We investigate ultrarelativistic freeze-out (UFO) in the context of generic reheating scenarios. While the standard WIMP dark matter paradigm has been extensively studied, UFO has so far only been analyzed within the specific reheating channel $\varphi \to f\overline{f}$. Unlike in the standard WIMP mechanism, where dark matter can only be diluted after freeze-out at $T_{\mathrm{FO}}\sim m_{\chi }/\mathcal{O}(10)$, UFO dark matter can undergo freeze-in like phases following the initial freeze-out, driven by the nontrivial temperature evolution. The exact temperature evolution then needs to be accounted for, as a change in the temperature scaling can modify the IR/UV nature of UFO, impacting the relic abundance. We first generalize UFO to an arbitrary temperature profile $T\sim a^{-\xi }$, making explicit the UV and IR regimes for a thermally averaged cross section $⟨\sigma v⟩\sim T^n/{\mathrm{\Lambda }}^{n+2}$. Then, as a concrete example, we consider the minimal scenario in which gravitational particle production at the onset of reheating sources an initial radiation abundance, and show that this early hot bath changes the UFO parameter space. We refer to this effect as GUFO. Specializing to $n=2$, we find that matter-like reheating ($V\sim {\varphi }^2$) accommodates dark matter masses up to ${10}^7\mathrm{GeV}$ for $\mathrm{\Lambda }\lesssim {10}^9\mathrm{GeV}$ as thermalization becomes less stringent, while radiation-like reheating ($V\sim {\varphi }^4$) is compatible with GUFO across all reheating channels only if gravitational processes are taken into account.

Assuming that a fraction of the dark matter is an ultralight pseudoscalar particle which couples to electromagnetism like an axion (an axionlike particle, ALP), we demonstrate that the coupling of the cosmological magnetic field produced by the ALP field oscillations to the primordial dark matter fluctuations yields a spectrum of gauge field fluctuations that can produce a sufficient flux of Lyman-Werner photons to enable the direct collapse black hole formation scenario. The induced flux is consistent with the bounds on the excess flux of radio photons from ARCADE2 and EDGES measurements.

Recent anomalous compact-star observations challenge the conventional neutron-star interpretation in complementary ways: HESS J1731–347 and XTE J1814–338 favor unusually small radii at low mass, while the secondary component of GW190814 appears too massive for an ordinary neutron star under GW170817-based maximum-mass inferences. We examine whether neutron stars with a strong first-order hadron-quark phase transition can address these tensions via the extended stable hybrid branches that arise when the phase conversion is slow compared to radial oscillations, while remaining consistent with GW170817 and NICER constraints. Using a piecewise-polytropic (PP) benchmark, supplemented by an independent speed-of-sound (CS) parametrization comparison, we find two viable scenarios after a general parameter scan over both the hadronic and hybrid branches. Scenario 1 realizes an all-at-once solution: the same equation of state has a pure hadronic branch compatible with both the GW190814-scale mass and HESS J1731–347, while its slow stable hybrid branch reaches XTE J1814–338. Scenario 2 retains the GW190814-scale hadronic branch but reaches XTE J1814–338 and HESS J1731–347 on slow stable hybrid branches in very different transition-strength regimes.

Gravitational lensing is an invaluable probe of the nature of dark matter, and the structures it forms. Lensed gravitational waves in particular allow for unparalleled sensitivity to small-scale structures within the lenses, due to the precise time resolution in combination with the continuous monitoring of the entire sky. In this work, we show two distinct ways of using strongly lensed gravitational waves to identify the presence of dark matter subhalos; (i) through higher order caustics generating high relative magnification (${\mu }_r>2$), short time-delay image pairs that break the caustic universality relations of single dark matter halos, which occur for $\sim 1–10$ percent of strongly lensed events in our cold dark matter models, and (ii) through the presence of more than three highly magnified images, which occur for $\sim 0.01–1$ percent of the same simulated events. We find that these results are highly sensitive to the concentrations of subhalos in our simulations, and more mildly to their number densities. The presence of low-mass subhalos increases the probability of observing wave-optics lensing in lensed gravitational waves, which is studied by solving the diffraction integral with the stationary phase approximation, as well as numerically. We also report distinct quantitative and qualitative differences in the distributions of relative magnifications and time delays for subhalo populations with increased number densities or concentrations. With the upcoming detection of strongly lensed events by ground- and space-based detectors, comparisons against these simulated distributions will provide insight into the nature of dark matter.

