We compute tree-level $n$-point scattering amplitudes in scalar field theories in terms of geometric invariants on a fiber bundle. All 0- and 2-derivative interactions are incorporated into a metric on this bundle. The on-shell amplitudes can be efficiently pieced together from covariant Feynman rules, and we present a general closed formula for obtaining the $n$-point amplitude in this way. The covariant Feynman rules themselves can be derived using a generalization of the normal coordinate expansion of the fiber bundle metric. We demonstrate the efficiency of this approach by computing the covariant Feynman rules up to $n=10$ points, from which one can obtain the full amplitudes using our general formula. The formalism offers a prototype for obtaining geometric amplitudes in theories with higher-derivative interactions, by passing from the fiber bundle to its jet bundles.

We present the strongest experimental limits to date on the mixing angle, $\theta$, with which a new scalar particle, $S$, mixes with the Higgs field in the mass range $110\mathrm{MeV}<m_S<155\mathrm{MeV}$. This result uses the MicroBooNE liquid argon time projection chamber to search for decays of these Higgs-portal scalar particles through the $S\to e^{+}{e}^{-}$ channel with the decays of kaons in the NuMI neutrino beam acting as the source of the scalar particles. The analysis uses an exposure of $2.01\times {10}^{21}$ protons on target of NuMI beam data including periods when the beam focusing system was configured to focus positively charged hadrons and separate periods when negatively charged hadrons were focused. The analysis searches for scalar particles produced from kaons decaying in flight in the beam’s decay volume and at rest in the target and absorber. At $m_S=125\mathrm{MeV}$ ($m_S=150\mathrm{MeV}$) we set a limit of $\theta <3.19\times {10}^{-4}$ ($\theta <2.79\times {10}^{-4}$) at the 95% confidence level.

We investigate the phenomenology of the charged Higgs boson within the two-Higgs-doublet type-II seesaw model (2HDMcT type-III) at future ${\mu }^{+}{\mu }^{-}$ colliders. Focusing on $2\to 2$ processes such as ${\mu }^{+}{\mu }^{-}\to H_1^{+}{S}^{-}$ ($S^{-}=H_1^{-},H_2^{-}$) and ${\mu }^{+}{\mu }^{-}\to H_1^{+}{W}^{-}$, we incorporate both theoretical and experimental constraints to assess their production prospects. We find that $\sigma ({\mu }^{+}{\mu }^{-}\to H_1^{+}{H}_1^{-})$ can reach or exceed the corresponding $e^{+}{e}^{-}$ cross section, while $\sigma ({\mu }^{+}{\mu }^{-}\to H_1^{+}{H}_2^{-})$ is roughly 18 times smaller but can be enhanced up to 0.62 fb upon relaxing electroweak precision observables constraints. Moreover, we observe that the cross section for ${\mu }^{+}{\mu }^{-}\to H_1^{+}{W}^{-}$ is significantly enhanced due to the large $\tan \beta$ amplification characteristic of the type-III scenario. Furthermore, we conduct a signal-background analysis and determine the discovery ($5\sigma$) regions at a 3 TeV muon collider for the ${\mu }^{+}{\mu }^{-}\to H_1^{+}{H}_1^{-}$, ${\mu }^{+}{\mu }^{-}\to H_1^{+}{H}_2^{-}$, and ${\mu }^{+}{\mu }^{-}\to W^{\pm }{H}_1^{\mp }$ processes.

We present a generalization of the $x$-space candia algorithm to next-to-next-to-next-to-leading order (${\mathrm{N}}^3\mathrm{LO}$) accuracy in quantum chromodynamics (QCD) for solving the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) evolution equations for unpolarized parton densities in the nucleon. The algorithm is based on logarithmic expansions of the solution and can be extended to all orders in QCD. An expansion equivalent to the exact solution of the DGLAP equation at ${\mathrm{N}}^3\mathrm{LO}$ is presented in the non-singlet sector. Results for approximate ${\mathrm{N}}^3\mathrm{LO}$ parton distribution functions, evolved using the most recent approximations to the ${\mathrm{N}}^3\mathrm{LO}$ DGLAP splitting functions, are provided for benchmarking. The new version of the code, candia-v2, is publicly available [github, candia-v2, (2025).].

In this paper, we propose a new mechanism for charmonium scattering that utilizes the internal structure of charmonium. By capturing light flavor quarks and antiquarks from the vacuum, charm, and anticharm quarks form virtual charmed mesons, which mediate an effective one-boson exchange process. This approach accurately reproduces the di-$J/\psi$ invariant mass spectrum observed by CMS and LHCb, demonstrating its validity. Our mechanism offers a comprehensive framework for understanding charmonium scattering and is applicable to the scattering problems involving all fully heavy hadrons, an area of increasing interest.

