Recently, electron scattering fixed-target experiments came into focus to search for $U(1)$ extensions of the Standard Model of particle physics at low energies. These extensions are motivated from anomalies in astrophysical observations as well as from deviations from Standard Model predictions, such as the discrepancy between the experimental and theoretical determinations of the anomalous magnetic moment of the muon. In such $U(1)$ extensions, a new, light messenger particle ${\gamma }^{\prime }$, the hidden photon, couples to the hidden sector as well as to the electromagnetic current of the Standard Model by kinetic mixing, which allows for a search for this particle, e.g., in the invariant mass distribution of the process $e(A,Z)\to e(A,Z)l^{+}{l}^{-}$. In this process the hidden photon is emitted by bremsstrahlung and decays into a pair of Standard Model leptons. In this work we study the applicability of the Weizsäcker–Williams approximation to calculate the signal cross section of the process, which is widely used to design such experimental setups. Furthermore, we study the influence of the contribution from doubly virtual Compton scattering on experiments performed with proton targets. Based on a previous work, we investigate the discovery potential of future experimental setups at the Jefferson Lab and obtain a projected exclusion limit. We find that doubly virtual Compton scattering causes a 10% effect on the cross section for the kinematics of a planned experiment and therefore does not alter the obtained experimental reach significantly. In addition, we find that the Weizsäcker–Williams approximation can be applied to calculate the underlying cross sections in the kinematic range of most of the present and upcoming experiments, provided that the dependence on the energy and emission angle of the hidden photon is accounted for.

We study the anomalous production of a single top quark in association with a Higgs boson at the LHC originating from flavor-changing neutral current interactions in $tqg$ and $tqH$ vertices. We derive the discovery potentials and 68% C.L. upper limits considering leptonic decay of the top quark and the Higgs boson decay into a $b\overline{b}$ pair with $10{\mathrm{fb}}^{-1}$ integrated luminosity of data in proton-proton collisions at the center-of-mass energy of 14 TeV. We propose a charge ratio for the lepton in top quark decay in terms of lepton $p_T$ and $\eta$ as a strong tool to observe the signal. In particular, we show that the charge ratio increases significantly at large $p_T$ of the charged lepton, while the main background from $t\overline{t}$ is nearly charge symmetric and the $W+\text{jets}$ background has much smaller charge ratio with respect to the signal. We show that this feature can also be used in the probe of anomalous single top production with a $Z$ boson or a photon that is under the attention of the experimental collaborations.

Proof of transverse-momentum-dependent factorization for hadron-hadron collisions is given in this paper. We focus on processes without detected soft final hadrons or detected final hadrons that are collinear to the initial hadrons. This contradicts the widely accepted viewpoint that transverse-momentum-dependent factorization does not hold in such processes even in the generalized sense. The key point is that singular points of the type $l^{+}=0$ can be absorbed into the Wilson lines of soft gluons, where $l$ is collinear to the plus direction. Thus, one should subtract such singular points from the collinear region of $l$. After such subtraction, one can make use of the Ward identity to absorb the effects of scalar-polarized collinear gluons into Wilson lines.

In brane-world models, combining the extradimensional field modes with the standard four-dimensional ones yields interesting physical consequences that have been proved from high-energy physics to cosmology. Even some low-energy phenomena have been considered along these lines to set bounds on the brane model parameters. In this work, we extend to the gravitational realm a previous result which gave finite electromagnetic and scalar potentials and self-energies for a source looking pointlike to an observer sitting in a 4D Minkowski subspace of the single brane of a Randall-Sundrum spacetime including compact dimensions. We calculate here the gravitational field for the same type of source by solving the linearized Einstein equations. Remarkably, it also turns out to be nonsingular. Moreover, we use gravitational experimental results of the Cavendish type and the parameterized post-Newtonian coefficients to look for admissible values of the brane model parameters. The anti–de Sitter radius hereby obtained is concordant with previous results based on the Lamb shift in hydrogen. However, the resulting parameterized post-Newtonian parameters lie outside the acceptable value domain.

We study the structure of neutron stars in the $f(R)=R+\alpha R^2$ theory of gravity (the Starobinsky model) in an exact and nonperturbative approach. In this model, apart from the standard general relativistic junction conditions, two extra conditions—namely, the continuity of the curvature scalar and its first derivative—need to be satisfied. For an exterior Schwarzschild solution, the curvature scalar and its derivative must be zero at the stellar surface. We show that for some equation of state (EoS) of matter, matching all conditions at the surface of the star is impossible. Hence the model brings two major fine-tuning problems: (i) only some particular classes of EoS are consistent with Schwarzschild at the surface, and (ii) given the EoS, only a very particular set of boundary conditions at the center of the star will satisfy the given boundary conditions at the surface. Hence we show that this model [and subsequently many other $f(R)$ models where the uniqueness theorem is valid] is highly unnatural for the existence of compact astrophysical objects. This is because the EoS of a compact star should be completely determined by the physics of nuclear matter at high density and not the theory of gravity.

