' K. Hicks5, K. Ardashev5, D. Babusci', M. Blecher", A. Caracappa', A. Cichocki", C. Commeaux7, A. D'Angelo', R. Deininger', J.-P. Didelez7, G: Giorda-ilo', S. Hoblit', P. Hoffmann-Rothe7, A. Honig", M. Khandaker4, 0. Kistner', A. Kuczewski', A. Lewis", R,. Lindgren'l, M. Lomy', M. Lucasg, G. Matone2, L. Miceli', T. Nakano', B. Norum", A. Opper', B. Preedomg, A. Sandofi', C. Schaerf8, H. Stroeher3, J. Tonnison'', C. Thorn1, K. Wang", X. Wei", S. Whisnantg

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.

We report preliminary results of pi photo-production using polarized gamma beams and a polarized HD target. Four observables can be extracted simultaneously from the data, the cross section, the beam asymmetry Sigma, and the double-polarization observables G and E. The latter determines the GDH sum rule integral.

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.

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.

We report new high-precision measurements of p(<(<gamma>)over right arrow>,gamma), p(<(<gamma>)over right arrow>,pi (o)) and p(<(<gamma>)over right arrow>,pi (+)) cross section and beam asymmetry angular distributions for photon beam energies in the range from 213 MeV to 333 MeV. The cross sections for all three channels are locked together with a small common systematic scale uncertainty of 2%. A large overdetermination of kinematic parameters was used to achieve the first complete separation of the Compton scattering and pi (o)-production channels. This has also allowed all detector efficiencies for the p(<(<gamma>)over right arrow>,gamma) and p(<(<gamma>)over right arrow>,pi (o)) channels to be measured directly from the data itself without resorting to simulations. The new Compton results are approximately 30% higher than previous Bonn data near the peak of the Delta resonance, resolving a long-standing unitarity puzzle. However, our p(<(<gamma>)over right arrow>,pi (o)) and p(<(<gamma>)over right arrow>,pi (+)) cross sections are also about 10% higher than both earlier Bonn data and recent Mainz measurements, while our p(<(<gamma>)over right arrow>,pi (+)) cross sections are in good agreement with results from Tokyo. Our polarization asymmetry data are of the highest precision yet available and have considerable impact upon multipole analyses. These new data have been combined with other polarization ratios in a simultaneous analysis of both Compton scattering and pi production, with Compton scattering providing two new constraints on the photopion amplitude, This analysis has improved the accuracy in the E2/M1 mixing ratio for the N-->Delta transition, EMR = -[3.07+/-0.26(stat+syst)+/-0.24(model)](%), and the corresponding N-->Delta transverse helicity amplitudes, A(1/2)= -[135.7+/-1.3(stat+syst) +/- 3.7(model)](10(-3) GeV-1/2) and A(3/2)=-[266.9+/-1.6(stat+syst) +/-7.8(model)](10(-3) GeV-1/2). From these we deduce an oblate spectroscopic deformation for the Delta (+). The same simultaneous analysis has been used to extract the proton dipole polarizabilities, <(<alpha>)over bar>-<(<beta>)over bar>= +[10.39 +/- 1.77( stat + syst) (+1.02)(-1.87)(model)](10(-4) fm(3)) in agreement with previous low energy measurements, and <(<alpha>)over bar>+<(<beta>)over bar>=+[13.25+/-0.86(stat+syst) (+0.23)(-0.58)(model)](10(-4) fm(3)) in agreement with recent evaluations of the Baldin sum rule. Our simultaneous analysis has also provided the first determination of the proton spin polarizabilities, gamma (pi)=-[27.23+/-2.27(stat+syst) (+2,24)(-2.10) (model)](10(-4) fm(4)), gamma (o)=-[1.55 +/-0.15(stat+syst) (+0.03)(-0.03)(model)](10(-4) fm(4)) gamma (13) = + [3.94 +/- 0.53(stat + syst) (+0.20)(-0.18)(model)](10(-4) fm(4)), and gamma (14) = -[2.20 +/- 0.27(stat + syst) (+0.05)(-0.09)(model)](10(-4) fm(4)). The extracted value of the backward spin polarizability, gamma (pi) is considerably different from other analyses and this has been instrumental in bringing the value of <(<alpha>)over bar> - <(<beta>)over bar> extracted from high energy data into agreement with low energy experiments.

