In recent years, the formation and evolution of rapidly accreting supermassive stars has received significant attention in the hope of better understanding the origin of high redshift quasars. It is often taken for granted that, once formed, these supermassive stars will encounter the general relativistic radial instability and collapse to form massive black holes. Here, we present the first ever general relativistic hydrodynamical simulations of the collapse of rapidly accreting supermassive stars. We find that black hole formation is in many cases prevented by nuclear burning due to the long timescales of the collapse of these stars (${10}^6\mathrm{s}$). Consequently, this is a novel astrophysical site for hot CNO burning and hydrogen burning via proton captures. For Pop III accreting supermassive stars, we find that only stars with very high ($100M_{\odot }/\mathrm{yr}$) or low ($0.1M_{\odot }/\mathrm{yr}$) accretion rates can form black holes, with models in between undergoing energetic thermonuclear pulsations. The final fate of these pulsating models may be to undergo subsequent pulsations or explosions or to collapse to black holes. For metal rich accreting supermassive stars ($Z\ge 0.1Z_{\odot }$), we do not find any black hole formation, with some models undergoing extremely energetic explosions (${10}^{55}\mathrm{ergs}$). Our results invite further study on the formation of massive black holes from rapidly accreting supermassive stars which have reached the general relativistic radial instability.

In six dimensions there exists a unique one-parameter family of nonlinear conformal electrodynamics for a chiral 2-form gauge field that includes (in a free-field limit) the linear chiral 2-form theory and is related, by dimensional reduction, to the four-dimensional ModMax electrodynamics. In this work, we present the first exact solutions of this theory in presence of gravity. In particular, we will consider plane wave, Robinson-Trautman, and self-dual dyonic string configurations, generalizing well-known solutions of this type in six-dimensional supergravities with linear chiral 2-form multiplets.

This work aims to review several aspects of the current status of the rare ${\pi }^0\to e^{+}{e}^{-}$ decay. A particular emphasis is made on radiative corrections and detailed interpretation of related quantities, some numbers appearing in the literature are updated, and the connection with the Dalitz decay, ${\pi }^0\to e^{+}{e}^{-}\gamma$, is discussed. This comes timely as it is aligned with an announcement of a preliminary result of a new branching-ratio measurement done by the NA62 Collaboration, which brings new light into an earlier-reported discrepancy between the Standard Model prediction and the (until-recently latest) precise KTeV result.

Photon rings are key targets for near-future space-based very long-baseline interferometry missions. The ratio of flux measured between successive light rings is characterized by the Lyapunov exponents of the corresponding nearly bound null geodesics. Therefore, understanding Lyapunov exponents in this environment is of crucial importance to understanding black hole observations in general, and in particular, they may offer a route for constraining modified theories of gravity. While recent work has made significant progress in describing these geodesics for Kerr, a theory-agnostic description is complicated by the fact that Lyapunov exponents are time-parametrization-dependent, which necessitates care when comparing these exponents in two different theories. In this work, we present a robust numerical framework for computing and comparing the Lyapunov exponents of null geodesics in Kerr with those in an arbitrary modified theory. We then present results obtained from calculating the Lyapunov exponents for null geodesics in two particular effective theories: scalar Gauss-Bonnet gravity and dynamical Chern-Simons gravity. Using this framework, we determine accuracy lower bounds required before a very long-baseline interferometry observation can constrain these theories.

We present details on a new measurement of the muon magnetic anomaly, $a_{\mu }=(g_{\mu }-2)/2$. The result is based on positive muon data taken at Fermilab’s Muon Campus during the 2019 and 2020 accelerator runs. The measurement uses $3.1\mathrm{GeV}/c$ polarized muons stored in a 7.1-m-radius storage ring with a 1.45 T uniform magnetic field. The value of $a_{\mu }$ is determined from the measured difference between the muon spin precession frequency and its cyclotron frequency. This difference is normalized to the strength of the magnetic field, measured using nuclear magnetic resonance. The ratio is then corrected for small contributions from beam motion, beam dispersion, and transient magnetic fields. We measure $a_{\mu }=116592057(25)\times {10}^{-11}$ (0.21 ppm). This is the world’s most precise measurement of this quantity and represents a factor of 2.2 improvement over our previous result based on the 2018 dataset. In combination, the two datasets yield $a_{\mu }(\mathrm{FNAL})=116592055(24)\times {10}^{-11}$ (0.20 ppm). Combining this with the measurements from Brookhaven National Laboratory for both positive and negative muons, the new world average is $a_{\mu }(\exp )=116592059(22)\times {10}^{-11}$ (0.19 ppm).

