Fri·May 22, 2015Friday·May 22, 2015

1. 01*

   Enhanced Gamma-Ray Emission from Neutron Unbound States Populated in Beta Decay

   J. L. Tain🇪🇸 · E. Valencia🇪🇸 · A. Algora🇪🇸 · J. Agramunt🇪🇸 · B. Rubio🇪🇸 · S. Rice🇬🇧 · W. Gelletly🇬🇧 · P. Regan🇬🇧 · A.-A. Zakari-Issoufou🇫🇷 · M. Fallot🇫🇷 · A. Porta🇫🇷 · J. Rissanen🇫🇮 and 26 other authors

   Total absorption spectroscopy was used to investigate the beta-decay intensity to states above the neutron separation energy followed by gamma-ray emission in 87,88Br and 94Rb. Accurate results were obtained thanks to a careful control of systematic errors. An unexpectedly large gamma intensity was observed in all three cases extending well beyond the excitation energy region where neutron penetration is hindered by low neutron energy. The gamma branching as a function of excitation energy was compared to Hauser-Feshbach model calculations. For 87Br and 88Br the gamma branching reaches 57% and 20% respectively, and could be explained as a nuclear structure effect. Some of the states populated in the daughter can only decay through the emission of a large orbital angular momentum neutron with a strongly reduced barrier penetrability. In the case of neutron-rich 94Rb the observed 4.5% branching is much larger than the calculations performed with standard nuclear statistical model parameters, even after proper correction for fluctuation effects on individual transition widths. The difference can be reconciled introducing an enhancement of one order-of-magnitude in the photon strength to neutron strength ratio. An increase in the photon strength function of such magnitude for very neutron-rich nuclei, if it proved to be correct, leads to a similar increase in the (n,gamma) cross section that would have an impact on r-process abundance calculations.

   nucl-exPRL(2015)·48 citations
2. 02*

   Electroproduction of the $N^{\ast }(1535)$ nucleon resonance in QCD

   I.V. Anikin🇷🇺 · V.M. Braun🇩🇪 · N. Offen🇩🇪

   Following the 12 GeV upgrade, a dedicated experiment is planned with the Hall B CLAS12 detector at Jefferson Lab, with the aim to study electroproduction of nucleon resonances at high photon virtualities up to $Q^2=12$ GeV${}^2$. In this work we present a QCD-based approach to the theoretical interpretation of these upcoming results in the framework of light-cone sum rules that combine perturbative calculations with dispersion relations and duality. The form factors are thus expressed in terms of $N^{\ast }(1535)$ light-front wave functions at small transverse separations, called distribution amplitudes. The distribution amplitudes can therefore be determined from the comparison with the experimental data on form factors and compared to the results of lattice QCD simulations. The results of the corresponding next-to-leading order calculation are presented and compared with the existing data. We find that the form factors are dominated by the twist-four distribution amplitudes that are related to the $P$-wave three-quark wave functions of the $N^{\ast }(1535)$, i.e. to contributions of orbital angular momentum.

   ↳ hep-phnucl-exnucl-thPRD(2015)·41 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.