We report new high-precision measurements of $p(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},\ensuremath{\gamma}),$ $p(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},{\ensuremath{\pi}}^{0})$ and $p(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},{\ensuremath{\pi}}^{+})$ cross section and beam asymmetry angular distributions for photon beam energies in the range from 213 MeV to 333 MeV. The cross sections for all three channels are locked together with a small common systematic scale uncertainty of 2%. A large overdetermination of kinematic parameters was used to achieve the first complete separation of the Compton scattering and ${\ensuremath{\pi}}^{0}$-production channels. This has also allowed all detector efficiencies for the $p(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},\ensuremath{\gamma})$ and $p(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},{\ensuremath{\pi}}^{0})$ channels to be measured directly from the data itself without resorting to simulations. The new Compton results are approximately 30% higher than previous Bonn data near the peak of the $\ensuremath{\Delta}$ resonance, resolving a long-standing unitarity puzzle. However, our $p(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},{\ensuremath{\pi}}^{0})$ and $p(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},{\ensuremath{\pi}}^{+})$ cross sections are also about 10% higher than both earlier Bonn data and recent Mainz measurements, while our $p(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},{\ensuremath{\pi}}^{+})$ cross sections are in good agreement with results from Tokyo. Our polarization asymmetry data are of the highest precision yet available and have considerable impact upon multipole analyses. These new data have been combined with other polarization ratios in a simultaneous analysis of both Compton scattering and $\ensuremath{\pi}$ production, with Compton scattering providing two new constraints on the photopion amplitude. This analysis has improved the accuracy in the $E2/M1$ mixing ratio for the $\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\Delta}$ transition, $EMR=\ensuremath{-}[3.07\ifmmode\pm\else\textpm\fi{}0.26(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}+\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\ifmmode\pm\else\textpm\fi{}0.24(\mathrm{model})](%),$ and the corresponding $\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\Delta}$ transverse helicity amplitudes, ${A}_{1/2}=\ensuremath{-}[135.7\ifmmode\pm\else\textpm\fi{}1.3(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}+\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\ifmmode\pm\else\textpm\fi{}3.7(\mathrm{model})]{(10}^{\ensuremath{-}3}{\mathrm{GeV}}^{\ensuremath{-}1/2})$ and ${A}_{3/2}=\ensuremath{-}[266.9\ifmmode\pm\else\textpm\fi{}1.6(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}+\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\ifmmode\pm\else\textpm\fi{}7.8(\mathrm{model})]{(10}^{\ensuremath{-}3}{\mathrm{GeV}}^{\ensuremath{-}1/2}).$ From these we deduce an oblate spectroscopic deformation for the ${\ensuremath{\Delta}}^{+}.$ The same simultaneous analysis has been used to extract the proton dipole polarizabilities, $\overline{\ensuremath{\alpha}}\ensuremath{-}\overline{\ensuremath{\beta}}=+[10.39\ifmmode\pm\else\textpm\fi{}1.77(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}+\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}{)}_{\ensuremath{-}1.87}^{+1.02}(\mathrm{model})]{(10}^{\ensuremath{-}4}{\mathrm{fm}}^{3})$ in agreement with previous low energy measurements, and $\overline{\ensuremath{\alpha}}+\overline{\ensuremath{\beta}}=+[13.25\ifmmode\pm\else\textpm\fi{}0.86(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}+\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}{)}_{\ensuremath{-}0.58}^{+0.23}(\mathrm{model})]{(10}^{\ensuremath{-}4}{\mathrm{fm}}^{3})$ in agreement with recent evaluations of the Baldin sum rule. Our simultaneous analysis has also provided the first determination of the proton spin polarizabilities, ${\ensuremath{\gamma}}_{\ensuremath{\pi}}=\ensuremath{-}[27.23\ifmmode\pm\else\textpm\fi{}2.27(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}+\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}{)}_{\ensuremath{-}2.10}^{+2.24}(\mathrm{model})]{(10}^{\ensuremath{-}4}{\mathrm{fm}}^{4}),$ ${\ensuremath{\gamma}}_{0}=\ensuremath{-}[1.55$$\ifmmode\pm\else\textpm\fi{}0.15(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}+\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}{)}_{\ensuremath{-}0.03}^{+0.03}(\mathrm{model})]{(10}^{\ensuremath{-}4}{\mathrm{fm}}^{4}),$ ${\ensuremath{\gamma}}_{13}=+[3.94\ifmmode\pm\else\textpm\fi{}0.53(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}+\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}{)}_{\ensuremath{-}0.18}^{+0.20}(\mathrm{model})]{(10}^{\ensuremath{-}4}{\mathrm{fm}}^{4}),$ and ${\ensuremath{\gamma}}_{14}=\ensuremath{-}[2.20\ifmmode\pm\else\textpm\fi{}0.27(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}+\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}{)}_{\ensuremath{-}0.09}^{+0.05}(\mathrm{model})]{(10}^{\ensuremath{-}4}{\mathrm{fm}}^{4}).$ The extracted value of the backward spin polarizability, ${\ensuremath{\gamma}}_{\ensuremath{\pi}},$ is considerably different from other analyses and this has been instrumental in bringing the value of $\overline{\ensuremath{\alpha}}\ensuremath{-}\overline{\ensuremath{\beta}}$ extracted from high energy data into agreement with low energy experiments.

