Azimuthal anisotropies in heavy-ion collisions are conventionally interpreted as signatures of hydrodynamic flow. We demonstrate that in peripheral collisions, a significant $\cos 2\varphi$ asymmetry in the decay leptons of coherently photoproduced $J/\psi$ mesons arises purely from the initial-state geometry of the nuclear electromagnetic field. This modulation originates from the linear polarization of coherent photons, which is radially aligned in impact parameter space and transferred to the vector meson. By employing light-cone perturbation theory within the dipole formalism, we calculate the centrality dependence of this asymmetry for collisions at RHIC and LHC energies. Our predictions quantitatively reproduce STAR data. This observable thus provides a rigorous benchmark for distinguishing electromagnetic initial-state effects from collective medium dynamics.

Using data samples corresponding to an integrated luminosity of $0.9{\mathrm{fb}}^{-1}$ collected with the BESIII detector operating at the BEPCII storage ring at the center-of-mass energies of 4.84, 4.92, and 4.95 GeV, we search for the exotic charmoniumlike state with quantum numbers $J^{PC}=0^{--}$, ${\psi }_0(4360)$, in the process $e^{+}{e}^{-}\to \eta {\psi }_0(4360)$ with ${\psi }_0(4360)\to \eta \psi (2S)$. No significant signal of the ${\psi }_0(4360)$ resonance state is observed. Upper limits on $\sigma (e^{+}{e}^{-}\to \eta {\psi }_0(4360))·\mathcal{B}({\psi }_0(4360)\to \eta \psi (2S))$ and $\sigma (e^{+}{e}^{-}\to \eta \eta \psi (2S))$ at the 90% confidence level are determined for each energy point.

We report the closed-form expression for Hill’s surfaces in the circular restricted three-body problem. The solution $\varphi (r,\theta )$, derived in the primary-centric spherical coordinate system, is deduced from a cubic equation delivering at most two roots on each side of a separatrix. The famous patterns (tadpole, horseshoe, and peanut shapes, Roche lobes, and Hill’s quasispheres) are exactly produced.

Axion models often face the domain wall problem, which threatens the standard big bang cosmology. The Lazarides-Shafi mechanism attempts to resolve this by identifying degenerate vacua through a continuous gauge symmetry. We formulate a topological quantum field theory to isolate the essential structure of the mechanism and analyze its generalized symmetry structure, including higher-form symmetries and higher-group. This framework yields a master formula for computing the domain wall number and clarifies the higher-form symmetry conditions required for complete vacuum identification in a model-independent way. Moreover, while a domain-wall-number-one scenario eliminates all higher-form global symmetries, the theory nevertheless exhibits a nontrivial four-group structure and realizes a symmetry-protected topological phase.

We advance the study of flat space holography by computing the entanglement entropy of highly excited states in two-dimensional Carrollian/Galilean Conformal Field Theories ($\mathrm{C}/\mathrm{G}$ CFTs). Our approach is centered on a novel, physically intuitive derivation of the heavy-light conformal block in the large central charge limit, where the backreaction of heavy operators is absorbed by a $\mathrm{C}/\mathrm{G}$ conformal coordinate transformation. Using this result and the replica trick, we find that the entanglement entropy of highly excited states assumes a thermal form, providing a concrete realization of the eigenstate thermalization hypothesis. This field-theoretic result perfectly reproduces the holographic entanglement entropy computed via the swing surface proposal in three-dimensional Einstein gravity, for backgrounds corresponding to spinning particles and flat space cosmological solutions. This agreement establishes a precise dictionary relating the weight $\mathrm{\Delta }$ and charge $\xi$ of the boundary state to the mass $m$ and angular momentum $j$ of the dual spacetime, offering a powerful consistency check for the Flat/Carrollian conformal field theory correspondence.

The Lorentzian-Euclidean black hole has been recently introduced as a geodesically complete spacetime featuring a signature shift at the event horizon where causal geodesics are precluded from reaching the central $r=0$ singularity. In this paper, we investigate the shadows produced by this geometry to identify deviations from the standard Schwarzschild solution. Our analysis reveals an excess intensity in the inner shadow region that points to a potential observational signature of the novel behavior of light rays propagating near the event horizon. This excess could be a probe for horizon-scale modifications of black hole geometries. Furthermore, although the horizon surface of the Lorentzian-Euclidean black hole continuously accumulates photons and energy, we show that its backreaction response differs from that of stable light rings found in various exotic compact objects.

