Following previous work in vacuum spacetimes, we investigate the constraint-damping properties in the presence of matter of the recently developed traceless, conformal and covariant Z4 (CCZ4) formulation of the Einstein equations. First, we evolve an isolated neutron star with an ideal gas equation of state and subject to a constraint-violating perturbation. We compare the evolution of the constraints using the CCZ4 and Baumgarte-Shibata-Shapiro-Nakamura-Oohara-Kojima (BSSNOK) systems. Second, we study the collapse of an unstable spherical star to a black hole. Finally, we evolve binary neutron star systems over several orbits until the merger, the formation of a black hole, and up to the ringdown. We show that the CCZ4 formulation is stable in the presence of matter and that the constraint violations are 1 or more orders of magnitude smaller than for the BSSNOK formulation. Furthermore, by comparing the CCZ4 and the BSSNOK formulations also for neutron star binaries with large initial constraint violations, we investigate their influence on the errors on physical quantities. We also give a new, simple and robust prescription for the damping parameter that removes the instabilities found when using the fully covariant version of CCZ4 in the evolution of black holes. Overall, we find that at essentially the same computational costs, the CCZ4 formulation provides solutions that are stable and with a considerably smaller violation of the Hamiltonian constraint than the BSSNOK formulation. We also find that the performance of the CCZ4 formulation is very similar to another conformal and traceless, but noncovariant formulation of the Z4 system, i.e., the Z4c formulation.

In Einstein-Maxwell theory, according to classic uniqueness theorems, the most general stationary black-hole solution is the axisymmetric Kerr-Newman metric, which is defined by three parameters: mass, spin and electric charge. The radial and angular dependence of gravitational and electromagnetic perturbations in the Kerr-Newman geometry do not seem to be separable. In this paper we circumvent this problem by studying scalar, electromagnetic and gravitational perturbations of Kerr-Newman black holes in the slow-rotation limit. We extend (and provide details of) the analysis presented in a recent Letter [P. Pani, E. Berti, and L. Gualtieri, Phys. Rev. Lett. 110, 241103 (2013)]. Working at linear order in the spin, we present the first detailed derivation of the axial and polar perturbation equations in the gravito-electromagnetic case, and we compute the corresponding quasinormal modes for any value of the electric charge. Our study is the first self-consistent stability analysis of the Kerr-Newman metric, and in principle it can be extended to any order in the small rotation parameter. We find numerical evidence that the axial and polar sectors are isospectral at first order in the spin and speculate on the possible implications of this result.

According to folklore in general relativity, the Weyl tensor can be decomposed into parts corresponding to Newton-like, incoming and outgoing wavelike field components. It is shown here that this one-to-one correspondence does not hold for space-time geometries with cylindrical isometries. This is done by investigating some well-known exact solutions of Einstein’s field equations with whole-cylindrical symmetry, for which the physical interpretation is very clear, but for which the standard Weyl interpretation would give contradictory results. For planar or spherical geometries, however, the standard interpretation works for both static and dynamical space-times. It is argued that one reason for the failure in the cylindrical case is that for waves spreading in two spatial dimensions there is no local criterion to distinguish incoming and outgoing waves already at the linear level. It turns out that Thorne’s local energy notion, subject to certain qualifications, provides an efficient diagnostic tool to extract the proper physical interpretation of the space-time geometry in the case of cylindrical configurations.

The Majoron provides an attractive dark matter candidate, directly associated with the mechanism responsible for spontaneous neutrino mass generation within the standard model $\mathrm{SU}(3{)}_{\mathrm{c}}\otimes \mathrm{SU}(2{)}_{\mathrm{L}}\otimes \mathrm{U}(1{)}_{\mathrm{Y}}$ framework. Here we update the cosmological and astrophysical constraints on Majoron dark matter coming from the cosmic microwave background and a variety of x- and $\gamma$-ray observations.

