Semileptonic $B_s\to K\ell \nu$ decays provide an alternative $b$-decay channel to determine the Cabibbo-Kobayashi-Maskawa (CKM) matrix element $|V_{ub}|$ and to obtain a $R$-ratio to investigate lepton-flavor-universality violations. Results for the CKM matrix element may also shed light on the discrepancies seen between analyses of inclusive or exclusive decays. We calculate the decay form factors using lattice QCD with domain-wall light quarks and a relativistic $b$-quark. We analyze data at three lattice spacings with unitary pion masses down to 268 MeV. Our numerical results are interpolated/extrapolated to physical quark masses and to the continuum to obtain the vector and scalar form factors $f_{+}(q^2)$ and $f_0(q^2)$ with full error budgets at $q^2$ values spanning the range accessible in our simulations. We provide a possible explanation of tensions found between results for the form factor from different lattice collaborations. Model- and truncation-independent $z$-parametrization fits following a recently proposed Bayesian-inference approach extend our results to the entire allowed kinematic range. Our results can be combined with experimental measurements of $B_s\to D_s$ and $B_s\to K$ semileptonic decays to determine $|V_{ub}|=3.8(6)\times {10}^{-3}$. The error is currently dominated by experiment. We compute differential branching fractions and two types of $R$ ratios, the one commonly used as well as a variant better suited to test lepton-flavor universality.

We demonstrate the quantum mean estimation algorithm on Euclidean lattice field theories. This shows a quadratic advantage over Monte Carlo methods which persists even in presence of a sign problem, and is insensitive to critical slowing down. The algorithm is used to compute $\pi$ with and without a sign problem, a toy U(1) gauge theory model, and the Ising model. The effect of $R_Z$-gate synthesis errors on a future fault-tolerant quantum computer is investigated.

We study self-gravitating multipion systems (pion stars) in a state of Bose condensate. To ensure stability of such stars, it is assumed that they are immersed in the lepton background. Two different phenomenological equations of state (EoS) for the pion matter are used, some of them having the first-order phase transition. The model parameters are chosen to reproduce the recent lattice QCD data at zero temperature, but nonzero isospin chemical potential. It is shown that the mass-radius diagrams of pion stars obtained with phenomenological EoS are close to ones calculated in the ideal gas model. We analyze properties of neutrino clouds which are necessary for stabilizing the pion stars.

The forward scattering amplitude of a small dipole at high energies is given in the mean field approximation by the Balitsky-Kovchegov (BK) evolution equation. It requires an initial condition $N(r;x_0)$ describing the scattering of a dipole with size $r$ off the target that is probed at momentum fraction $x_0$. Rather than using ad hoc parametrizations tuned to high-energy data at $x\ll x_0$, here we attempt to construct an initial scattering amplitude that is consistent with low-energy, large-$x$ properties of the proton. We start from a nonperturbative three quark light-cone model wave function from the literature. We add $\mathcal{O}(g)$ corrections due to the emission of a gluon, and $\mathcal{O}(g^2)$ virtual corrections due to the exchange of a gluon, computed in light-cone perturbation theory with exact kinematics. We provide numerical data as well as analytic parametrizations of the resulting $N(r;x_0)$ for $x_0=0.01–0.05$. Solving the BK equation in the leading logarithmic approximation towards lower $x$, we obtain a fair description of the charm cross section in deeply inelastic scattering measured at HERA by fitting one parameter, the coupling constant ${\alpha }_s\simeq 0.2$. However, without the option to tune the initial amplitude at $x_0$, the fit of the high precision data results in ${\chi }^2/N_{\mathrm{dof}}=2.3$ at $N_{\mathrm{dof}}=38$, providing clear statistical evidence for the need of systematic improvement, e.g., of the photon wave function, evolution equation, and initial condition.