We report new high-precision measurements of p(g ,g), p(g ,p 0 ) and p(g ,p 1 ) cross section and beam asymmetry angular distributions for photon beam energies in the range from 213 MeV to 333 MeV. The cross sections for all three channels are locked together with a small common systematic scale uncertainty of 2%. A large overdetermination of kinematic parameters was used to achieve the first complete separation of the Compton scattering and p 0 -production channels. This has also allowed all detector efficiencies for the p(g W ,g) and p(g W ,p 0 ) channels to be measured directly from the data itself without resorting to simulations. The new Compton results are approximately 30% higher than previous Bonn data near the peak of the D resonance, resolving a long-standing unitarity puzzle. However, our p(g ,p 0 ) and p(g ,p 1 ) cross sections are also about 10% higher than both earlier Bonn data and recent Mainz measurements, while our p(g ,p 1 ) cross sections are in good agreement with results from Tokyo. Our polarization asymmetry data are of the highest precision yet available and have considerable impact upon multipole analyses. These new data have been combined with other polarization ratios in a simultaneous analysis of both Compton scattering and p production, with Compton scattering providing two new constraints on the photopion amplitude. This analysis has improved the accuracy in the E2/M1 mixing ratio for the N!D transition, EMR 52@3.0760.26(stat1syst) 60.24(model)#(%), and the corresponding N!D transverse helicity amplitudes, A1/252@135.761.3(stat1syst) 63.7(model)#(10 23 GeV 21/2 ) and A3/252@266.961.6(stat1syst) 67.8(model)#(10 23 GeV 21/2 ). From these we deduce an oblate spectroscopic deformation for the D 1 . The same simultaneous analysis has been used to extract the proton dipole polarizabilities, a2b51@10.39

