Break-up and Coherent Photoproduction ofηMesons on the Deuteron

Abstract

We present new break-up and coherent data for h meson photoproduction on the deuteron, using a deuterium target and tagged bremsstrahlung photons up to 1 GeV. The differential cross sections for the coherent process were measured from threshold to 800 MeV. They are much smaller than those previously reported. The break-up channel provides a direct measurement of the neutron to proton differential cross section ratios. At the S 11 ͑1535͒ resonance peak, s n ͞s p 0.68 6 0.06 leading to an isoscalar to isovector amplitude ratio of A s ͞A y 0.096 6 0.02. [S0031-9007 (97)03324-3]

Production of η and η′ mesons on nucleons and nuclei

“…The cross section for the coherent reaction γd → dη will be much larger when the coupling is dominantly isoscalar. The experimental results [52,63] clearly favour a dominant isovector coupling and thus a negative relative sign. An alternative access to the sign comes from the interference term between the S 11 and D 13 states.…”

“…(2.1), this indicated that either the isoscalar or the isovector photons dominate the reaction. The small cross sections reported for the coherent γd → dη reaction [51,52,63] established the dominance of the isovector part (see [49] for details).…”

“…This has been exploited for η production, where measurements of the breakup reaction [51,52,53] revealed a neutron/proton cross section ratio of 2/3. According to Eq.…”

“…We now discuss in greater detail the results for η and η production. Inclusive photoproduction of η mesons from nuclei has been measured for the deuteron [43,51,52,53,55], for 3 He [45], and for heavier nuclei such as 12 C, 40 Ca, 63 Cu, 93 Nb, nat Pb [151,152,158,159] at MAMI, at KEK, in Tohoku, and at ELSA. In general, the agreement between the different measurements is good (see comparisons in [152]).…”

“…Early experiments measured only the inclusive reaction γd → η(np) (without the detection of recoil nucleons) [51] or detected only the recoil nucleons using missing-mass analysis [52]. The first fully exclusive measurement, which already used a complete reconstruction of the final state kinematics to remove the Fermi motion effects, was done in the threshold region [53].…”