He6 breakup dynamic polarization potential reexamined

The dynamic polarization potential contribution to the effective interaction between 6He and 208Pb at 27 MeV, due to breakup channels, is recalculated exploiting a recently developed improved model for 6He. The most general features of the long-range attractive and absorptive components remain the same as were found in an earlier study, but the asymptotic magnitudes are reduced by factors of about 2.5 and 4.5, respectively. We draw conclusions from these results, as well as from further calculations at 22 MeV, closer to the Coulomb barrier, and at 32 MeV

Coupling effects in proton scattering from ⁴⁰Ca

Recent studies showed that neutron pickup makes a substantial contribution to the proton optical model potential (OMP) for light, mostly halo, target nuclei. Here, we extend those studies to a more “normal” target nucleus: 40Ca. We present coupled reaction channel (CRC) calculations with the coupling of 30.3 MeV incident protons to deuterons and up to 12 states of 39Ca. The proton elastic scattering S matrix from the CRC calculation is subject to Slj→V(r)+l·s VSO(r) inversion and the bare potential of the CRC calculation is subtracted, directly yielding a local and L-independent representation of the dynamic polarization potential (DPP). This is appropriate for comparison with phenomenological OMPs and local OMPs derived in local density folding models. The real-central part of the DPP is repulsive and cannot be represented as a uniform normalization of the bare potential, changing the rms radius. A series of model calculations reveal the dependence of the DPP on a range of parameters illuminating (i) departures of nucleon potentials of specific nuclei from global properties, (ii) the generation of repulsion, and (iii) the requirements for all-order CRC and deuteron breakup. Light is thrown on the nonlocality of the underlying DPP

Strong pickup-channel coupling effects in proton scattering: the case of p + Be-10

The dynamic polarization potential (DPP) contribution to the effective proton-nucleus interaction, that is due to the coupling of deuteron channels, is evaluated by applying $S_{lj} \to V(r)$ inversion to the elastic channel $S$-matrix from coupled reaction channel calculations of proton elastic scattering. This was done for protons scattering from $^{10}$Be at 12, 13, 14, 15, and 16 MeV; non-orthogonality corrections were included. We find a consistent pattern of a repulsive real and an absorptive imaginary DPP, with the absorption shifted to a larger radius. This is consistent with what has been found for proton scattering from the neutron skin nucleus $^8$He. The DPP is not of a form that can be represented by a renormalization of the bare potential, and has properties suggesting an underlying non-local process. We conclude that deuteron channels cannot be omitted from a full theoretical description of the proton-nucleus interaction (optical potential).Comment: 14 pages, 4 figures, RevTeX4, accepted by Phys Rev

Bright gamma-ray Galactic Center excess and dark dwarfs: Strong tension for dark matter annihilation despite Milky Way halo profile

We incorporate Milky Way dark matter halo profile uncertainties, as well as an accounting of diffuse gamma-ray emission uncertainties in dark matter annihilation models for the Galactic Center Extended gamma-ray excess (GCE) detected by the Fermi Gamma Ray Space Telescope. The range of particle annihilation rate and masses expand when including these unknowns. However, two of the most precise empirical determinations of the Milky Way halo's local density and density profile leave the signal region to be in considerable tension with dark matter annihilation searches from combined dwarf galaxy analyses for single-channel dark matter annihilation models. The GCE and dwarf tension can be alleviated if: one, the halo is very highly concentrated or strongly contracted; two, the dark matter annihilation signal differentiates between dwarfs and the GC; or, three, local stellar density measures are found to be significantly lower, like that from recent stellar counts, increasing the local dark matter density.Comment: 9 pages, 3 figures; v3 & v4: match version to appear in PRD; analysis code available at https://github.com/rekeeley/GCE_error

A Cautionary Tale: The Coulomb Modified ANC for the 1/22+\mathbf{1/2^+_2} State in 17^\mathbf{17}O

We discuss the impact of the uncertainty ($\pm 8$ keV) in the excitation energy of the astrophysically important 6.356 MeV $1/2^+_2$ state of $^{17}$O on the precision with which the Coulomb reduced ANC ($\widetilde{C}$) for the \left<^{17}\mathrm{O}(1/2^+_2) \mid \protect{^{13}\mathrm{C}} + \alpha \right> overlap can be extracted from direct reaction data. We find a linear dependence of $\widetilde{C}^2$ on the binding energy, the value extracted varying by a factor of 4 over the range $E_{\mathrm{ex}} = 6.356$ -- $6.348$ MeV. This represents an intrinsic limit on the precision with which $\widetilde{C}^2$ can be determined which cannot be improved unless or until the uncertainty in $E_{\mathrm{ex}}$ is reduced

