The KM3NeT Collaboration has recently reported the detection of a neutrino event with energy in excess of 100 PeV. This detection is in $2.5-3\sigma$ statistical tension with the upper limit on the neutrino flux at this energy imposed by IceCube and the Pierre Auger Observatory, if the event is considered part of the diffuse all-sky neutrino flux. We explore an alternative possibility that the event originates from a flare of a rare transient source. We show that the data of KM3NeT, IceCube and the Pierre Auger Observatory are consistent with the possibility of a source flare of duration $T\lesssim 2\mathrm{yr}$ with muon neutrino flux $F\sim 3\times {10}^{-10}(1\mathrm{yr}/T)\mathrm{erg}/({\mathrm{cm}}^2\mathrm{s})$. Constraints on the neutrino spectrum indicate that the protons responsible for the neutrino emission have a very hard spectrum in the $E_p\gtrsim {10}^{19}\mathrm{eV}$ energy range, or otherwise that the neutrinos are produced by photohadronic interactions with infrared photons. The all-sky rate of similar neutrino flaring sources is constrained to be $\mathcal{R}\lesssim 0.4/\mathrm{yr}$.

In recent years, a number of experiments dedicated to searches for long-lived particles (LLPs) have been proposed or approved, or have entered operation. While the sensitivities of these experiments to various LLP scenarios have been extensively studied, key aspects—such as detector geometries, background estimates, and projected operational durations—for several facilities, including MATHUSLA, ANUBIS, and SHiP, have undergone significant updates. In this work, we implement the latest experimental designs in the Displaced Decay Counter tool for calculating detector acceptances and signal-event yields, and reevaluate their sensitivity reach to one of the most widely studied LLP scenarios, namely minimal heavy neutral leptons.

In 2023, the Belle II Collaboration announced the observation of the $B^{+}\to K^{+}\nu \overline{\nu }$ decay channel for the first time. This decay channel provides a clean signal with high precision in theoretical calculations. However, we encounter a $2.8\sigma$ deviation from the Standard Model prediction. To resolve this excess, we study a scalar dark matter model with local discrete $Z_3$ symmetry. Assuming dark $U(1{)}_X\equiv U(1{)}_{L_{\mu }-L_{\tau }}$ symmetry, this $U(1{)}_{L_{\mu }-L_{\tau }}$ symmetry is spontaneously broken into local discrete $Z_3$ by nonzero vacuum expectation value of dark Higgs boson. Considering a dark Higgs mass is 2 GeV, we can explain the recent $\mathrm{Br}(B^{+}\to K^{+}\nu \overline{\nu })$ excess reported from the Belle II Collaboration and relic abundance at the same time.

We chart new-physics models that produce exotic, high-multiplicity muon decays featuring prompt or displaced $e^{+}{e}^{-}$ pairs and/or photons, with or without missing energy, such as $\mu \to 5e$, $\mu \to 7e$, etc. Starting from an effective-field-theory perspective, we estimate the reach on the ultraviolet scale and identify conditions under which lower-multiplicity modes are suppressed or occur at comparable rates. We then construct explicit realizations in minimal dark-sector models with light, feebly interacting particles, such as flavor-protected scalars, dark photons, inelastic dark matter, and axionlike particles. The predicted novel signatures can be probed at MEG II and Mu3e, as well as during calibration runs of COMET and Mu2e. A future discovery would provide valuable insights into short-distance dynamics and the mechanism of lepton-flavor symmetry breaking.

Motivated by recent HAL QCD simulations of the $\varphi N$ interaction in the ${{}^{4}S}_{3/2}$ channel and its modification in the ${{}^{2}S}_{1/2}$ channel, we investigate light $\varphi$-mesic systems with total number of particles $A=3–5$ using configuration-space Faddeev and Faddeev-Yakubovsky formalism. Binding energies of $\varphi NN$, $\varphi NNN$, and $\varphi NNNN$ systems are evaluated for both spin-dependent and spin-independent $\varphi N$ interactions controlled by a phenomenological strength parameter. The $\varphi N$, $\varphi NN$, $\varphi NNN$, and $\varphi NNNN$ systems are analyzed under different mass scenarios and varying spin splitting of the $\varphi N$ interaction. The bound states in the $\varphi$-mesic nuclei ${}_{\varphi }{}^4\mathrm{H}$, ${}_{\varphi }{}^4\mathrm{He}$, and ${}_{\varphi }{}^5\mathrm{He}$ were predicted. In particular, the strongly attractive ${{}^{2}S}_{1/2}$ component of the $\varphi N$ potential generates deeply bound $\varphi$-mesic nuclear states. The low-energy correlation effects in the $\varphi$-mesic nuclei are described. Coulomb shifts of the binding energies are evaluated. Our findings clarify the binding mechanism and the structure of $\varphi$-mesic nuclei, as well as the role of short-range $\varphi N$ interaction.