We discuss numerical aspects of instantons in two- and three-dimensional ${\varphi }^4$ theories with an internal $O(N)$ symmetry group, the so-called $N$-vector model. By combining asymptotic transseries expansions for large arguments with convergence acceleration techniques, we obtain high-precision values for certain integrals of the instanton that naturally occur in loop corrections around instanton configurations. Knowledge of these numerical properties is necessary in order to evaluate corrections to the large-order factorial growth of perturbation theory in ${\varphi }^4$ theories. The results contribute to the understanding of the mathematical structures underlying the instanton configurations.

In-ice radio detection is a promising technique to discover and characterize ultrahigh-energy (UHE) neutrinos, with energies above ${10}^{17}\mathrm{eV}$, adopted by present—ARA, ARIANNA, and RNO-G—and planned—IceCube-Gen2—experiments. So far, their ability to measure neutrino flavor had remained unexplored. We show and quantify how the neutrino flavor can be measured with in-ice radio detectors using two complementary detection channels. The first channel, sensitive to ${\nu }_e$, identifies them via their charged-current interactions, whose radio emission is elongated in time due to the Landau-Pomeranchuk-Migdal effect. The second channel, sensitive to ${\nu }_{\mu }$ and ${\nu }_{\tau }$, identifies events made up of multiple showers generated by the muons and taus they generate. We show this in state-of-the-art forecasts geared at IceCube-Gen2, for representative choices of the UHE neutrino flux. This newfound sensitivity could allow us to infer the UHE neutrino flavor composition at their sources—and thus the neutrino production mechanism—and to probe UHE neutrino physics.

Primordial black holes (PBHs) can efficiently form black hole binaries in the early Universe. We update the resulting constraints on PBH abundance using data from the third observational run of LIGO-Virgo-KAGRA. To capture a wide range of PBH scenarios, we consider a variety of mass functions, including critical collapse in the QCD epoch and primordial non-Gaussianities. Applying hierarchical Bayesian analysis to a population of binaries consisting of primordial and astrophysical black holes, we find that, in every scenario, the PBHs can make up at most $f_{\mathrm{PBH}}\lesssim {10}^{-3}$ of dark matter in the mass range $1-200M_{\odot }$. The shape and strength of the constraints are not significantly affected by the type of non-Gaussianities, the modifications to the mass function during the QCD epoch, or the modeling of the astrophysical PBH population.

Infrared and collinear (IRC) safety has long been used a proxy for robustness when developing new jet substructure observables. This guiding philosophy has been carried into the deep learning era, where IRC-safe neural networks have been used for many jet studies. For graph-based neural networks, the most straightforward way to achieve IRC safety is to weight particle inputs by their energies. However, energy-weighting by itself does not guarantee that perturbative calculations of machine-learned observables will enjoy small nonperturbative corrections. In this paper, we demonstrate the sensitivity of IRC-safe networks to nonperturbative effects, by training an energy flow network (EFN) to maximize its sensitivity to hadronization. We then show how to construct Lipschitz energy flow networks ($L$-EFNs), which are both IRC safe and relatively insensitive to nonperturbative corrections. We demonstrate the performance of $L$-EFNs on generated samples of quark and gluon jets, and showcase fascinating differences between the learned latent representations of EFNs and $L$-EFNs.

