The two-loop renormalization constants $Z_m$ and $Z_1=Z_2$ of Feynman gauge quantum electrodynamics (QED) in the on-shell scheme are computed using dimensional regularization. Analytic results are obtained for these constants. In addition, analytic expressions are obtained for their expansion coefficients in the dimensional regularization parameter $\epsilon$ from order $1/{\epsilon }^2$ through order ${\epsilon }^5$. The high-order expansion coefficients will be needed for future multiloop calculations in QED and will provide a useful check for multiloop calculations in QCD.

We study dark photon dark matter $X$ associated with a dark $U(1{)}_X$ gauge symmetry. To evade laboratory and cosmological constraints on kinetic mixing with the Standard Model $U(1{)}_Y$, we assign a $Z_2$-odd dark parity to $X$ that forbids such mixing. The leading interactions then arise from gauge-invariant dimension-6 Higgs-portal operators, including both parity-even and parity-odd terms. We assume that the dark matter mass is generated via the Stueckelberg mechanism. In the interacting Stueckelberg EFT, a gauge-invariant dimension-6 parent reduces in unitary gauge to the nominal dimension-4 Higgs-portal interaction $(H^{†}H)X_{\mu }{X}^{\mu }$, with its coefficient correlated with $m_X^2/{\mathrm{\Lambda }}^2$, while additional derivative operators are organized by the same longitudinal-mode power counting. We investigate freeze-in production of $X$ from Higgs-pair annihilations after reheating, incorporating both field-strength and Stueckelberg-allowed operators. First, we consider the case in which the Wilson coefficients of the gauge-invariant dimension-6 operators, $C$ and $\tilde{C}$, are of order unity. Under a conservative single-cutoff power counting in which the strong-coupling scales of the longitudinal mode lie above the cutoff of the field-strength effective theory, we find that the Stueckelberg contributions remain subdominant in dark matter production. We emphasize that this ordering of scales is an explicit assumption rather than a universal consequence of perturbative unitarity. Next, we explore a more general situation in which $C$ and $\tilde{C}$ are smaller than unity. In this second case, Stueckelberg-allowed operators can become comparable and generate contributions proportional to $T_{\mathrm{reh}}^3$, $T_{\mathrm{reh}}^5$, and $T_{\mathrm{reh}}^7$, including sign-dependent interference effects. For both cases, we show that there exists a wide parameter space consistent with the observed dark matter relic density.

High-energy $\gamma \gamma$ collisions, realizable as an operational mode of future lepton linear colliders such as the ILC and CLIC, provide a promising environment to probe extended Higgs sectors. We investigate the sensitivity of such colliders to doubly charged Higgs bosons within the 2-Higgs doublet model with type-II seesaw (2HDMcT). Focusing on the three-body production channels $\gamma \gamma \to H^{\pm \pm }{H}_1^{\mp }{H}_1^{\mp }$ and $\gamma \gamma \to H^{\pm \pm }{H}_1^{\mp }{W}^{\mp }$, we perform a parameter space scan consistent with theoretical constraints as well as current collider, flavor, and electroweak precision observables (EWPOs). We show that $\gamma \gamma$ collisions can rival the discovery potential of the conventional $e^{+}{e}^{-}$ mode for probing doubly charged Higgs bosons through a $4\ell +{\cancel{E}}_T$ signature ($\ell =e$, $\mu$). Despite the reduced effective luminosity resulting from the photon spectrum, the significantly enhanced production cross sections, exceeding those in electron-positron collisions by more than one order of magnitude, compensate for this limitation. By performing a detailed signal-to-background analysis at center-of-mass energies of $\sqrt{s}=830$ and 1245 GeV, we demonstrate that a discovery significance at the $5\sigma$ level can be achieved for viable benchmark points (BPs).