We investigate charged lepton-flavor violation (LFV) induced by dimension-six four-lepton operators within the Standard Model effective field theory (SMEFT) at a proposed high-energy muon collider. We study the processes ${\mu }^{+}{\mu }^{-}\to e^{\pm }{\tau }^{\mp }$, ${\mu }^{+}{\mu }^{-}\to e^{\pm }{\mu }^{\mp }$, and ${\mu }^{+}{\mu }^{-}\to {\mu }^{\pm }{\tau }^{\mp }$ at $\sqrt{s}=3$, 10, and 14 TeV, incorporating beam polarization and hadronic $\tau$ reconstruction. Using an optimal-observable analysis of the angular distributions, we perform a global fit to the relevant set of four-lepton operators. Projected sensitivities reach $C/{\mathrm{\Lambda }}^2\sim (0.5–1.2)\times {10}^{-4}{\mathrm{TeV}}^{-2}$, depending on the flavor and chiral structure of the operator, exceeding current limits by up to an order of magnitude. A combined analysis of multiple center-of-mass energies and beam polarizations significantly improves the resolution of correlations among the Wilson coefficients. These results highlight the strong sensitivity of a future multi-TeV muon collider to charged LFV four-lepton interactions, establishing it as a powerful probe of the SMEFT parameter space.

We study the effects of dimension-four and dimension-six nonminimal Higgs couplings to the Ricci scalar $R$ in the $R^2$-Higgs inflation model in light of the recent ACT and SPT observations. We show that the dimension-six operators $|\mathrm{\Phi }{|}^2{R}^2$ and $|\mathrm{\Phi }{|}^{4}R$ can accommodate the enhanced scalar spectral index $n_s$ preferred by the combined $\mathrm{CMB}+\mathrm{BAO}$ analyses. Using a doubly covariant formalism, we find that the same region of parameter space that explains the observed value of $n_s$ can also induce rapid preheating through the production of the Goldstone modes. If thermalization proceeds efficiently through this preheating mechanism, it may help match the inflationary scale with the CMB reference scale.

Recent results from the DAMIC-M and PandaX collaborations have excluded the standard freeze-in production of dark matter for masses in the range $3\mathrm{MeV}\lesssim m_{\chi }\lesssim 1\mathrm{GeV}$ in the context of extensions of the Standard Model featuring an additional ultralight $U(1{)}_{\mathrm{X}}$ gauge boson. In this work, we analyze the constraints induced by DAMIC-M and PandaX results on the reheating temperature in freeze-in models at stronger coupling, or when a nonthermal source (such as inflaton decay) comes into play. We identify viable scenarios in which the DM relic abundance is correctly reproduced while evading current experimental bounds on the electron-scattering cross section, ${\overline{\sigma }}_{\mathrm{e}}$. In particular, we show that for reheating temperatures below the electroweak scale, Boltzmann-suppressed production can be compensated by stronger couplings, bringing freeze-in scenarios within present experimental reach. Finally, we study a hybrid scenario in which a small branching ratio of inflaton decay seeds a nonzero initial dark-matter abundance. We show that such contributions can significantly modify freeze-in predictions across broad regions of parameter space, offering an additional pathway for probing extremely feeble interactions.

The prospects for observing small departures from Lorentz invariance are studied using effective interactions of the Higgs boson with photons. A single operator of mass-dimension five suffices to describe all leading signatures of Lorentz violation in the diphoton decay channel. Comparison of the theoretical modified decay rates with observed signal strengths measured by the ATLAS and CMS Collaborations at the Large Hadron Collider indicates an estimated attainable sensitivity to the corresponding coefficients for Lorentz violation of order ${10}^{-4}{\mathrm{GeV}}^{-1}$.