Quantum vacuum fluctuations tend to be strongly anticorrelated, which reduces their observable effects. However, time dependence can upset the cancellation of these anticorrelated fluctuations and greatly enhance their effects. This form of noncancellation is investigated for spacetime geometry fluctuations driven by the vacuum stress tensor fluctuations of the electromagnetic field. The time dependence can take the form of sinusoidal conformal metric perturbations or of time variation of the gravitational constant, both of which can arise from modifications of the gravitational action. We examine two observable quantities, luminosity fluctuations and redshift fluctuations, both of which can arise from stress tensor fluctuations. We find that both quantities can grow with increasing distance between a source and an observer. This secular growth raises the possibility of observing spacetime geometry fluctuation effects at length scales far longer than the Planck scale.

The time-domain multiple-slit interference effect for boson pair production in an alternating-sign $N$-pulse electric field is investigated. An approximate solution of boson momentum spectrum is deduced, and it is very consistent with the exact numerical calculation, especially for a two-pulse field and for small longitudinal momentum. The relationship between the number density of created bosons $n$ and the pulse number of electric field $N$ is also studied, and it can be described with a quadratic parabola. Moreover, we compare the differences of pair creation between bosons and fermions for their longitudinal momentum spectra. It is found that the oscillation behaviors of momentum spectrum for bosons and fermions have a phase difference of an odd multiple of $\pi /2$.

An interesting consequence of string dualities is that they reveal situations where the geometry of a string background appears to be globally ill defined, a phenomenon usually referred to as nongeometry. On the other hand, string theory contains extended objects with nontrivial monodromy around them, often dubbed defect or exotic branes in codimension-2. We determine and examine the worldvolume actions and the couplings of certain such branes. In particular, based on specific chains of T- and S-dualities, we derive the Dirac-Born-Infeld and Wess-Zumino actions, which describe the dynamics of type IIB five-branes as well as their couplings to the appropriate gauge potentials associated to mixed symmetry tensors. Based on these actions we discuss how these branes act as sources of nongeometric fluxes. In one case this flux is what is usually termed $Q$ flux, associated to a T-fold compactification, while in the S-dual case a type of nongeometry related to the Ramond-Ramond sector is encountered.

We investigate the holographic renormalization of the Einstein-Maxwell-dilaton theory which provides an asymptotic Lifshitz geometry dual to a Lifshitz field theory. In this case, the existence of a field combination with zero scaling dimension causes an ambiguity in fixing local counterterms. Nevertheless, we show that all possible local counterterms give rise to consistent thermodynamic quantities with the Lifshitz black brane results. In addition, we also study the retarded Green functions of the current and momentum operator of a nonrelativistic Lifshitz field theory. In the nonzero momentum regime, the results show intriguingly that there exists a massive quasinormal mode whose effective mass is linearly proportional to temperature and that even at zero temperature there exists a quasinormal mode in the nonrelativistic Lifshitz medium.

We study the properties of strongly interacting massive quantum fields in space-time as resulting from a parametric decay of the fields with a large decay width $\gamma$. The resulting imaginary part of the retarded and advanced propagators in this case is of Lorentzian form, and the theory conserves microcausality; i.e., the commutator between the fields vanishes for spacelike distances in space-time. However, when considering separately spacelike and timelike components of the spectral function in momentum space, we find microcausality to be violated for each component separately. This implies that the modeling of effective field theories for strongly interacting systems has to be considered with great care, and restrictions to timelike four-momenta in case of broad spectral functions have to be ruled out. Furthermore, when employing effective propagators with a width $\gamma ({\mathbf{p}}^2)$ depending explicitly on three-momentum $\mathbf{p}$, the commutator of the fields no longer vanishes for $r>t$, since the related field theory becomes nonlocal and violates microcausality.

We investigate the analytic structure of the $2\to 5$ production amplitude in the planar limit of $\mathcal{N}=4$ SYM in the multi-Regge kinematics in all physical regions. We demonstrate the close connection between Regge pole and Regge cut contributions: in a selected class of kinematic regions (Mandelstam regions), the usual factorizing Regge pole formula develops unphysical singularities that have to be absorbed and compensated by Regge cut contributions. This leads, in the corrections to the Bern-Dixon-Smirnov formula, to conformal invariant “renormalized” Regge pole expressions in the remainder function. We compute these renormalized Regge poles for the $2\to 5$ production amplitude.

We present black hole type solutions in the scalar-tensor theory with nonminimal derivative coupling to the Einstein tensor. The effects of the nonminimal derivative coupling appear in the large scales, while the solutions approach those in the Einstein gravity in the small scales. For the particular coupling constant tuned to the inverse of the cosmological constant, the scalar field becomes trivial and the solutions in the Einstein gravity are recovered. For the other coupling constant, more general solutions can be obtained. If the two-dimensional maximally symmetric space is a two-sphere, the spacetime structure approaches anti–de Sitter (AdS) spacetime in the large scales. On the other hand, if the two-dimensional space is a two-hyperboloid, the spacetime approaches de Sitter spacetime. If it is a flat space, the cosmological constant affects only the amplitude of the scalar field. The extension to the higher-dimensional case is also straightforward. For a certain range of the negative cosmological constant, thermodynamic properties of a black hole are very similar to those of the Schwarzschild-AdS black hole in the Einstein gravity.

