ENIGMA: Eccentric, Non-spinning, Inspiral Gaussian-process Merger Approximant for the characterization of eccentric binary black hole mergers [CL]


We present $\texttt{ENIGMA}$, a time domain, inspiral-merger-ringdown waveform model that describes non-spinning binary black holes systems that evolve on moderately eccentric orbits. The inspiral evolution is described using a consistent combination of post-Newtonian theory, self-force and black hole perturbation theory. Assuming moderately eccentric binaries that circularize prior to coalescence, we smoothly match the eccentric inspiral with a stand-alone, quasi-circular merger, which is constructed using machine learning algorithms that are trained with quasi-circular numerical relativity waveforms. We show that $\texttt{ENIGMA}$ reproduces with excellent accuracy the dynamics of quasi-circular compact binaries. We validate $\texttt{ENIGMA}$ using a set of $\texttt{Einstein Toolkit}$ eccentric numerical relativity waveforms, which describe eccentric binary black hole mergers with mass-ratios between $1 \leq q \leq 5.5$, and eccentricities $e_0 \lesssim 0.2$ ten orbits before merger. We use this model to explore in detail the physics that can be extracted with moderately eccentric, non-spinning binary black hole mergers. In particular, we use $\texttt{ENIGMA}$ to show that the gravitational wave transients GW150914, GW151226, GW170104 and GW170814 can be effectively recovered with spinning, quasi-circular templates if the eccentricity of these events at a gravitational wave frequency of 10Hz satisfies $e_0\leq {0.175,\, 0.125,\,0.175,\,0.175}$, respectively. We show that if these systems have eccentricities $e_0\sim 0.1$ at a gravitational wave frequency of 10Hz, they can be misclassified as quasi-circular binaries due to parameter space degeneracies between eccentricity and spin corrections.

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E. Huerta, C. Moore, P. Kumar, et. al.
Mon, 20 Nov 17

Comments: 17 pages, 9 figures, 1 Appendix. Submitted to Phys. Rev. D

Cosmological abundance of the QCD axion coupled to hidden photons [CL]


We study cosmological evolution of the QCD axion coupled to hidden photons. For a moderately strong coupling, the motion of the axion field leads to explosive production of hidden photons by tachyonic instability. We use lattice simulations to evaluate the cosmological abundance of the QCD axion. In doing so we incorporate the backreaction of the produced hidden photons on the axion dynamics, which becomes significant in the non-linear regime. We find that the axion abundance is suppressed by at most ${\cal O}(10^{3})$ for the decay constant $f_a = 10^{16}$ GeV, compared to the case without the coupling. For a sufficiently large coupling, the motion of the QCD axion becomes strongly damped, and as a result the axion abundance is enhanced. Our results show that the cosmological upper bound on the axion decay constant can be relaxed by a few hundred for a certain range of the coupling to hidden photons.

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N. Kitajima, T. Sekiguchi and F. Takahashi
Mon, 20 Nov 17

Comments: 10 pages, 3 figures

The Higgs properties in the MSSM after the LHC Run-2 [CL]


We scrutinize the parameter space of the SM-like Higgs boson in the minimal supersymmetric standard model (MSSM) under current experimental constraints. The constraints are from (i) the precison electroweak data and various flavor observables; (ii) the direct 22 separate ATLAS searches in Run-1; (iii) the latest LHC Run-2 Higgs data and tri-lepton search of electroweakinos. We perform a scan over the parameter space and find that the Run-2 data can further exclude a part of parameter space. For the property of the SM-like Higgs boson, its gauge couplings further approach to the SM values with a deviation below 0.1\%, while its Yukawa couplings $hb\bar{b}$ and $h\tau\bar\tau$ can still sizably differ from the SM predictions by several tens percent.

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J. Zhao
Mon, 20 Nov 17

Comments: 12 pages, 4 figures

Cosmological Axion and Quark Nugget Dark Matter Model [CL]


