Fri·Apr 10, 2026Friday·April 10, 2026

1. 01*

   Axion-like Particles and Lepton Flavor Violation in Muonic Atoms

   Girish Kumar🇺🇸 · Alexey A. Petrov🇺🇸

   We explore the potential of the Mu2e experiment to probe the lepton-flavor-violating process ${\mu }^{-}{e}^{-}\to e^{-}{e}^{-}$ in a muonic atom within a simplified axion-like particle (ALP) framework featuring flavor-violating $e$-$\mu$ couplings and a flavor-diagonal pseudoscalar coupling to electrons, which also allows for possible invisible ALP decays into a dark sector. We compute the ALP-mediated contribution to the transition rate and show that, at fixed couplings, the branching ratio increases for lighter mediators and scales as $(Z-1{)}^3$, favoring heavier nuclei. We compare the model against constraints from $\mu \to e\gamma$, $\mu \to 3e$, $\mu \to e\gamma \gamma$, $\mu \to e+\mathrm{inv}$, and muonium-antimuonium conversion, as well as from the anomalous magnetic moments of the electron and muon. Additional astrophysical and beam-dump limits on the electron coupling are also discussed. A key result is that $\Delta a_e$ provides one of the most stringent probes of the parameter space and, in the global scan, excludes the largest fraction of sampled points. After applying the laboratory constraints used in the scan, the viable branching ratio for ${\mu }^{-}{e}^{-}\to e^{-}{e}^{-}$ in aluminum drops to at most $\mathcal{O}(10^{-20})$, while the resonant region $2m_e<m_a<m_{\mu }-m_e$ is much more heavily suppressed. The highest achievable values are closely tied to $\mathcal{B}(\mu \to 3e)$ near its current limit, indicating that the upcoming Mu3e experiment will explore the most promising region relevant for this muonic-atom signal. Our analysis shows that, although a light ALP can parametrically enhance ${\mu }^{-}{e}^{-}\to e^{-}{e}^{-}$ at fixed couplings, existing bounds -- especially $\Delta a_e$, $\mu \to 3e$, $\mu \to e\gamma$, and muonium oscillations -- severely limit the observable rate.

   hep-phhep-ex0 citations
2. 02*

   Excitation function for global \Lambda polarization in relativistic heavy ion collisions with the Core Corona model

   Alejandro Ayala🇲🇽 · José Jorge Medina Serna🇲🇽 · Isabel Domínguez🇲🇽 · María Elena Tejeda-Yeomans🇲🇽

   We compute the excitation function of the global $\Lambda$ polarization in semicentral heavy-ion collisions within a Core--Corona framework, where the interaction region is described as a dense core and a dilute corona separated by a critical value of the participant density. An important ingredient in the model are the intrinsic polarization functions in each of the two regions. These are computed from a field-theoretical approach where the vortical motion of the medium is included in an effective fermion propagator, which we derive explicitly. The interactions in the core and the corona are transmitted by suitable mediators at finite temperature and baryon chemical potential; gluons for the former and $\sigma$-mesons for the latter. The temperatures and baryon chemical potentials are related to the collision energies along the chemical freeze-out curve. By allowing the cross section for $\Lambda$ production in the nuclear environment to take on values below the nucleon-nucleon threshold cross section, the calculation describes the lowest energy polarization data point. For the centralities corresponding to the experimental data, we find that the contribution from the corona is the dominant one and that a lifetime, and correspondingly a volume of this region, which becomes larger for the smaller energies due to stopping, is an essential ingredient in the calculation. Overall, the model provides a good description of the excitation function across the full experimental range and predicts a robust maximum near $\sqrt{s_{NN}}\sim$ 3 GeV that remains stable under reasonable variations of the freeze-out curve and the proton-proton $\Lambda$ production threshold to account for subthreshold production in a nuclear environment.

   hep-phnucl-th0 citations
3. 03*

   Forward trijet production in proton-nucleus collisions: gluon initiated channel

   Paul Caucal🇫🇷 · Marcos Guerrero Morales🇺🇸 · Farid Salazar🇺🇸

   In this paper, we present results for the forward three-parton production differential cross section in the gluon-initiated channel in proton-nucleus collisions. This result is the leading-order contribution to forward trijet production and provides the real-emission building block required for the NLO dijet/dihadron cross section. The calculations are carried out within the Color Glass Condensate (CGC) effective theory, and in the dilute-dense approximation, using effective vertices for the quark and gluon propagators interacting with the small-$x$ background gluon field. We employ the covariant perturbation theory approach and disentangle the amplitudes into regular and instantaneous contributions. Our results are expressed as convolutions of multiparton color correlators of light-like Wilson lines and perturbative impact factors, organized in compact expressions in terms of the ``bare" topologies of the contributing diagrams. The gluon-initiated channel receives contributions from a $q\bar{q}g$ and a $ggg$ final state. Interestingly, when considering the $ggg$ final state, we observe, for the first time, that the four-gluon vertex topology follows a structure similar to the instantaneous contributions. This observation suggests a simplification in the organization of multigluon CGC amplitudes and may prove useful for future one-loop calculations. Furthermore, when integrating (one of) the real gluon(s) in the final state, we identify that the rapidity divergence is absorbed into the real part of the JIMWLK evolution of the leading-order Wilson-line correlator. In addition, we isolate the divergences arising when the unobserved parton becomes collinear to one of the observed hadrons in the final state. These divergences are absorbed by renormalizing the initial-state parton distribution functions and final-state fragmentation functions, which are shown to obey DGLAP evolution.

