Isospin-forbidden electric-dipole capture and the $α(d,γ)^6$Li reaction [CL]

At the long-wavelength approximation, $E1$ transitions are forbidden between isospin-zero states. Hence $E1$ radiative capture is strongly hindered in reactions involving $N = Z$ nuclei but the $E1$ $S$ factor may remain comparable to, or larger than, the $E2$ one. Theoretical expressions of the isoscalar and isovector contributions to $E1$ capture are analyzed in microscopic and three-body approaches in the context of the $\alpha(d,\gamma)^6$Li reaction. The lowest non-vanishing terms of the operators are derived and the dominant contributions to matrix elements are discussed. Some of these contributions computed in a three-body model are compatible with an interpretation of the low-energy experimental data in terms of dominant isovector transitions involving small isospin-one admixtures in the wave functions. This suggests that the exact-masses prescription which is often used to avoid the disappearance of the $E1$ matrix element in potential models is not founded at the microscopic level. The importance of capture components from an initial $S$ scattering wave is also discussed.

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D. Baye and E. Tursunov
Wed, 18 Oct 2017

Comments: 23 pages, 1 table, 2 figures

Phase transitions between dilute and dense axion stars [CL]

We study the nature of phase transitions between dilute and dense axion stars interpreted as self-gravitating Bose-Einstein condensates. We develop a Newtonian model based on the Gross-Pitaevskii-Poisson equations for a complex scalar field with a self-interaction potential $V(|\psi|^2)$ involving an attractive $|\psi|^4$ term and a repulsive $|\psi|^6$ term. Using a Gaussian ansatz for the wave function, we analytically obtain the mass-radius relation of dilute and dense axion stars for arbitrary values of the self-interaction parameter $\lambda\le 0$. We show the existence of a critical point $|\lambda|c\sim (m/M_P)^2$ above which a first order phase transition takes place. We qualitatively estimate general relativistic corrections on the mass-radius relation of axion stars. For weak self-interactions $|\lambda|<|\lambda|_c$, a system of self-gravitating axions forms a stable dilute axion star below a general relativistic maximum mass $M{\rm max,GR}^{\rm dilute}\sim M_P^2/m$ and collapses into a black hole above that mass. For strong self-interactions $|\lambda|>|\lambda|c$, a system of self-gravitating axions forms a stable dilute axion star below a Newtonian maximum mass $M{\rm max,N}^{\rm dilute}=5.073 M_P/\sqrt{|\lambda|}$, collapses into a dense axion star above that mass, and collapses into a black hole above a general relativistic maximum mass $M_{\rm max,GR}^{\rm dense}\sim \sqrt{|\lambda|}M_P^3/m^2$. Dense axion stars explode below a Newtonian minimum mass $M_{\rm min,N}^{\rm dense}\sim m/\sqrt{|\lambda|}$ and form dilute axion stars of large size or disperse away. We determine the phase diagram of self-gravitating axions and show the existence of a triple point $(|\lambda|*,M*/(M_P^2/m))$ separating dilute axion stars, dense axion stars, and black holes. We make numerical applications for QCD axions and ultralight axions.

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P. Chavanis
Wed, 18 Oct 2017

Comments: N/A

Observational signatures of the parametric amplification of gravitational waves during reheating after inflation [CL]

We study the evolution of Gravitational Waves (GWs) during and after inflation as well as the resulting observational consequences in a Lorentz-violating massive gravity theory with one scalar (inflaton) and two tensor degrees of freedom. We consider two explicit examples of the tensor mass $m_g$ that depends either on the inflaton field $\phi$ or on its time derivative $\dot{\phi}$, both of which lead to parametric excitations of GWs during reheating after inflation. The first example is Starobinsky’s $R^2$ inflation model with a $\phi$-dependent $m_g$ and the second is a low-energy-scale inflation model with a $\dot{\phi}$-dependent $m_g$. We compute the energy density spectrum $\Omega_{\rm GW}(k)$ today of the GW background. In the Starobinsky’s model, we show that the GWs can be amplified up to the detectable ranges of both CMB and DECIGO. In low-scale inflation with a fast transition to the reheating stage driven by the potential $V(\phi)=M^2 \phi^2/2$ around $\phi \approx M_{\rm pl}$ (where $M_{\rm pl}$ is the reduced Planck mass), we find that the peak position of $\Omega_{\rm GW}(k)$ induced by the parametric resonance can reach the sensitivity region of advanced LIGO for the Hubble parameter of order 1 GeV at the end of inflation. Thus, our massive gravity scenario offers exciting possibilities for probing the physics of primordial GWs at various different frequencies.

