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2 years ago I. INTRODUCTION Cold dark matter (CDM), favored by most astrophysical and cosmological observations, can be realized in symmetric or asymmetric scenarios. In the first class of models, dark matter is made of stable X particles and an equal amount of stable X¯ antiparticles of mass mX. In the early universe, these were in thermal equilibrium and their residual abundance ΩX is fixed, at the “freeze-out” value, when the rate of the Hubble expansion overcomes that of X¯ X annihilation. A prototypical example, which has been extensively studied, is provided by supersymmetric models with R-parity conservation, where the lightest superpartner is stable and plays the role of the dark matter of the universe. As yet no light sub-TeV SUSY partners have been discovered at the LHC, and searches for electrons, positrons or photons from annihilations in clumps of DM in and around our Galaxy, do not provide solid “indirect” evidence for symmetric massive DM. Moreover, the ongoing direct underground searches put very strong bounds on the scattering cross sections of massive X’s on nuclei. In the symmetric case, accretion of DM particles onto the sun accelerates the rate of particle-antiparticle annihilation. The resulting photons, electrons, etc., are all trapped in the star. However, for massive DM particles, looking in ICE-CUBE (the large km3 Cerenkov radiation detector near the south-pole) for the resulting UHE neutrinos is an excellent indirect detection method. The fact that no such energetic neutrinos have been detected constrains symmetric DM models. Consequently there has been, in recent years, a renewed interest in a second class of models: the asymmetric dark matter (ADM) models. In such models, the relic ADM density is determined in a manner analogous to that of ordinary baryonic matter, not by the freeze-out of DM annihilation, as in the symmetric case. An excess of dark fermions (over the antifermions) remains after the annihilation of most antiparticles. The required dark matter density in such models is readily achieved if the ratio of the ADM particles mass and that of ordinary baryons is tuned inversely with the corresponding ratio of asymmetries. Many examples of such models have been proposed over the years [1]. Here we consider variants in which the dark matter particle is rather light with mass in the sub- GeV range. Scattering of such light CDM on most detector materials yields
Possible Implications of Asymmetric Fermionic Dark Matter for Neutron Stars I. Goldmana , R. N. Mohapatrab , S. Nussinovc,d, D. Rosenbaume and V. Teplitze,f aDepartment of Exact Sciences, Afeka Tel Aviv Academic Engineering College, Tel Aviv, Israel bMaryland Center for Fundamental Physics and Department of Physics, University of Maryland, College Park, USA cSchool of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel dSchmid College of Science,Chapman University, Orange, California 92866, USA e Physics Department, Southern Methodist University, Dallas and fNASA Goddard Space Flight Center, Greenbelt, MD, USA Abstract We consider the implications of fermionic asymmetric dark matter (ADM) for a “mixed neutron star” composed of ordinary baryons and dark fermions. We find examples, where for a certain range of dark fermion mass – when it is less than that of ordinary baryons – such systems can reach higher masses than the maximal values allowed for ordinary (“pure”) neutron stars. This is shown both within a simplified, heuristic Newtonian analytic framework with non-interacting particles and via a general relativistic numerical calculation, under certain assumptions for the dark matter equation of state. Our work applies to various dark fermion models such as mirror matter models and to other models where the dark fermions have self interactions.