. The photon asymmetry data are compared with calculations in a DWIA framework. The agreement between the present data and calculations is reasonable at quasifree kinematics, but disagree as the momentum transfer to the residual nucleus becomes large.

Measurements of the spin structure of the nucleon using SPHICE: A strongly polarized hydrogen and deuterium ice target

Frozen‐spin HD polarized targets operating between 0.4 and 4K, used with cold‐transfer (4K) techniques, provide great configurational flexibility. Their long depolarization times under target usage conditions assure reasonable match between polarization production and usage times, for weakly ionizing beam fluxes, and the very long relaxation times at fields above 7T (∼1 yr.) provide an economical storage mode and ‘‘off‐the‐shelf’’ availability.

The E2/M1 ratio in Δ photoproduction

New high-precision measurements of p (γ,π) and p (γ,γ) cross sections and beam asymmetries have been combined with other polarisation ratios in a simultaneous analysis of both reactions. The E 2/ M 1 mixing ratio for the N →Δ transition extracted from this analysis is EMR =−3.0%±0.3( stat+sys )±0.2 (model).

Status of the legs polarized HD target

A solid, polarized HD target has been developed for the measurement of double-polarization observables in the Delta resonance region. Since the use of the first target in-beam in November 2001, dramatic improvements have been made to the in-beam and transfer cryostats and the NMR circuit has been redesigned and is well understood.

Double-Polarization Experiments using Polarized HD at LEGS

A solid, polarized HD target has been developed for the measurement of double-polarization observables in the A resonance region. The focus of the experimental program is a comparison of pion photo-production amplitudes for the neutron and proton and the associated spin-dependent sum rules.

First pion photoproduction double-polarization results with polarized (H)over-right-arrow(D)over-right-arrow at LEGS

A new polarized target using HD in the solid phase has been developed for studies of the nucleon spin structure at Q(2) = 0 using pion photo-production. In combination with the high quality LEGS photon beam and a large solid angle spectrometer this target allows practically background-free measurements on the proton and on the neutron. The first beam-target double-polarization data taken with this target are reported here.

NEUTRAL PION PRODUCTION FROM DEUTERIUM AT THE LEGS FACILITY

Neutral pion photoproduction from a liquid deuterium target was measured in the energy region near 300 MeV at the LEGS facility of Brookhaven National Laboratory. The inclusive cross sections from deuterium are in agreement with measurments from Mainz, yet the exclusive cross sections and spin asymmetries for neutral pion production in coincidence with a detected nucleon are much smaller than expected from a quasi-free approximation. This may indicate that substantial final state interactions play a significant role, which will complicate the extraction of the desired amplitudes that would be measured if a free neutron target could be used.

The first pion photoproduction results from polarized HD at legs

A solid, polarized HD target has been developed for the measurement of double-polarization observables in the Delta resonance region. We report here the inaugural data obtained with this target. This new polarized target technology, combined with the high quality LEGS photon beam and the large acceptance spectrometer, SASY, provides a unique facility for studying the spin structure of the nucleon below 500 MeV. Pion production data collected on a longitudinally polarized target with six gamma-ray polarization states provides the first simultaneous measurement of d sigma/d Omega, as well as the Sigma, G and E asymmetries. With the future addition of magnetic analysis to SASY, a complete set of pion production observables on the proton and the deuteron (neutron) will be obtained.