The nature of dark matter remains an outstanding problem in particle physics and cosmology. Hidden-sector extensions of the Standard Model predict a neutral partner of the neutron, whose weak mixing with ordinary neutrons induces oscillations between visible and dark baryonic states. We propose and analyze a macroscopic quantum interferometric strategy that could provide a direct and experimentally accessible probe of this phenomenon. In particular, a Mach-Zehnder interferometer with very cold neutrons converts neutron–hidden neutron oscillations into measurable phase-dependent intensity modulations. By combining controlled phase shifts with tunable magnetic fields and material potentials, the setup enables a resonant exploration of the hidden-sector parameter space. Under representative experimental conditions on neutron flux, magnetic-field homogeneity, vertical-aperture and beam-overlap control in the presence of gravity, and interferometric stability, the expected reach extends to mixing amplitudes down to ${\epsilon }_{n{n}^{\prime }}\sim {10}^{-14}\mathrm{eV}$ for mass splittings $\delta m\sim {10}^{-9}\mathrm{eV}$, accessing a previously unexplored region of parameter space and relevant to baryonic dark matter scenarios. These results suggest that neutron interferometry may become a precision laboratory tool for testing hidden-sector physics.

Ultralight dark matter can interact with Standard Model particles via gravitational and nongravitational interactions. Through such interactions, it can leave distinctive signals in gravitational-wave experiments. In this work, we investigate signals induced by ultralight dark matter quadratically coupled to the Standard Model in the future space-borne gravitational-wave detector, the Laser Interferometer Space Antenna (LISA). Due to the quadratic nature of the coupling, dark matter signals appear at two distinct frequencies: the frequency corresponding to twice the dark matter mass, and frequencies below the typical dark matter kinetic energy. We analyze both contributions and show that LISA can surpass current constraints from terrestrial and astrophysical probes in certain mass ranges. We also find that dark matter signals in LISA are free from screening effects which significantly limit the sensitivity of terrestrial experiments.

We investigate small-instanton contributions to the axion potential across a range of Kim-Shifman-Vainshtein-Zakharov models containing vector-like quarks (VQs), using naive dimensional analysis. We consider scenarios containing a single VQ, multiple identical copies, and sets of distinct VQs, requiring in each case that the gauge couplings remain perturbative up to the Planck scale under the two-loop gauge running. The associated fermion zero-mode content varies between these cases, requiring different combinations of mass insertions and scalar-Yukawa contractions for its saturation. Increasing the copies of VQs can render the instanton-size integral dominated by instantons of the smallest size, corresponding to the scale near the ultraviolet (UV) cutoff. The resulting contribution then becomes sensitive to the UV completion, and the induced potential can compete with, or dominate over, the ordinary QCD contribution. Assuming that the QCD and small-instanton potentials are aligned, we determine the resulting axion-mass shift and its consequences for the axion-photon coupling. When the small-instanton-induced susceptibility becomes comparable to or larger than the QCD susceptibility, the physical axion mass $m_a$ is enhanced at fixed decay constant $f_a$, while the axion-photon coupling $g_{a\gamma \gamma }$ remains controlled by $f_a$ and the anomaly ratio of $E/N$. The standard QCD relation among $m_a$, $f_a$, and $g_{a\gamma \gamma }$ is consequently modified, opening new regions of the $(m_a,g_{a\gamma \gamma })$ plane for axion searches.

The standard type-I seesaw can also be regarded as a low-scale seesaw by using the freedom of the Casas-Ibarra parametrization. In this framework, radiative corrections to the neutrino mass matrix can dominate over the tree-level contribution, so that the tree-level Casas-Ibarra construction no longer solves the equation that determines the physical neutrino masses and mixings. We show how the Casas-Ibarra parametrization can be consistently extended to this loop-corrected equation, by reabsorbing the one-loop corrections into the right-handed neutrino mass matrix, so that the resulting light neutrino mass matrix is consistent with experimental values. On the other hand, we show that physical processes related to right-handed neutrino propagation, such as heavy neutral lepton searches, do not depend on the one-loop corrections. Moreover, we show that $\text{Br}(\mu \to e\gamma )$ provides competitive constraints on the parameter space of heavy neutral lepton search experiments for masses above 100 GeV.