   hep-phhep-thnucl-thJHEP(2026)·3 citations
4. 04*

   Lecture notes on Machine Learning applications for global fits

   Jorge Alda🇮🇹

   These lecture notes provide a comprehensive framework for performing global statistical fits in high-energy physics using modern Machine Learning (ML) surrogates. We begin by reviewing the statistical foundations of model building, including the likelihood function, Wilks' theorem, and profile likelihoods. Recognizing that the computational cost of evaluating model predictions often renders traditional minimization prohibitive, we introduce Boosted Decision Trees to approximate the log-likelihood function. The notes detail a robust ML workflow including efficient generation of training data with active learning and Gaussian processes, hyperparameter optimization, model compilation for speed-up, and interpretability through SHAP values to decode the influence of model parameters and interactions between parameters. We further discuss posterior distribution sampling using Markov Chain Monte Carlo (MCMC). These techniques are finally applied to the $B^{\pm }\to K^{\pm }\nu \bar{\nu }$ anomaly at Belle II, demonstrating how a two-stage ML model can efficiently explore the parameter space of Axion-Like Particles (ALPs) while satisfying stringent experimental constraints on decay lengths and flavor-violating couplings.

   hep-phcs.LGSciPost Phys. Lect. Notes 135 (2026)·1 citation
5. 05*

   Transverse energy-momentum tensor distributions in polarized nucleons

   Ho-Yeon Won🇫🇷 · Cédric Lorcé🇫🇷

   We complete our study of the relativistic spatial distributions of the energy-momentum tensor inside polarized nucleons within the quantum phase-space formalism. In the present work, we focus on the components of the energy-momentum tensor involving at least one transverse index. We also explore the multipole structure of the transverse distributions in a moving nucleon. In the infinite-momentum frame, we show that the formalism reproduces the standard light-front distributions, including those with a ``bad'' component, and explains the origin of their structure.

   hep-phnucl-thPRD(2026)·2 citations
6. 06*

   Multi Component Dark Matter in a Minimal Model

   Karim Ghorbani🇮🇷

   We study a minimal ${\mathbb{Z}}_2$-symmetric extension of the Standard Model containing two singlet fermions and a singlet scalar that interact with the SM particles through the Higgs-portal. We identify regions of parameter space in which all three new particles are kinematically stable, giving rise to a multi component dark matter (DM) scenario. The parameter space consistent with the observed dark matter relic abundance are determined, and the contribution of each component to the total relic density is evaluated. While the DM-nucleon elastic scattering cross sections of the two fermionic dark matter components are loop-suppressed, the corresponding cross section of the scalar dark matter particle arises at tree level and is therefore expected to dominate. We find a viable region of the parameter space in which the scalar dark matter candidate with mass range of approximately $125-400$ GeV, evades current direct detection (DD) bounds while contributing only a small fraction of the observed relic density. In contrast, the fermionic dark matter possesses a loop-suppressed DD cross section that lies below the neutrino floor and can constitute a substantial fraction of the total relic density.

   hep-phEPJC(2026)·1 citation
7. 07*

   Memory effect on the heavy quark dynamics in hot QCD matter

   Jai Prakash🇨🇳 · Ling Hai Li🇨🇳 · Ying Shan Zhao🇨🇳 · Yifeng Sun🇨🇳

   We study the heavy quark dynamics in the presence of memory within the framework of a generalized Langevin equation. Time correlated thermal noise with power-law decay is generated by a fractional differential equation, formulated using the Caputo fractional derivative with order parameter $\nu$. The effect of memory is calculated through the momentum correlation, the time evolution of the average squared momentum, the average squared displacement, and the average kinetic energy. The effect of memory is further studied for the higher normalised central moments of the heavy quark transverse-momentum distribution. The results indicate that time correlated thermal noise substantially influences heavy quark dynamics in the quark gluon plasma.

   hep-phhep-thnucl-thPRD(2026)·1 citation
8. 08*

   Constraining Ultralight Scalar Dark Matter in the Galactic Center with the S2 Orbit

   Jiang-Chuan Yu🇨🇳 · Yan Cao🇨🇳 · Lijing Shao🇨🇳

   The dense environment of our Galactic Center (GC) offers a unique laboratory for probing ultralight dark matter (ULDM). We explore the prospect of detecting a scalar ULDM field through its effects on the orbital dynamics of S-stars around the supermassive black hole in the GC, Sgr A${}^{\ast }$. We consider both linear and quadratic couplings between the real scalar field $\varphi$ and Standard Model particles, and analyze two representative ULDM structures: the scalar gravitational atom and the spherical soliton. We first find that quadratic coupling induces a non-oscillatory perturbation, leading to a long-term secular orbital evolution. We use the observed periastron precession rate of S2 star to put stringent constraints on the total ULDM mass in the GC and the quadratic coupling constant. For the gravitational atom $|211⟩$ state, assuming an equatorial S2 orbit, we constrain the mass ratio of ULDM to Sgr A${}^{\ast }$ to $\beta \lesssim 10^{-3}$ at $m\sim 10^{-18}$ eV, and for the spherical soliton which extends to $\sim 0.2$pc, the mass ratio is limited to $\beta \lesssim 1$ at $m\sim 3\times 10^{-20}$ eV. Notably, over part of the mass range $10^{-20}\mathrm{eV}\lesssim m\lesssim 10^{-18}\mathrm{eV}$, the S2 constraints are stronger than the corresponding bounds from Cassini and MICROSCOPE.

