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Register a new userMomentum dependence of the imaginary part of the $omega$- and $eta^prime$-nucleus optical potential
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The photoproduction of $omega$ and $eta^prime$ mesons off carbon and niobium nuclei has been measured as a function of the meson momentum for incident photon energies of 1.2-2.9 GeV at the electron accelerator ELSA. The mesons have been identified via the $omega rightarrow pi^0 gamma rightarrow 3 gamma$ and $eta^primerightarrow pi^0 pi^0eta rightarrow 6 gamma$ decays, respectively, registered with the CBELSA/TAPS detector system. From the measured meson momentum distributions the momentum dependence of the transparency ratio has been determined for both mesons. Within a Glauber analysis the in-medium $omega$ and $eta^prime$ widths and the corresponding absorption cross sections have been deduced as a function of the meson momentum. The results are compared to recent theoretical predictions for the in-medium $omega$ width and $eta^prime$-N absorption cross sections. The energy dependence of the imaginary part of the $omega$- and $eta^prime$-nucleus optical potential has been extracted. The finer binning of the present data compared to the existing data allows a more reliable extrapolation towards the production threshold. The modulus of the imaginary part of the $eta^prime$ nucleus potential is found to be about three times smaller than recently determined values of the real part of the $eta^prime$-nucleus potential, which makes the $eta^prime$ meson a suitable candidate for the search for meson-nucleus bound states. For the $omega$ meson, the modulus of the imaginary part near threshold is comparable to the modulus of the real part of the potential. As a consequence, only broad structures can be expected which makes the observation of $omega$ mesic states very difficult experimentally.
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The excitation function and momentum distribution of $eta^prime$ mesons have been measured in photon induced reactions on $^{12}{}$C in the energy range of 1250-2600 MeV. The experiment was performed with tagged photon beams from the ELSA electron accelerator using the Crystal Barrel and TAPS detectors. The data are compared to model calculations to extract information on the sign and magnitude of the real part of the $eta^prime$-nucleus potential. Within the model, the comparison indicates an attractive potential of -($37 pm 10(stat)pm10(syst)$) MeV depth at normal nuclear matter density. Since the modulus of this depth is larger than the modulus of the imaginary part of the $eta^prime$-nucleus potential of -($10pm2.5$) MeV, determined by transparency ratio measurements, a search for resolved $eta^prime$-bound states appears promising.
The excitation function and momentum distribution of {eta} mesons have been measured in photoproduction off 93^Nb in the energy range of 1.2-2.9 GeV. The experiment has been performed with the combined Crystal Barrel and MiniTAPS detector system, using tagged photon beams from the ELSA electron accelerator. Information on the sign and magnitude of the real part of the {eta}-Nb potential has been extracted from a comparison of the data with model calculations. An attractive potential of -(41 pm 10(stat) pm 15(syst)) MeV depth at normal nuclear matter density is deduced within model uncertainties. This value is consistent with the potential depth of -(37 pm 10(stat) pm 10(syst)) MeV obtained in an earlier measurement for a light nucleus (carbon). This relatively shallow {eta}-nucleus potential will make the search for {eta}-nucleus bound states more difficult.
In a search for $omega$ mesic states, the production of $omega$-mesons in coincidence with forward going protons has been studied in photon induced reactions on $^{12}$C for incident photon energies of 1250 - 3100 MeV. The $pi^0 gamma$ pairs from decays of bound or quasi-free $omega$-mesons have been measured with the CBELSA/TAPS detector system in coincidence with protons registered in the MiniTAPS forward array. Structures in the total energy distribution of the $pi^0 gamma$ pairs, which would indicate the population and decay of bound $omega~^{11}$B states, are not observed. The $pi^0 gamma$ cross section of 0.3 nb/MeV/sr observed in the bound state energy regime between -100 and 0 MeV may be accounted for by yield leaking into the bound state regime because of the large in-medium width of the $omega$-meson. A comparison of the measured total energy distribution with calculations suggests the real part $V_0$ of the $omega~^{11}$B potential to be small and only weakly attractive with $V_0(rho=rho_0) = -15pm$ 35(stat) $pm$20(syst) MeV in contrast to some theoretical predictions of attractive potentials with a depth of 100 - 150 MeV.
A comparison of the close-to-threshold total cross section for the eta prime production in pp --> pp eta prime and pn --> pn eta prime reactions constitutes a tool to investigate the eta prime meson structure and the reaction mechanism in the channels of isospin I=0 and I=1 and may provide insight into the flavour-singlet (perhaps also into gluonium) content of the eta prime meson. In this contribution we present preliminary results of measurement of the quasi-free production of the eta prime meson in the proton-neutron collisions conducted using the COSY-11 facility.
Due to their short life-time, flavour-neutral mesons cannot be utilized as free secondary beams or targets, and therefore a study of their interaction with nucleons is not possible via direct scattering experiments. This interaction is, however, accessible via its influence on the energy dependence - and on the phase space distributions of the cross sections for reactions in which these mesons are produced. In case of the p p --> p p eta reaction the experimentally determined distributions of the differential cross sections close to the production threshold cannot be described by taking into account the S-wave proton-proton and proton-eta interaction only. Here we show that the angular distributions determined at the COSY-11 facility reveal some evidence for P-wave admixture in the proton-proton subsystem already at an excess energy as low as Q = 15.5 MeV. We also present that one can estimate the relative strength of the eta-nucleon and eta-prime-nucleon interactions by comparison of the eta and eta-prime production yield.
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