Modern machine learning techniques have become increasingly important in particle physics because of their powerful pattern-recognition capabilities, including in real-time data acquisition where stringent runtime constraints apply. This paper details the performance of deep-learning-based trigger algorithms for a large water Cherenkov detector such as Hyper-Kamiokande, aimed at low-energy neutrino events (below 7 MeV). The performance of custom neural-network supervised classifiers is shown alongside two anomaly-detection approaches trained solely on detector noise: a pure autoencoder and a model based on manifold projection-diffusion recovery. The supervised model shows signal identification efficiencies of 76.7% for single electrons of 3 MeV kinetic energy, significantly exceeding signal efficiencies obtained from a traditional hit-count-based trigger of 26.4%, while the manifold projection-diffusion recovery approach reaches 35.4% at the same operating point. Runtime evaluations on graphical processing unit yield per-window inference latencies well below the millisecond scale.

We express the scalar four-point correlator of the $Q$-model associated with the proposed minimal higher-spin de Sitter/Sp($N$) correspondence as an integral over the orthogonal Grassmannian $\mathrm{OGr}(4,8)$. The full, crossing-symmetric higher-spin Grassmannian correlator is given by $(S^2+T^2+U^2)/(STU)$, where $S$, $T$, $U$ are the Grassmannian Mandelstam variables. Remarkably, this has the same form as the field-theory limit of the Veneziano amplitude, despite encoding an infinite massless higher-spin tower associated with the opposite, tensionless regime. For the scalar sector considered here, the Grassmannian contour gives the full correlator, with no dispersive reconstruction required. We verify the formula by evaluating the contour integral and matching it to the complete momentum-space result, and analyze its singularities and higher-spin residues directly in Grassmannian space.

High-energy physics often motivates multifield inflationary scenarios where stochastic effects play a crucial role. Peculiar to multifield models, the noise-induced centrifugal force results in a longer duration of inflation depending on the number of fields, even when the stochastic noises themselves are small. We show that, in such small-noise regimes, the number of fields generically discriminates whether inflation successfully terminates or lasts forever. Our results indicate that inflation with an extremely large number of fields may fail to realize our observable Universe.

KM3NeT’s detection of a muon track produced by a $\sim 220\mathrm{PeV}$ neutrino provides an opportunity to probe physics at center-of-momentum energies greater than those probed by the LHC or other existing particle accelerators. In this study, we use this single event to place an upper limit on the neutrino-nucleon cross section of ${\sigma }_{\nu N}<26{\sigma }_{\nu N}^{\mathrm{SM}}$ at $E_{\mathrm{CM}}\sim 20\mathrm{TeV}$. This result can be used to constrain a variety of scenarios beyond the Standard Model. With future very large volume neutrino telescopes, constraints on the neutrino-nucleon scattering cross section are expected to become substantially more stringent and, in some scenarios, could become competitive with accelerator probes of new physics.

We investigate the quantum stability of a timelike topological wormhole with a simple geometry $M_2\times S^2$, supported classically by anisotropic fluid. We compute the one-loop quantum backreaction generated by the vacuum fluctuations of a minimally coupled, massive scalar field propagating on the wormhole background. Using dimensional regularization we renormalize the one-loop energy-momentum tensor and identify the necessary gravitational counterterms. We then solve the semiclassical Einstein equations to linear order in $\hslash$ for both time-dependent and static metric Ansätze. Depending on the choice of finite counterterms, the quantum effects can induce either negative or positive angular pressure, which will tend to destabilize or stablize the wormhole, respectively. We also show that a classically traversable wormhole will remain traversable when the quantum backreaction is taken into account. It would be of interest to investigate whether these conclusions remain true when the equations of semiclassical gravity are self-consistently solved.