   hep-phgr-qc2 citations
9. 09*

   Probing Majoron Dark Matter with Gravitational Wave Detectors

   Ippei Obata🇯🇵 · Tsutomu T. Yanagida🇯🇵

   The Majoron is a hypothetical (pseudo) Nambu-Goldstone boson arising from the spontaneous breaking of a global lepton number symmetry, and is known as a candidate for dark matter in our Universe. In this paper, we investigate the possibility of probing the Majoron dark matter with a linear optical cavity used in the interferometric gravitational wave detectors. We consider a scenario in which the Majoron dark matter couples to photons through a QED anomaly, leading to an oscillatory photon birefringence induced by the coherent dark matter background. The anomaly coefficient is fixed by requiring the model to simultaneously reproduce the electroweak Higgs scale and a typical right-handed Majorana neutrino mass scale, and the resulting dark matter-photon coupling naturally falls within the sensitivity range of optical interferometers. By incorporating additional optics to extract the birefringence signal, we find that ground-based laser interferometers such as Advanced LIGO, KAGRA, as well as future detectors, can probe a region of the parameter space of Majoron dark matter.

   hep-phastro-ph.COastro-ph.IMphysics.ins-det+11 citation
10. 10*

    Fermion Multiplicities at the GUT Scale: A Statistical Study of Unification and Proton Decay

    Akifumi Chitose🇯🇵 · Ko Hirooka🇯🇵 · Masahiro Ibe🇯🇵 · Satoshi Shirai🇯🇵

    We study the impact of multiple vector-like fermions in SU(5) grand unified theory (GUT). Threshold effects from extra fermions allow the observed gauge couplings to be consistently matched to a single unified gauge coupling, and typically raise the unification scale to $M_{\mathrm{GUT}}\simeq 10^{15.5}\mathrm{GeV}$. Because the Standard Model fermions arise as admixtures of several GUT multiplets, the nucleon decay operator coefficients are further suppressed, leading to longer proton lifetimes than in conventional GUTs. We also find that the admixture of multiple GUT multiplets relaxes the rigid Yukawa relations of conventional GUTs and alleviates the bottom-tau unification problem. Overall, our analysis demonstrates that multi-fermion SU(5) GUTs provide a testable framework that simultaneously reconciles gauge coupling unification, realistic flavor structures, and proton stability. Our results highlight the importance of probing multiple proton-decay channels in next-generation experiments such as Hyper-Kamiokande to critically test this scenario.

    hep-phJHEP(2026)·0 citations
11. 11*

    $\bar{B}\to D^{(\ast )}\ell \bar{\nu }$ Branching Ratios and Evidence for Isospin Breaking in ${\rm Y}(4S)$ Decays

    Martin Jung🇮🇹 · Stefan Schacht🇬🇧

    We introduce a new method for the determination of the ratio of production fractions $R^{\pm 0}=\mathcal{B}({\rm Y}(4S)\to B^{+}{B}^{-})/\mathcal{B}({\rm Y}(4S)\to B^0{\bar{B}}^0)$ based on $\bar{B}\to D^{(\ast )}\ell \bar{\nu }$ decays. Given the importance of these modes, we perform a comprehensive analysis of the available data, extracting the information on their branching fractions and \Rpmz in parallel and providing their correlations in order to avoid double-use of this information in phenomenological analyses. We obtain the most precise value for $R^{\pm 0}$ from a single channel so far, about 2$\sigma$ from unity. The combination with previously available determinations from other channels yields $R^{\pm 0}=1.062(19)$, constituting evidence for isospin violation in ${\rm Y}(4S)$ decays. This demonstrates the necessity to take this effect into account in experimental and phenomenological analyses. The results for the $\bar{B}\to D^{(\ast )}\ell \bar{\nu }$ branching fractions are up to $1.6\sigma$ larger compared to averages available in the literature, owing to the removal of overlooked inconsistencies in the treatment of older analyses and correcting for d'Agostini bias where possible, thereby reducing the $V_{cb}$ puzzle.

    hep-phhep-ex4 citations
12. 12*

    LFV decays in a 3-4-1 model with minimal inverse seesaw neutrinos

    N.H.T. Nha🇻🇳 · L.T. Hue🇻🇳 · L.T.T. Phuong🇻🇳 · T.T. Hong🇻🇳

    We investigate an extended 3-4-1 model consisting of a new singly charged Higgs boson, implementing the minimal inverse seesaw mechanism to account for the current $(g-2{)}_{e,\mu }$ constraints as well as the lepton-flavor-violating decay rates of charged leptons, the Standard Model-like Higgs boson, and the $Z$ boson, all consistent with current experimental data. Unlike the previously studied 3-4-1 realization, the model considered here predicts strong correlations among these observables that can be tested in future experiments. In particular, the current upper bound on Br$(\tau \to \mu \gamma )$ imposes a stringent constraint compatible with the $1\sigma$ experimental range of $(g-2{)}_{\mu }$, corresponding to an allowed deviation of order $10^{-9}$ from the SM prediction. The forthcoming experimental sensitivity to Br$(\tau \to \mu \gamma )$ will reduce this deviation to $5\times 10^{-10}$.

