Neutron stars exclude light dark baryons [CL]

Exotic new particles carrying baryon number and with mass of order the nucleon mass have been proposed for various reasons including baryogenesis, dark matter, mirror worlds, and the neutron lifetime puzzle. We show that the existence of neutron stars with mass greater than 0.7 $M_\odot$ places severe constraints on such particles, requiring them to be heavier than 1.2 GeV or to have strongly repulsive self-interactions.

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D. McKeen, A. Nelson, S. Reddy, et. al.
Fri, 23 Feb 18

Comments: 5 pages, 1 figure

A new class of compact stars: pion stars [CL]

Compact stellar objects offer deep insight into the physics of elementary particles in dense environments through the imprint left by merger events on the electromagnetic and gravitational wave spectra. The theoretical description of compact star interiors requires full knowledge of the equation of state (EoS) of nuclear matter and involves the non-perturbative solution of quantum chromodynamics (QCD), the theory of strongly interacting quarks and gluons. However, first-principle methods (most notably, lattice QCD simulations) are not available for high neutron densities – consequently, the EoS of neutron stars necessarily relies on a modeling of the nuclear force. Here we propose a different scenario, where the neutron density vanishes and a Bose-Einstein condensate of charged pions (the lightest excitations in QCD) plays the central role instead. This setting can be approached by first-principle methods and leads to a new class of compact stars: pion stars. As we demonstrate, pion star matter exhibits gravitationally bound configurations and is metastable against electroweak decays. If pion stars indeed exist in our Universe, this result constitutes the first occasion that the EoS and the mass-radius relation of a compact stellar object is determined from first principles within the Standard Model.

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B. Brandt, G. Endrodi, E. Fraga, et. al.
Tue, 20 Feb 18

Comments: 6 pages, 4 figures

Impacts of nuclear-physics uncertainties in the s-process determined by Monte-Carlo variations [SSA]

The s-process, a production mechanism based on slow-neutron capture during stellar evolution, is the origin of about half the elements heavier than iron. Abundance predictions for s-process nucleosynthesis depend strongly on the relevant neutron-capture and $\beta$-decay rates, as well as on the details of the stellar model being considered. Here, we have used a Monte-Carlo approach to evaluate the nuclear uncertainty in s-process nucleosynthesis. We considered the helium burning of massive stars for the weak s-process and low-mass asymptotic-giant-branch stars for the main s-process. Our calculations include a realistic and general prescription for the temperature dependent uncertainty for the reaction cross sections. We find that the adopted uncertainty for (${\rm n},\gamma$) rates, tens of per cent on average, effects the production of s-process nuclei along the line of $\beta$-stability, and that the uncertainties in $\beta$-decay from excited state contributions, has the strongest impact on branching points.

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N. Nishimura, G. Cescutti, R. Hirschi, et. al.
Mon, 19 Feb 18

Comments: 6 pages, 4 figures, 2 tables, the Proceedings of “the 2017 Symposium on Nuclear Data”; a supplementary article of arXiv:1701.00489

Sensitivity to neutron captures and beta-decays of the enhanced s-process in rotating massive stars at low metallicities [SSA]

The s-process in massive stars, producing nuclei up to $A\approx 90$, has a different behaviour at low metallicity if stellar rotation is significant. This enhanced s-process is distinct from the s-process in massive stars around solar metallicity, and details of the nucleosynthesis are poorly known. We investigated nuclear physics uncertainties in the enhanced s-process in metal-poor stars within a Monte-Carlo framework. We applied temperature-dependent uncertainties of reaction rates, distinguishing contributions from the ground state and from excited states. We found that the final abundance of several isotopes shows uncertainties larger than a factor of 2, mostly due to the neutron capture uncertainties. A few nuclei around branching points are affected by uncertainties in the $\beta$-decay.

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N. Nishimura, R. Hirschi and T. Rauscher
Mon, 19 Feb 18

Comments: 3 pages, 3 figures, published in the Proceedings of “Nuclear Physics in Astrophysics Conference (NPA VII)”; see arXiv:1701.00489, for the completed results

Neutron star equation of state from the quark level in the light of GW170817 [CL]

Matter state inside neutron stars is an exciting problem in astrophysics, nuclear physics and particle physics. The equation of state (EOS) of neutron stars plays a crucial role in the present multi-message astronomy, especially after the event of GW170817. We propose a new neutron star EOS “QMF18” from the quark level, which describe well robust observational constraints from free-space nucleon, nuclear matter saturation, heavy pulsar measurements and the tidal deformability of the very recent observation of the GW170817 event. For this purpose, we employ the quark-mean-field (QMF) model, allowing one to tune the density dependence of the symmetry energy and study effectively its correlations with the Love number and the tidal deformability. The positive correlations between the slope parameter and the tidal deformability are found to be closely related to the sensitive dependence of the star radius on the slope. We provide tabulated data for the new EOS and compare it with other recent EOSs from various many-body frameworks.

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Z. Zhu, E. Zhou and A. Li
Fri, 16 Feb 18

Comments: 10 pages, 5 figures, 4 tables

Neutron Star Mergers Chirp About Vacuum Energy [HEAP]

Observations of gravitational waves from neutron star mergers open up novel directions for exploring fundamental physics: they offer the first access to the structure of objects with a non-negligible contribution from vacuum energy to their total mass. The presence of such vacuum energy in the inner cores of neutron stars occurs in new QCD phases at large densities, with the vacuum energy appearing in the equation of state for a new phase. This in turn leads to a change in the internal structure of neutron stars and influences their tidal deformabilities which are measurable in the chirp signals of merging neutron stars. By considering three commonly used neutron star models we show that for large chirp masses the effect of vacuum energy on the tidal deformabilities can be sizable. Measurements of this sort have the potential to provide a first test of the gravitational properties of vacuum energy independent from the acceleration of the Universe, and to determine the size of QCD contributions to the vacuum energy.

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C. Csaki, C. Eroncel, J. Hubisz, et. al.
Thu, 15 Feb 18

Comments: 28 pages, 10 figures, 1 table

Discriminating WIMP-nucleus response functions in present and future XENON-like direct detection experiments [CL]

The standard interpretation of direct-detection limits on dark matter involves particular assumptions of the underlying WIMP-nucleus interaction, such as, in the simplest case, the choice of a Helm form factor that phenomenologically describes an isoscalar spin-independent interaction. In general, the interaction of dark matter with the target nuclei may well proceed via different mechanisms, which would lead to a different shape of the corresponding nuclear structure factors as a function of the momentum transfer $q$. We study to what extent different WIMP-nucleus responses can be differentiated based on the $q$-dependence of their structure factors (or “form factors”). We assume an overall strength of the interaction consistent with present spin-independent limits and consider an exposure corresponding to XENON1T-like, XENONnT-like, and DARWIN-like direct detection experiments. We find that, as long as the interaction strength does not lie too much below current limits, the DARWIN settings allow a conclusive discrimination of many different response functions based on their $q$-dependence, with immediate consequences for elucidating the nature of dark matter.

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A. Fieguth, M. Hoferichter, P. Klos, et. al.
Wed, 14 Feb 18

Comments: 10 pages, 7 figures