We compute the weak-coupling free energy density of pure four-dimensional $\mathcal{N}=2$ supersymmetric Yang-Mills (SYM) theory through second order in the ’t Hooft coupling $\lambda$ at finite temperature using effective-field-theory (EFT) methods. The contribution from the hard scale $T$ is obtained from massless three-loop vacuum diagrams in the full theory and that from the electric scale $\sqrt{\lambda }T$ from a two-loop calculation in the dimensionally reduced three-dimensional effective theory. In contrast to $\mathcal{N}=4$ SYM, the theory is asymptotically free; the coupling-renormalization counterterm, together with the EFT unit-operator counterterm, cancels all $1/\epsilon$ poles. The remaining renormalization-scale dependence is fixed by the one-loop beta function. The expansion reproduces earlier results through order ${\lambda }^{3/2}$, and the order-${\lambda }^2$ term is new.

A compact form of the stress-tensor $⟨T^{ab}{T}^{cd}⟩$ correlation function of a quantum scalar field of arbitrary mass $m$ and curvature coupling $\xi$ is presented, with particular emphasis on decoupling in the $m\to \infty$ limit and the IR origin of the conformal anomaly as $m\to 0$. The Ward identity (WI) of covariant conservation is verified for the full second-order metric variation of the one-loop effective action, of which $⟨T^{ab}{T}^{cd}⟩$ is part, including local contact terms. This WI is satisfied by two tensors, one spin-2 (traceless) and the second spin-0 (traceful), each multiplied by a Lorentz-invariant form factor computed in $n$-dimensional regularization. Minimal subtraction of pole terms at $n=4$ produces form factors that fail to satisfy decoupling for general $\xi \ne 1/6$, but can be simply amended to do so. Particular attention is given to the $\xi =1/6$ case, in which the spin-0 form factor at $n=4$ is completely finite, requires no UV regularization or subtractions, and satisfies decoupling directly. Its imaginary part defines a spectral function that obeys a UV-finite sum rule for any $m$, while its real part unambiguously determines the $\square R$ term in the conformal anomaly in the massless limit. Its $m^2$ dependence describes a Wilsonian renormalization group flow to an effective field theory limit at large distances. At $m=0$, the spin-0 spectral function becomes a Dirac $\delta$ function at zero energy, demonstrating the existence of a massless scalar Goldstone collective excitation due to the conformal anomaly in the low-energy effective theory of gravity.

The characterization of the photon region, i.e., the region of space-time filled with unstable bound null geodesics, is critical to understand the behavior of radiation near a compact-enough object, such as black holes. However, its study has been focused on stationary space-times, neglecting dynamical scenarios such as collapsing, accreting, or evaporating black holes. In this work, we present a novel covariant approach to describe radiation in dynamical spherical space-times. Using a $2+2$ decomposition, we characterize a dynamical photon sphere as the two-sphere associated with instantaneous critical turning points of nonradial null motion, where both the radial momentum and its derivative along the null ray vanish. A smooth time-dependent family of these spheres defines a photon sphere tube, which is not assumed to be a photon surface generated by null geodesics. The resulting condition is quasilocal and can be written algebraically in terms of the areal radius, the Misner-Sharp mass, and the radial pressure. We further use geodesic deviation to characterize the local radial deviation between neighboring null geodesics at each critical turning point, while distinguishing this local deviation from the global stability of an evolving null orbit. Our formulation recovers the standard photon sphere in the static limit, and we show dynamical examples like stellar collapse and accreting/evaporating scenarios.

The origin of black hole entropy remains one of the deepest mysteries in modern physics. Two recent developments offer complementary perspectives on this puzzle: the entropy release from regular Schwarzschild black holes with a de Sitter core (the Ovalle, Casadio, and Kamenshchik [OCK] construction), and the local thermodynamics of the de Sitter vacuum (Volovik). In the OCK framework, the inner horizon carries positive Bekenstein-Hawking entropy $S_{\text{inner}}=\pi h_c^2$, whose formal contribution decreases as the core shrinks; the $n=0$ Minkowski breaking obstructs a regular continuation within the ansatz. In Volovik’s framework, the signed entropy assignment for a homogeneous de Sitter Hubble volume is $S_{\text{dS}}=\mathrm{sgn}(H)\pi /H^2=\mathrm{sgn}(H)A/4$: expanding de Sitter ($H>0$) carries positive entropy, while contracting de Sitter ($H<0$) carries negative entropy. We show that the sign of the Hubble parameter is the unifying element linking the two pictures. The OCK core is de Sitter only for $n>2$; the Minkowski breaking is a classical kinematic obstruction, not a dynamical transition to a contracting de Sitter phase. Any connection between the expanding and contracting regimes might involve a quantum mechanism that remains speculative. Nevertheless, both frameworks embody the same thermodynamic principle: interior shrinkage changes the assigned entropy, with total entropy increase requiring an appropriate physical balance, and together they provide a consistent picture of entropy flow during gravitational collapse. The integer nature of the OCK parameter suggests a quantized entropy spectrum and a statistical interpretation of black hole entropy, while compatibility with the generalized second law is conditional on the entropy prescription and a justified environmental contribution.