Faraday rotation (FR) of CMB polarization, as measured through mode-coupling correlations of E and B modes, can be a promising probe of a stochastic primordial magnetic field (PMF). While the existence of a PMF is still hypothetical, there will certainly be a contribution to CMB FR from the magnetic field of the Milky Way. We use existing estimates of the Milky Way rotation measure (RM) to forecast its detectability with upcoming and future CMB experiments. We find that the Galactic RM will not be seen in polarization measurements by Planck, but that it will need to be accounted for by CMB experiments capable of detecting the weak lensing contribution to the B mode. We then discuss prospects for constraining the PMF in the presence of FR due to the Galaxy under various assumptions that include partial delensing and partial subtraction of the Galactic FR. We find that a realistic future suborbital experiment, covering a patch of the sky near the Galactic poles, can detect a scale-invariant PMF of 0.1 nanogauss at better than a 95% confidence level, while a dedicated space-based experiment can detect even smaller fields.

We revisit the dynamics of the axial coupling between many N-flatons and an Abelian gauge field, with special attention to its statistically anisotropic signal. The anisotropic power spectrum of curvature perturbations associated with the large-wavelength modes of the gauge vector field is generally undetectable, since the anisotropy is confined to small scales. If the gauge field is the electromagnetic field, provided that the number of fields participating in the exponential expansion is large, it could be possible to generate sizable large-scale magnetic fields. However, its spectrum is blue, and appreciable power on large scales implies an overly strong field on smaller scales, incompatible with observations. Furthermore, the anisotropy is also markedly enhanced, and might be at odds with the isotropic observed sky. These aspects further demand that the scale of inflation be kept to a minimum.

We discuss weak lensing characteristics in the gravitational field of a compact object in the low-energy approximation of fourth order $f(R)$-gravity theory. The particular solution is characterized by a gravitational strength parameter $\sigma$ and a distance scale $r_c$ much larger than the Schwarzschild radius. Above $r_c$ gravity is strengthened and as a consequence weak lensing features are modified compared to the Schwarzschild case. We find a critical impact parameter (depending upon $r_c$) for which the behavior of the deflection angle changes. Using the Virbhadra-Ellis lens equation we improve the computation of the image positions, Einstein ring radii, magnification factors and the magnification ratio. We demonstrate that the magnification ratio as function of image separation obeys a power law depending on the parameter $\sigma$, with a double degeneracy. No $\sigma \ne 0$ value gives the same power as the one characterizing Schwarzschild black holes. As the magnification ratio and the image separation are the lensing quantities most conveniently determined by direct measurements, future lensing surveys will be able to constrain the parameter $\sigma$ based on this prediction.

The form factors, as well as the helicity amplitudes for the ${\gamma }^{*}N\to N(1535)$ transition, are calculated in the framework of the QCD light-cone sum rules. The contamination coming from diagonal transition is eliminated by considering combinations of the sum rules corresponding to different Lorentz structures. A similar procedure is applied for the determination of the residue and mass of the negative-parity baryon. A comparison of our predictions on the helicity amplitudes with the existing theoretical and experimental results is also presented.

A variational method is discussed, extending the Gaussian effective potential to higher orders. The single variational parameter is replaced by trial unknown two-point functions, with infinite variational parameters to be optimized by the solution of a set of integral equations. These stationary conditions are derived by the self-energy without having to write the effective potential, making use of a general relation between self-energy and functional derivatives of the potential. This connection is proven to any order and verified up to second order by an explicit calculation for the scalar theory. Among several variational strategies, the methods of minimal sensitivity and of minimal variance are discussed in some detail. For the scalar theory, at variance with other post-Gaussian approaches, the pole of the second-order propagator is shown to satisfy the simple first-order gap equation that seems to be more robust than expected. By the method of minimal variance, nontrivial results are found for gauge theories containing fermions, where the first-order Gaussian approximation is known to be useless.