This paper presents the first numerical study of black hole thermodynamics in causal set theory, focusing on the entropy of a Schwarzschild black hole as embodied in the distribution of proposed horizon molecules. To simulate causal sets we created a highly parallelized computational framework in c++ which allowed for the generation of causal sets with over a million points, the largest causal sets in a nonconformally flat spacetime to date. Our results confirm that the horizon molecules model is consistent with the Bekenstein-Hawking formula up to a dimensionless constant that can be interpreted as the fundamental discreteness scale in the order of a Planck length. Furthermore, the molecules are found to straddle the horizon of the black hole to within a few Planck lengths, indicating that entropy lives on the surface of the black hole. Finally, possible implications for the information paradox are drawn. In particular, we show how the horizon molecules model could yield a finite black hole temperature cutoff or even prevent full black hole evaporation.

We explore the utility of $d=8$ qudits, qu8its, for quantum simulations of the dynamics of $1+1\mathrm{D}$ SU(3) lattice quantum chromodynamics, including a mapping for arbitrary number of flavors and lattice size and a reorganization of the Hamiltonian for efficient time evolution. Recent advances in parallel gate applications, along with the shorter application times of single-qudit operations compared with two-qudit operations, lead to significant projected advantages in quantum simulation fidelities and circuit depths using qu8its rather than qubits. The number of two-qudit entangling gates required for time evolution using qu8its is found to be more than a factor of 5 fewer than for qubits. We anticipate that the developments presented in this work will enable improved quantum simulations to be performed using emerging quantum hardware.

The matter power spectrum is only weakly constrained on subgalactic scales, while physics beyond the Standard Model can leave unique imprints, especially on sub-parsec scales. We propose measuring the arrival-time difference of fast radio bursts (FRBs) along two adjacent sightlines as a new probe to dark matter substructures on scales down to $\sim 1\mathrm{AU}$. We discuss two observational scenarios in which it may be possible to place interesting constraints on such models through the monitoring of repeating FRB sources: (i) By sending radio receivers to space to form a baseline of tens of AU or more and measuring the temporal variation of the arrival-time difference between receivers. (ii) By measuring the temporal variation of the arrival-time difference between two lensed images of one strongly lensed repeater. In both scenarios, obtaining interesting constraints requires correlating the voltage time series to measure the radio signal arrival time to sub-nanosecond precision. We find that two radio dishes separated by 20 AU may be sensitive to the enhancement of small-scale structures at $\sim {10}^{-8}{M}_{\odot }$ masses in the QCD axion dark matter scenario, or from an early epoch of matter domination with a reheating temperature up to 60 MeV. Other dark matter models, such as those composed of $\sim {10}^{-13}{M}_{\odot }$ primordial black holes produced during inflation, would also be probed by this method. We further show that a strong lensing situation of multiple images provides an equivalent $\sim 2000\mathrm{AU}({\sigma }_v/{10}^3\mathrm{km}{\mathrm{s}}^{-1})(\delta t/10\mathrm{yr})$ baseline, for a typical velocity of dark matter substructures ${\sigma }_v$ and an observational time span $\delta$. This is much more sensitive, but with the uncertainty that intervening decoherence from the interstellar medium may degrade the timing precision, and that spatial variation in the FRB emission spot may result in confounding signals. We show that the lensing magnifications of Type Ia supernovae constrain a similar quantity to such FRB timing, with present limits being equivalent to ruling out the same parameter space that would be probed by a 0.14 AU baseline.

The explosion of core-collapse supernovae (CCSNe) is an extremely challenging problem, and there are still large uncertainties regarding which stars lead to successful explosions that leave behind a neutron star, and which ones will form a black hole instead. In this paper, we simulate 341 progenitors at three different metallicities using spherically symmetric simulations that include neutrino-driven convection via a mixing-length theory. We use these simulations to improve previously derived explosion criteria based on the density and entropy profiles of the presupernova progenitor. We also provide numerical fits to calculate the final mass of neutron stars based on either compactness, the location of the $\mathrm{Si}/\mathrm{Si}\text{−}\mathrm{O}$ interface, or the Chandrasekhar mass. The neutron star birth mass distribution derived from our $1\mathrm{D}+$ simulations is bimodal, contrary to what the most popular 1D CCSN simulations have shown so far. We compare the theoretically derived neutron star mass distributions with the observed ones and discuss potential implications for population synthesis studies. We also analyze the black hole mass distribution predicted by our simulations. To be consistent with current models of matter ejection in failed SNe, a large fraction of the envelope must be expelled, leading to small black holes in the low-mass gap. One black hole in this mass region has recently been observed in the GW230529 event by the LIGO-Virgo-KAGRA collaboration. Our results naturally agree with this detection, which the most popular prescriptions for explodability and remnant masses are not able to reproduce. In general, we find that the explosion outcome and mass of the remnant strongly depend on the precollapse structure of the progenitor. However, their dependence on the initial mass of the star and the mass of the CO core is highly uncertain and nonlinear.

