Beam Asymmetry of Coherentπ° Photoproduction on4Hein theΔ(1232) Range M

Abstract

Using coherent bremsstrahlung at MAMI (Mainz), the beam asymmetry of the differential cross sections for coherent p ± photoproduction on 4 He has been measured in the energy interval from 200 to 500 MeV at an angle of u lab 37 ± . The coherent (g, p ± ) reaction is used as a precise linear polarimeter for the first time. Agreement with predictions has been obtained on a *3% level of precision. [S0031-9007 (97)04489-X] PACS numbers: 25.20.Lj, 24.70. + s, 29.27.Hj Linearly polarized photons from backscattered laserlight or coherent bremsstrahlung [1] are indispensable tools of intermediate-energy nuclear and hadron physics.In parallel to their applications there is a need to investigate these processes in order to achieve the highest possible precision in predicting the degrees of linear polarization. In this connection the beam asymmetry of coherent p ± photoproduction on 4 He is of special interest because this quantity fulfills all the conditions required for a precise polarimeter. Both the nucleus and the p ± meson have no spins, so that p ± mesons are exclusively emitted as p waves through M1 excitation of the D resonance, a channel which is already strongly favored for the free nucleon. As a consequence, the degree of linear polarization of the photon beam is completely transferred to the azimuthal asymmetry of the p ± mesons. In the cm system the differential cross section is given bywhere e denotes the polarization vector, k the direction of the photon, and q the direction of the pion. The form factor F͑ D͒ is related to the density of nucleon centers. C͑v͒ is a polarization independent normalization factor.The present experiment has been carried out using the Glasgow tagged-photon facility [2] at MAMI (Mainz) [3]. A 100 mm diamond crystal has been used to produce coherent bremsstrahlung [4,5]. The beam was collimated using different opening angles u coll . In this way the increase of the degree of linear polarization with decreasing u coll was investigated. Though improvements on the degree of linear polarization of coherent bremsstrahlung as a consequence of collimation have been discussed already for a long time [6], experiments have been carried out only to find collimation effects on the photon intensity [7-10], whereas the degree of linear polarization of collimated coherent bremsstrahlung has never been measured.The tagged photon facility at MAMI is especially suited for experiments of this type because the divergence if the electron beam is small (#0.1 mrad) and the electron energy is comparatively low (E 0 855 MeV) so that collimation effects can be seen at technically feasible collimation angles.The experimental arrangement was the same as described previously [11]. It consisted of the Mainz 48 cm diam 3 64 cm modular NaI(Tl) detector [12] positioned at a scattering angle of u lab 37 ± . This angle is close to the maximum of the angular dependent differential cross section for coherent p ± photoproduction. In order to suppress the cosmic-ray background the NaI(Tl) detector is surrounded by a modul...

Compton scattering by nuclei

“…Using Eqs. (3.70), (3.71) and (4.16) we arrive at the multipole decompositions 57) where the sum for the resonance amplitude is written with taking into account only the first three multipoles (E1, M 1 and E2), as in the order ω 2 higher multipoles do not contribute. Here we used the retarded formR GR of the resonance amplitude, as it is most appropriate at energies below the resonance region.…”

“…Brookhaven [50], Lund [51,52], Mainz [53,54], Saskatoon [55] -where experiments on Compton scattering by nuclei are carried out. Back scattering of laser light [50] and coherent bremsstrahlung from diamond crystals [56][57][58] are also efficient sources of linearly polarized photons. At low energies (5-100 MeV) also the method of off-axis tagging [33,59] leads to reasonable degrees of linear polarization.…”