We study a dark matter (DM) model offering a very natural explanation of two (naively unrelated) problems in cosmology: the observed relation $\Omega_{\rm DM}\sim\Omega_{\rm visible}$ and the observed asymmetry between matter and antimatter in the Universe, known as the “baryogenesis” problem. In this framework, both types of matter (dark and visible) have the same QCD origin, form at the same QCD epoch, and both proportional to one and the same dimensional parameter of the system, $\Lambda_{\rm QCD}$, which explains how these two, naively distinct, problems could be intimately related, and could be solved simultaneously within the same framework. More specifically, the DM in this model is composed by two different ingredients: the (well- studied) DM axions and (less-studied) the quark nuggets made of matter or antimatter. The focus of the present work is the quantitative analysis of the relation between these two distinct components contributing to the dark sector of the theory determined by $\Omega_{\rm DM}\equiv [\Omega_{\rm DM}(\rm nuggets)+ \Omega_{\rm DM}(\rm axion)]$. We argue that the nugget’s DM component always traces the visible matter density, i.e. $\Omega_{\rm DM}(\rm nuggets)\sim\Omega_{\rm visible}$ and this feature is not sensitive to the parameters of the system such as the axion mass $m_a$ or the misalignment angle $\theta_0$. It should be contrasted with conventional axion production mechanism due to the misalignment when $\Omega_{\rm DM}(\rm axion)$ is highly sensitive to the axion mass $m_a$ and the initial misalignment angle $\theta_0$. We also discuss the constraints on this model related to the inflationary scale $H_I$, non-observation of the isocurvature perturbations, $r_T < 0.12$, and also, varies axions search experiments.

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S. Ge, X. Liang and A. Zhitnitsky
Mon, 20 Nov 17

Comments: 18 pages, 12 figures

Neutron skins and neutron stars in the multi-messenger era [CL]


The historical first detection of a binary neutron star merger by the LIGO-Virgo collaboration [B. P. Abbott {\sl et al.} Phys. Rev. Lett. 119, 161101 (2017)] is providing fundamental new insights into the astrophysical site for the $r$-process and on the nature of dense matter. A set of realistic models of the equation of state (EOS) that yield an accurate description of the properties of finite nuclei, support neutron stars of two solar masses, and provide a Lorentz covariant extrapolation to dense nuclear matter are used to confront its predictions against tidal polarizabilities extracted from the gravitational-wave data. Given the sensitivity of the gravitational-wave signal to the underlying EOS, limits on the tidal polarizabilities inferred from the observation translate into stringent constraints on the neutron-star radius. Based on these constraints, models that predict a stiff symmetry energy, and thus large stellar radii, can be ruled out. Indeed, under a particular binary-mass scenario, we deduce an upper limit on the radius of a $1.6\,M_{\odot}$ neutron star of $R_{\star}^{1.6}!<!13.25\,{\rm km}$. Given the sensitivity of the neutron-skin thickness of ${}^{208}$Pb to the symmetry energy, albeit at a lower density, we infer a corresponding upper limit of $R_{\rm skin}^{208}!\lesssim!0.25\,{\rm fm}$. However, if the upcoming PREX-II experiment measures a significantly thicker skin, this may be evidence of a softening of the symmetry energy at high densities—likely indicative of a phase transition in the interior of neutron stars.

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F. Fattoyev, J. Piekarewicz and C. Horowitz
Mon, 20 Nov 17

Comments: 6 pages, 4 figures

Exact relations for energy transfer in self-gravitating isothermal turbulence [CL]


Self-gravitating isothermal supersonic turbulence is analyzed in the asymptotic limit of large Reynolds numbers. Based on the inviscid invariance of total energy, an exact relation is derived for homogeneous, (not necessarily isotropic) turbulence. A modified definition for the two-point energy correlation functions is used to comply with the requirement of detailed energy equipartition in the acoustic limit. In contrast to the previous relations (Galtier and Banerjee, Phys. Rev. Lett., 107, 134501, 2011; Banerjee and Galtier, Phys. Rev. E, 87, 013019, 2013), the current exact relation shows that the pressure dilatation terms plays practically no role in the energy cascade. Both the flux and source terms are written in terms of two-point differences. Sources enter the relation in a form of mixed second-order structure functions. Unlike kinetic and thermodynamic potential energy, gravitational contribution is absent from the flux term. An estimate shows that for the isotropic case, the correlation between density and gravitational acceleration may play an important role in modifying the energy transfer in self-gravitating turbulence. The exact relation is also written in an alternative form in terms of two-point correlation functions, which is then used to describe scale-by-scale energy budget in spectral space.

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S. Banerjee and A. Kritsuk
Mon, 20 Nov 17

Comments: N/A

The cosmological dark sector as a scalar $σ$-meson field [CL]


Previous quantum field estimations of the QCD vacuum in the expanding space-time lead to a dark energy component scaling linearly with the Hubble parameter, which gives the correct figure for the observed cosmological term. Here we show that this behaviour also appears at the classical level, as a result of the chiral symmetry breaking in a low energy, effective $\sigma$-model. The dark sector is described in a unified way by the $\sigma$ condensate and its fluctuations, giving rise to a decaying dark energy and a homogeneous creation of non-relativistic dark particles. The creation rate and the future asymptotic de Sitter horizon are both determined by the $\sigma$ mass scale.

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S. Carneiro
Fri, 17 Nov 17

Comments: 4 pages, no figures