Investigating neutron-proton pairing in sd -shell nuclei via (p, He 3) and (He 3,p) transfer reactions

Neutron-proton pairing correlations are investigated in detail via np transfer reactions in N = Z sd-shell nuclei. In particular, we study the cross-section ratio of the lowest 0+ and 1+ states as an observable to quantify the interplay between T = 0 (isoscalar) and T = 1 (isovector) pairing strengths. The experimental results are compared to second-order distorted-wave Born approximation calculations with proton-neutron amplitudes obtained in the shell-model formalism using the universal sd-shell interaction B. Our results suggest underestimation of the nonneglible isoscalar pairing strength in the shell-model descriptions at the expense of the isovector channel.Séptimo Programa Marco de la Comisión Europea-FP7/2007-2013 00376National Science Foundation (NSF) de los Estados Unidos-PHY-1404442US Department of Energy, Office of Science, Office of Nuclear Physics-DE-AC02-05CH1123

Research with real photons at the MAMI 1.6 GeV electron accelerator

The A2-CB Collaboration at Mainz is studying the structure of hadrons by meson photoproduction using unpolarised, linearly polarised and circularly polarised photons with energies up to 1.6 GeV. Photons are energy-tagged using the Glasgow-Mainz tagged photon spectrometer and a new high-energy end-point tagger which allows η’ reactions to be studied. Reaction products are detected in a ~4π detector consisting of the Crystal Ball detector and TAPS forward wall. Transverse or longitudinally polarised proton targets are available and new techniques have been developed to measure the polarisation of recoiling protons. These facilities have allowed an extensive programme of double-polarisation meson-photoproduction experiments to be carried out to search for so-called “missing baryon resonances” on proton and deuteron targets. Searches have also been carried out to investigate narrow resonances in the η-photoproduction channel at invariant masses around 1680 MeV. Coherent π0 production measurements have been used to estimate the neutron skin thickness in 208Pb. This paper presents selected highlights from the A2-CB collaboration research programme at MAMI

Uncomputably noisy ergodic limits

V'yugin has shown that there are a computable shift-invariant measure on Cantor space and a simple function f such that there is no computable bound on the rate of convergence of the ergodic averages A_n f. Here it is shown that in fact one can construct an example with the property that there is no computable bound on the complexity of the limit; that is, there is no computable bound on how complex a simple function needs to be to approximate the limit to within a given epsilon

Low-energy electric dipole response in 120Sn

The electric dipole strength in 120Sn has been extracted from proton inelastic scattering experiments at E_p = 295 MeV and at forward angles including 0 degree. Below neutron threshoild it differs from the results of a 120Sn(gamma,gamma') experiment and peaks at an excitation energy of 8.3 MeV. The total strength corresponds to 2.3(2)% of the energy-weighted sum rule and is more than three times larger than what is observed with the (gamma,gamma') reaction. This implies a strong fragmentation of the E1 strength and/or small ground state branching ratios of the excited 1- states.Comment: 7 pages, 6 figure