We study the electromagnetic (EM) decays of $X(3823)$ as the ${\psi }_2(1{{}^{3}D}_2)$ state by using the relativistic Bethe-Salpeter method. Our results are $\mathrm{\Gamma }[X(3823)\to {\chi }_{c0}\gamma ]=1.2\mathrm{keV}$, $\mathrm{\Gamma }[X(3823)\to {\chi }_{c1}\gamma ]=265\mathrm{keV}$, $\mathrm{\Gamma }[X(3823)\to {\chi }_{c2}\gamma ]=57\mathrm{keV}$, and $\mathrm{\Gamma }[X(3823)\to {\eta }_c\gamma ]=1.3\mathrm{keV}$. The ratio $\mathcal{B}[X(3823)\to {\chi }_{c2}\gamma ]/\mathcal{B}[X(3823)\to {\chi }_{c1}\gamma ]=0.22$, agrees with the experimental data. Similarly, the EM decay widths of ${\psi }_2(n^3{D}_2)$, $n=2$, 3, are predicted, and we find the dominant decays channels are ${\psi }_2(n{{}^{3}D}_2)\to {\chi }_{c1}(nP)\gamma$, where $n=1$, 2, 3. The wave function include different partial waves, which means the relativistic effects are considered. We also study the contributions of different partial waves.

We outline an algorithm to numerically compute the black-to-white hole transition amplitude using the loop quantum gravity covariant formulation and the Lorentzian Engle-Pereira-Rovelli-Livine model. We apply the algorithm to calculate the crossing time of the transition in the deep quantum regime, comparing our result with previous analytical estimates of the same physical observable in the semiclassical limit. Furthermore, we show how to evaluate the crossing time analytically using an alternative approach to the one currently in the literature. This method requires much easier calculations and emphasizes that the crossing time does not depend on the extrinsic geometry of the transition.

We holographically calculate the partition functions of certain types of isotropic sectors of the CFTs dual to Bruhat-Tits trees and $p$-adic Bañados-Teitelboim-Zanelli (BTZ) black holes. Along the way, we propose new spectral decompositions of the Laplacian operator other than the plane-wave basis on these two types of background, with both analytical and numerical evidence. We extract the density of states and hence entropy from the BTZ partition function via the inverse Laplace transform. Then the one-loop Witten diagram is computed in the $p$-adic BTZ black hole background, yielding constraints on the heavy-heavy-light averaged three-point coefficient of its boundary $p$-adic CFT. Finally, for general $p$-adic CFTs (not necessarily holographic), we analyze the representation theory of their global conformal group $PGL(2,{\mathbb{Q}}_p)$, and discuss the suitability of different representations as Hilbert spaces of $p$-adic CFT.

We revisit the problem of analytically computing the one-point functions for scalar fields in planar AdS black holes of arbitrary dimension, which are sourced by the Weyl squared tensor. We analyze the problem in terms of power series expansions around the boundary using the method of Frobenius. We clarify the pole structure of the final answer in terms of operator mixing, as argued previously by Grinberg and Maldacena. We generalize the techniques to also obtain analytic results for slowly modulated spatially varying sources to first nontrivial order in the wave vector for arbitrary dimension. We also study the first-order corrections to the one-point function of the global AdS black hole at large mass, where we perturb in terms that correspond to the curvature of the horizon.

We investigate an interacting supersymmetric gradient flow in the Wess-Zumino model. Thanks to the nonrenormalization theorem and an appropriate initial condition, we find that any correlator of flowed fields is ultraviolet finite. This is shown at all orders of the perturbation theory using the power counting theorem for one-particle irreducible supergraphs. Since the model does not have the gauge symmetry, the mechanism of realizing the ultraviolet finiteness is quite different from that of the Yang-Mills flow, and this could provide further understanding of the gradient flow approach.

We show that the Wilsonian formulation of the renormalization group (RG) defines a quantum channel acting on the momentum-space density matrices of a quantum field theory. This information theoretical property of the RG allows us to derive a remarkable consequence for the vacuum of theories at a fixed point: they have no entanglement between momentum scales. Our result can be understood as deriving from the scale symmetry of such theories and leads to constraints on the form of the ground state and on expectation values of momentum-space operators.

We present new numerical relativity calculations of the scattering angle $\chi$ between two, equal-mass, black holes on hyperbolic-like orbits. We build upon previous work considering, for the first time, spinning black holes, with equal spins either aligned or antialigned with the orbital angular momentum. We detail the numerical techniques used in the computation of $\chi$. Special care is taken in estimating error uncertainties on the quantities computed. The numerical values are compared with analytical predictions obtained using a new, state-of-the-art, effective-one-body model valid on generic orbits that incorporates post-Newtonian analytic information up to 5PN in the nonspinning, conservative sector and that has been additionally informed by numerical relativity simulations of quasicircular coalescing black hole binaries. Our results indicate that the spin sector of the analytic model should be improved further in order to achieve satisfactory consistency with the most relativistic spinning configurations.

