It is well known that atoms can change their shape when subject to external electromagnetic fields. Analogous phenomena are expected for particles that are much smaller in size, with the corresponding shape change much harder to observe. We point out that the shape change of particles can be accessed by measurements of their spin alignment. Motivated by recent measurements of spin alignment in relativistic heavy ion collisions, we consider quarkonia in electromagnetic fields as an example. We show that the quarkonia spin alignment receives both spin contributions from spin state mixing and orbital contributions from shape changes. By a proper choice of quantization axis, it is possible to switch off the spin contribution, leaving only the orbital contribution. This makes the spin alignment measurement a valuable probe of quarkonium structure.

We analyze the mass spectra of hadrons and demonstrate that the physical spectra of mesons and baryons are well described by the exponential spectrum of open strings. The open-string spectrum, derived from string theory, is characterized by a unique Hagedorn temperature $T_{\mathrm{H}}$ and free from any other parameters. Notably, our fitting to the physical spectra yields consistent values for both mesons and baryons, $T_{\mathrm{H}}\simeq 0.34\mathrm{GeV}$, which contrasts with previous phenomenological analyses that suggested different values. This obtained value aligns well with typical string tension derived from lattice-QCD calculations and the Regge slope. In the baryonic sector, our results indicate that diquarks play a crucial role in describing the mass spectrum, implying that baryons can be understood as a quark-diquark system, as anticipated by Regge phenomenology. These findings have significant implications for our understanding of quark deconfinement, especially in the possibly existing regime at high temperature and small baryon chemical potential within the QCD phase diagram.

In this work, we investigate the productions of ${\mathrm{\Lambda }}_c(2910)$ and ${\mathrm{\Lambda }}_c(2940)$ in the $p\overline{p}\to {\overline{\mathrm{\Lambda }}}_c{D}^{0}p$ process by utilizing an effective Lagrangian approach, where both ${\mathrm{\Lambda }}_c(2910)$ and ${\mathrm{\Lambda }}_c(2940)$ are considered as $D^{*}N$ molecular states with $J^P=1/2^{-}$ and $3/2^{-}$, respectively. In addition to the $t$-channel $D$ and $D^{*}$ exchanges, the contribution from light-meson exchange is also considered. At $\sqrt{s}=10\mathrm{GeV}$, our estimations indicate that the total cross sections for $p\overline{p}\to {\overline{\mathrm{\Lambda }}}_c{D}^{0}p$ are $({130.7}_{-103.9}^{+397.1})\mathrm{nb}$, where the central value is estimated with ${\mathrm{\Lambda }}_r=1.1\mathrm{GeV}$, and the uncertainties are resulted from the variation of parameter ${\mathrm{\Lambda }}_r$ from 1.0 to 1.2 GeV. Our results indicate that the light-meson exchange diagrams are very important, which provide a very large, smooth background. Moreover, the estimations of the $D^{0}p$ invariant mass spectrum reveal that the peak structure between 2.9 and 3.0 GeV primarily originates from ${\mathrm{\Lambda }}_c(2910)$, while the signal of ${\mathrm{\Lambda }}_c(2940)$ is about 1 order smaller than that of ${\mathrm{\Lambda }}_c(2910)$. Furthermore, the resulting Dalitz plot is estimated with $\sqrt{s}=8\mathrm{GeV}$. It is expected that our estimations in the present work can be tested by future experiments at $\overline{\mathrm{P}}\mathrm{ANDA}$.

Within the framework of SU(2) chiral perturbation theory, we derive the general solution of the QCD $\theta$ vacuum for an arbitrary vacuum phase, explicitly incorporating isospin-breaking effects from the light quark mass difference, and compute the temperature dependence of the topological susceptibility, higher-order cumulants, and the domain wall tension up to next-to-leading order. We find that the topological susceptibility agrees with lattice data at low temperatures but deviates at higher temperatures as expected from the breakdown of the chiral expansion; moreover, we demonstrate that the normalized fourth-order cumulant and the domain wall tension decrease monotonically with increasing temperature, while the normalized sixth-order cumulant exhibits the opposite behavior. These results extend earlier analyses by showing how isospin breaking reshapes the full hierarchy of topological charge cumulants and the dynamics of $\theta$-vacuum domain walls, thereby offering new theoretical input on the $\theta$-vacuum properties, which are relevant for axion-related effective theories in hot QCD matter.

