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Zheng, Y. et al, & Gadea, A. (2013). gamma-ray linear polarization measurements and (g(9/2))(-3) neutron alignment in Ru-91. Phys. Rev. C, 87(4), 044328–10pp.
Abstract: Linear polarization measurements have been performed for gamma rays in Ru-91 produced with the Ni-58(Ar-36,2p1n gamma)Ru-91 reaction at a beam energy of 111 MeV. The EXOGAM Ge clover array has been used to measure the gamma-gamma coincidences, gamma-ray linear polarization, and gamma-ray angular distributions. The polarization sensitivity of the EXOGAM clover detectors acting as Compton polarimeters has been determined in the energy range 0.3-1.3 MeV. Several transitions have been observed for the first time. Measurements of linear polarization and angular distribution have led to the firm assignments of spin differences and parity of high-spin states in Ru-91. More specifically, calculations using a semiempirical shell model were performed to understand the structures of the first and second (21/2(+)) and (17/2(+)) levels. The results are in good agreement with the experimental data, supporting the interpretation of the nonyrast (21/2(+)) and (17/2(+)) states in terms of the J(max) and J(max) – 2 members of the seniority-three nu(g(9/2))(-3) multiplet.
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Navarro, J., & Polls, A. (2013). Spin instabilities of infinite nuclear matter and effective tensor interactions. Phys. Rev. C, 87(4), 044329–11pp.
Abstract: We study the effects of the tensor force, present in modern effective nucleon-nucleon interactions, in the spin instability of nuclear and neutron matter. Stability conditions of the system against certain very low energy excitation modes are expressed in terms of Landau parameters. It is shown that in the spin case, the stability conditions are equivalent to the condition derived from the spin susceptibility, which is obtained as the zero-frequency and long-wavelength limit of the spin response function calculated in the random phase approximation. Zero-range forces of the Skyrme type and finite-range forces of M3Y and Gogny type are analyzed. It is shown that for the Skyrme forces considered, the tensor effects are sizable and tend to increase the spin instability, which appears at smaller densities than in the case that the tensor is not taken into account. On the contrary, the tensor contribution of finite-range forces to the spin susceptibility is small or negligible for both isospin channels of symmetric nuclear matter as well as for neutron matter. A comparison with the spin susceptibility provided by realistic interactions is also presented.
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Garzon, E. J., Xie, J. J., & Oset, E. (2013). Case in favor of the N*(1700)(3/2(-)). Phys. Rev. C, 87(5), 055204–12pp.
Abstract: Using an interaction extracted from the local hidden-gauge Lagrangians, which brings together vector and pseudoscalar mesons, and the coupled channels rho N (s wave), pi N (d wave), pi Delta (s wave), and pi Delta (d wave), we look in the region ofv root s = 1400-1850 MeV and find two resonances dynamically generated by the interaction of these channels, which are naturally associated to N*(1520)(3/2(-)) and N*(1700)(3/2(-)). N*(1700)(3/2(-)) appears neatly as a pole in the complex plane. The free parameters of the theory are chosen to fit the pi N (d-wave) data. Both the real and imaginary parts of the pi N amplitude vanish in our approach in the vicinity of this resonance, which is similar to what happens in experimental determinations and which makes this signal very weak in this channel. This feature could explain why this resonance does not show up in some experimental analyses, but the situation is analogous to that of the f(0)(980) resonance, the second scalar meson after sigma[f(0)(500)] in the pi pi(d-wave) amplitude. The unitary coupled channel approach followed here, in connectionwith the experimental data, leads automatically to a pole in the 1700-MeV region and makes this second 3/2-resonance unavoidable.
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Roca, L., & Oset, E. (2013). Lambda(1405) poles obtained from pi(0)Sigma(0) photoproduction data. Phys. Rev. C, 87(5), 055201–8pp.
Abstract: We present a strategy to extract the position of the two Lambda(1405) poles from experimental photoproduction data measured recently at different energies in the gamma p -> K+pi(0)Sigma(0) reaction at Jefferson Laboratory. By means of a chiral dynamics motivated potential with free parameters, we solve the Bethe-Salpeter equation in the coupled channels (K) over barN and pi Sigma in isospin I = 0 and parametrize the amplitude for the photonuclear reaction in terms of a linear combination of the pi Sigma -> pi Sigma and (K) over barN -> pi Sigma scattering amplitudes in I = 0, with a different linear combination for each energy. Good fits to the data are obtained with some sets of parameters, by means of which one can also predict the cross section for the K- p -> pi(0)Sigma(0) reaction. These later results help us decide among the possible solutions. The result is that the different solutions lead to two poles similar to those found in the chiral unitary approach. With the best result we find the two Lambda(1405) poles at 1385 – 68i MeV and 1419 – 22i MeV.
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Bayar, M., & Oset, E. (2013). (K)over-bar N N absorption within the framework of the fixed-center approximation to Faddeev equations. Phys. Rev. C, 88(4), 044003–8pp.
Abstract: We present a method to evaluate the (K) over bar absorption width in the bound (K) over bar N N system. Most calculations of this system ignore this channel and only consider the (K) over bar N -> pi Sigma conversion. Other works make a qualitative calculation using perturbative methods. Since the (1405) resonance is playing a role in the process, the same resonance is changed by the presence of the absorption channels andwe find that a full nonperturbative calculation is called for, which we present here. We employ the fixed center approximation to Faddeev equations to account for (K) over bar rescattering on the (NN) cluster and we find that the width of the states found previously for S = 0 and S = 1 increases by about 30 MeV due to the (K) over bar N N absorption, to a total width of about 80 MeV.
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