Gamma-ray Observatory INTEGRAL reloaded [CL]

A new lease on life was given to ESA’s International Gamma-ray Astrophysics Laboratory, because of its unique capability to identify electromagnetic counterparts to sources of gravitational waves and ultra-high energy neutrinos.

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E. Heuvel
Fri, 22 Sep 17

Comments: 5 pages, 2 Figures, published in Nature Astronomy Volume 1, 4 April 2017

Thermal Dark Matter Through the Dirac Neutrino Portal [CL]

We study a simple model of thermal dark matter annihilating to standard model neutrinos via the neutrino portal. A (pseudo-)Dirac sterile neutrino serves as a mediator between the visible and the dark sectors, while an approximate lepton number symmetry allows for a large neutrino Yukawa coupling and, in turn, efficient dark matter annihilation. The dark sector consists of two particles, a Dirac fermion and complex scalar, charged under a symmetry that ensures the stability of the dark matter. A generic prediction of the model is a sterile neutrino with a large active-sterile mixing angle that decays primarily invisibly. We derive existing constraints and future projections from direct detection experiments, colliders, rare meson and tau decays, electroweak precision tests, and small scale structure observations. Along with these phenomenological tests, we investigate the consequences of perturbativity and scalar mass fine tuning on the model parameter space. A simple, conservative scheme to confront the various tests with the thermal relic target is outlined, and we demonstrate that much of the cosmologically-motivated parameter space is already constrained. We also identify new probes of this scenario such as multi-body kaon decays and Drell-Yan production of $W$ bosons at the LHC.

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B. Batell, T. Han, D. McKeen, et. al.
Fri, 22 Sep 17

Comments: 12 pages, 2 figures

Relativistic effects due to gravimagnetic moment of a rotating body [CL]

We compute exact Hamiltonian (and corresponding Dirac brackets) for spinning particle with gravimagnetic moment $\kappa$ in an arbitrary gravitational background. $\kappa=0$ corresponds to the Mathisson-Papapetrou-Tulczyjew-Dixon (MPTD) equations. $\kappa=1$ leads to modified MPTD equations with reasonable behavior in the ultrarelativistic limit. So we study the modified equations in the leading post-Newtonian approximation. Rotating body with unit gravimagnetic moment has qualitatively different behavior as compared with MPTD body: A) If a number of gyroscopes with various rotation axes are freely traveling together, the angles between the axes change with time. B) For specific binary systems, gravimagnetic moment gives a contribution to frame-dragging effect with the magnitude, that turns out to be comparable with that of Schiff frame dragging.

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W. Ramirez and A. Deriglazov
Fri, 22 Sep 17

Comments: 21 pages

Non-standard neutrino self-interactions in a supernova and fast flavor conversions [CL]

We study the effects of non-standard self-interactions (NSSI) of neutrinos streaming out of a core-collapse supernova. We show that with NSSI, the standard linear stability analysis gives rise to linearly as well as exponentially growing solutions. We demonstrate analytically that flavor-preserving NSSI lead to suppression of collective oscillations, while flavor-violating NSSI allow an earlier onset with enhanced growth rate of oscillations. In the intersecting four-beam model, we show that flavor-violating NSSI can lead to fast oscillations even when the angle between the neutrino and antineutrino beams is obtuse, which is forbidden in the standard model. This leads to the new possibility of fast oscillations in a two-beam system with opposing neutrino-antineutrino fluxes, even in the absence of any spatial inhomogeneities. Finally, we solve the full non-linear equations of motion in the four-beam model numerically, and explore the interplay of fast and slow flavor conversions in the long-time behavior, in the presence of NSSI.

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A. Dighe and M. Sen
Fri, 22 Sep 17

Comments: 12 pages, Revtex style, 15 figures

Axion as a non-WIMP dark matter candidate [CL]

The axion arises in well-motivated extensions of the Standard Model of particle physics and is regarded as an alternative to the weakly interacting massive particle paradigm to explain the nature of dark matter. In this contribution, we review theoretical aspects of dark matter axions, particularly focusing on recent developments in the estimation of their relic abundance. A closer look at their non-thermal production mechanisms in the early universe reveals the possibility of explaining the observed dark matter abundance in various mass ranges. The mass ranges predicted in various cosmological scenarios are briefly summarized.

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K. Saikawa
Fri, 22 Sep 17

Comments: 6 pages, 2 figures, contribution to the proceedings of the European Physical Society Conference on High Energy Physics, 5-12 July 2017, Venice, Italy

Solving the flatness problem with an anisotropic instanton in Hořava-Lifshitz gravity [CL]

In Ho\v{r}ava-Lifshitz gravity a scaling isotropic in space but anisotropic in spacetime, often called anisotropic scaling with the dynamical critical exponent z=3, lies at the base of its renormalizability. This scaling also leads to a novel mechanism of generating scale-invariant cosmological perturbations, solving the horizon problem without inflation. In this paper we propose a possible solution to the flatness problem, in which we assume that the initial condition of the Universe is set by a small instanton respecting the same scaling. We argue that the mechanism may be more general than the concrete model presented here, and rely simply on the deformed dispersion relations of the theory, and on equipartition of the various forms of energy at the starting point.

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S. Bramberger, A. Coates, J. Magueijo, et. al.
Fri, 22 Sep 17

Comments: 10 pages, 5 figures

Hidden Sector Dark Matter and the Galactic Center Gamma-Ray Excess: A Closer Look [CL]

Stringent constraints from direct detection experiments and the Large Hadron Collider motivate us to consider models in which the dark matter does not directly couple to the Standard Model, but that instead annihilates into hidden sector particles which ultimately decay through small couplings to the Standard Model. We calculate the gamma-ray emission generated within the context of several such hidden sector models, including those in which the hidden sector couples to the Standard Model through the vector portal (kinetic mixing with Standard Model hypercharge), through the Higgs portal (mixing with the Standard Model Higgs boson), or both. In each case, we identify broad regions of parameter space in which the observed spectrum and intensity of the Galactic Center gamma-ray excess can easily be accommodated, while providing an acceptable thermal relic abundance and remaining consistent with all current constraints. We also point out that cosmic-ray antiproton measurements could potentially discriminate some hidden sector models from more conventional dark matter scenarios.

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M. Escudero, S. Witte and D. Hooper
Fri, 22 Sep 17

Comments: 28 pages, 35 figures