Breakup coupling effects on near-barrier ⁶Li, ⁷Be and ⁸B + ⁵⁸Ni elastic scattering compared

New data for near-barrier 6Li, 7Be and 8B + 58Ni elastic scattering enable a comparison of breakup coupling effects for these loosely-bound projectiles. Coupled Discretised Continuum Channels (CDCC) calculations suggest that the large total reaction cross sections for 8B + 58Ni are dominated by breakup at near-barrier energies, unlike 6Li and 7Be where breakup makes a small contribution. In spite of this, the CDCC calculations show a small coupling influence due to breakup for 8B, in contrast to the situation for 6Li and 7Be. An examination of the S matrices gives a clue to this counter-intuitive behaviour

Strong coupling effects in near-barrier heavy-ion elastic scattering

Accurate elastic scattering angular distribution data measured at bombarding energies just above the Coulomb barrier have shapes that can markedly differ from or be the same as the expected classical Fresnel scattering pattern depending on the structure of the projectile, the target or both. Examples are given such as 18O + 184W and 16O + 148,152Sm where the expected rise above Rutherford scattering due to Coulomb-nuclear interference is damped by coupling to the target excited states, and the extreme case of 11Li scattering, where coupling to the 9Li + n + n continuum leads to an elastic scattering shape that cannot be reproduced by any standard optical model parameter set. The recent availability of high quality 6He, 11Li and 11Be data provides further examples of the influence that coupling effects can have on elastic scattering. Conditions for strong projectile-target coupling effects are presented with special emphasis on the importance of the beam-target charge combination being large enough to bring about the strong coupling effects. Several measurements are proposed that can lead to further understanding of strong coupling effects by both inelastic excitation and nucleon transfer on near-barrier elastic scattering. A final note on the anomalous nature of 8B elastic scattering is presented as it possesses a more or less normal Fresnel scattering shape whereas one would a priori not expect this due to the very low breakup threshold of 8B. The special nature of 11Li is presented as it is predicted that no matter how far above the Coulomb barrier the elastic scattering is measured, its shape will not appear as Fresnel like whereas the elastic scattering of all other loosely bound nuclei studied to date should eventually do so as the incident energy is increased, making both 8B and 11Li truly "exotic".Comment: Review articl

Reaction channel coupling effects for nucleons on <b>¹⁶O</b>: Induced undularity and proton-neutron potential differences

Background: Precise fitting of scattering observables suggests that the nucleon-nucleus interaction is l dependent. Such l dependence has been shown to be S-matrix equivalent to an undulatory l-independent potential. The undulations include radial regions where the imaginary term is emissive. Purpose: To study the dynamical polarization potential (DPP) generated in proton-16O and neutron-16O interaction potentials by coupling to pickup channels. Undulatory features occurring in these DPPs can be compared with corresponding features of empirical optical model potentials (OMPs). Furthermore, the additional inclusion of coupling to vibrational states of the target will provide evidence for dynamically generated nonlocality. Methods: The FRESCO code provides the elastic channel S-matrix Slj for chosen channel couplings. Inversion, Slj -> V(r)+ 1 ⋅ s VSO (r), followed by subtraction of the bare potential, yields an l-independent and local representation of the DPP due to the chosen couplings. Results: The DPPs have strongly undulatory features, including radial regions of emissivity. Certain features of empirical DPPs appear, e.g., the full inverted potential has emissive regions. The DPPs for different collective states are additive except near the nuclear center, whereas the collective and reaction channel DPPs are distinctly nonadditive over a considerable radial range, indicating dynamical nonlocality. Substantial differences between the DPPs due to pickup coupling for protons and neutrons occur; these imply a greater difference between proton and neutron OMPs than the standard phenomenological prescription. Conclusions: The onus is on those who object to undularity in the local and l-independent representation of nucleon elastic scattering to show why such undulations do not occur. This work suggests that it is not legitimate to halt model-independent fits to high-quality data at the appearance of undularity