In double charmonium production, a long-standing challenge is that the theoretical predictions are not consistent with the measurements at $B$ factories. Within the Non-Relativistic Quantum Chromodynamics (NRQCD) framework, the next-to-leading order calculation has proved its power to cut down the discrepancy between theory and experiments. To further clarify this puzzle, we have performed the next-to-next-to-leading order calculation. The amplitude is obtained as an analytical asymptotic expansion in the ratio of the squared charm-quark mass over the squared center-of-mass energy, $m_c^2/s$. We investigate the origin of the leading logarithms by performing a region analysis, revealing the intricate factorization structure in this process. We provide numerical predictions on the total cross sections of $J/\psi +{\eta }_c$ production, which agree with the experimental results. Extension of our computation to $\mathrm{\Upsilon }+{\eta }_b$ production is also discussed.

In this paper, we develop a quantum-optical treatment of acceleration radiation for atoms freely falling into a Schwarzschild black hole when the ambient field is a massive spin-1 (Proca) field. Building on the HBAR framework of Scully and collaborators, we analyze two detector realizations: a charged-monopole current coupling and a physical electric-dipole coupling, both within a cavity that isolates a single outgoing Schwarzschild mode prepared in the Boulware state. Using a near-horizon stationary-phase analysis, we show that the thermal detailed-balance factor governing excitation versus absorption is universal and depends only on the near-horizon Rindler coordinate transformation. At the same time, the absolute spectra acquire distinctive Proca signatures: a hard mass threshold, polarization-dependent prefactors, and axial/polar graybody transmissions. Promoting single-pass probabilities to escaping rates yields a master equation whose steady state is geometric and whose entropy flux obeys an horizon brightened acceleration radiation-style area entropy relation identical in form to the scalar case, with all vector-field specifics entering through the radiative area change. Our results provide a controlled pathway to probe longitudinal versus transverse responses, mass thresholds, and the role of polarization-resolved graybody transmission in acceleration radiation. More precisely, we derive the universal near-horizon kernel and show how the Proca transmission data enter the escaping probabilities, rates, and entropy flux; a dedicated numerical computation of the axial/polar graybody profiles is left for future work. This sets the stage for extensions to rotating backgrounds, alternative exterior states, and detector-engineering strategies.

We develop a covariant differential-form framework to define scalar charges for stationary, asymptotically flat black holes in 4-dimensional Einstein-scalar-Gauss-Bonnet gravity with a general scalar coupling function. Contracting the scalar field equation of motion with the horizon generator $k$ yields a nonclosed-form scalar charge, revealing a bulk contribution encoded in a 3-form, which measures the obstruction to its closedness. In the presence of shift-symmetry, this obstruction vanishes and the 2-form scalar charge satisfies a Gauss law, depending solely on boundary data. Geometrically, this reproduces known topological results in the shift-symmetric limit. This framework allows us to analyze the role of the nonclosed scalar charges in black hole thermodynamics through the Smarr formula for more general couplings and provide a covariant, charge-based interpretation of the spontaneous scalarization mechanism, showing how the behavior of the scalar charge and the bulk term capture the instability of scalar-free black holes and the emergence of scalar hair. Our results offer a unified geometric understanding of the role of scalar charges and a physical interpretation to its nonclosedness in terms of the spontaneous scalarization mechanism.