We investigate extremal black holes with scalar hair in the generic Einstein-Euler-Heisenberg (EEH)-scalar theory with two scalar couplings to the Maxwell term. One is an exponential form with coupling constant $\alpha$ and the other is the polynomial coupling. We consider the cold horizon of Einstein–Euler–Heisenberg black holes described by mass $M$ and magnetic charge $P$. Fixing the Euler-Heisenberg parameter $\epsilon =0.03$ with $M=1$, we study spontaneous scalarization at the linear level, through the existence curves ${\alpha }_n(q)$ with $q=P/M$ and construct fully backreaction solutions. As extremality ($q=q_e$) is approached, all ${\alpha }_n(q)$ accumulate, following ${\alpha }_n-{\alpha }_c\sim 1/{\ln }^2(q_e-q)$, at the critical branch ${\alpha }_c$ fixed by the Breitenlohner-Freedman bound for the near-horizon throat. We obtain scalarized extremal black hole (SEBH) with constant scalar hair and it is recovered exactly from the entropy function approach on the near-horizon throat. Its entropy is an attractor invariant, being independent of the asymptotic modulus, so the scalar hair remains secondary. Exploiting this constant scalar, we seek SEBHs with scalar hair keeping its charge $Q_s$ for the zero asymptotic scalar (${\varphi }_{\infty }=0$). In the $(q,\alpha )$ plane, we observe that these extremal solutions occupy $q\ge q_e$. Hence, the existence curves ${\alpha }_n(q)$ existing for $q\le q_e$ and the extremal branches bound the scalarized domain from opposite sides and they meet only at $q=q_e$.

Accurate reconstruction of peculiar velocities from galaxy positions is important for probing the motion and evolution of large-scale structure. They are sensitive to the cosmological effects of gravity and dark sector matter, and they complement other velocity inference methods such as the kinematic Sunyaev-Zel’dovich (kSZ) imprinted in the cosmic microwave background. We show that machine learning methods improve velocity reconstruction by capturing nonlinear contributions. Specifically, we train both a gradient boosting decision tree (GBDT) and a transformer using multiscale features to predict the residual between the actual velocity and the estimate from linear theory for both the line-of-sight and transverse components. We evaluate our approach in both periodic box and, more realistically, light cone settings using mock galaxy catalogs from the abacussummit simulations tailored to DESI spectroscopic surveys of luminous red galaxies and emission line galaxies. We also assess the impact of redshift uncertainties such as those in Rubin LSST photometry. Both models significantly outperform the linear theory, with the transformer achieving the best performance for the simulation considered here. They more accurately recover the density-weighted momentum power spectrum and maintain a higher cross-correlation with the true velocities across a wider range of spatial scales. Finally, we demonstrate two applications relevant to kSZ analyses: estimating cluster pairwise velocity correlations and stacked cluster density profiles. This machine learning framework for nonlinear velocity reconstruction opens up powerful new applications of survey data from DESI, Rubin LSST, Euclid and the Roman Space Telescope.