We point out that existing measurements of $pp\to ZZ$ cross sections at the LHC in a broad range of $ZZ$ invariant masses allow one to derive a model-independent upper bound on the Higgs boson width, thanks to a strongly enhanced off-shell Higgs contribution. Using CMS data and considering events in the interval of $ZZ$ invariant masses from 100 to 800 GeV, we find ${\Gamma }_H\le 38.8{\Gamma }_H^{\mathrm{SM}}\approx 163\mathrm{MeV}$, at the 95% confidence level. Restricting $ZZ$ invariant masses to the $M_{ZZ}\ge 300\mathrm{GeV}$ range, we estimate that this bound can be improved to ${\Gamma }_H\le 21{\Gamma }_H^{\mathrm{SM}}\approx 88\mathrm{MeV}$. Under the assumption that all couplings of the Higgs boson to Standard Model particles scale in a universal way, our result can be translated into an upper limit on the branching fraction of the Higgs boson decay to invisible final states. We obtain $\mathrm{Br}(H\to \mathrm{inv})<0.84(0.78)$, depending on the range of $ZZ$ invariant masses that are used to constrain the width. We believe that an analysis along these lines should be performed by experimental collaborations in the near future and also in run II of the LHC. We estimate that such analyses can, eventually, be sensitive to a Higgs boson width as small as ${\Gamma }_H\sim 10{\Gamma }_H^{\mathrm{SM}}$.

We study vacuum stability of $B-L$ extension of the Standard Model (SM) and its supersymmetric version. We show that the generation of nonvanishing neutrino masses through TeV inverse seesaw mechanism leads to a cutoff scale of SM Higgs potential stability of order ${10}^5\mathrm{GeV}$. However, in the nonsupersymmetric $B-L$ model, we find that the mixing between the SM-like Higgs boson and the $B-L$ Higgs boson plays a crucial role in alleviating the vacuum stability problem. We also provide the constraints of stabilizing the Higgs potential in the supersymmetric $B-L$ model.

Short gamma-ray bursts (GRBs), brief intense emission of $\gamma$ rays characterized by a duration shorter than 2 s that are plausibly powered by the coalescence of binary neutron stars, are believed to be strong gravitational wave radiation (GWR) sources. The test of such a speculation has been thought to be impossible until the performance of the detectors like advanced LIGO. Recently there has been growing evidence for the formation of a highly magnetized neutron star (i.e., magnetar) in the double neutron star mergers. In this work we reexamine the interpretation of the x-ray plateau followed by an abrupt decline detected in some short GRB afterglows within the supramassive magnetar model and find that the maximum gravitational mass of the nonrotating neutron stars is $\sim 2.3M_{\odot }$ and the observed duration of some x-ray plateaus are significantly shorter than that expected in the magnetic dipole radiation scenario, suggesting that the collapse of the supramassive magnetars has been considerably enhanced by the energy loss via GWR. Such a result demonstrates that the signature of GWR may have already existed in current electromagnetic data of short GRBs.

We study the correlators of two circular Wilson loops of different radii at strong coupling. In our setup one Wilson loop is located inside the other. We use holography to calculate the connected two-point function. Both an anti-de Sitter background and a confining background are considered. As the computation for the confining case cannot be carried out analytically, we solve the problem numerically. In the anti-de Sitter case, our results agree with similar holographic calculations. In the case of a confining background, we find an asymptotic area law, in agreement with the result of the lattice strong coupling expansion. We also elaborate on the subtle issue of the interplay between connected and disconnected string world sheets.

We explore the phase diagram of the four-dimensional compact $U(1)$ gauge theory at finite temperature as a function of the gauge coupling and of the compactified Euclidean time dimension $L_t$. We show that the strong-to-weak coupling transition, which is first order at $T=0$ ($L_t=\infty$), becomes second order for high temperatures, i.e. for small values of $L_t$, with a tricritical temporal size ${\overline{L}}_t$ located between 5 and 6. The critical behavior around the tricritical point explains and reconciles previous contradictory evidences found in the literature.

Motivated by the scenario of resonant leptogenesis in which lepton number creation in the electroweak scale is relevant, we investigate the spectral properties and possible collective nature of the standard model neutrinos at electroweak scale temperature ($T$). We adopt the $R_{\xi }$ gauge fixing, which includes the unitary gauge as a limiting case, and allows us to study the broken as well as the restored phases of the gauge symmetry in a unified way. We show that the spectral density of the neutrino has a three-peak structure in the low-momentum region due to the scattering with the thermally excited particles (i.e., Landau damping) for $T\gtrsim M_{\mathrm{W},\mathrm{Z}}(T)$, with $M_{\mathrm{W},\mathrm{Z}}(T)$ being the weak-boson masses in the plasma. The three peaks are identified with a novel ultrasoft mode, the usual quasiparticle, and antiplasmino modes. Varying the gauge-fixing parameter $\xi$, we show that the three-peak structure appears independently of the gauge fixing and thus has a physical significance. We discuss possible implications of the neutrino spectral density obtained in the present work on particle cosmology, in particular in the context of resonant leptogenesis.