We discuss the fate of classical type IV singularities in quantum cosmology. The framework is Wheeler-DeWitt quantization applied to homogeneous and isotropic universes with a perfect fluid described by a generalized Chaplygin gas. Such a fluid can be dynamically realized by a scalar field. We treat the cases of a standard scalar field with positive kinetic energy and of a scalar field with negative energy (phantom field). We first present the classical solutions. We then discuss in detail the Wheeler-DeWitt equation for these models. We are able to give analytic solutions for a special case and to draw conclusions for the general case. Adopting the criterion that singularities are avoided if the wave function vanishes in the region of the classical singularity, we find that type IV singularities are avoided only for particular solutions of the Wheeler-DeWitt equation. We compare this result with earlier results found for other types of singularities.

When black holes are dynamical, event horizons are replaced by apparent and trapping horizons. Conformal and Kerr–Schild transformations are widely used in relation to dynamical black holes, and we study the behavior under such transformations of quantities related to the thermodynamics of these horizons, such as the Misner–Sharp–Hernandez mass (internal energy), the Kodama vector, surface gravity, and temperature. The transformation properties are not those expected on the basis of naive arguments.

In this paper we study the scalar Green function in the Kerr spacetime using Wentzel-Kramers-Brillouin (WKB) methods. The Green function can be expressed by Fourier-transforming to its frequency-domain counterpart, and with the help of complex analysis it can be divided into parts: 1) the “direct part,” which propagates on the light cone and dominates at very early times; 2) the “quasinormal-mode part,” which represents the waves traveling around the photon sphere and is important at early and intermediate times; and 3) the “tail part,” which is due to scattering by the Coulomb-type potential and becomes more important at later times. We focus on the “quasinormal-mode part” of the Green function and derive an approximate analytical formula for it using WKB techniques. This approximate Green function diverges at points that are connected by null geodesics, and it recovers the fourfold singular structure of Green functions that are seen in Schwarzschild and other spacetimes. It also carries unique signatures of the Kerr spacetime such as frame dragging. Along the way, we also derive approximate quasinormal-mode wave functions and expressions for the black hole excitation factors in the Kerr spacetime. We expect this work to benefit the understanding of both wave propagation and the problem of self-force in the Kerr spacetime.

In this paper, we analyze a possible formalization of the deformed special relativity as a five-dimensional theory. This is not the first attempt to do so, but we feel that either these previous treatments are too arbitrary in the choice of the new enlarged space, or they lack a satisfactory physical interpretation. In this work, we propose an algorithm which fixes the shape of the enlarged space. Afterwards, we focus our attention on the consequences of our formalism, proposing a physical interpretation.

We study certain aspects of the recently proposed notion of nonrelativistic diffeomorphism invariance. In particular, we consider specific examples of invariant actions, extended gauge symmetry as well as an application to the theory of quantum Hall effect. We also discuss an alternative approach based on Hořava-Lifshitz gravity.

We calculate the effect of self-interaction on the “geodetic” spin precession of a compact body in a strong-field orbit around a black hole. Specifically, we consider the spin precession angle $\psi$ per radian of orbital revolution for a particle carrying mass $\mu$ and spin $s\ll (G/c){\mu }^2$ in a circular orbit around a Schwarzschild black hole of mass $M\gg \mu$. We compute $\psi$ through $\mathcal{O}(\mu /M)$ in perturbation theory, i.e, including the correction $\delta \psi$ (obtained numerically) due to the torque exerted by the conservative piece of the gravitational self-field. Comparison with a post-Newtonian (PN) expression for $\delta \psi$, derived here through 3PN order, shows good agreement but also reveals strong-field features which are not captured by the latter approximation. Our results can inform semianalytical models of the strong-field dynamics in astrophysical binaries, important for ongoing and future gravitational-wave searches.

We consider Lorentzian wormholes supported by a phantom field and threaded by nontrivial Yang-Mills fields, which may be regarded as hair on the Ellis wormhole. Like the Bartnik-McKinnon solutions and their associated hairy black holes, these hairy wormholes form infinite sequences, labeled by the node number $k$ of their gauge field function. We discuss the throat geometry of these wormholes, showing that odd-$k$ solutions may exhibit a double throat, and evaluate their global charges. We analyze the limiting behavior exhibited by wormhole solutions as the gravitational coupling becomes large. The even-$k$ solutions approach smoothly the Bartnik-McKinnon solutions with $k/2$ nodes, while the odd-$k$ solutions develop a singular behavior at the throat in the limit of large coupling. In the limit of large $k$, on the other hand, an embedded Abelian wormhole solution is approached, when the throat is large. For smaller throats the extremal Reissner-Nordström solution plays a fundamental role in the limit.