The effective number of neutrino species, $N_{\mathrm{eff}}$, serves as a key fitting parameter extensively employed in cosmological studies. In this work, we point out a fundamental inconsistency in the conventional treatment of $N_{\mathrm{eff}}$ in big bang nucleosynthesis (BBN), particularly regarding its applicability to new physics scenarios where $\mathrm{\Delta }{N}_{\mathrm{eff}}$, the deviation of $N_{\mathrm{eff}}$ from the standard BBN prediction, is negative. To ensure consistent interpretation, it is imperative to either restrict the allowed range of $N_{\mathrm{eff}}$ or systematically adjust neutrino-induced reaction rates based on physically motivated assumptions. As a concrete example, we consider a simple scenario in which a negative $\mathrm{\Delta }{N}_{\mathrm{eff}}$ arises from entropy injection into the electromagnetic sector due to the decay of long-lived particles after neutrino decoupling. This process dilutes the neutrino density and suppresses the rate of neutrino-driven neutron-proton conversion. Under this assumption, we demonstrate that the resulting BBN constraints on $N_{\mathrm{eff}}$ deviate significantly from those obtained by the conventional, but unphysical, extrapolation of dark radiation scenarios into the $\mathrm{\Delta }{N}_{\mathrm{eff}}<0$ regime.

We revisit the extraction of parton-to-eta meson fragmentation functions at next-to-leading order accuracy in QCD in the light of the recent hadroproduction measurements in proton-proton collisions obtained by the PHENIX, LHCb, and ALICE collaborations. In addition to an increased precision, the data explore complementary rapidity ranges and center-of-mass system energies. The analysis exploits the theoretical scale dependence to ease tensions among the datasets at different energies that are potentially caused by QCD corrections beyond the next-to-leading order. The resulting set of fragmentation functions yields a consistent description of all available data. Estimates of uncertainties are obtained with the Monte Carlo replica method.

We present the first measurements of the forward and midrapidity $\eta$-meson cross sections from $p+p$ collisions at $\sqrt{s}=500$ and 510 GeV, respectively. We also report the midrapidity $\eta /{\pi }^0$ ratio at 510 GeV. The forward cross section is measured differentially in $\eta$-meson transverse momentum ($p_T$) from 1.0 to $6.5\mathrm{GeV}/c$ for pseudorapidity $3.0<|\eta |<3.8$. The midrapidity cross section is measured from 3.5 to $44\mathrm{GeV}/c$ for pseudorapidity $|\eta |<0.35$. Both cross sections serve as critical inputs to an updated global analysis of the $\eta$-meson fragmentation functions.

We study a model of ($1+1$)-dimensional dilaton gravity derived from the four-dimensional Einstein-Hilbert action by dimensional reduction in a semiclassical approximation including backreaction. The reduced action involves the cosmological constant and admits black hole solutions; among these, the solutions of interest are the evaporating black holes. We solve the equations of motion perturbatively by demanding that the initial state geometry is a Minkowski space-time. When the infalling matter intersects the space-time boundary, the black hole forms and begins to evaporate. We find that as the black hole evaporates, its horizon shrinks and at a finite space-time point, it meets the singularity. Along this hypersurface, the metric can be continuously matched to a static end-state geometry. This end-state geometry is a quantum-corrected Minkowski space-time within the first order of perturbation theory.

Scalar-tensor theories predict fundamental scalar fields of considerable interest in astrophysics and cosmology. We investigate the superradiant instability of scalar clouds around Kerr black holes, showing that stimulated decay generates detectable electromagnetic signals. The growth of the superradiant scalar cloud differs from that of other bosonic fields and depends sensitively on the matter distribution surrounding the black hole, which originates from the scalar-matter coupling realized by the chameleon mechanism in modified gravity theories. In nonuniform matter distributions, stimulated emission from scalar clouds offers an observational signature that distinguishes fundamental scalars from other light bosonic fields.

We develop a reference-renormalized (photon-sphere-free) normalization scheme for Gauss-Bonnet gravitational lensing at finite distance in static, spherically symmetric spacetimes. The method treats the curvature primitive used to reduce the Gauss-Bonnet curvature-area integral as a quantity defined only modulo an additive constant (an additive gauge freedom). We fix this gauge by matching to a physically chosen reference optical geometry in an outer regime where the physical geometry approaches that reference, thereby defining a unique renormalized discrepancy primitive ${\mathcal{P}}_e(r)$ by reference subtraction. The resulting master formula yields the Ishihara-Li finite-distance deflection angle without invoking any circular null orbit, while remaining fully compatible with orbit-normalized prescriptions whenever a suitable photon sphere exists (the two gauges differ only by a constant shift and give identical $\alpha$). In asymptotically flat settings the canonical reference is Minkowski, while in Kottler-type backgrounds the canonical reference is de Sitter within the static patch, making the operational fiducial explicit. We validate the method by reproducing Ishihara’s finite-distance weak-deflection formulas for Schwarzschild, Reissner-Nordström, and Kottler spacetimes, including the mixed $r_g\mathrm{\Lambda }$ term in the Kottler case within the static-patch fiducial. We also present a demonstrative example in which orbit normalization is genuinely inapplicable because no circular null orbit exists in the physical optical region (the Janis-Newman-Winicour spacetime for $\gamma \le \frac{1}{2}$). The result is a unified, geometrically transparent route to finite-distance lensing that preserves compatibility with orbit-normalized prescriptions whenever those apply.