    hep-phNPB(2026)·3 citations
13. 13*

    Kinetic and canonical momentum broadening in the Glasma

    Dana Avramescu🇫🇮 · Carlos Lamas🇪🇸 · Tuomas Lappi🇫🇮 · Meijian Li🇪🇸 · Carlos A. Salgado🇪🇸

    We lay the foundations for a quantum formalism describing the real-time evolution of particles in the Glasma phase of a heavy-ion collision, focusing on the implications of gauge invariance in the definition of the momentum of a particle in a classical background field. We first establish the correspondence between the classical Wong's equations and the Heisenberg equations of motion for a particle in a classical non-Abelian background field. Using this correspondence, we obtain equations of motion for both the kinetic momentum -- the gauge invariant, physically measurable quantity -- and the canonical momentum, which is conjugate to the coordinates in the Hamiltonian. In particular, the kinetic momentum broadening receives non-trivial contributions from the transverse field components, even in the eikonal limit. Finally, we demonstrate that imposing a transverse Coulomb gauge condition at the initial time significantly reduces the accumulation of numerical errors, thereby providing an optimized framework for the forthcoming quantum implementation.

    hep-phnucl-th2 citations
14. 14*

    How well can the QCD axion hide?

    Sung Mook Lee🇨🇭 · Maria Ramos🇨🇭 · Fuensanta Vilches🇪🇸

    Motivated UV frameworks generically predict the existence of multiple axion fields. Their interplay gives rise to novel collective phenomena - including level crossings and the formation of string bundles - which modify the predicted mass and couplings of the QCD axion as a solution to both the strong CP problem and the observed dark matter abundance. Among these effects, the domain wall number is determined by the full anomaly structure of the theory: in the single axion case, the absence of long-lived domain walls imposes $E/N\ge 8/3$ as a theoretical bound on the QCD axion photon coupling, assuming the global structure of the Standard Model gauge group is minimal. We show that this bound can be relaxed in the multi-axion framework. Combined with the fact that the QCD axion can become a subdominant dark matter component, this might render multi-axion scenarios experimentally challenging. Nevertheless, a careful analysis of the parameter space reveals that in most regions where the QCD axion evades detection, an axion-like particle remains visible to next-generation experiments. When all signals fall below future projections, we identify the most promising regions of parameter space to probe in an illustrative two-axion setup.

    hep-ph5 citations
15. 15*

    On Exclusive Coherent Production of Bosons in Electron-Proton Collisions

    Reuven Balkin🇺🇸 · Ta'el Coren🇮🇱 · Alexander Jentsch🇺🇸 · Hongkai Liu🇺🇸 · Maksym Ovchynnikov🇨🇭 · Yotam Soreq🇮🇱 · Sokratis Trifinopoulos🇨🇭

    We study the exclusive electroproduction process $e+p\to e^{\prime }+p^{\prime }+X$, with $X$ a single-particle final state, in the forward-proton kinematics relevant for the future Electron-Ion Collider (EIC). We develop a unified $2\to 3$ framework that provides the full event kinematics and incorporates pseudoscalar and vector mesons, as well as axion-like particles and vector mediators such as dark photons. It is based on phenomenological amplitudes constrained by existing photo- and electroproduction data and constructed to admit systematic refinement as new measurements become available. To benchmark the framework, we compare its predictions to flux-factorized descriptions based on the equivalent-photon approximation, demonstrating close agreement for total rates and selected single-differential distributions in the near-real regime, while highlighting the role of finite-$Q^2$ correlations for multi-differential observables at larger photon virtualities. As a case study, we perform a detailed kinematic analysis of the missing-proton-energy signature, illustrating how the full $2\to 3$ treatment informs forward-proton acceptance and signal selection in realistic EIC configurations.

    hep-ph2 citations
16. 16*

    The fall and the rise of Weyl gauge theory

    D. M. Ghilencea🇷🇴

    In 1918 Weyl introduced Weyl conformal geometry and its associated quadratic action which was the first gauge theory, of the Weyl group (of dilatations and Poincaré symmetry). The initial physical interpretation of his theory was however short-lived and led to the downfall of Weyl geometry as a physical theory. We review how this action was re-born into a physical Weyl quadratic gauge theory of gravity. This is the only (quadratic) gauge theory of a spacetime symmetry with a physical gauge boson, is Weyl anomaly-free, has {\it exact} geometric interpretation, with all scales of geometric origin, and generates Einstein-Hilbert action and a positive cosmological constant in its Stueckelberg broken phase. A more fundamental Weyl gauge theory is the Weyl-Dirac-Born-Infeld (WDBI) action of Weyl geometry, that is Weyl gauge invariant in arbitrary $d$ dimensions and that does not need a UV regulator (!), of which the (geometrically regularised) Weyl quadratic gauge theory is the leading order. For $d=4$ the WDBI action can include SM operators alongside gravitational terms into a unified description, both geometric and by the gauge principle, of SM and Einstein-Hilbert gravity, which are recovered in the leading order of this action.