We present an extensive analysis of the in-medium masses and decay constants of the $B_s^{*}(5415)$ and $B^{*}(5325)$ multiplets, including both particles and antiparticles, using QCD sum rules at finite temperature and density. The operator product expansion incorporates the full temperature- and density-dependent contributions from the quark, gluon, and mixed condensates. Computing the strange ($B_s^{*0}$, ${\overline{B}}_s^{*0}$), charged ($B^{*\pm }$), and neutral ($B^{*0}$, ${\overline{B}}^{*0}$) doublet properties allows us to study the effects of flavor symmetry breaking, strangeness, and heavy-quark decoupling on the beauty vector mesons in the medium. Our results indicate that the mass is remarkably resistant to the medium across the entire multiplet: no state loses more than $\sim 13\%$ of its vacuum value, even at $T=T_c$ and $n=5n_0$, the extreme conditions explored here. The decay constant is far more sensitive, losing up to $\sim 78\%$ at the same point. Baryon density clearly dominates the medium response, while temperature plays a secondary role until the system approaches the deconfinement crossover. At zero density, every state loses almost the same fraction of its mass and decay constant: mass shifts lie between $-(0.5–1.1)\%$ and decay-constant shifts between $-(3.9–5.3)\%$, regardless of charge or flavor, so temperature alone does not distinguish a particle from its antiparticle. At finite baryon density, a clear particle-antiparticle asymmetry emerges: at $T=0$ and $n=5n_0$, the ${\overline{B}}^{*0}$ mass decreases by 12.9%, whereas the $B^{*0}$ mass shifts by only 6.1%, a gap of nearly seven percentage points driven entirely by the vector self-energy. This provides a theoretical basis for the future heavy-ion collision program at RHIC, LHC, FAIR, and NICA.

In this work, we extend our previous diabatic framework for the charmoniumlike spectrum below 4.3 GeV by introducing the complex scaling method. Unlike the previous framework, the present approach explicitly incorporates couplings to the meson-meson continuum, comprehensively accounting for its contributions to the physical states. Consequently, it allows bound and resonant states to be treated on an equal footing, enabling the direct extraction of decay widths from complex energy eigenvalues without introducing any additional free parameters. Using the obtained solutions, we calculate the channel weights and complex root-mean-square radii ($r_{\mathrm{rms}}$) to elucidate their internal structures. Specifically, we find that the ${\chi }_{c1}(3872)$, $\psi (4040)$, and $\psi (4230)$ states exhibit significant molecular characteristics. Furthermore, we propose the $X(3940)$ as a candidate for a $J^{PC}=1^{++}$ state and discuss the nature of the ${\chi }_{c0}(2P)$ resonance in detail.

We perform a theoretical study of the $B^{+}\to D^{+}{D}^{-}{K}^{+}$, $B^0\to D^{+}{D}^{-}{K}^0$ and ${\mathrm{\Lambda }}_b\to D^{+}{D}^{-}\mathrm{\Lambda }$ reactions by looking at the production mechanisms, with special emphasis in the final state interaction of the charmed mesons, which gives rise to the $X(3930)$, coupling strongly to $D_s{\overline{D}}_s$, and another state that we call $X(3700)$, coupling strongly to $D\overline{D}$ that qualifies as a $D\overline{D}$ bound state. The combined study of all these reactions shows that the final state interaction responsible for the production of these resonances is more important in the $B^{+}\to D^{+}{D}^{-}{K}^{+}$ reaction than in the ${\mathrm{\Lambda }}_b\to D^{+}{D}^{-}\mathrm{\Lambda }$ one. We have taken this into account and shown that normalizing the $D^{+}{D}^{-}$ mass distributions to the same value at the peak of the $\psi (3770)$ production, the two mass distributions are quite different in a range of 10 MeV above the $D^{+}{D}^{-}$ threshold, with a value about an order of magnitude bigger for the $B^{+}\to D^{+}{D}^{-}{K}^{+}$ reaction than for the ${\mathrm{\Lambda }}_b\to D^{+}{D}^{-}\mathrm{\Lambda }$ one. The $D^{+}{D}^{-}$ mass distribution by itself also shows an enhancement of about one order of magnitude in the presence of the $X(3700)$ with respect to a calculation where this resonance is absent. We make a call to measure these magnitudes in the coming LHCb upgrades, which would bring great support to the existence of the bound $D^{+}{D}^{-}$ state.