We complete the determination of the parapositronium decay into three photons by evaluating amplitudes mediated by the $Z$ boson. We show that, contrary to the expectation that the extra mass scale $m_e$ may bring an enhancement to the overall scaling, the amplitude turns out to start at $1/m_Z^6$ order, similarly to the $W$ boson mediated amplitude. The decay rate with both $W$ and $Z$ boson contributions is found to be $1.1\times {10}^{-80}\mathrm{eV}$, about 47 orders of magnitude smaller than previously estimated. We also discuss its implication for the ${\pi }^0\to 3\gamma$ amplitude.

Using $(10087\pm 44)\times {10}^{6}J/\psi$ events collected with the BESIII detector at the BEPCII collider, we report the first observation of the decay ${\eta }_c\to {\mathrm{\Xi }}^0{\overline{\mathrm{\Xi }}}^0$. The interference between $J/\psi \to \gamma {\eta }_c\to \gamma {\mathrm{\Xi }}^0{\overline{\mathrm{\Xi }}}^0$ and $J/\psi \to \gamma {\mathrm{\Xi }}^0{\overline{\mathrm{\Xi }}}^0{|}_{\mathrm{non}-\mathrm{resonance}}$ is considered in the ${\mathrm{\Xi }}^0{\overline{\mathrm{\Xi }}}^0$ mass spectrum fits. The branching fractions are measured to be $\mathcal{B}({\eta }_c\to {\mathrm{\Xi }}^0{\overline{\mathrm{\Xi }}}^0)=(1.33\pm 0.03\pm 0.18)\times {10}^{-3}$ for the constructive interference and $\mathcal{B}({\eta }_c\to {\mathrm{\Xi }}^0{\overline{\mathrm{\Xi }}}^0)=(1.63\pm 0.04\pm 0.21)\times {10}^{-3}$ for the destructive interference, where the first uncertainties are statistical and the second are systematic.

We analyze the decay of $\eta \to {\ell }^{+}{\ell }^{-}(\ell =e,\mu )$ via $J/\psi \to \gamma {\eta }^{\prime }$ and ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\eta$, based on ($10087\pm 44$) $\times {10}^{6}J/\psi$ events collected with the BESIII detector at the BEPCII storage rings. The branching fraction of $\eta \to {\mu }^{+}{\mu }^{-}$ is measured to be $(5.8\pm {1.0}_{\mathrm{stat}}\pm {0.2}_{\mathrm{syst}})\times {10}^{-6}$, which is consistent with the previous measurements and theoretical expectations. In addition, no significant $\eta \to e^{+}{e}^{-}$ signal is observed in the $e^{+}{e}^{-}$ invariant mass spectrum, and an improved upper limit of $\mathcal{B}(\eta \to e^{+}{e}^{-})<2.2\times {10}^{-7}$ is set at 90% confidence level.

Understanding how hydrodynamics emerges rapidly in the medium produced by relativistic heavy-ion collisions remains a key theoretical challenge. While the attractor solution—manifesting as a nonthermal fixed point during the early evolution stage—offers a potential explanation, it does not fully account for how far-from-equilibrium systems quickly approach near-equilibrium states. In this Letter, we focus on systems that are boost invariant and homogeneous in the transverse plane and demonstrate that the attractor in a higher-order hydrodynamic theory converges to the same solution as a second-order theory before reaching the Navier-Stokes limit. This finding suggests that commonly used second-order hydrodynamic equations, provided they incorporate the correct transport coefficients, are adequate to approximate the system’s behavior starting from an intermediate time—even when using a higher-order theory that would be more suitable for describing the far-from-equilibrium evolution.

We consider a spacetime singularity at $t=0$ arising in a Kasner-type metric that solves the gravitational equations modified by quantum effects of a conformal field theory (CFT). The resulting constraints can be solved efficiently when expressed in terms of symmetric polynomials. Focusing first on the trace part of the modified gravitational equation, we determine the corresponding solution surfaces in the Kasner parameter space. The geometry of these surfaces depends sensitively on the ratio $\eta =A/C$, the quotient of the conformal charges characterizing the underlying CFT. We then fully integrate the conformal anomaly near the singularity for a generic Kasner-type metric and obtain the corresponding stress-energy tensor. Its components are expressed in terms of three symmetric polynomials (of degrees 2, 3, and 4) and depend on seven arbitrary constants, which may be interpreted as parametrizing different choices of the quantum state at the singularity. By imposing a set of constraints, we reduce this parameter space to a single free constant. Subsequently, we solve, at leading order near the singularity, the modified gravitational equations. Among the admissible solutions, we identify, in particular, those that develop a curvature singularity while remaining geodesically complete.

