Isospin-symmetry breaking in superallowed Fermi beta-decay due to isospin-nonconserving forces

We investigate isospin-symmetry breaking effects in the sd-shell region with large-scale shell-model calculations, aiming to understand the recent anomalies observed in superallowed Fermi beta-decay. We begin with calculations of Coulomb displacement energies (CDE's) and triplet displacement energies (TDE's) by adding the T=1,J=0 isospin nonconserving (INC) interaction into the usual isospin-invariant Hamiltonian. It is found that CDE's and TDE's can be systematically described with high accuracy. A total number of 122 one- and two-proton separation energies are predicted accordingly, and locations of the proton drip-line and candidates for proton-emitters are thereby suggested. However, attempt to explain the anomalies in the superallowed Fermi beta-decay fails because these well-fitted T=1,J=0 INC interactions are found no effects on the nuclear matrix elements. It is demonstrated that the observed large isospin-breaking correction in the 32Cl beta-decay, the large isospin-mixing in the 31Cl beta-decay, and the small isospin-mixing in the 23Al beta-decay can be consistently understood by introducing additional T=1,J=2 INC interactions related to the s1/2 orbit.Comment: 7 pages, 3 figures, accepted in Phys. Lett.

The Water-ice Feature in Near-infrared Disk-scattered Light around HD 142527:Micron-sized Icy Grains Lifted up to the Disk Surface?

We study the $3~\mu$m scattering feature of water ice detected in the outer disk of HD 142527 by performing radiative transfer simulations. We show that an ice mass abundance at the outer disk surface of HD 142527 is much lower than estimated in a previous study. It is even lower than inferred from far-infrared ice observations, implying ice disruption at the disk surface. Next, we demonstrate that a polarization fraction of disk-scattered light varies across the ice-band wavelengths depending on ice grain properties; hence, polarimetric spectra would be another tool for characterizing water-ice properties. Finally, we argue that the observed reddish disk-scattered light is due to grains with a few microns in size. To explain the presence of such grains at the disk surface, we need a mechanism that can efficiently oppose dust settling. If we assume turbulent mixing, our estimate requires $\alpha\gtrsim2\times10^{-3}$, where $\alpha$ is a non-dimensional parameter describing the vertical diffusion coefficient of grains. Future observations probing gas kinematics would be helpful to elucidate vertical grain dynamics in the outer disk of HD 142527.Comment: 21 pages, 14 figures, 1 table; Accepted for publication in Ap

Large mirror asymmetry in Gamow-Teller β-decay in the A = 26 isobaric multiplet

We investigate a striking example of mirror asymmetry in Gamow-Teller (GT) β-decay using the nuclear shell-model including isospin-nonconserving (INC) forces in the sd-shell region. We show that the large mirror asymmetry between 26 P and 26 Na GT β-decay can be accurately reproduced by introducing T=1, J≠0 INC forces related to the s 1/2 orbit, while the usual J=0 INC force, commonly adopted to describe isospin-symmetry breaking, does not work. We further show that the calculated distribution of summed GT strength for the 26 P β-decay is in good agreement with the experimental data. Our results support the conclusion that 26 P is a nucleus with proton-halo structure