Hadronic transport approaches used in heavy-ion collision simulations rely on a consistent and accurate hadron list with decay channels. Hadron lists in common use are often experimentally outdated, or, as with the Particle Data Group (PDG) compilations, incompatible with transport codes without further adaptation. We construct $\mathrm{PDG}2021+$, an updated hadron list including all states from the 2021 Particle Data Booklet, together with a binary-decay list designed for direct use in the SMASH transport framework. Using the hadron resonance gas model, we validate the $\mathrm{PDG}2021+$ list against lattice quantum chromodynamics results and experimental yield data. We show that employing $1\to 2$-body decay chains as a proxy for the full decay processes has a suppressing effect in the low-$p_T$ region of the pion spectrum and introduces a $\sim 3\%$ systematic uncertainty in the pion $⟨p_T⟩$. Moreover, the inclusion of additional states in $\mathrm{PDG}2021+$ further shifts the pion $⟨p_T⟩$. These result establish $\mathrm{PDG}2021+$ as a robust, transport-ready hadron list and quantify the systematic effects of decay modeling on key heavy-ion observables.

We introduce a novel method to compute structure constants from Q functions in the scalar sector of planar $\mathcal{N}=4$ super Yang-Mills (SYM) and related theories. The method derives from operatorial as well as functional separation of variables, and the structure constants are expressed as determinants of matrices whose entries are integrals over products of Q functions. In this framework, each operator is twisted by an external angle, mirroring the cusped Maldacena-Wilson loop. The structure constants of local single-trace operators in $\mathcal{N}=4$ SYM are recovered in the untwisting limit, where we obtain a one-to-one correspondence between our key building blocks and those of the hexagon formalism. Retaining appropriate twists, our structure constants also perfectly match those of the orbifold points of $\mathcal{N}=4$ SYM. Our results thus far are valid at leading order in the weak-coupling expansion, but their formulation in terms of Q functions provides a natural starting point for including loop corrections. Many of the methods we develop in this work apply more generally to the computation of correlation functions in integrable models.

We show that operator growth in large-central-charge conformal field theories with ${\mathcal{W}}_3$ symmetry can violate the universal operator growth hypothesis once the Liouvillian is enlarged to probe the higher-spin generators. For the generalized Liouvillian $\mathcal{L}={\kappa }_1(L_1+L_{-1})+{\kappa }_2(W_2+W_{-2})$, we compute the Lanczos coefficients in the descendant module of a heavy primary and find several classes with faster-than-linear growth in the descendant level $N$, including maximally violating sectors with asymptotic behavior $b_N\sim N^2$. This superlinear growth exceeds the conjectured bound and renders the Krylov complexity divergent. We further show that the same quadratic asymptotic growth already arises in the global $SL(3,\mathbb{R})$ subalgebra, indicating that the violation is rooted in the extended higher-rank symmetry itself. Our results demonstrate that extended $\mathcal{W}$ symmetries can qualitatively modify operator growth and evade conventional bounds on information scrambling.

This is the companion paper to Pan and Yan [arXiv:2410.15695 (2024)], containing all the details and series of examples on a 4D mirror symmetry for the class-$\mathcal{S}$ theories which relates the representation theory of the chiral quantization of the Higgs branch and the geometry of the Coulomb branch. We study the representation theory by using the 4D/vertex operator algebra correspondence, (defect) Schur indices, and (flavor) modular differential equations, and match the data with the fixed manifolds of the Hitchin moduli spaces. This correspondence extends the connection between the Higgs and Coulomb branch of Argyres-Douglas theories, and can provide systematic guidance for the study of the representation theory of vertex operator algebras by exploiting results from Hitchin systems.

In this paper, we investigate the interaction between stationary pointlike external sources within the framework of pseudoquantum electrodynamics (PQED). Besides ordinary electric charges, we consider configurations involving Dirac points and generalized topological sources. For the configurations analyzed, we compute the corresponding interaction energies, forces, torques, and electromagnetic field configurations. Furthermore, we compare the results obtained in PQED with those arising in ordinary planar Maxwell electrodynamics, showing that the nonlocal character of PQED gives rise to physical effects with no counterpart in the conventional local planar theory. In particular, our results show that topological sources induce intrinsically anisotropic and noncentral interactions, leading to the emergence of nontrivial torques between the interacting objects. The directional character of these interactions suggests possible applications in the effective description of topological defects and anisotropic phenomena in planar condensed matter systems.

