Incoherent φ -meson photoproduction from deuterium 146 Nuclear Physics A strong attenuation was observed in the φ -meson photoproduction from the Li, C, Al and Cu nuclei, compared to its production from hydrogen, the free proton case. It has been interpreted as the nuclear density effect due to a large φ -N cross section in nuclear matter. To clarify the underlying physics, it is essential to construct a baseline in a low density region, using the simplest nucleus –the deuteron– made of one proton and one neutron. We measured the differential cross sections and decay asymmetries of incoherent φ -meson photoproduction from a liquid deuterium target near threshold in the very forward direction with linearly polarized photons using the LEPS spectrometer at beamline BL33LEP . Highly polarized photons were produced by backward Compton scattering with an ultra-violet Ar laser from 8 GeV electrons in the storage ring of SPring-8. The photon energy was event-by-event determined by measuring the scattering angle of recoil electrons. A liquid deuterium target with an effective length of 16 cm was employed. Charged particles produced at the target were detected at forward angles with the LEPS spectrometer, which consisted of a dipole magnet, a silicon-strip vertex detector, three multi-wire drift chambers, a plastic scintillator behind the target, and a time-of-flight hodoscope placed downstream of the tracking detectors. The production of φ mesons was identified via the charged kaon decay mode with detection of K + and K – in the final state. A clear signal of φ mesons could be identified in the invariant mass of K + K – pairs. The separation of coherent ( g d → φ d) and incoherent ( g d → φ pn) interactions were done by fitting the missing mass spectrum, where the reaction of coherent production from deuterons, g d → φ d, is peaking at the mass of deuterons 1.875 GeV/c 2 . Figure 1 shows the disentanglement done in the missing mass spectra for 8 bins of photon energy in the range of 1.57-2.57 GeV. The overall shapes were nicely reproduced by the sum of individual ones generated by Monte Carlo simulations. The zero-degree cross sections of incoherent production from deuterium as a function of the photon energy are shown in Fig. 2(a), together with data from hydrogen. The nuclear transparency ratios for deuterium, Td = σ d/(2* σ p), is evaluated by the ratio of cross sections at zero degrees and are shown in 0 2.5 0 10 0 20 0 20 0 50 0 100 0 100 1.8 1.9 2.0 2.1 1.8 1.9 2.0 2.1 0 100 MM d ( γ , φ )(GeV/c 2 ) Counts 1.57
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