We show that it is possible to accommodate physical scalar resonances within a minimal nonlinearly realized electroweak theory in a way compatible with a natural Hopf algebra selection criterion (weak power-counting) and the relevant functional identities of the model (local functional equation, Slavnov-Taylor identity, ghost equations, $b$-equations). The beyond-the-Standard-Model (BSM) sector of the theory is studied by Becchi-Rouet-Stora-Tyutin techniques. The presence of a mass generation mechanism à la Stückelberg allows for two mass invariants in the gauge boson sector. The corresponding ’t Hooft gauge-fixing is constructed by respecting all the symmetries of the theory. The model interpolates between the Higgs and a purely Stückelberg scenario. Despite the presence of physical scalar resonances, we show that tree-level violation of unitarity in the scattering of longitudinally polarized charged gauge bosons occurs at sufficiently high energies, if a fraction of the mass is generated by the Stückelberg mechanism. The formal properties of the physically favored limit after LHC7-8 data, where BSM effects are small and custodial symmetry in the gauge boson sector is respected, are studied.

The geometric approach based on parallel transport and Wilson lines to gain insight into the decomposition of the gluon gauge 4-vector potential is being developed in the literature to address the proton spin puzzle. Unfortunately, conflicting claims on the uniqueness of the decomposition have been made. We reexamine this controversy and argue that Hodge decomposition and the de Rham theorem resolve this issue in a natural way such that the topological defects acquire a fundamental significance. It is suggested that qualitatively angular momentum holonomy could be an alternative new mechanism to account for the deficit proton spin.

Compact bosonic field configurations, or boson stars, are promising dark matter candidates which have been invoked as an alternative description for the supermassive compact objects in active galactic nuclei. Boson stars can be comparable in size and mass to supermassive objects, and they might be hard to distinguish by electromagnetic observations. However, boson stars do not possess an event horizon, and their global spacetime structure is different from that of a black hole. This leaves a characteristic imprint in the gravitational-wave emission, which can be used as a discriminant between black holes and other horizonless compact objects. Here we perform a detailed study of boson stars and their gravitational-wave signatures in a fully relativistic setting, a study which was lacking in the existing literature in many respects. We construct several fully relativistic boson star configurations, and we analyze their geodesic structure and free oscillation spectra, or quasinormal modes. We explore the gravitational and scalar response of boson star spacetimes to an inspiraling stellar-mass object and compare it to its black hole counterpart. We find that a generic signature of compact boson stars is the resonant-mode excitation by a small compact object on stable quasicircular geodesic motion.

Since black holes can be formed through widely varying processes, the horizon structure is highly complicated in the dynamical phase. Nonetheless, as numerical simulations show, the final state appears to be universal, well described by the Kerr geometry. How are all these large and widely varying deviations from the Kerr horizon washed out? To investigate this issue, we introduce a well-suited notion of horizon multipole moments and equations governing their dynamics, thereby providing a coordinate- and slicing-independent framework to investigate the approach to equilibrium. In particular, our flux formulas for multipoles can be used as analytical checks on numerical simulations and, in turn, the simulations could be used to fathom possible universalities in the way black holes approach their final equilibrium.

In this paper we study dynamical compactification in Einstein-Gauss-Bonnet gravity from an arbitrary dimension for generic values of the coupling constants. We show that, when the curvature of the extra-dimensional space is negative, for any value of the spatial curvature of the four-dimensional space-time one obtains a realistic behavior in which for asymptotic time both the volume of the extra dimension and expansion rate of the four-dimensional space-time tend to a constant. Remarkably, this scenario appears within the open region of parameters space for which the theory does not admit any maximally symmetric ($4+D$)-dimensional solution, which gives to the dynamical compactification an interpretation as geometric frustration. In particular there is no need to fine-tune the coupling constants of the theory so that the present scenario does not violate the “naturalness hypothesis.” Moreover we show that with an increase of the number of extra dimensions the stability properties of the solution are increased.