An accelerated charge interacts with its own electromagnetic field, a phenomenon known as electromagnetic radiation reaction. The DeWitt-Brehme-Hobbs (DWBH) equation describes the motion of a charged mass in the presence of combined electromagnetic and gravitational fields, taking into account electromagnetic radiation-reaction effects. Here, we find the first exact analytical solution of the DWBH equation in the case of a charged mass in the presence of copropagating and otherwise arbitrary electromagnetic and gravitational plane waves. As a consequence of the Penrose limit, the scenario considered here can be seen as a local limit around ultrarelativistic trajectories in a general curved spacetime. Finally, the paradigmatic example of an electromagnetic wave in the presence of a constant-amplitude gravitational wave is worked out explicitly and it is shown how the presence of the gravitational wave can qualitatively change electromagnetic radiation-reaction effects.

In this work, we study the effective running coupling constant of heavy quark pair and jet quenching parameter in the spinning background. Ultralocally, the boosted fluid is described by the boosted parameter and dual to a globally rotating system. Our results show that the angular momentum suppresses the effective running coupling constant and reduces its maximum value. The results demonstrate that the angular momentum promotes the dissociation of quarkonium and has a stronger effect on the effective running coupling constant when the axis of $Q\overline{Q}$ is transverse to the direction of the angular momentum. We also find that the angular momentum enhances the jet quenching parameter and has a stronger effect when the jet moves transversely to the direction of the angular momentum, namely ${\widehat{q}}_{\perp }>{\widehat{q}}_{\parallel }$. We discuss the dependence of the jet quenching parameter on the $\eta /s$ at strong coupling in the presence of the angular momentum.

In higher-dimensional theories, we often assume that the extra dimensions form an orientable space, perhaps with singularities. However, many physical theories are well defined on nonorientable spaces, and many spaces are not orientable, so it is reasonable to explore what happens if the assumption of orientability is relaxed. Here we consider the simplest example of free 6D theories compactified on a flat Klein bottle. We focus on a Dirac fermion in 6D, with boundary conditions that define ${\mathrm{pin}}^{+}$ and ${\mathrm{pin}}^{-}$ structures. Translation invariance is broken by the boundary conditions, which leads to sharp features localized near the parity walls (fixed points of the reflection used to construct the Klein bottle). For a scalar field, there is a position-dependent energy density, peaked near the parity walls. A Dirac fermion can lead to breaking of parity, charge conjugation, and $CP$ in $3+1$ dimensions. Order parameters for this breaking are provided by the vacuum expectation values of certain fermion bilinears, again peaked near the parity walls. As one potential application, these results suggest mechanisms for $CP$ violation and baryogenesis, enabled by compactification on a Klein bottle.

This work aims to develop a global formulation for $\mathcal{N}=2$ harmonic/projective anti–de Sitter (AdS) superspace ${\mathrm{AdS}}^{4|8}\times S^2\simeq {\mathrm{AdS}}^{4|8}\times \mathbb{C}{P}^1$ that allows for a simple action of superconformal (and hence AdS isometry) transformations. First of all, we provide an alternative supertwistor description of the $\mathcal{N}$-extended AdS superspace in four dimensions, ${\mathrm{AdS}}^{4|4\mathcal{N}}$, which corresponds to a realization of the connected component ${\mathsf{O}\mathsf{S}\mathsf{p}}_0(\mathcal{N}|4;\mathbb{R})$ of the AdS isometry supergroup as $\mathsf{S}\mathsf{U}(2,2|\mathcal{N})\cap \mathsf{O}\mathsf{S}\mathsf{p}(\mathcal{N}|4;\mathbb{C})$. The proposed realization yields the following properties: (i) ${\mathrm{AdS}}^{4|4\mathcal{N}}$ is an open domain of the compactified $\mathcal{N}$-extended Minkowski superspace, ${\overline{\mathbb{M}}}^{4|4\mathcal{N}}$; (ii) the infinitesimal $\mathcal{N}$-extended superconformal transformations naturally act on ${\mathrm{AdS}}^{4|4\mathcal{N}}$; and (iii) the isometry transformations of ${\mathrm{AdS}}^{4|4\mathcal{N}}$ are described by those superconformal transformations which obey a certain constraint. The obtained results for ${\mathrm{AdS}}^{4|4\mathcal{N}}$ are then applied to develop a supertwistor formulation for an AdS flag superspace ${\mathrm{AdS}}^{4|8}\times {\mathbb{F}}_1(2)$ that we identify with the $\mathcal{N}=2$ harmonic/projective AdS superspace. This construction makes it possible to read off the superconformal and AdS isometry transformations acting on the analytic subspace of the harmonic superspace.

This paper introduces differential-geometric methods to study $n$-dimensional locally conformally flat spaces as submanifolds in ${\mathbb{R}}^{n+2}$. We derive explicit formulas relating intrinsic and ambient differential-geometric objects, including curvature tensors, the codifferential, and Laplacian operators. We apply this approach to Friedmann-Lemaître-Robertson-Walker (FLRW) spaces using newfound embedding formulas, obtaining new and simplified expressions for the photon propagator in four dimensions.