We report new high-precision measurements of p({rvec {gamma}},{gamma}), p({rvec {gamma}},{pi}{sup 0}) and p({rvec {gamma}},{pi}{sup +}) cross section and beam asymmetry angular distributions for photon beam energies in the range from 213 MeV to 333 MeV. The cross sections for all three channels are locked together with a small common systematic scale uncertainty of 2%. A large overdetermination of kinematic parameters was used to achieve the first complete separation of the Compton scattering and {pi}{sup 0}-production channels. This has also allowed all detector efficiencies for the p({rvec {gamma}},{gamma}) and p({rvec {gamma}},{pi}{sup 0}) channels to be measured directly from the data itself without resorting to simulations. The new Compton results are approximately 30% higher than previous Bonn data near the peak of the {Delta} resonance, resolving a long-standing unitarity puzzle. However, our p({rvec {gamma}},{pi}{sup 0}) and p({rvec {gamma}},{pi}{sup +}) cross sections are also about 10% higher than both earlier Bonn data and recent Mainz measurements, while our p({rvec {gamma}},{pi}{sup +}) cross sections are in good agreement with results from Tokyo. Our polarization asymmetry data are of the highest precision yet available and have considerable impact upon multipole analyses. These new data have been combined with other polarization ratios in a simultaneous analysismore » of both Compton scattering and {pi} production, with Compton scattering providing two new constraints on the photopion amplitude. This analysis has improved the accuracy in the E2/M1 mixing ratio for the N{yields}{Delta} transition, EMR=-[3.07{+-}0.26(stat+syst){+-}0.24(model)](%), and the corresponding N{yields}{Delta} transverse helicity amplitudes, A{sub 1/2}=-[135.7{+-}1.3(stat+syst){+-}3.7(model)](10{sup -3}GeV{sup -1/2}) and A{sub 3/2}=-[266.9{+-}1.6(stat+syst){+-}7.8(model)](10{sup -3}GeV{sup -1/2}). From these we deduce an oblate spectroscopic deformation for the {Delta}{sup +}. The same simultaneous analysis has been used to extract the proton dipole polarizabilities, {bar {alpha}}-{bar {beta}}=+[10.39{+-}1.77(stat+syst){sub -1.87}{sup +1.02}(model)](10{sup -4}fm{sup 3}) in agreement with previous low energy measurements, and {bar {alpha}}+{bar {beta}}=+[13.25{+-}0.86(stat+syst){sub -0.58}{sup +0.23}(model)](10{sup -4}fm{sup 3}) in agreement with recent evaluations of the Baldin sum rule. Our simultaneous analysis has also provided the first determination of the proton spin polarizabilities, {gamma}{sub {pi}}=-[27.23{+-}2.27(stat+syst){sub -2.10}{sup +2.24}(model)](10{sup -4}fm{sup 4}), {gamma}{sub 0}=-[1.55{+-}0.15(stat+syst){sub -0.03}{sup +0.03}(model)](10{sup -4}fm{sup 4}), {gamma}{sub 13}=+[3.94{+-}0.53(stat+syst){sub -0.18}{sup +0.20}(model)](10{sup -4}fm{sup 4}), and {gamma}{sub 14}=-[2.20{+-}0.27(stat+syst){sub -0.09}{sup +0.05}(model)](10{sup -4}fm{sup 4}). The extracted value of the backward spin polarizability, {gamma}{sub {pi}}, is considerably different from other analyses and this has been instrumental in bringing the value of {bar {alpha}}-{bar {beta}} extracted from high energy data into agreement with low energy experiments.« less

Cross sections are presented for the ${}^{16}\mathrm{O}(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},{\ensuremath{\pi}}^{\ensuremath{-}}p)$ reaction at incident photon energies between 290 and 325 MeV. The data are presented for specific proton and pion angles as a function of proton energy, which are compared with calculations in a local distorted wave impulse approximation model. The results are in agreement at most kinematics, although at some kinematics the data and calculations disagree by a factor of 2 or more. These data do not support the conclusion of a large modification to the mass of the $\ensuremath{\Delta}$ resonance in the nucleus.

Cross sections are presented for the {sup 16}O({gamma}(vector sign),{pi}{sup -}p) reaction at incident photon energies between 290 and 325 MeV. The data are presented for specific proton and pion angles as a function of proton energy, which are compared with calculations in a local distorted wave impulse approximation model. The results are in agreement at most kinematics, although at some kinematics the data and calculations disagree by a factor of 2 or more. These data do not support the conclusion of a large modification to the mass of the {delta} resonance in the nucleus. (c) 2000 The American Physical Society.