We consider a setup to detect stimulated photon-photon scattering using high-power lasers. Signal photons are emitted from an overlap of the incoming intense laser pulses focused in vacuum from three sides. We derive and justify a general approximate analytical formula for the angular distribution and total yield of such signal photons in terms of the parameters of the incoming pulses, including their intensity, carrier frequencies, durations, focusing, polarizations, mutual orientation and overlap. Using the obtained formula a parametric study of the signal is carried out and optimization is performed.

Neutron stars cooling passively since their birth may be reheated in their late-stage evolution by a number of possible phenomena; rotochemical, vortex creep, crust cracking, magnetic field decay, or more exotic processes such as removal of neutrons from their Fermi seas (the nucleon Auger effect), baryon number-violating nucleon decay, and accretion of particle dark matter. Using Exposure Time Calculator tools, we show that reheating mechanisms imparting effective temperatures of 2000–40000 Kelvin may be uncovered with excellent sensitivities at the James Webb Space Telescope (JWST), the Extremely Large Telescope (ELT), and the Thirty Meter Telescope (TMT), with imaging instruments operating from visible-edge to near-infrared. With a day of exposure, they could constrain the reheating luminosity of a neutron star up to a distance of 500 pc, within which about ${10}^5$ (undiscovered) neutron stars lie. Detection in multiple filters could overconstrain a neutron star’s surface temperature, distance from Earth, mass, and radius. Using publicly available catalogs of newly discovered pulsars at the FAST and CHIME radio telescopes and the Galactic electron distribution models YMW16 and NE2001, we estimate the pulsars’ dispersion measure distance from Earth, and find that potentially 30–40 of these may be inspected for late-stage reheating within viable exposure times, in addition to a few hundred candidates already present in the ATNF catalog. Whereas the coldest neutron star observed (PSR $\mathrm{J}2144-3933$) has an upper limit on its effective temperature of about 33000 Kelvin set with the Hubble Space Telescope, we show that the effective temperature may be constrained down to 20000 Kelvin with JWST-NIRCam, 15000 Kelvin with ELT-MICADO, and 9000 Kelvin with TMT-IRIS. Campaigns to measure thermal luminosities of old neutron stars would be transformative for astrophysics and fundamental physics.

We investigate exact and approximate techniques to calculate the emission of gravitational radiation from cosmic string loops in order to generate beam models covering the entire celestial sphere for a wide range of modes $m$. One approach entails summing over contributions of stationary and nearly stationary points of individual, factorized, left- and right-moving modes. This “multipoint method” generalizes traditional methods that rely on expansions around exact stationary points of mode products. A second complementary approach extends the method of steepest descent to generate an asymptotic description of the beam as $m\to \infty$. We present example calculations of the emission of power from cusp-containing loops and compare the results with those obtained by numerically exact techniques as well as by previous approaches. The multipoint method achieves its best results at an intermediate range of modes, improving over previous methods in terms of accuracy. It handles the emission from all regions of the loop not just those near cusps. We demonstrate this capability by making a detailed study of the “pseudocusp” phenomenon.