Besides the transient effect, the passage of a gravitational wave also causes a persistent displacement in the relative position of an interferometer’s test masses through the nonlinear memory effect. This effect is generated by the gravitational backreaction of the waves themselves, and encodes additional information about the source. In this work, we explore the implications of using this information for the parameter estimation of massive binary black holes with LISA. Based on a Fisher analysis for nonprecessing black hole binaries, our results show that the memory can help to reduce the degeneracy between the luminosity distance and the inclination for binaries observed only for a short time ($\sim \mathrm{few}\mathrm{hours}$) before merger. To assess how many such short signals will be detected, we utilized state-of-the-art predictions for the population of massive black hole binaries and models for the gaps expected in the LISA data. We forecast from tens to few hundreds of binaries with observable memory, but only $\sim \mathcal{O}(0.1)$ events in 4 years for which the memory helps to reduce the degeneracy between distance and inclination. Based on this, we conclude that the new information from the nonlinear memory, while promising for testing general relativity in the strong field regime, has probably a limited impact on further constraining the uncertainty on massive black hole binary parameters with LISA.

We develop a novel technique through spectral decompositions to study the gravitational perturbations of a black hole, without needing to decouple the linearized field equations into master equations and separate their radial and angular dependence. We first spectrally decompose the metric perturbation in a Legendre and Chebyshev basis for the angular and radial sectors respectively, using input from the asymptotic behavior of the perturbation at spatial infinity and at the black hole event horizon. This spectral decomposition allows us to then transform the linearized Einstein equations (a coupled set of partial differential equations) into a linear matrix equation. By solving the linear matrix equation for its generalized eigenvalues, we can estimate the complex quasinormal frequencies of the fundamental mode and various overtones of the gravitational perturbations simultaneously and to high accuracy. We apply this technique to perturbations of a nonspinning, Schwarzschild black hole in general relativity and find the complex quasinormal frequencies of two fundamental modes and their first two overtones. We demonstrate that the technique is robust and accurate, in the Schwarzschild case leading to relative fractional errors of $\le {10}^{-10}–{10}^{-8}$ for the fundamental modes, $\le {10}^{-7}–{10}^{-6}$ for their first overtones, $\le {10}^{-7}–{10}^{-4}$ for their second overtones. This method can be applied to any black hole spacetime, irrespective of its Petrov type, making the numerical technique extremely powerful in the study of black hole ringdown in and outside general relativity.

We investigate whether photon ring observations in black hole imaging are able to distinguish between the Kerr black hole in general relativity and alternative black holes that deviate from Kerr. Certain aspects of photon rings have been argued to be robust observables in very-long-baseline interferometry black hole observations which carry imprints of the underlying spacetime. The photon ring shape, as well as its Lyapunov exponent (which encodes the narrowing of successive photon subrings), are detailed probes of the underlying geometry; measurements thereof have been argued to provide a strong null test of general relativity and the Kerr metric. However, a more complicated question is whether such observations of the photon ring properties can distinguish between Kerr and alternative black holes. We provide a first answer to this question by calculating photon rings of the Johannsen, Rasheed-Larsen, and Manko-Novikov black holes. We find that large deviations from Kerr and large observer inclinations are needed to obtain measurable differences in the photon ring shape. In other words, the Kerr photon ring shape appears to be the universal shape even for deviating black holes at low inclinations. On the other hand, the Lyapunov exponent shows more marked variations for deviations from the Kerr metric. Our analysis lays out the groundwork to determine deviations from the Kerr spacetime in photon rings that are potentially detectable by future observing missions.

This paper presents and investigates non-Gaussian perturbations for the warm $k$-inflation model that is driven by pure kinetic energy. The two complementary components of the overall non-Gaussianity are the three-point and four-point correlations. The intrinsic non-Gaussian component, denoted as the nonlinear parameter $f_{NL}^{\mathrm{int}}$, is rooted in the three-point correlation for the inflaton field. Meanwhile, the $\delta N$ part non-Gaussianity, denoted as $f_{NL}^{\delta N}$, is the contribution attributed to the four-point correlation function of the inflaton field. In this paper, the above two components in warm $k$-inflation are individually computed and analyzed under the condition that the dissipative coefficient in warm inflation is temperature independent. Then, comparisons and discussions between them are conducted, and the non-Gaussian theoretical results are compared with experimental observations to determine the range of model parameters within the allowable range of observation.