In this paper, we investigate the quantum field theory in Klein space that has two time directions. To study the canonical quantization, we select the “length of time” $q$ as the evolution direction of the system. In our novel construction, some additional modes beyond the plane wave modes are crucial in the canonical quantization and the later derivation of the Lehmann-Symanzik-Zimmermann reduction formula. We also derive the free two-point function by using Wick contraction in the canonical quantization formalism. Moreover, we introduce the path-integral formalism in which we can redefine the vacuum states and rederive the correlation functions. We show that all the results in the Klein space derived in our novel approach match those obtained via analytical continuation from the Minkowski spacetime.

Nonrelativistic quantum particles in Earth’s gravitational field are successfully described by the Schrödinger equation with Newton’s gravitational potential. Particularly, quantum mechanics is in agreement with such experiments as free fall and quantum interference induced by gravity. However, quantum mechanics is a low-energy approximation to quantum field theory. The latter is successful by the description of high-energy experiments. Gravity is embedded in quantum field theory through the general-covariance principle. This framework is known in the literature as quantum field theory in curved spacetime, where the concept of a quantum particle is, though, ambiguous. In this article, we study in this framework how a Hawking particle moves in the far-horizon region of Schwarzschild spacetime by computing its propagator. We find this propagator differs from that which follows from the path-integral formalism—the formalism which adequately describes both free fall and quantum interference induced by gravity.

We derive the gauge-general expressions of the two gyro-gravitomagnetic functions entering the spin-orbit sector of the effective-one-body (EOB) Hamiltonian up to the fifth-and-half post-Newtonian (5.5PN) order. Our results include both local and nonlocal-in-time contributions, providing the most general analytical formulation of the linear-in-spin conservative dynamics within the EOB framework. These expressions are then employed to compute two gauge-invariant observables for quasicircular orbits: the binding energy and the fractional periastron advance. We also use them to compare two spin gauge choices: the well-known Damour-Jaranowski-Schäfer (DJS) gauge, in which the gyro-gravitomagnetic functions are independent of the orbital angular momentum, and the alternative anti-DJS (or $\overline{\mathrm{DJS}}$) gauge, designed to reproduce in the test-mass limit the spin-orbit interaction of a spinning test particle in a Kerr background. For a circular, equal-mass, equal-spin binary, our analysis indicates that the $\overline{\mathrm{DJS}}$ gauge provides a slightly improved description of the inspiral dynamics, suggesting potential advantages for its use in future EOB waveform models.

We study generalizations of Buchdahl’s compactness limits for perfect-fluid star solutions of $D$-dimensional Einstein gravity coupled to higher-curvature corrections. We focus on quasitopological theories involving infinite towers of terms for which the unique vacuum spherically symmetric solutions correspond to regular black holes. We solve analytically the problem of constant-density stars and find that the space of solutions is bounded by: configurations with divergent central pressure, corresponding to the most compact stars; configurations that possess zero central pressure; and configurations for which the sizes of the stars coincide with the inner-horizon radii of the would-be regular black holes. In the more general case of perfect-fluid stars for which the mean density decreases with increasing radius, we show that, for each density profile, maximum compactness is reached when the metric becomes singular at the center. Under certain additional conditions, we find a novel Buchdahl limit for the maximum compactness of stars, attained by a specific constant-density profile. We show, in particular, that stars in these theories may be more compact than in Einstein gravity. While the vacuum solutions of these theories are such that all curvature invariants take mass-independent maximum finite values, we argue that there exist ordinary matter stars with finite central pressures for which such bounds can be violated—namely, arbitrarily high curvatures can be reached—unless additional constraints, such as the dominant energy condition, are imposed on the fluid.