In view of the new Dark Energy Spectroscopic Instrument 2025 results, we analyze three types of Padé cosmology, based on rational series making use of Padé approximants over the equations of state, namely ${\mathrm{Padé}}^{\omega }$ (0, 1) and Padé (1, 1), plus a ${\mathrm{Padé}}^q$ (0, 1), i.e., a rational expansion on the dark energy deceleration parameter, in which the numerator and denominator orders are incorporated into the above brackets. These scenarios appear as alternative dark energy parametrizations with respect to the well-known ${\omega }_0{\omega }_a\mathrm{CDM}$ model. Accordingly, we perform Monte Carlo Markov chain analyses with the publicly available class Boltzmann code, including the three Padé parametrizations, along with the ${\omega }_0{\omega }_a\mathrm{CDM}$ and $\mathrm{\Lambda }\mathrm{CDM}$ standard pictures. To this end, we combine independent probes from high to low redshifts to obtain reliable constraints on the cosmological parameters of these models and compare them using statistical selection criteria. Our results show that ${\omega }_0{\omega }_a\mathrm{CDM}$ parametrization is not significantly preferred over the Padé cosmology. On the contrary, the deviance information criterion identifies Padé (1, 1) as the best-fit model along with the ${\omega }_0{\omega }_a\mathrm{CDM}$ parametrization. Based on our bounds, we further investigate the evolution of the squared sound speed, revealing that the ${\mathrm{Padé}}^q$ (0, 1) and ${\mathrm{Padé}}^{\omega }$ (0, 1) parametrizations exhibit enhanced stability compared with the other cases here considered and, therefore, describe robust alternatives for the cosmological background.

From catalogs of gravitational-wave transients, the population-level properties of their sources and the formation channels of merging compact binaries can be constrained. However, astrophysical conclusions can be biased by misspecification or misestimation of the population likelihood. Despite detection thresholds on the false-alarm rate (FAR) or signal-to-noise ratio (SNR), the current catalog is likely contaminated by noise transients. Further, computing the population likelihood becomes less accurate as the catalog grows. Current methods to address these challenges often scale poorly with the number of events and potentially become infeasible for future catalogs. Here, we evaluate a simple remedy: increasing the significance threshold for including events in population analyses. To determine the efficacy of this approach, we analyze simulated catalogs of up to 1600 gravitational-wave signals from black-hole mergers using full Bayesian parameter estimation with current detector sensitivities. We show that the growth in statistical uncertainty about the black-hole population, as we analyze fewer events but with higher SNR, depends on the source parameters of interest. When the SNR threshold is raised from 11 to 15—reducing our catalog size by two–thirds—we find that statistical uncertainties on the mass distribution only grow by a few 10% and constraints on the spin distribution are essentially unchanged; meanwhile, uncertainties on the high-redshift cosmic merger rate more than double. Simultaneously, numerical uncertainty in the estimate of the population likelihood more than halves, allowing us to ensure unbiased inference without additional computational expense. Our results demonstrate that focusing on higher-significance events is an effective way to facilitate robust astrophysical inference with growing gravitational-wave catalogs.

In [M. Du et al., [Phys. Rev. D 109, 055041 (2024)] ] it has been argued that production of dark photons—hypothetical massive vectors—in nuclear reactors via mixing with the visible photons is considerably enhanced (by a factor of 10) due to Compton scattering of the latter. We revisit the production of dark photons in the reactor environment and find that although the scattering of photons indeed leads to a larger number of produced dark photons, the overall enhancement is considerably smaller than what has been obtained in the above article. Our findings are validated using geant simulations, which take into account oscillations between ordinary and visible photons and interaction of the latter with matter. The correction to the limit on mixing parameter between dark and visible photons is expected to be below 30% for all masses. Its application must be accompanied with an update on the original spectrum of photons produced in nuclear reactions inside the nuclear reactor.