We construct a position-space cosmological perturbation theory around spatially flat Friedmann-Lemaître-Robertson-Walker geometries that allows one to model localized primordial sources of gravitational waves. The equations of motion are decoupled using a generalized harmonic gauge, which avoids the use of a scalar-vector-tensor decomposition. We point out that sources cannot generically be defined in a compact domain due to fluctuations of the cosmic perfect fluid. For power-law cosmologies, we obtain the exact Green’s function necessary to solve for all metric perturbations in terms of a hypergeometric function, which matches with a Green’s function derived earlier by Chu [Classical Quantum Gravity 34, 035009 (2017), Classical Quantum Gravity 34, 194001 (2017)]. This allows us to derive the closed form expression of the linearized metric perturbation generated by sources up to quadrupolar order in the multipolar expansion.

Within scalar-tensor (ST) theories, neutron stars in binary systems experience tidal deformations caused by both their companion and the scalar field. These deformations are strongly correlated to the star’s internal structure and composition. Accurately modeling their imprint on the emitted gravitational waves will be essential for interpreting the high-precision data expected from future detectors and for disentangling potential signatures of modified gravity from those arising due to the properties of neutron star matter. Using the post-Newtonian multipolar-post-Minkowskian formalism adapted to ST theories, and working within the adiabatic approximation, we compute the tidal corrections to the total energy flux, accounting for both gravitational and scalar radiation, and to the waveform phasing, at the next-to-next-to-leading order (NNLO). This corresponds to second post-Newtonian (2PN) order beyond the leading-order dipolar tidal contribution. At this accuracy, three independent types of tidal deformability (scalar, tensorial, and mixed scalar-tensorial) contribute to the signal. We also derive the full waveform amplitude modes, including gravitational and scalar modes, as well as the memory ($\mathrm{m}=0$) ones, to the ${\mathrm{N}}^{1.5}\mathrm{LO}$ (i.e., to relative 1.5PN order).

Magnetic fields in the Universe potentially serve as messengers of primordial physics. The observationally suggested intergalactic magnetic fields may be a relic of helical primordial $\mathrm{U}(1{)}_Y$ magnetic fields, which may also explain the origin of the baryon asymmetry of the Universe. This scenario has been considered not to be viable, which we revisit as well as the baryon isocurvature problem for nonhelical primordial $\mathrm{U}(1{)}_Y$ magnetic fields, based on the recent discussion on the hot electroweak theory. We find that maximally helical fields can be the origin of both the intergalactic magnetic fields and the baryon asymmetry of the Universe and that there can be a window for nonhelical fields to explain the origin of the intergalactic magnetic fields if the Higgs dynamics during the electroweak crossover compensate the helicity decay with a $\lesssim {10}^{-9--10}$ precision.

Horizon-scale images of black holes provide a potential probe of fundamental physics, including tests of gravity and black-hole hair. To assess the impact of scalar hair on accretion-flow imaging self-consistently, we construct an analytical model of a geometrically thick, magnetized disk around a rotating hairy black hole in Horndeski theory and analyze its 230 GHz image morphology. We find that scalar hair modestly alters the inflow and magnetic-field structure but strengthens gravitational redshift, markedly reducing the total flux and lensed ring brightness through relativistic transfer and spectral-shift effects. Moreover, we identify a novel, spatially resolved imaging feature: the maximum interferometric diameter of the first photon ring responds strongly to the hair parameter but shows little dependence on accretion-flow details, making it a promising observable for constraining black-hole hair with future space-based interferometry.

We evaluate the coupling of quantum electrodynamics (QED) in the gradient-flow scheme in three and four space-time dimensions. Our general result applies to any theory with a U(1) gauge field coupled to arbitrary other fields via arbitrary interactions. As an example, we consider QED with $n_{\mathrm{f}}$ flavors in three and four space-time dimensions and evaluate the corresponding $\beta$ functions. In four dimensions, we find that the perturbative expansion of the $\beta$ function behaves much better than the corresponding expression in quantum chromodynamics. In three dimensions, we recover both the ultraviolet as well as the infrared fixed points of the QED coupling in the large-$n_{\mathrm{f}}$ limit.

