We study the Sivers asymmetry in semi-inclusive hadron production in deep inelastic scattering. We concentrate on the contribution from the photon-gluon fusion channel at $\mathcal{O}({\alpha }_{em}^2{\alpha }_s)$, where three-gluon correlation functions play a major role within the twist-3 collinear factorization formalism. We establish the correspondence between such a formalism with three-gluon correlation functions and the usual transverse momentum-dependent (TMD) factorization formalism at moderate hadron transverse momenta. We derive the coefficient functions used in the usual TMD evolution formalism related to the quark Sivers function expansion in terms of the three-gluon correlation functions. We further perform the next-to-leading order calculation for the transverse momentum-weighted spin-dependent differential cross section and identify the off-diagonal contribution from the three-gluon correlation functions to the QCD collinear evolution of the twist-3 Qiu-Sterman function.

New data from the BABAR Collaboration, on azimuthal asymmetries measured in $e^{+}{e}^{-}$ annihilations into pion pairs at $Q^2=112{\mathrm{GeV}}^2$, allow us to take the first, direct glance at the $p_{\perp }$ dependence of the Collins functions, in addition to their $z$ dependence. These data, together with previous $e^{+}{e}^{-}\to h_1{h}_2$ Belle measurements and the available semi-inclusive deep inelastic scattering (SIDIS) data on the Collins asymmetry, are simultaneously analyzed in the framework of the generalized parton model, assuming two alternative $Q^2$ evolution schemes and exploiting two different parametrizations for the Collins functions. The corresponding results for the transversity distributions are presented. Analogous data, newly released by the BESIII Collaboration, on $e^{+}{e}^{-}$ annihilations into pion pairs at the lower $Q^2$ of $13{\mathrm{GeV}}^2$, offer the possibility to explore the sensitivity of these azimuthal correlations on transverse-momentum-dependent evolution effects.

We study the exclusive leptonic and semileptonic $B$ decays $B\to {\ell }^{-}{\overline{\nu }}_{\ell }$ and $B\to D^{(*)}{\ell }^{-}{\overline{\nu }}_{\ell }$ in the framework of the covariant quark model with built-in infrared confinement. We compute the relevant form factors in the full kinematical momentum transfer region. The calculated form factors are used to evaluate branching fractions and polarization observables of the above transitions. We compare our results with experimental data and results from other theoretical studies.

The hypothesis of nonradiative neutrino decay is tested using the latest IceCube data. The track-to-shower ratio expected in IceCube is calculated in a complete decay scenario, taking into account the uncertainties in neutrino oscillation parameters and assuming only one stable neutrino. The subset of high-energy neutrino events observed by IceCube above 60 TeV, combined with the information provided by passing muons, actually disfavors the possibility of neutrinos decay at about a $2\sigma$ level of significance for both the normal and inverted neutrino mass hierarchy.

A search is presented for pair production of a new heavy quark ($Q$) that decays into a $W$ boson and a light quark ($q$) in the final state where one $W$ boson decays leptonically (to an electron or muon plus a neutrino) and the other $W$ boson decays hadronically. The analysis is performed using an integrated luminosity of $20.3{\mathrm{fb}}^{-1}$ of $pp$ collisions at $\sqrt{s}=8\mathrm{TeV}$ collected by the ATLAS detector at the LHC. No evidence of $Q\overline{Q}$ production is observed. New chiral quarks with masses below 690 GeV are excluded at 95% confidence level, assuming $\mathrm{BR}(Q\to Wq)=1$. Results are also interpreted in the context of vectorlike quark models, resulting in the limits on the mass of a vectorlike quark in the two-dimensional plane of $\mathrm{BR}(Q\to Wq)$ versus $\mathrm{BR}(Q\to Hq)$.

We present measurements of Collins asymmetries in the inclusive process $e^{+}{e}^{-}\to h_1{h}_{2}X$, $h_1{h}_2=KK$, $K\pi$, $\pi \pi$, at the center-of-mass energy of 10.6 GeV, using a data sample of $468{\mathrm{fb}}^{-1}$ collected by the BABAR experiment at the PEP-II $B$ factory at SLAC National Accelerator Center. Considering hadrons in opposite thrust hemispheres of hadronic events, we observe clear azimuthal asymmetries in the ratio of unlike sign to like sign, and unlike sign to all charged $h_1{h}_2$ pairs, which increase with hadron energies. The $K\pi$ asymmetries are similar to those measured for the $\pi \pi$ pairs, whereas those measured for high-energy $KK$ pairs are, in general, larger.