    ↳ gr-qchep-phhep-th2 citations
17. 17*

    Which Neutron Stars Reach the Stiffening Regime? Multimessenger Constraints on Core Sound Speed and Stellar-Mass Thresholds

    Nicolás Viaux🇨🇱 · Sebastián Mendizabal🇨🇱

    We present a concise multimessenger inference of the neutron-star core sound-speed profile using GW170817 and three \textit{NICER} mass--radius posteriors (PSR J0030$+$0451, PSR J0740$+$6620, and PSR J0437$-$4715). The main result is not only a preference for intermediate-density stiffening within smooth equation-of-state families, but a translation of that inference into the stellar masses that access the relevant density regime. In the baseline smooth peaked family, the posterior probability that $c_s^2>1/3$ at $3.5n_{\mathrm{sat}}$ is $85.4\%$, while equal-prior averaging over peaked, monotonic, piecewise, and transition-capable families gives a more conservative $79.0\%$. Posterior-resampled exact Tolman--Oppenheimer--Volkoff solutions show that the onset density of the inferred stiffening is typically reached near $1.6M_{\odot }$, whereas the peak region is accessed only near $2.1M_{\odot }$. A J0740-like $2.07M_{\odot }$ pulsar reaches the onset in $91\%$ of posterior draws but the peak in only $46\%$, showing that current data mainly constrain whether massive stars have entered the stiffening regime rather than traversed its full peak.

    ↳ astro-ph.HEastro-ph.SRhep-phnucl-th0 citations
18. 18*

    Gaussian pseudogauge invariant hydrodynamics with spin

    David Montenegro🇧🇷 · Mariana Julia Pereira Dos Dores Savioli · Giorgio Torrieri🇧🇷

    Extending the Gaussian covariant hydrodynamics approach [1] using torsion as an auxiliary field we formulate a fluctuating hydrodynamics with spin which is covariant with respect to pseudo-gauge transformations as well as generally covariant with respect to foliations. This is done via the second order gravitational Ward identities, derived here in the torsionful case. This ensures that, while angular momentum observables depend covariantly on the pseudo-gauge, the dynamics is pseudo-gauge independent, thus clarifying the role of the pseudo-gauge in hydrodynamics with spin

    ↳ hep-thhep-phnucl-th2 citations
19. 19*

    Classification of Pati--Salam Asymmetric ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ Heterotic String Orbifolds

    Luke A. Detraux🇬🇧 · Alon E. Faraggi🇬🇧 · Benjamin Percival🇬🇧

    We develop a systematic classification of asymmetric ${\mathbb{Z}}_2$ orbifold actions in Pati-Salam heterotic string vacua constructed in the free fermionic formulation. Starting from symmetric ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ orbifold vacua with an $SO(10)$ GUT, we allow the Pati--Salam breaking vector to act asymmetrically on the internal degrees of freedom. The asymmetric orbifold action freezes geometrical moduli whilst inducing doublet--triplet splitting in the untwisted sector. Notably, this doublet-triplet splitting operates for any asymmetric action, including pure asymmetric shifts that preserve all geometric moduli, and is therefore independent of moduli stabilisation. Classifying the breaking vector according to its twist action, we find six inequivalent classes of geometric moduli spaces characterised by 12, 8, 4 or 0 real untwisted moduli. Through combining these asymmetric twists with all compatible asymmetric shifts, 24 inequivalent cases are identified and characterised by their residual moduli content and internal Narain lattice. For each case we construct representative basis sets admitting three chiral generations, providing the starting point for further classification within each class. We perform explicit GGSO (generalized Gliozzi-Scherk-Olive) phase enumerations in representative model classes with 12, 8, 4 and 0 moduli, classify the resulting $\mathcal{N}=1$ and $\mathcal{N}=0$ vacua according to phenomenological criteria and identify exophobic, phenomenologically viable models. We compute the partition function and corresponding one-loop vacuum energy at the free fermionic point in moduli space for each phenomenologically viable model across the four classes. As the number of geometrical moduli decreases, the number of distinct partition functions for these vacua collapses to a small number, reflecting a pronounced degeneracy under GGSO phase variations.

    ↳ hep-thhep-phPRD(2026)·2 citations
20. 20*

    Oblique Shocks at Supernova Remnants in Massive Star Clusters: A Model for the Cosmic-Ray Knee Observed by LHAASO

    Luana N. Padilha🇧🇷 · Rita C. Anjos🇧🇷

    This work establishes oblique shocks in Massive Star Clusters (MSC) as a primary mechanism for accelerating cosmic rays (CR) up to the knee of the energy spectrum. We develop a model that incorporates the combined contribution of supernova and collective wind shocks, emphasizing the critical role of the shock obliquity angle in determining the maximum particle energy. We illustrate, within our model that oblique shocks can significantly enhance acceleration efficiency, allowing particles to reach multi-PeV energies in a rigidity-dependent manner. Our preferred model, which incorporates oblique shocks, reproduces the all-particle spectrum and composition observed by The Large High Altitude Air Shower Observatory (LHAASO), interpreting the knee as arising from a sequence of rigidity-dependent cutoffs. The model also predicts subdominant but detectable gamma-ray and neutrino emissions. This study provides an attempt at building a unified framework connecting MSC particle acceleration to the observed features of the cosmic-ray knee.