Building upon the work of Hu, Paz, and Zhang [Phys. Rev. D 45, 2843 (1992) and Phys. Rev. D 47, 1576 (1993)] on open quantum systems we consider the quantum Brownian motion model with one oscillator (position variable $x$) as the system, nonlinearly coupled to an environment of $N$ harmonic oscillators (with mass $m_n$, natural frequency ${\omega }_n$, position $q_n$, and momentum $p_n$ variables) in the form ${\sum }_n(v_{n1}(x)q_n^k+v_{n2}(x)p_n^l)$ where $k$, $l$ are integers (the present work only considers the $k=l=2$ cases). The vertex functions $v_{n1},v_{n2}$ are of the form $v_{n1}=\lambda C_{n1}f(x),v_{n2}(x)=-\lambda C_{n2}{m}_n^{-2}{\omega }_n^{-2}f(x)$ where $C_{n1,2}$ are the coupling constants with the $n$th oscillator, $f(x)$ is any arbitrary function of $x$, and $\lambda$ is a dimensionless constant. Employing the closed-time-path formalism the influence action $S_{IF}$ is calculated using a perturbative expansion in $\lambda$. It is possible to identify the terms in $S_{IF}$ quadratic or higher in $\mathrm{\Delta }(s)\equiv f(x_{+}(s))-f(x_{-}(s))$ to constitute the noise kernel, while terms linear in $\mathrm{\Delta }$ to that of the dissipation kernel. The non-Gaussian noise kernel gives rise to nonzero three-point correlation function of the corresponding stochastic force. The pathway presented here should be useful for the exploration of non-Gaussian properties of systems nonlinearly coupled with their environments; examples in early universe cosmology and in quantum optomechanics are mentioned. A fluctuation-dissipation relation is also established, which ensures the consistency of the model and the accuracy of results even at higher perturbative orders. Another result of significance is the derivation of a nonlinear Langevin equation which is expected to be useful for many open quantum system applications.

We revisit the one-loop calculation of the Higgs-mass spectrum of the scale-invariant two-Higgs-doublet model, relying on a direct calculation of the relevant Feynman diagrams. We highlight a number of incorrect assumptions in earlier calculations that relied on the effective-potential approach. In contrast with the earlier findings, we show that the one-loop corrections can have an effect of $\mathcal{O}(10\%)$ on the predictions for the beyond the standard model (BSM) Higgs masses, and they can also induce non-negligible mixing between the standard model–like and BSM states in the neutral scalar sector.

We search for 15 rare decays of $D$ mesons to hadrons accompanied by an electron-positron pair $D\to h(h^{(\prime )})e^{+}{e}^{-}$, based on $20.3{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ collision data collected at the center-of-mass energy of 3.773 GeV with the BESIII detector at BEPCII. No significant signals are observed, and the corresponding upper limits on the branching fractions at the 90% confidence level are determined. The sensitivities of the results are at the level of ${10}^{-6}\sim {10}^{-7}$. The upper limits on the branching fractions for the $D^{+}\to {\rho }^{+}{e}^{+}{e}^{-}$, $D^{+}\to K^{*+}{e}^{+}{e}^{-}$, $D^0\to K_S^0{K}_S^0{e}^{+}{e}^{-}$, $D^0\to {\pi }^0{\pi }^0{e}^{+}{e}^{-}$ and $D^0\to {\eta }^{\prime }{e}^{+}{e}^{-}$ decay channels are measured for the first time. For the $D^0\to {\pi }^0{e}^{+}{e}^{-}$, $D^0\to \eta e^{+}{e}^{-}$, $D^0\to \omega e^{+}{e}^{-}$, $D^0\to K_S^0{e}^{+}{e}^{-}$, $D^{+}\to {\pi }^{+}{\pi }^0{e}^{+}{e}^{-}$, $D^{+}\to K^{+}{\pi }^0{e}^{+}{e}^{-}$, $D^{+}\to {\pi }^{+}{K}_S^0{e}^{+}{e}^{-}$ and $D^{+}\to K^{+}{K}_S^0{e}^{+}{e}^{-}$ decay channels, the upper limits on the branching fractions are determined, with an improvement of at least a factor of four compared to previous searches. The upper limits on the branching fractions for the $D^0\to {\rho }^0{e}^{+}{e}^{-}$ and $D^0\to \varphi e^{+}{e}^{-}$ decay channels are set at $0.7\times {10}^{-6}$ and $4.6\times {10}^{-6}$, respectively.

The Sun can efficiently capture leptophilic dark matter that scatters with free electrons. If this dark matter subsequently annihilates into leptonic states, it can produce a detectable neutrino flux. Using 10 years of Super-Kamiokande observations, we set constraints on the dark matter–electron scattering cross section that exceed terrestrial direct detection searches by more than an order of magnitude for dark matter masses below 100 GeV and reach cross sections as low as $\sim 4\times {10}^{-41}{\mathrm{cm}}^2$.