Intensity interferometry is a technique developed many decades ago, that has recently enjoyed a renaissance thanks in part to advances in photodetector technology. We investigate the potential for long-baseline optical intensity interferometry to observe bright, active galactic nuclei (AGN) associated with rapidly accreting supermassive black holes. We argue that realistic telescope arrays similar in area to existing Cherenkov arrays, if equipped with modern high-precision single photon detectors, can achieve a sufficiently high signal to noise ratio not only to detect distant AGN, but also to study them in great detail. We explore the science potential of such observations by considering two examples. First, we find that intensity interferometric observations of bright nearby AGN can allow detailed studies of the central accretion disks powering the AGN, allowing reconstruction of many disk properties like the radial profile. Next, we argue that intensity interferometers can spatially resolve the broad-line regions of AGN at cosmological distances, and thereby provide a geometric determination of the angular diameter distances to those AGN when combined with reverberation mapping. Since this measurement can be performed for AGN at distances of hundreds of megaparsecs, this directly measures the Hubble expansion rate $H_0$, with a precision adequate to resolve the recent Hubble tension. Finally, we speculate on future applications that would be enabled by even larger intensity interferometer arrays.

It is commonly believed that the dissipative properties of superdense matter play a negligible role in modeling gravitational waveforms from neutron star inspirals. This study aims to investigate whether this presumption holds true for the often neglected dissipative process associated with particle diffusion in superconducting neutron stars. As we demonstrate, diffusion effects can significantly impact the phase of the gravitational wave from the inspiral, manifesting at a magnitude of a few tens of milliradians at large orbit separations, equivalent to orbital frequencies of a few hertz. We also find that dissipation resulting from particle diffusion might increase the neutron star’s temperature to approximately ${10}^7\mathrm{K}$ during the inspiral.

We perform a nonperturbative calculation of the shear viscosity for ($2+1$)-dimensional SU(2) gauge theory by using the lattice Hamiltonian formulation. The retarded Green’s function of the stress-energy tensor is calculated from real time evolution via exact diagonalization of the lattice Hamiltonian with a local Hilbert space truncation, and the shear viscosity is obtained via the Kubo formula. When taking the continuum limit, we account for the renormalization group flow of the coupling but no additional operator renormalization. We find the ratio of the shear viscosity and the entropy density $\frac{\eta }{s}$ is consistent with a well-known holographic result $\frac{1}{4\pi }$ at several temperatures on a $4\times 4$ honeycomb lattice with the local electric representation truncated at $j_{\max }=\frac{1}{2}$. We also find the ratio of the spectral function and frequency $\frac{{\rho }^{xy}(\omega )}{\omega }$ exhibits a peak structure when the frequency is small. Both the exact diagonalization method and simple matrix product state classical simulation method beyond $j_{\max }=\frac{1}{2}$ on bigger lattices require exponentially growing resources. So we develop a quantum computing method to calculate the retarded Green’s function and analyze various systematics of the calculation including $j_{\max }$ truncation and finite size effects, Trotter errors and the thermal state preparation efficiency. Our thermal state preparation method still requires resources that grow exponentially with the lattice size, but with a very small prefactor at high temperature. We test our quantum circuit on both the Quantinuum emulator and the IBM simulator for a small lattice and obtain results consistent with the classical computing ones.

In order to explore the spectrum of hidden-charm scalar and tensor resonances, we study meson-meson scattering with $J^{PC}=0^{++},2^{++}$ in the charmonium energy region using lattice QCD. Employing a light-quark mass corresponding to $m_{\pi }\approx 391\mathrm{MeV}$, we determine coupled-channel scattering amplitudes up to around 4100 MeV considering all kinematically relevant channels consisting of a pair of open-charm mesons or a charmonium meson with a light meson. A single isolated scalar resonance near 4000 MeV is found with large couplings to $D\overline{D}$, $D_s{\overline{D}}_s$ and the kinematically closed $D^{*}{\overline{D}}^{*}$ channel. A single tensor resonance at a similar mass couples strongly to $D\overline{D}$, $D{\overline{D}}^{*}$ and $D^{*}{\overline{D}}^{*}$. We compare the extracted resonances to contemporary experimental candidate states, previous lattice results and theoretical modeling. In contrast to several other studies, we do not find any significant feature in the scalar amplitudes between the ground state ${\chi }_{c0}(1P)$ and the resonance found around 4000 MeV.