We present upper limits on the axionlike particle (ALP) photon coupling constant $g_{a\gamma \gamma }$ from a broadband spectral analysis of the Coma cluster (Abell 1656) radio halo. We have used flux density measurements spanning from 30.9 MHz to 6.6 GHz, which probe ALP masses from 0.26 to $55.9\mathrm{\mu }\mathrm{eV}$, the widest continuous mass coverage from any single cluster to date. We model the synchrotron foreground with the physically motivated in situ reacceleration spectrum, which provides a substantially better description of the strongly curved Coma halo spectrum than the simple power law adequate for the Bullet cluster. We adopt an aperture-matched profile-likelihood analysis as our primary statistical estimator, fitting the full 23-point radio spectrum simultaneously at each tested axion mass. For the robust CMB-only and $\mathrm{CMB}+\text{extragalactic}$ radio background photon baths, the $2\sigma$ upper end points at $m_a=55.9\mathrm{\mu }\mathrm{eV}$ are both $\simeq 4.35\times {10}^{-8}{\mathrm{GeV}}^{-1}$. Under the conditional $g_{a\gamma \gamma }$ derived under a specific, conditional modeling framework of the intracluster medium (ICM) kinetic-temperature prescription for the stimulating photon bath, the corresponding limit strengthens to $g_{a\gamma \gamma }<7.33\times {10}^{-12}{\mathrm{GeV}}^{-1}$, lying below the CAST helioscope and horizontal-branch stellar bounds at the high-frequency end of the Coma spectrum, while the lowest-frequency bins remain weaker than these stellar-evolution limits. At $m_a>40\mathrm{\mu }\mathrm{eV}$, the conditional ICM-enhanced limits are comparable to the sensitivity reached by the MADMAX prototype, while providing continuous broadband coverage across this mass range. We further forecast the sensitivity achievable with the Square Kilometre Array (SKA) in its high-frequency bands. Under the same conditional ICM photon-bath prescription, an ideal thermal-noise-limited SKA midfrequency telescope (SKA-MID) band 5b observation with 100 h of integration on Coma would reach $g_{a\gamma \gamma }\sim 7.5\times {10}^{-13}{\mathrm{GeV}}^{-1}$ across $70–130\mathrm{\mu }\mathrm{eV}$, extending broadband sensitivity into a mass range that remains sparsely constrained by present experiments. A systematic comparison across 14 galaxy clusters demonstrates that the luminosity distance to the observer is the dominant figure of merit, with the nearby Ophiuchus cluster ($z=0.028$) emerging as the premier SKA-MID target in our forecast analysis, at a projected limit of $g_{a\gamma \gamma }\sim 2.70\times {10}^{-13}{\mathrm{GeV}}^{-1}$.

Twin stars (TSs) are predicted to exist in nature if the hadron-to-quark phase transition is strong enough to form a new branch of hybrid stars, separated from the branch of neutron stars. It is essential to study the hybrid equation of states (EoSs) that can form TSs, with a primary focus on estimating the onset of the phase transition and its strength. We systematically study the TS scenario by adopting a four-parameter Maxwell-CSS approach to construct a large ensemble of Maxwell-type hybrid EoSs. Within this framework, we report the conditions on our parameter space imposed by the Tolman-Oppenheimer-Volkoff (TOV) solutions. For a fixed transition energy density and speed of sound for quark matter, we define distinct ranges of transition pressures based on the allowed strengths of discontinuity. Below a maximum transition pressure, a range of discontinuity exists that increases as the transition pressure decreases. Thereby, we identify the loci of the limits on discontinuities as the TS existence boundaries. Based on the causality limit, the TS existence boundaries can be truncated. Strong constraints on this picture are drawn from the CET, GW170817 and NICER. Within our considered model, we computed conditional extrema surviving the adopted $2\sigma$ filters—the maximum mass for TSs to be $2.05M_{\odot }$, the allowed strongest discontinuity in rest-mass density to be $7.76{\rho }_{\mathrm{sat}}$, and the upper bound on transition rest-mass density to be $4.03{\rho }_{\mathrm{sat}}$. Subsequently, we compute the implications of the stiffness of the quark matter EoS on this picture. Different confidence levels for observational constraints are considered to assess the extent to which Maxwell-type EOSs remain compatible with the observations.