The question whether energy is conserved in an expanding universe remains one of the central conceptual issues of relativistic cosmology. The absence of a global timelike Killing vector and the lack of a local gravitational energy density often lead to the conclusion that cosmological redshift represents a genuine violation of energy conservation. In this work we reconsider this problem within the framework of Brown-York quasilocal energy and its associated balance law. For comoving spherical boundaries in Friedmann–Lemaître–Robertson–Walker spacetimes we show that the ordinary stress-energy flux through the boundary vanishes identically for arbitrary perfect fluids, while the unreferenced Brown-York energy evolves entirely due to a purely geometrical boundary contribution. We further demonstrate that the timelike boundary world tube possesses a natural dynamical reference geometry. The corresponding reference subtraction removes the universal background contribution of the unreferenced Brown-York energy and leads to a quasilocal energy whose leading nontrivial term exactly reproduces the enclosed matter energy. For radiation dominated cosmologies, the resulting expansion is $E_{\mathrm{BY}}=H^2{R}^3/2-H^4{R}^5/8+\mathcal{O}(R^7)$, where the leading term coincides precisely with the energy of the enclosed radiation responsible for cosmological redshift. Higher-order contributions naturally possess the structure expected for gravitational self-energy corrections. These results suggest that cosmological redshift can be understood within a fully quasilocal conservation framework. The apparent loss of photon energy therefore need not indicate a breakdown of energy conservation but may instead reflect the fundamentally quasilocal character of gravitational energy in general relativity.

Tidal dissipation in spinning compact binaries imprints characteristic corrections on the late-inspiral gravitational-wave signal. We develop a next-to-leading order post-Newtonian description of dissipative, electric-quadrupolar tides in spinning compact binaries, deriving the center-of-mass equations of motion, a generalized energy-balance law, and the corresponding Fourier-phase correction for quasicircular orbits with spins aligned or antialigned with the orbital angular momentum. Using the most general, low-frequency, linear tidal response compatible with rotational symmetry, we show that spin-induced tidal dissipation enters the gravitational-wave phase at 2.5 post-Newtonian order and carries a logarithmic frequency dependence, so it is not degenerate with the coalescence phase. For binary black holes, our dissipative flux reproduces horizon absorption in the extreme-mass-ratio limit. These results provide new waveform ingredients for precision modeling of spinning compact binaries in the high-signal-to-noise era.

Modified gravity is often formulated as an effective field theory (EFT), where higher-order corrections parametrize departures from general relativity (GR). We argue that such corrections should be constrained by the global causal structure of curved spacetime, in addition to the usual flat-space requirements such as positivity and unitarity. We propose that within the domain of validity of the EFT, the onset of closed timelike curves should not happen in a parametrically more accessible region than in the corresponding GR background. We test this diagnostic in the quadratic k-essence sector of scalar-tensor gravity. For stationary and axisymmetric spacetimes, the invariant test for closed axial orbits is the sign of the azimuthal component of the metric $g_{\phi \phi }$. We supplement this test by requiring a local time function in the space of Killing vectors. We apply these conditions to quadratic k-essence on Kerr-(A)dS backgrounds, with and without scalar charge. The zero-charge branch is exact Kerr-(A)dS, and we treat the charged branch perturbatively in scalar charge and in Hartle-Thorne slow rotation. Expanding for small spin $\chi =a/(GM)\ll 1$, frame dragging begins at $\mathcal{O}(\chi )$, while the quadrupolar backreaction relevant for circular closed timelike curves enters at second order in both rotation and charge. We find that, in the truncation used here, any occurrence of $g_{\phi \phi }<0$ also lies outside EFT control. A higher-order calculation or a fully nonlinear treatment is therefore needed. Finally, we discuss how quasinormal modes and black-hole echoes could probe such causal structure.