In this work, we investigate relativistic tidal forces around a black hole sourced by a cloud of strings, described by the generalized Letelier-Alencar solution. We first review the original Letelier spacetime and its recent generalization, computing the Kretschmann scalar and showing that the generalized model exhibits a stronger curvature divergence at $r\to 0$ than both Letelier and Schwarzschild cases. We then analyze geodesic motion in this background. For massless particles, we focus on circular photon orbits, while for massive particles, we consider both radial infall and circular motion. We find that the radii of the photon sphere and of the innermost stable circular orbit increase with the cloud of strings parameter $g_s$ and decrease with the length scale $l_s$, and circular orbits cease to exist in certain regions of the parameter space. For radial motion, we compute the radial acceleration and the corresponding tidal forces. In this case, we show that an inversion between stretching and compression may occur, although this regime is typically hidden inside the event horizon. Once the tidal forces are known, we computed the behavior of the displacement vector in order to verify whether the usual stretching behavior induced by tidal forces is preserved. Finally, we study tidal forces for observers in circular motion, showing that the cloud of strings modifies the Keplerian frequency and the tidal force profile even at large distances, and that in this case there is no sign change of the tidal components.

We develop a fully relativistic approach for determining the response of a compact star to a time/frequency dependent (tidal) environment. The strategy involves matching the solution for the linearized fluid dynamics in the star’s interior to the spacetime perturbations in the near-zone surrounding the body, along with an identification of the tidal driving and the star’s response. Notably, this identification is exact in Newtonian gravity and we provide strong evidence that it remains robust also in the relativistic case. The argument does not involve a sum over the star’s quasinormal modes and hence circumvents one of the obstacles that have held up the development of models for relativistic tides. Numerical results are provided, at the proof-of-principle level, for a realistic matter equation of state from the Brussels-Skyrme models of the Equation of State family, including composition stratification leading to the presence of low-frequency gravity modes. We also sketch the connection with the field-theory inspired approach to the problem, in which the tidal response is expressed in terms of asymptotic scattering amplitudes. We argue that progress in this direction is essential in order to complete the description of tides in relativistic stars.

We show that the stochastic background of gravitational wave memory of growing type leads to a fractional Brownian motion increasing at the order of $t^H$ for large $t$ where $\frac{1}{2}<H<1$. This beats the scaling law of Brownian motion. In this article, we investigate sources of gravitational waves in the early Universe as well as in astrophysical settings. Cosmological sources may include primordial black holes or other sources immediately after the big bang when there were pockets of hot material and large density fluctuations. Gravitational waves from mergers of primordial black holes produce memory. We show that, due to the conditions in which these are taking place, the gravitational wave memory will be increasing in time following a certain power law. Corresponding results hold for any gravitational wave memory from a cosmological source where the surrounding conditions are similar. The stochastic limit of these memories is a stochastic process growing in time faster than the $\sqrt{t}$ scaling law of Brownian motion. The latter is also typical for noise and for the limit of memory events as they have been mostly considered in the literature. In an expanding Universe, the memory is enhanced by the expansion itself. Our results provide a tool to extract gravitational wave sources of this type from data using this memory signature. This would be particularly useful for the pulsar timing array data that have been already observed, answering the long-standing question about how to extract memory signals from the data. Further, the new results open up a new door to explore the conditions right after the big bang using the long-range dependence and further probability analysis.

The conjecture by two of the authors (N. E. M. and S. S.) that a $\mathcal{PT}$-symmetric phase plays a role in understanding singular renormalization group (RG) flows for a Chern-Simons (CS) gauge theory of axions, has been reexamined and significantly improved. We have used the more complete Wetterich equation, which includes gravitational couplings in a systematic way from the start, to understand the emergence of this phase. The singular structure of the RG flows has persisted on including gravitational couplings, thereby offering further support to the conjecture that $\mathcal{PT}$ -symmetric phases of (repulsive) gravity characterize string-effective CS gravitational theories, where the axion is the massless string-model independent axion, which can also play a role of a totally antisymmetric torsion degree of freedom. This has suggested a novel interpretation of the currently observed acceleration of the expansion of the Universe in terms of such a phase at large (cosmological) scales.