We present the gravitational wave predictions from two-dimensional core collapse supernova simulations initiated from two nearly identical progenitors that have significantly different internal structures due to their late-stage stellar evolution. At the time of collapse, the $15.78M_{\odot }$ and $15.79M_{\odot }$ progenitors have compactness parameters ${\xi }_{2.5}$ of 0.136 and 0.206, respectively. We connect several features of the gravitational wave signal from each model to its previously explored explosion dynamics. In particular, the greater accretion onto the proto-neutron star of the more compact model is evident in broadband frequency features with larger amplitude gravitational wave strains and greater gravitational wave energy release when compared to the less compact model. Additionally, the faster contraction rate of the more compact model is reflected in the $\sim 26\%$ greater slope of the $g$-/$f$-mode feature evolution of the gravitational wave signal. This work shows that in principle gravitational wave detection may provide information about interior stellar structure.

The complex processes of baryonic feedback associated with galaxy evolution are still poorly understood, and their impact on the clustering of matter on small scales remains difficult to quantify. While many fitting functions and emulators exist to model the matter power spectrum, their input parameters are not directly observable. However, recent studies using hydrodynamical simulations have identified a promising correlation between the gas content of halos and changes to the matter power spectrum from feedback. Building on these findings, we create the first fully data-driven power spectrum emulator. We utilize the kinematic Sunyaev-Zel’dovich (kSZ) effect, a secondary anisotropy in the cosmic microwave background, as a tracer of free electrons in and around halos. We train a neural network to learn the mapping between the suppression of the matter power spectrum and the shape of the kSZ power spectrum extracted with a radial velocity template. We train and validate our algorithm using the FLAMINGO suite of hydrodynamical simulations, which encompasses a wide range of feedback models. Our emulator can reconstruct the matter power spectrum at the subpercent level for scales $k\le 5h/\mathrm{Mpc}$ and $0.2\le z\le 1.25$ directly from the data. Our model is robust and retains percent-level accuracy even for feedback models and cosmological parameter values not seen during training (except in a few extreme cases drastically different from the fiducial model). Due to its robustness, our algorithm offers a new way to identify the sources of suppression in the matter power spectrum, breaking the degeneracies between baryonic feedback and new physics. Finally, we present a forecast for reconstruction of the matter power spectrum combining maps of the microwave background anisotropies from a Simons Observatory-like experiment and galaxy catalogs from the Dark Energy Spectroscopic Instrument.

Supernova (SN) and baryon acoustic oscillation (BAO) distance measures have recently provided hints that the dark energy is not only dynamical but apparently evolves from normal to phantom dark energy between redshifts $0<z<1$. A normal axion dark energy component in the mass range just below the Hubble scale can mimic a phantom component by appearing as dark energy at $z=1$ and dark matter at $z=0$, raising the possibility of a phantom mirage. We show that there is a wide range of axion dark energy contributions that can resolve the SN-BAO tension as well as thawing quintessence does, leaving BAO tension with the cosmic microwave background (CMB) for the distance measures from $z\sim 1$ to recombination to be resolved at high redshifts. With axions, raising the optical depth to reionization to $\tau \approx 0.1$ works essentially as well as $w_0-w_a$ phantom dark energy for all but the lowE CMB data, with a remaining $\mathrm{\Delta }{\chi }^2\sim -16$ compared with $\mathrm{\Lambda }$ cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$), whereas a small spatial curvature of ${\mathrm{\Omega }}_K\sim 0.003$ can largely relax the full SN-BAO-CMB tension with a total $\mathrm{\Delta }{\chi }^2\sim -12$.

Dark matter (DM) can form dense condensates around black holes (BHs), such as superradiant clouds and ultracompact minihalos, which can significantly affect the orbital evolution of their companion objects through dynamical friction. In this work, we define a novel quantity to quantify such effects in emitted gravitational waves (GWs) in terms of GW amplitude, frequency, and their time derivatives. The information about the density profile can be extracted from this quantity, which characterizes the type of condensate and, therefore, the corresponding DM property. This quantity allows us to probe the dark dense environment by multiwavelength GW observation with existing ground-based and future space-based GW detectors, potentially revealing the properties of the dark sector and shedding light on the primordial origin of the stellar mass BHs. A null detection can place strong constraints on the relevant DM parameters.

Using magnetic white dwarfs as a case study, we show that the emission of sub-MeV bosons from stellar plasmas can be substantially modified in the presence of a magnetic field. In particular, the magnetic field-induced anisotropy and cyclotron resonance both significantly affect the in-medium dispersion relations of the Standard Model photon. As a result, resonant level crossing between photons and other light bosons occurs under environmental conditions that differ from the resonance criteria in unmagnetized environments. We find that the magnetic field opens additional regions within magnetic white dwarfs where resonance can occur. These findings motivate revisiting astrophysical constraints on light bosons in systems where the cyclotron frequency is comparable to or larger than the plasma frequency.