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S. Kuroyanagi, C. Lin, M. Sasaki, et. al.
Thu, 19 Oct 17

Comments: 17 pages, 8 figures

Dark Matter Freeze-out During Matter Domination [CL]

We highlight the general scenario of dark matter freeze-out whilst the energy density of the universe is dominated by a decoupled non-relativistic species. Decoupling during matter domination changes the freeze-out dynamics, since the Hubble rate is parametrically different for matter and radiation domination. Furthermore, for successful Big Bang Nucleosynthesis the state dominating the early universe energy density must decay, this dilutes (or repopulates) the dark matter. As a result, the masses and couplings required to reproduce the observed dark matter relic density can differ significantly from radiation dominated freeze-out.

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S. Hamdan and J. Unwin
Thu, 19 Oct 17

Comments: 5 pages, 2 figures

Space-time slicing in Horndeski theories and its implications for non-singular bouncing solutions [CL]

In this paper, we show how the proper choice of gauge is critical in analyzing the stability of non-singular cosmological bounce solutions based on Horndeski theories. We show that it is possible to construct non-singular cosmological bounce solutions with classically stable behavior for all modes with wavelengths above the Planck scale where: (a) the solution involves a stage of null-energy condition violation during which gravity is described by a modification of Einstein’s general relativity; and (b) the solution reduces to Einstein gravity both before and after the null-energy condition violating stage. Similar considerations apply to galilean genesis scenarios.

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A. Ijjas
Thu, 19 Oct 17

Comments: 33 pages, 3 figures

Charged $ρ$-meson condensation in neutron stars [CL]

We extend relativistic mean-field models with hadron masses and meson-baryon coupling constants dependent on the scalar field $\sigma$, including hyperons and $\Delta(1232)$ baryons, to incorporate a possibility of the charged $\rho$ meson condensation in neutron star matter. The influence of the $\rho^-$ condensation on the equation of state proves to be strongly model dependent. In our models of one type (KVORcut-based ones) the $\rho^-$ condensation arises by a second-order phase transition above a critical density and the maximum value of the neutron star mass diminishes only slightly. The matter composition changes more significantly. In our models of other type (MKVOR-based ones), if the system is considered at fixed density, the $\rho^-$ condensation arises by a second-order phase transition at the baryon density $n=n_{c,\rho}^{(\rm II)}$ and at a slightly higher density $n=n_{c,\rho}^{(\rm I)}$ there occurs a first-order phase transition. In a neutron star matter starting with a density $n<n_{c,\rho}^{(\rm II)}$ there appears a region of a mixed phase, or the system is described by Maxwell construction, that results in a substantial decrease of the value of the maximum neutron star mass. Nevertheless in the models under consideration the observational constraint on the maximum neutron star mass is fulfilled. Besides, in MKVOR-based models the appearance of the $\rho^-$ condensate is accompanied by a strong rearrangement of the matter composition. Dependence of the results on a choice of the $\rho$ meson scaling functions for the effective $\rho$ meson mass and coupling constants is also investigated.

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E. Kolomeitsev, K. Maslov and D. Voskresensky
Thu, 19 Oct 17

Comments: 33 pages, 8 figures

A quantum heating as an alternative of reheating [CL]

To model a realistic situation for the beginning we consider massive real scalar $\phi^4$ theory in a (1+1)-dimensional asymptotically static Minkowski spacetime with an intermediate stage of expansion. To have an analytic headway we assume that scalars have a big mass. At past and future infinities of the background we have flat Minkowski regions which are joint by the inflationary expansion region. We use the tree-level Keldysh propagator in the theory in question to calculate the expectation value of the stress-energy tensor which is, thus, due to the excitations of the zero-point fluctuations. Then we show that even for large mass, if the de Sitter expansion stage is long enough, the quantum loop corrections to the expectation value of the stress-energy tensor are not negligible in comparison with the tree-level contribution. That is revealed itself via the excitation of the higher-point fluctuations of the exact modes: During the expansion stage a non-zero particle number density for the exact modes is generated. This density is not Plankian and serves as a quench which leads to a thermalization in the out Minkowski stage.

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E. Akhmedov and F. Bascone
Thu, 19 Oct 17

Comments: 31 pages, 1 fig