Analog E1 transitions and isospin mixing

We investigate whether isospin mixing can be determined in a model-independent way from the relative strength of E1 transitions in mirror nuclei. The specific examples considered are the A=31 and A=35 mirror pairs, where a serious discrepancy between the strengths of 7/2--->5/2+ transitions in the respective mirror nuclei has been observed. A theoretical analysis of the problem suggests that it ought to be possible to disentangle the isospin mixing in the initial and final states given sufficient information on experimental matrix elements. With this in mind, we obtain a lifetime for the relevant 7/2- state in 31S using the Doppler-shift attenuation method. We then collate the available information on matrix elements to examine the level of isospin mixing for both A=31 and A=35 mirror pairs

Evidence Against Instanton Dominance of Topological Charge Fluctuations in QCD

The low-lying eigenmodes of the Dirac operator associated with typical gauge field configurations in QCD encode, among other low-energy properties, the physics behind the solution to the $U_A(1)$ problem (i.e. the origin of the $\eta'$ mass), the nature of spontaneous chiral symmetry breaking, and the physics of string-breaking, quark-antiquark pair production, and the OZI rule. Moreover, the space-time chiral structure of these eigenmodes reflects the space-time topological structure of the underlying gauge field. We present evidence from lattice QCD on the local chiral structure of low Dirac eigenmodes leading to the conclusion that topological charge fluctuations of the QCD vacuum are not instanton-dominated. The result supports Witten's arguments that topological charge is produced by confinement-related gauge fluctuations rather than instantons.Comment: 35 pages, 11 figure

Genome-wide differentiation in closely related populations: the roles of selection and geographic isolation.

Population divergence in geographic isolation is due to a combination of factors. Natural and sexual selection may be important in shaping patterns of population differentiation, a pattern referred to as 'Isolation by Adaptation' (IBA). IBA can be complementary to the well-known pattern of 'Isolation by Distance' (IBD), in which the divergence of closely related populations (via any evolutionary process) is associated with geographic isolation. The barn swallow Hirundo rustica complex comprises six closely related subspecies, where divergent sexual selection is associated with phenotypic differentiation among allopatric populations. To investigate the relative contributions of selection and geographic distance to genome-wide differentiation, we compared genotypic and phenotypic variation from 350 barn swallows sampled across eight populations (28 pairwise comparisons) from four different subspecies. We report a draft whole genome sequence for H. rustica, to which we aligned a set of 9,493 single nucleotide polymorphisms (SNPs). Using statistical approaches to control for spatial autocorrelation of phenotypic variables and geographic distance, we find that divergence in traits related to migratory behavior and sexual signaling, as well as geographic distance together, explain over 70% of genome-wide divergence among populations. Controlling for IBD, we find 42% of genome-wide divergence is attributable to IBA through pairwise differences in traits related to migratory behavior and sexual signaling alone. By (i) combining these results with prior studies of how selection shapes morphological differentiation and (ii) accounting for spatial autocorrelation, we infer that morphological adaptation plays a large role in shaping population-level differentiation in this group of closely related populations. This article is protected by copyright. All rights reserved

γ-ray spectroscopy of the A=23, T=1/2 nuclei 23Na and 23Mg: High-spin states, mirror symmetry, and applications to nuclear astrophysical reaction rates