    ↳ astro-ph.HEhep-phApJ(2026)·1 citation
21. 21*

    Leading low-temperature correction to the Heisenberg-Euler Lagrangian

    Felix Karbstein🇩🇪

    In this note, we show that the well-known leading low-temperature correction to the Heisenberg-Euler Lagrangian in a constant electromagnetic field arising at two loops can be efficiently extracted from its one-loop zero-temperature analogue. Resorting to the real-time formalism of equilibrium quantum field theory that explicitly separates out the zero-temperature contribution from the finite-temperature corrections the determination becomes essentially trivial. In essence, it only requires taking derivatives of the Heisenberg-Euler Lagrangian at one loop and zero temperature for the field strength. As a bonus, we then effectively dress the low-temperature contribution at two loops by one-particle reducible tadpole structures. This generates a subset of higher-loop contributions to the Heisenberg-Euler Lagrangian in the limit of low temperatures. We extract their leading strong-field behavior at a given loop order, and finally resum these to all loop orders.

    ↳ hep-thhep-phquant-phParticles(2026)·0 citations
22. 22*

    Detecting Chiral Gravitational Wave Background with a Dipole Pulsar Timing Array

    Baoyu Xu🇨🇳 · Hanyu Jiang🇨🇳 · Rong-Gen Cai🇨🇳 · Misao Sasaki🇯🇵 · Yun-Long Zhang🇨🇳

    The pulsar timing array (PTA) is a powerful technique for detecting nanohertz gravitational wave backgrounds (GWBs). However, conventional PTAs lack sensitivity to parity violation in the GWB. In this work, we propose a dipole pulsar timing array system (dPTA). By deriving the overlap reduction functions (ORFs) from the cross-correlation of timing signals, we find that this system exhibits sensitivity to chiral GWBs in the nanohertz regime. Furthermore, through numerical calculations of its sensitivity curves, we demonstrate that the dPTA extends the detectable frequency range of PTAs for GWBs from the nanohertz to the microhertz regime.

    ↳ gr-qcastro-ph.COhep-ph1 citation
23. 23*

    Caustic formation in DBI models: Wave propagation on planar domain walls

    E. Babichev🇫🇷 · B. Gafarov🇷🇺 · S. Ramazanov🇷🇺 · M. Valencia-Villegas🇷🇺

    We investigate propagation of generic waves on thin planar domain walls effectively described by the scalar Dirac-Born-Infeld model (DBI). We pay a particular attention to the possibility of caustic formation - the process, which may lead to intensive particle emission by domain walls. It is demonstrated that no singularities arise in DBI in 2D flat spacetime in the hyperbolic case, if one starts from smooth initial conditions. Technically, this happens because the same family characteristics of the relevant partial differential equation remain parallel at all the times, albeit not being straight lines generically. Crucially, characteristic curves cease to be parallel beyond the simplified setup of DBI in 2D flat spacetime. In particular, this is shown to be the case in $D>2$ for spherical waves, in an expanding Universe, and in the case of a minimal deformation of DBI necessary for avoiding the domain wall problem in cosmology. However, we prove that DBI remains caustic free in the hyperbolic case in all these physically relevant situations. This strongly suggests that caustics can form on planar domain walls only due to the loss of hyperbolicity, and they have a cusp profile. We demonstrate, how the non-trivial structure of DBI characteristics beyond the 2D flat spacetime setup uncovered in this work can significantly affect cusp formation.

    ↳ hep-thastro-ph.COgr-qchep-phPRD(2026)·2 citations
24. 24*

    How many VHE gamma-ray binaries with young pulsars can be observed?

    A. M. Bykov (Ioffe PTI, StPetersburg)🇷🇺 · A. G. Kuranov (Sternberg Astronomical Institute, Moscow)🇷🇺 · A. E. Petrov (Ioffe PTI, SPb)🇷🇺 · K. A. Postnov (SAI Moscow)🇷🇺

    A population of Galactic gamma-ray binaries is currently emerging due to ever increasing sensitivity of gamma-ray observatories. The detection of very high energy (VHE) photons with energies well above 10 TeV from a dozen of sources and the estimated power of those sources make them potentially interesting cosmic ray accelerators. Multi-wavelength observations of gamma-ray binaries revealed that most of them include a young massive star in pair with a relativistic companion, either a black hole or energetic pulsar. Fast stellar winds interacting with powerful relativistic outflows from pulsars or the black hole jets in microquasars are favorable sites for VHE particle acceleration. To estimate the expected number of gamma-ray binaries, we present results of population synthesis calculations of Galactic binaries in which a young massive OB- or Be-star is accompanied by a pulsar capable of producing a powerful relativistic outflow. The distributions over the binary eccentricities, orbital periods, Be-disk inclinations, and the pulsar braking energy losses are taken into account. Conditions for a binary to accelerate VHE particles, radiate and absorb the non-thermal photons that may reach the observer are discussed. We model the anisotropic structure of the zone of interaction of the relativistic pulsar wind with the strongly magnetized massive star's wind. The stellar winds with strong ($\sim$ Gauss) magnetic fields at $\sim$ AU distances colliding with powerful pulsar outflows are capable of accelerating particles up to PeV energies at some orbital configurations and phases. The strong magnetic field in the interaction region produces a highly anisotropic structure of the particle accelerator and emitter in the pulsar outflow. The anisotropic radiation pattern may affect the gamma-ray photon absorption and the number of the observed gamma-ray loud systems.