We compute meson spectral functions at finite temperature and density in the quark-meson model, supplemented with a computation of the phase diagram. In particular, we provide a detailed analysis of the nonanalytic structure of the meson two-point functions which is of great relevance for phenomenological applications, such as moat regimes and inhomogeneous phases. Furthermore, it is also relevant from a field-theoretical standpoint as it provides an insight into the applicability of derivative expansions of the effective action to studies of general fermion-boson models, both at zero and finite chemical potential. Our computation is based on a functional renormalization group setup that preserves causality, all spacetime symmetries, and the Silver-Blaze property. The combination of these properties can only be achieved by a Callan-Symanzik regulator. Instead of momentum shell integrations, renormalization group flows generated by such a regulator describe the change of the theory induced by a change of the masses of the mesons and quarks. A particular focus of our work lies on the construction of controlled Callan-Symanzik flows in the presence of spontaneous and explicit chiral symmetry breaking by means of chiral Ward-Takahashi identities.

Type I seesaw models with a spontaneously broken $B-L$ symmetry provide a natural framework for spontaneous leptogenesis driven by a Majoron. The kinetic background of the Majoron acts as a $CP$-violating source, generating a lepton asymmetry both through the decay of right-handed neutrinos and through equilibration via inverse-decay processes. We construct the Boltzmann equations in a fully consistent manner, incorporating both effects, to enable a quantitative analysis. When the neutrino Yukawa coupling is large enough to maintain $B-L$ violating interactions in thermal equilibrium, the resulting asymmetry closely tracks its equilibrium value. In contrast, when this condition is not satisfied, a nontrivial interplay emerges between decay and inverse-decay dynamics, determined by the Yukawa coupling strength and the initial abundance of right-handed neutrinos.

We propose a mechanism for relaxing a gauge theory $CP$ violating phase in discrete steps to very small values. The idea is that the $CP$ violating phase includes the magnetic dual of a 4-form flux that can discharge by the nucleation of membranes. Inside the bubbles surrounded by the membranes, the total $CP$ violating phase is reduced. When the bubbles are produced rapidly during radiation domination in the early Universe, near the chiral symmetry breaking scale, they will collide and percolate, melting away into gauge theory radiation and dramatically relaxing $CP$ violation.

We investigate the spin-parity ($J^P$) assignments of experimentally observed hidden-charm pentaquark states within a baryon-meson molecular framework. The pentaquark mass spectrum is obtained using the Gürsey-Radicati mass formula, with parameters fixed through a global fit to 41 experimentally established hadron masses. The resulting spectrum is then used to assign $J^P$ quantum numbers to the observed pentaquark candidates. Within this framework, the nonstrange states $P_c(4312)$, $P_c(4440)$, and $P_c(4457)$ are identified with the $J^P=1/2^{-}$, $3/2^{-}$, and $5/2^{-}$ configurations, respectively. The recently reported Belle state $P_{cs}(4459)$, which carries strangeness, is interpreted as the strange member of the SU(3) flavor octet with $J^P=3/2^{-}$. Magnetic moments are subsequently evaluated using explicitly constructed wave functions. Their systematic behavior across SU(3) flavor multiplets and different spin-parity assignments satisfies the expected sum-rule relations and indicates that magnetic moments can serve as a useful observable for refining the quantum-number identification of hidden-charm pentaquark states in future studies.

We present a straightforward derivation of the spectral representation of a scalar field at nonzero temperature, assuming that the field is relativistically invariant in vacuum. This form was first derived by Bros and Buchholz.

We investigate the QCD phase diagram and the associated Lee–Yang edge singularities using the two-flavor nonlocal Nambu–Jona-Lasinio model extended to complex chemical potential. We show explicitly there exists a strong correlation between the chiral phase transition and the patterns in the effective potential in the complex plane of order parameter, serving as a criterion to differentiate crossover from first-order transitions. Typically, the Lee–Yang edge singularities can be understood as a generalization of the critical end point (CEP) separating the crossover and the first-order transition, where the positive Nambu phase and the Wigner phase coalesce. We further analyze the scaling behavior near the CEP by extracting the critical exponent associated with the Lee–Yang singularities. Additionally, we confirm that the extrapolation of the Lee–Yang edge singularity trajectories provides an efficient method of determining the CEP location, even at a small real chemical potential. This provides a viable method for exploring regions of the QCD phase diagram that remain inaccessible to lattice QCD.