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Register a new userDetermination of the {eta}-nucleus optical potential
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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.
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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.
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.
Taking advantage of both the low-emittance proton-beam of the Cooler Synchrotron COSY and the high momentum precision of the COSY-11 detector system, the mass distribution of the eta meson was measured with a resolution of 0.33 MeV/c^2 (FWHM), improving the experimental mass resolution by almost an order of magnitude with respect to previous results. Based on the sample of more than 2300 reconstructed pp --> pp eta events the total width of the eta meson was determined to be 0.226 +- 0.017(stat.) +- 0.014(syst.) MeV/c^2.
Taking advantage of both the high mass resolution of the COSY-11 detector and the high energy resolution of the low-emittance proton-beam of the Cooler Synchrotron COSY we determine the excitation function for the pp --> pp eta reaction close-to-threshold. Combining these data with previous results we extract the scattering length for the eta-proton potential in free space to be Re(a_{p eta}) = 0+-0.43 fm and Im(a_{p eta}) = 0.37(+0.40)(-0.16) fm.
The production of $eta^prime$ mesons in coincidence with forward-going protons has been studied in photon-induced reactions on $^{12}$C and on a liquid hydrogen (LH$_2$) target for incoming photon energies of 1.3-2.6 GeV at the electron accelerator ELSA. The $eta^prime$ mesons have been identified via the $eta^primerightarrow pi^0 pi^0eta rightarrow 6 gamma$ decay registered with the CBELSA/TAPS detector system. Coincident protons have been identified in the MiniTAPS BaF$_2$ array at polar angles of $2^{circ} le theta _{p} le 11^{circ}$. Under these kinematic constraints the $eta^prime$ mesons are produced with relatively low kinetic energy ($approx$ 150 MeV) since the coincident protons take over most of the momentum of the incident-photon beam. For the C-target this allows the determination of the real part of the $eta^prime$-carbon potential at low meson momenta by comparing with collision model calculations of the $eta^prime$ kinetic energy distribution and excitation function. Fitting the latter data for $eta^prime$ mesons going backwards in the center-of-mass system yields a potential depth of V = $-$(44 $pm$ 16(stat)$pm$15(syst)) MeV, consistent with earlier determinations of the potential depth in inclusive measurements for average $eta^prime$ momenta of $approx$ 1.1 GeV/$c$. Within the experimental uncertainties, there is no indication of a momentum dependence of the $eta^prime$-carbon potential. The LH$_2$ data, taken as a reference to check the data analysis and the model calculations, provide differential and integral cross sections in good agreement with previous results for $eta^prime$ photoproduction off the free proton.
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