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Register a new userExperimental constraints on the $omega$-nucleus real potential
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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.
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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.
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.
It is a well-known fact that a cluster of nucleons can be formed in the interior of an atomic nucleus, and such clusters may occupy molecular-like orbitals, showing characteristics similar to normal molecules consisting of atoms. Chemical molecules having a linear alignment are commonly seen in nature, such as carbon dioxide. A similar linear alignment of the nuclear clusters, referred to as linear-chain cluster state (LCCS), has been studied since the 1950s, however, up to now there is no clear experimental evidence demonstrating the existence of such a state. Recently, it was proposed that an excess of neutrons may offer just such a stabilizing mechanism, revitalizing interest in the nuclear LCCS, specifically with predictions for their emergence in neutron-rich carbon isotopes. Here we present the experimental observation of {alpha}-cluster states in the radioactive 14C nucleus. Using the 10Be+{alpha} resonant scattering method with a radioactive beam, we observed a series of levels which completely agree with theoretically predicted levels having an explicit linear-chain cluster configuration. We regard this as the first strong indication of the linear-chain clustered nucleus.
In order to study the Sigma-nucleus optical potential, we measured inclusive (pi^-,K^+) spectra on medium-to-heavy nuclear targets: CH_2, Si, Ni, In and Bi. The CH_2 target was used to calibrate the excitation energy scale by using the elementary process p + pi^- -> K^+ + Sigma^-, where the C spectrum was also extracted. The calibration was done with +-0.1 MeV precision. The angular distribution of the elementary cross section was measured, and agreed well with the previous bubble chamber data, but with better statistics, and the magnitudes of the cross sections of the measured inclusive (pi^-,K^+) spectra were also well calibrated. All of the inclusive spectra were found to be similar in shape at a region near to the Sigma^- binding energy threshold, showing a weak mass-number dependence on the magnitude of the cross section. The measured spectra were compared with a theoretical calculation performed within the framework of the Distorted Wave Impulse Approximation (DWIA). It has been demonstrated that a strongly repulsive sig-nucleus potential with a non-zero size of the imaginary part is required to reproduce the shape of the measured spectra.
Photoproduction of the $omega$ meson on the proton has been experimentally studied near the threshold. The total cross sections are determined at incident energies ranging from 1.09 to 1.15 GeV. The 1/2 and 3/2 spin-averaged scattering length $a_{omega p}$ and effective range $r_{omega p}$ between the $omega$ meson and proton are estimated from the shape of the total cross section as a function of the incident photon energy: $a_{omega p} = left(-0.97^{+0.16_{rm stat}}_{-0.16_{rm stat}}{}^{+0.03_{rm syst}}_{-0.00_{rm syst}}right)+i left(0.07^{+0.15_{rm stat}}_{-0.14_{rm stat}}{}^{+0.17_{rm syst}}_{-0.09_{rm syst}}right)$ fm and $r_{omega p}=left(+2.78^{+0.68_{rm stat}}_{-0.54_{rm stat}}{}^{+0.11_{rm syst}}_{-0.13_{rm syst}}right)+ileft(-0.01^{+0.46_{rm stat}}_{-0.50_{rm stat}}{}^{+0.07_{rm syst}}_{-0.00_{rm syst}}right)$ fm, resulting in a repulsive force. The real and imaginary parts for $a_{omega p}$ and $r_{omega p}$ are determined separately for the first time. A small $P$-wave contribution does not affect the obtained values.
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