We have studied the existence of topological Bogomol’nyi-Prasad-Sommerfield or self-dual configurations in a Lorentz-violating gauged $O(3)$ nonlinear sigma model, where $CPT$-even Lorentz-violating (LV) terms were introduced in both the gauge and $\sigma$-field sectors. As happens in the usual gauged $\sigma$ model, purely magnetic self-dual configurations are allowed, maintaining some qualitative features of the standard ones. In a more involved configuration, Lorentz violation provides new self-dual magnetic solutions carrying an electric field but a null total electric charge. In both cases, the total energy of the self-dual configurations turns out to be proportional to the topological charge of the model and to the LV parameters introduced in the $\sigma$ sector. It is shown that the LV terms yield magnetic flux reversion as well.

We calculate the effective vertex of the quark production in the collision of a Reggeized quark and a Reggeized gluon in the next-to-leading order (NLO). The vertex in question is the missing component of the multi-Regge NLO amplitudes with the quark and gluon exchanges in the $t_i$ channels. This multi-Regge form of the amplitudes is the important hypothesis which was recently proved for the gluon exchanges only and remains unverified within the next-to-leading-logarithmic approximation (NLA) for the general case including the quark exchanges. Our calculation allows one to develop the bootstrap approach to the quark Reggeization proof in NLA.

We use a Green’s function approach in order to develop a method for calculating the tunneling rate between radiatively generated nondegenerate vacua. We apply this to a model that exhibits spontaneous symmetry breaking via the Coleman-Weinberg mechanism, where we determine the self-consistent tunneling configuration and illustrate the impact of gradient effects that arise from accounting for the underlying space-time inhomogeneity.

We consider a system consisting of a quantum, massless, real scalar field, in the presence of nonlinear mirrors: infinite parallel planes, upon which the field satisfies nonlinear boundary conditions. These may appear, for example, in metamaterials having nonlinear response functions. The boundary conditions are implemented by nonquadratic interaction vertices, strictly localized on the mirrors. By using the appropriate perturbative expansions, we obtain approximate expressions for the Casimir energy corresponding to weak coupling, regarding the strength of the interaction terms. We also comment on an alternative expansion scheme that may be useful when the weak coupling expansion is not justified.

Can a black hole that suffers a superradiant instability evolve towards a “hairy” configuration which is stable? We address this question in the context of Einstein-charged scalar field theory. First, we describe a family of static black hole solutions which possess charged scalar-field hair confined within a mirror-like boundary. Next, we derive a set of equations which govern the linear, spherically symmetric perturbations of these hairy solutions. We present numerical evidence which suggests that, unlike the vacuum solutions, the (single-node) hairy solutions are stable under linear perturbations. Thus, it is plausible that stable hairy black holes represent the end point of the superradiant instability of electrically charged Reissner-Nordström black holes in a cavity; we outline ways to explore this hypothesis.

Using Monte Carlo simulations, we compute the integrated emission spectra of black holes in the framework of loop quantum gravity (LQG). The black hole emission rates are governed by the entropy whose value, in recent holographic loop quantum gravity models, was shown to agree at leading order with the Bekenstein-Hawking entropy. Quantum corrections depend on the Barbero-Immirzi parameter $\gamma$. Starting with black holes of initial horizon area $A\sim {10}^2$ in Planck units, we present the spectra for different values of $\gamma$. Each spectrum clearly decomposes into two distinct parts: a continuous background which corresponds to the semiclassical stages of the evaporation and a series of discrete peaks which constitutes a signature of the deep quantum structure of the black hole. We show that $\gamma$ has an effect on both parts that we analyze in detail. Finally, we estimate the number of black holes and the instrumental resolution required to experimentally distinguish between the considered models.

Generic scalar-tensor theories of gravity predict deviations from Newtonian physics inside astrophysical bodies. In this paper, we point out that low mass stellar objects, red and brown dwarf stars, are excellent probes of these theories. We calculate two important and potentially observable quantities: the radius of brown dwarfs and the minimum mass for hydrogen burning in red dwarfs. The brown dwarf radius can differ significantly from the general relativity prediction, and upcoming surveys that probe the mass-radius relation for stars with masses $<\mathcal{O}(0.1M_{\odot })$ have the potential to place new constraints. The minimum mass for hydrogen burning can be larger than several presently observed red dwarf stars. This places a new and extremely stringent constraint on the parameters that appear in the effective field theory of dark energy and rules out several well-studied dark energy models.

