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]

Spectroscopic measurements of low‐n Balmer line profiles of atomic hydrogen and deuterium, emitted within the edge region of the TEXTOR‐94 tokamak plasma, have revealed the existence of a class of cold excited atoms, whose probable origin has been ascribed to electron impact‐induced molecular dissociation. Associated with these cold radiators are a second group of ‘lukewarm’ atoms, i.e. atoms heated by elastic collisions with hot protons (deuterons) diffusing outward from the plasma interior, as well as a third group of ‘hot’ atoms, produced in the corresponding excited states directly by charge‐exchange recombination between protons (deuterons) and boundary region atoms. A mechanism recently proposed to explain the heating process quantitatively, in terms of elastic atom‐ion collisions, is applied and discussed in this paper.

At the tagged photon facility PHOENICS at the Bonn accelerator ELSA a measurement of the target asymmetry of the reaction gp ! ph from threshold to 1150 MeV has been performed. Simultaneously the reaction gp ! pp 0 has been measured in the first resonance region. Results are presented for both reactions. The target asymmetry data are suited to put considerable constraints on the model parameters used for the theoretical description of meson photoproduction. [S0031-9007(98)06654-X] PACS numbers: 25.20.Lj, 13.60.Le, 14.20.Gk, 24.70. + s Meson production from nucleons is a major tool for the investigation of nucleon resonances. The technical development of tagged photon beams at high duty cycle accelerators has initiated a series of high precision experiments dedicated to the study of meson photoproduction. A lot of data have been obtained for the production of pions, and theoretical models have been developed which describe the data by resonance excitation and Born terms. The application of these models to another meson provides a good test of the assumptions and the predictive power. Moreover, a model which covers different meson production channels leads to a consistent picture of meson nucleon dynamics.In the case of h photoproduction some special aspects occur. The eta has isospin I 0 and, accordingly, only I 1͞2 nucleon resonances N ‫ء‬ can be excited. Further, a resonance state has several decay channels with different decay probabilities. This offers the possibility that a resonance state is almost hidden in pion production while it is rather prominent in eta photoproduction, as is the case for the S 11 ͑1535͒ [1]. The dominance of the S 11 ͑1535͒ in the reaction gN ! hN has been seen in the total and in the differential cross section in the threshold region [2]. Consequently, the eta channel is well suited for the precise determination of the S 11 ͑1535͒ resonance parameters. The contribution of weakly excited resonances to the cross section is only small and is hard to disentangle [3]. One example is the resonance D 13 ͑1520͒ which might be the source of an observed small anisotropy of the gp ! hp cross section [1]. Here, polarization observables offer an enhanced sensitivity because they contain bilinear products of partial waves, respectively, of multipoles. For the D 13 ͑1520͒, an observable such as the target asymmetry, which contains an interference of the strong S 11 ͑1535͒ with the weak D 13 ͑1520͒, is very advantageous.The experiment presented here is part of the extended program of h photoproduction that has been carried out at the PHOENICS facility at the Bonn accelerator ELSA. It included measurements of differential and total cross sections on the proton, neutron, deuteron, and nitrogen over a wide kinematic range [4,5]. In this framework the reaction gp ! ph has been investigated using a polarized proton target.The electron beam extracted from ELSA was used to produce a photon beam by bremsstrahlung at a thin radiator. The tagging system in the PHOENICS area [6] determined the energy and fl...

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