We study the effect of the relative velocity between the dark matter (DM) and the baryon on the 21-cm forest signals. The DM-baryon relative velocity arises due to their different evolutions before the baryon-photon decoupling epoch, and it gives an additional anisotropic pressure that can suppress the perturbation growth. It is intriguing that the scale $k\sim \mathcal{O}(10\sim {10}^3)h/\mathrm{Mpc}$ at which the matter power spectrum is affected by such a streaming velocity turns out to be the scale which the 21-cm forest signal is sensitive to. We demonstrate that the 21-cm absorption line abundance can decrease by more than a factor of a few due to the small-scale matter power spectrum suppression caused by the DM-baryon relative velocity.

We probe four cosmological models which, potentially, can solve the Hubble tension according to the dark energy equation of state. In this context, we demonstrate that the Einstein Telescope is capable of achieving a relative accuracy below 1% on the Hubble constant independently of the specific dark energy model. We firstly build mock catalogs containing gravitational wave events for one, five, and ten years of observations, and above signal-to-noise ratio equal to nine. From these catalogs, we extract the events which are most likely associated with possible electromagnetic counterpart detected by Transient High Energy Sources and Early Universe Surveyor. Finally, we select four dark energy models, namely a nonflat $\omega$ cold dark matter, an interacting dark energy, an emergent dark energy, and a time varying gravitational constant model, to forecast the precision down to which the Einstein Telescope can bound the corresponding cosmological parameters. We foresee that the Hubble constant is always constrained with less than 1% uncertainty, thereby offering a potential solution to the Hubble tension. The accuracy on the other cosmological parameters is at most comparable with the one currently obtained using multiple probes, except for the emergent dark energy model for which the Einstein Telescope alone will be able to improve the current limits by more than one order of magnitude.

Dark photons could be produced resonantly by the oscillating axion field in the early universe. This resonant production mechanism has been used in various contexts, including dark photon dark matter and primordial magnetic field production. However, for this resonant production to work in an expanding universe, a large axion-dark photon coupling is required, which is not easy to realize in terms of model building and requires the introduction of many charged fermions and/or the complex clockwork mechanism. In this paper, we present a new scenario that efficiently produces dark photons from the axion with a much smaller coupling. This is possible by modifying the dynamics of axion and significantly delaying the onset of oscillations, as in the so-called trapped misalignment mechanism. As a specific example, we consider models in which dark photon production occurs efficiently despite the small axion-dark photon coupling by temporally trapping an axion in a wrong minimum and releasing it after the Hubble parameter becomes much smaller than the axion mass. In this scenario, it is expected that the polarization asymmetry of dark photons and gravitational waves generated from dark photons will be significantly reduced.

Dark radiation (DR) is generally predicted in new physics scenarios that address fundamental puzzles of the Standard Model or tensions in the cosmological data. Cosmological data have the sensitivity to constrain not only the energy density of DR but also whether it is interacting. In this paper, we present a systematic study of five types of interacting DR (free-streaming, fluid, decoupling, instantaneous decoupling, and recoupling DR) and their impact on cosmological observables. We modify the Boltzmann hierarchy to describe all these types of interacting DR under the relaxation time approximation. We, for the first time, robustly calculate the collision terms for recoupling scalar DR and provide a better estimation of the recoupling transition redshift. We demonstrate the distinct features of each type of DR on the cosmic microwave background and matter power spectra. We perform Markov-chain Monte Carlo scans using the Planck 2018 data and baryon acoustic oscillation data. Assuming no new physics in the standard model neutrino sector, we find no statistically significant constraints on the couplings of DR, although there is a slight preference for the fluidlike limit of all the cases. In the case of instantaneous decoupling DR, this limit corresponds to a late transition redshift around recombination. The $\mathrm{\Delta }{N}_{\mathrm{eff}}$ constraint varies marginally depending on the type of DR.