    ↳ astro-ph.HEhep-phJHEAp(2026)·2 citations
25. 25*

    Unifying topological, geometric, and complex classifications of black hole thermodynamics

    Shi-Hao Zhang🇨🇳 · Shao-Wen Wei🇨🇳 · Jing-Fei Zhang🇨🇳 · Xin Zhang🇨🇳

    Black hole thermodynamics has recently witnessed three distinct classification schemes: based on local geometric properties of the temperature function, global topological invariants, and Riemann surface foliations in the complex plane. We show that these schemes can be precisely mapped onto one another in the real domain via two dictionaries: one linking thermal stability to the monotonicity of the temperature curve, and the other connecting the number of black hole states to the foliation number of a Riemann surface. The number of extremal points of the temperature curve determines the classification in all three frameworks, tracing this unification to the critical point structure of the black hole solution space. As an illustration, several black holes demonstrate how counting extrema yields topological invariants and phase transition information. This unified framework simplifies black hole thermodynamic analysis and provides a foundation for exploring more complex black holes.

    ↳ gr-qchep-phhep-thPLB(2026)·3 citations
26. 26*

    Lattice determination of the higher-order hadronic vacuum polarization contribution to the muon $g-2$

    Arnau Beltran🇩🇪 · Alessandro Conigli🇩🇪 · Simon Kuberski🇨🇭 · Harvey B. Meyer🇩🇪 · Konstantin Ottnad🇩🇪 · Hartmut Wittig🇩🇪

    We present the first lattice QCD calculation of the next-to-leading order (NLO) hadronic vacuum polarization (HVP) contribution to the muon anomalous magnetic moment with sub-percent precision. We employ the time-momentum representation combined with the spatially summed vector correlator computed on CLS ensembles with $N_{\mathrm{f}}=2+1$ flavors of $\mathrm{O}(a)$-improved Wilson fermions, spanning six lattice spacings ($0.039$-$0.097$fm) and a range of pion masses including the physical value. After accounting for finite-size corrections and isospin-breaking effects, we obtain in the continuum limit $a_{\mu }^{\mathrm{hvp},\mathrm{nlo}}=(-101.57\pm 0.26_{\mathrm{stat}}\pm 0.54_{\mathrm{syst}})\times 10^{-11}$, corresponding to a total relative error of 0.6$\%$. Our result lies 1.4$\sigma$ below the estimate of the 2025 White Paper update and is two times more precise. It also shows a tension of $4.6\sigma$ with data-driven evaluations based on hadronic cross section measurements prior to the CMD-3 result.

    ↳ hep-lathep-phPoS(2026)·1 citation
27. 27*

    Coupled Dark Energy and Dark Matter for DESI: An Effective Guide to the Phantom Divide

    Stefan Antusch🇨🇭 · Stephen F. King🇬🇧 · Xin Wang🇮🇹

    Motivated by the recent Dark Energy Spectroscopic Instrument (DESI) DR2 preference for dynamical dark energy, we study interacting dark energy models in which a canonical quintessence field couples to cold dark matter through a field-dependent mass $m(\varphi )$. In such scenarios, the effective equation of state inferred under the assumption of non-interacting dark sectors, $w_{\mathrm{eff}}(z)$, can differ from the intrinsic scalar-field equation of state $w_{\varphi }(z)$, making an apparent phantom crossing $w_{\mathrm{eff}}<-1$ possible without introducing a phantom scalar. We show that a viable realization of this mechanism requires the scalar field to originate from a frozen phase deep in the radiation era, in order for the effective coupling to remain sufficiently suppressed before recombination to evade cosmic microwave background constraints, and for the late-time evolution to become strong enough to reproduce the apparent behavior of $w_{\mathrm{eff}}(z)$ preferred by DESI. We identify the general conditions that allow these requirements to be satisfied simultaneously, and present an illustrative phenomenological realization in which $w_{\mathrm{eff}}(z)$ evolves from $w_{\mathrm{eff}}\approx -1.2$ at $z\approx 1.0$ to $w_{\mathrm{eff}}\approx -0.9$ at $z\approx 0.4$. These conditions and requirements serve as a guide for designing future models of this kind which can safely navigate the phantom divide at $w=-1$ in an effective way without phantom fields.