We obtain the thermal corrections to the Casimir energy for the neutrino and electromagnetic fields in Einstein and closed Friedmann universes containing a static, infinitely straight and thin cosmic string. The Casimir free energy is also obtained as well as their low and high temperature limits. It is shown that the vacuum energies associated with these fields, in the background considered, are given simply by the vacuum energies in the absence of the cosmic string multiplied by a factor that codifies the presence of this topological defect.

This work was mainly driven by the desire to explore to what extent embedding some given geometry in a higher dimensional flat one is useful for understanding the causal structure of classical fields traveling in the former, in terms of that in the latter. We point out, in the four-dimensional (4D) spatially flat Friedmann-Lemaître-Robertson-Walker universe, that the causal structure of transverse-traceless (TT) gravitational waves can be elucidated by first reducing the problem to a two-dimensional (2D) Minkowski wave equation with a time-dependent potential, where the relevant Green’s function is a pure tail—waves produced by a physical source propagate strictly within the null cone. By viewing this 2D world as embedded in a 4D one, the 2D Green’s function can also be seen to be sourced by a cylindrically symmetric scalar field in three dimensions (3D). From both the 2D wave equation and the 3D scalar perspective, we recover the exact solution of the 4D graviton tail for the case where the scale factor written in conformal time is a power law. There are no TT gravitational-wave tails when the universe is radiation dominated because the background Ricci scalar is zero. In a matter-dominated one, we estimate the amplitude of the tail to be suppressed relative to its null counterpart by both the ratio of the duration of the (isolated) source to the age of the universe ${\eta }_0$ and the ratio of the observer-source spatial distance (at the observer’s time) to the same ${\eta }_0$. In a universe driven primarily by a cosmological constant, the tail contribution to the background geometry $a[\eta {]}^2{\eta }_{\mu \nu }$ after the source has ceased is the conformal factor $a^2$ times a spacetime-constant symmetric matrix proportional to the spacetime volume integral of the TT part of the source’s stress-energy-momentum tensor. In other words, massless spin-2 gravitational waves exhibit a tail-induced memory effect in 4D de Sitter spacetime.

Despite all attempts, exact solutions for black rings in more than five dimensions remain elusive. In this paper we clarify some of the reasons for that; in particular we show that a peculiar symmetry of the five-dimensional black ring—separability of the base—cannot occur in dimensions higher than five. We also construct supersymmetric solutions that have symmetries of five-dimensional supersymmetric black ring and show that they do not have regular horizons.

We consider the generalization of the “spinor approach” to the Lorentzian case, in the context of three-dimensional loop quantum gravity with cosmological constant $\mathrm{\Lambda }=0$. The key technical tool that allows this generalization is the recoupling theory between unitary infinite-dimensional representations and nonunitary finite-dimensional ones, obtained in the process of generalizing the Wigner–Eckart theorem to SU(1,1). We use SU(1,1) tensor operators to build observables and a solvable quantum Hamiltonian constraint, analogous to the one introduced by V. Bonzom and his collaborators in the Euclidean case (with both $\mathrm{\Lambda }=0$ and $\mathrm{\Lambda }\ne 0$). We show that the Lorentzian Ponzano-Regge amplitude is the solution of the quantum Hamiltonian constraint by recovering the Biedenharn-Elliott relation [generalized to the case where unitary and nonunitary SU(1,1) representations are coupled to each other]. Our formalism is sufficiently general that both the Lorentzian and the Euclidean case can be recovered (with $\mathrm{\Lambda }=0$).

A new model of nonlinear electrodynamics with a dimensional parameter $\beta$ coupled to gravity is considered. We show that an accelerated expansion of the universe takes place if the nonlinear electromagnetic field is the source of the gravitational field. A pure magnetic universe is investigated, and the magnetic field drives the universe to accelerate. In this model, after the big bang, the universe undergoes inflation and the accelerated expansion and then decelerates approaching Minkowski spacetime asymptotically. We demonstrate the causality of the model and a classical stability at the deceleration phase.

We investigate the ultraviolet behavior of quantum gravity within a functional renormalization group approach. The present setup includes the full ghost and graviton propagators and, for the first time, the dynamical graviton three-point function. The latter gives access to the coupling of dynamical gravitons and makes the system minimally self-consistent. The resulting phase diagram confirms the asymptotic safety scenario in quantum gravity with a nontrivial UV fixed point. A well-defined Wilsonian block spinning requires locality of the flow in momentum space. This property is discussed in the context of functional renormalization group flows. We show that momentum locality of graviton correlation functions is nontrivially linked to diffeomorphism invariance, and is realized in the present setup.