We model the general relativistic interaction of a small hyperelastic sphere with a Schwarzschild black hole as it follows an initially marginally bound orbit through a close encounter. While the interaction reveals effects that are encoded by the Mathisson-Papapetrou-Dixon multipolar equations through quadrupole order, the calculation is made using an independent general relativistic finite element scheme introduced in our earlier work [Phys. Rev. D 108, 084020 (2023).]. The finite element calculation is done in Schwarzschild coordinates, following a large and scalable number of mass elements in interaction with each other through elastic forces derived from a potential energy function and with the spacetime geometry. After the fact, we analyze the dynamics using a local Fermi coordinate system, computing (1) the deviation of the center of mass of the body relative to the initial marginally bound orbit, (2) changes in orbital and spin angular momenta, and (3) the decrease in orbital energy and accompanying deposition of energy into internal elastic dynamics. The interaction leads to the capture of the small body into a highly eccentric orbit ($e\simeq 0.99998$ in a sample calculation).

The interaction of gravitational waves (GWs) with matter is normally treated as being insignificant. However, recent work has shown that the interaction with a viscous fluid may be astrophysically important when the distance between the matter and GW source is somewhat smaller than the GW wavelength. Previous work has mainly considered perturbations on a Minkowski background, and here these results are extended to the case that the background is a general, nonvacuum, static, spherically symmetric spacetime. Expressions are obtained for GW damping and the consequent heating of the fluid, and implemented in computer code. The results are applied to astrophysical scenarios: core collapse supernovae, the postmerger signal from a binary neutron star merger, and matter accreting at a binary black hole merger. It is found that, compared to the Minkowski case, the damping and heating effects increase, in some cases by several orders of magnitude. It is possible for a GW signal to be completely damped, and for the heating to be such that a gamma-ray burst occurs.

Gravitational waveform modeling in self-force theory has reached a mature stage in recent years, with fast and accurate models emerging at both adiabatic (0PA) and first postadiabatic (1PA) orders in a multiscale expansion. Here, we provide a gauge-invariant 1PA waveform-generation framework that involves no direct calculation of the (gauge-dependent) self-force. To achieve this, we recast the multiscale framework in a pseudo-Hamiltonian form, working on the six-dimensional phase space intrinsic to the multiscale expansion. We characterize the gauge freedom on phase space and show how a localization procedure avoids nonlocal-in-time effects in the 1PA dynamics. We find a conservative Hamiltonian structure can be naturally embedded into the complete, dissipative 1PA pseudo-Hamiltonian dynamics, giving rise to natural definitions of the conserved energy, angular momentum, and radial and polar actions. As a by-product, we clarify that the on-shell value of the conservative Hamiltonian is equal to the mechanical energy historically predicted by the first law of binary black hole mechanics.

We extend the conformal dimensional-derivative regularization of four-dimensional Gauss-Bonnet gravity to Riemann-Cartan geometry, obtaining a regularized action whose torsionless limit equals the well-known regularized four-dimensional Einstein-Gauss-Bonnet model. Varying independently with respect to the scalar, tetrad, and spin connection yields field equations that remain strictly second order in covariant derivatives, thereby avoiding Ostrogradsky-type instabilities. Within this framework we obtain static, spherically symmetric black holes carrying torsion hair, showing that the regularized Gauss-Bonnet interaction can support long-range torsion hair without invoking extra dimensions.

We study the scattering of gravitational waves by axion domain walls in teleparallel gravity with the Nieh-Yan term. Since a domain wall causes the parity violation, the transmitted gravitational waves also exhibit the parity violation. We calculate the degree of circular polarization of gravitational waves. It turns out that gravitational waves could be chiral after going through the domain wall. The degree of circular polarization does not depend on the tension of the domain wall.

We derive an extended expression for the relaxation time of a barotropic Israel-Stewart (IS) fluid (BISF) using the nonlinear causality constraint, and propose a new formulation for modeling causal bulk viscous dissipation in barotropic fluids in the full nonlinear regime. With this generalized relaxation time, the covariant nonlinear IS equation reduces to a first-order nonlinear differential equation relating the bulk viscous pressure and the energy density, which remains valid in any homogeneous and isotropic spacetime. In a spatially flat Friedmann universe, adopting this extended relation within the generalized nonlinear IS theory yields a new class of analytical solutions in both the linear and certain truncated nonlinear regimes. We also find that the resulting effective equation of state in the linear regime naturally reproduces the generalized polytropic form often introduced phenomenologically in the literature. The dynamical implications of adopting the causal relaxation time for BISF are investigated in the linear, truncated nonlinear, and full nonlinear far-from-equilibrium regimes, and the constraints required to ensure physically viable evolution of the viscous fluid are derived. A detailed dynamical systems analysis of the coupled Einstein-Israel–Stewart (EIS) system is also performed. Finally, we solve the coupled EIS equations numerically in the full nonlinear regime and show that the model can support a transient Hubble slow-roll expansion phase with a smooth exit to a radiation-dominated universe, which is challenging to obtain in standard inflationary models.