    ↳ astro-ph.COhep-ph17 citations
28. 28*

    High-dimensional inference for the $\gamma$-ray sky with differentiable programming

    Siddharth Mishra-Sharma🇺🇸 · Tracy R. Slatyer🇺🇸 · Yitian Sun🇨🇦 · Yuqing Wu🇺🇸

    We motivate the use of differentiable probabilistic programming techniques in order to account for the large model-space inherent to astrophysical $\gamma$-ray analyses. Targeting the longstanding Galactic Center $\gamma$-ray Excess (GCE) puzzle, we construct differentiable forward model and likelihood that make liberal use of GPU acceleration and vectorization in order to simultaneously account for a continuum of possible spatial morphologies consistent with the GCE emission in a fully probabilistic manner. Our setup allows for efficient inference over the large model space using variational methods. Beyond application to $\gamma$-ray data, a goal of this work is to showcase how differentiable probabilistic programming can be used as a tool to enable flexible analyses of astrophysical datasets.

    ↳ astro-ph.HEastro-ph.IMcs.LGhep-ph0 citations
29. 29*

    Primordial Neutron Stars

    Gordan Krnjaic🇺🇸 · Duncan Rocha🇺🇸 · Huangyu Xiao🇺🇸

    We propose a novel cosmological scenario in which baryonic neutron stars could plausibly form in the early universe. If baryogenesis initially produces an excessively-large baryon asymmetry, $Y_B\gg 10^{-10},$ the baryonic mass inside the horizon can exceed the minimum neutron star mass before big bang nucleosynthesis (BBN). While this large asymmetry is present, non-relativistic baryons can dominate the universe and enhanced density perturbations on small scales can gravitationally collapse Hubble patches shortly after horizon re-entry. For some initial perturbations, just below the threshold for black hole formation, this collapse will be arrested only by nuclear pressure, possibly resulting in neutron star formation. Afterwards, there must be a large entropy injection to restore the observed baryon asymmetry, $Y_B\sim 10^{-10}$, and preserve the successful predictions of standard BBN. Unlike neutron stars that form from stellar collapse, primordial neutron stars can, in principle, be as light as $\sim 0.1M_{\odot }$, limited only by the nuclear equation of state.

    ↳ astro-ph.COhep-ph3 citations
30. 30*

    On quantum tunnelling in the presence of Noether charges

    Giulio Barni🇪🇸 · Thomas Steingasser🇪🇸

    We provide a complete first-principles based discussion of quantum tunnelling out of initial states carrying a conserved Noether charge. Our main result is a simple, unambiguous Euclidean-time prescription for the calculation of tunnelling rates out of such states. By relying on a combination of the direct approach and the steadyon framework for the evaluation of real-time path integrals, our derivation offers full transparency of its underlying assumptions, and is independent of any ad-hoc generalisations. This strategy also offers a simple explanation for the emergence of complex saddle points for such systems, justifying techniques postulated by earlier works. Our analysis furthermore offers the first results for initial states with both a conserved Noether charge and a non-trivial energy. We first illustrate the main conceptual points of our analysis for the simple example of a point particle in two spatial dimensions carrying a conserved angular momentum. Then, we generalise our results to the case of multiple dimensions and an arbitrary conserved Noether charge, providing an easy-to-implement prescription for the calculation of the tunnelling rate. We furthermore apply our results to the example of a complex scalar field subject to a global U(1)-symmetry with associated charge. These results provide a reliable foundation for the calculation of tunnelling rates in applications in finite-density and charge-asymmetric systems.

    ↳ hep-thhep-phPRD(2026)·4 citations
31. 31*

    Toward Neutrino and Dark Matter Detection with Ancient Minerals: TEM Study of Heavy-Ion Tracks in Olivine

    Andrew Calabrese-Day · Emilie LaVoie-Ingram🇺🇸 · Kathryn Ream · Hannah Ross · Joshua Spitz🇺🇸 · Patrick Stengel🇸🇮 · Kai Sun🇨🇳 · Alexander Takla🇺🇸

    Solar, supernova, and atmospheric neutrinos, and possibly weakly interacting massive particle (WIMP) dark matter, have been interacting in the Earth beneath our feet for billions of years. The ''paleo-detector'' technique seeks to detect and characterize the induced crystalline defects from these events, in particular from energetic nuclear recoils, which in some minerals can be preserved on these timescales. Such defects can manifest as nuclear recoil tracks, on the order of a few nanometers wide and extending up to hundreds of microns in length, which can be detected with nanoscale-resolution microscopy. In order to test the feasibility of the paleo-detector technique and to study the formation and morphology of track defects in promising mineral candidates like olivine, we use ion irradiation to artificially implant tracks to effectively mimic astrophysical particle interactions. We present a study of heavy-ion track width as a function of depth, which we relate to ion energy, in an olivine crystal irradiated with 15 MeV Au${}^{+5}$ using scanning transmission electron microscopy (STEM). Unlike previous studies, which measure tracks at the surface of the irradiated sample, we instead take measurements at various target depths via focused ion-beam sectioning of the irradiated sample. No etching techniques are used to enhance the tracks. In addition, we provide a comparison to predictions from simulations using SRIM software and previous results with a variety of ion species and energies. Notably, we find that a significant change in track continuity across the energy range studied (0.4-12.9 MeV) is indicative of the transition between electronic and nuclear stopping power dominance, consistent with the simulations' predictions. Overall, the tracks produced in olivine indicate that this mineral is an attractive candidate for paleo-detection, with robust track creation at the MeV scale.

    ↳ physics.ins-detastro-ph.IMhep-exhep-ph+15 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.