Background: Obtaining reaction rates for nuclear astrophysics applications is often limited by the availability of radioactive beams. Indirect techniques to establish reaction rates often rely heavily on the properties of excited states inferred from mirror symmetry arguments. Mirror energy differences can depend sensitively on nuclear structure effects. Purpose: The present work sets out to establish a detailed comparison of mirror symmetry in the A=23, T=1/2 mirror nuclei 23Na and 23Mg both to high spin, and high excitation energy, including beyond the proton threshold. These data can be used to benchmark state-of-the-art shell-model calculations of these nuclei. Methods: Excited states in 23Na and 23Mg were populated using the 12C(12C,p) and 12C(12C,n) reactions at beam energies of 16 and 22 MeV, and their resulting γ decay was measured with Gammasphere. Results: Level schemes for 23Na and 23Mg have been considerably extended; highly excited structures have been found in 23Na, as well as their counterparts in 23Mg for previously known rotational structures in 23Na. Mirror symmetry has been investigated up to an excitation energy of 8 MeV and spin-parity of 13/2+. Excited states in the region above the proton threshold have been studied in both nuclei. Conclusions: A detailed exploration of mirror symmetry has been performed which heavily constrains expectations as to how mirror energy differences should evolve for different structures. Agreement with shell-model calculations provides confidence in using such estimations where real data are absent

Mirror energy differences in the A=31 mirror nuclei, S31 and P31, and their significance in electromagnetic spin-orbit splitting

Excited states in S31 and P31 were populated in the C12(Ne20,n) and C12(Ne20,p) reactions, respectively, at a beam energy of 32 MeV. High spin states of positive and negative parity have been observed in S31 for the first time, and the yrast scheme of P31 has been extended. Large mirror energy differences between the first 9/2- and 13/2- states were observed, but only small differences for the first 7/2- and 11/2- levels. The significance of these observations is discussed in relation to the electromagnetic spin-orbit effect and the relative binding energy of the levels

Studies of neutron-deficient nuclei near the Z=82 shell closure via cold fusion reactions

Over the last decade, we have performed in-beam experiments using Gammasphere + FMA to measure excited states in proton-rich Au, Hg, Tl and Pb isotopes. In these studies, the use of the FMA is essential in order to differentiate evaporation residues from the large fission background which dominates the reaction cross-section. In addition, we have found that using near-symmetric reactions at bombarding energies near the Coloumb barrier is beneficial in performing these studies. By keeping the bombarding energy low, fission is minimized and the reaction products are concentrated in only a few channels. New results have recently been obtained using the 90Zr+92Mo reaction to study shape co-existence in 181T1 via the lp evaporation channel. In addition, we have measured the total γ-ray energy and multiplicity associated with the surviving compund system, 179Au, following the fusion reaction, 90Zr + 89Y

Candidate superdeformed band in 28Si

Recent antisymmetrized molecular dynamics (AMD) calculations for 28Si suggest the presence of a superdeformed (SD) band with a dominant 24Mg+α clustering for its configuration, with firm predictions for its location and associated moment of inertia. This motivates a review of the experimental results reported in the literature with a particular focus on 24Mg(α,γ) studies, as well as on α-like heavy-ion transfer reactions such as 12C(20Ne,α)28Si. Combining this information for the first time leads to a set of candidate SD states whose properties point to their α-cluster structure and strong associated deformation. Analysis of data from Gammasphere allows the electromagnetic decay of these candidate states to be probed and reveals further supporting evidence for such a structure. This paper appraises this body of information and finds the evidence for an SD band is strong

Reevaluation of the P30(p,γ)S31 astrophysical reaction rate from a study of the T=1/2 mirror nuclei, S31 and P31

The P30(p,γ)S31 reaction rate is expected to be the principal determinant for the endpoint of nucleosynthesis in classical novae. To date, the reaction rate has only been estimated through Hauser-Feschbach calculations and is unmeasured experimentally. This paper aims to remedy this situation. Excited states in S31 and P31 were populated in the C12(Ne20,n) and C12(Ne20,p) reactions, respectively, at a beam energy of 32 MeV, and their resulting γdecay was detected with the Gammasphere array. Around half the relevant proton unbound states in S31 corresponding to the Gamow window for the P30(p,γ)S31 reaction were identified. The properties of the unobserved states were inferred from mirror symmetry using our extended data on P31. The implications of this new spectroscopic information for the P30(p,γ)S31 reaction rate are considered and recommendations for future work with radioactive beams are discussed