We study the out-of-equilibrium dynamics of a Sachdev-Ye-Kitaev (SYK) model, $N$ fermions with a $q$-body interaction of infinite range, coupled to a Markovian environment. Close to the infinite-temperature steady state, the real-time Lindbladian dynamics of this system is identical to the near-zero-temperature dynamics in Euclidean time of a two-site non-Hermitian SYK with intersite coupling whose gravity dual has been recently related to wormhole configurations. We show that the saddle-point equations in the real-time formulation are identical to those in Euclidean time. Indeed, an explicit calculation of Green’s functions at low temperature, numerical for $q=4$ and analytical for $q=2$ and large $q$, illustrates this equivalence. Only for very strong coupling does the decay rate approach the linear dependence on the coupling characteristic of a dissipation-driven approach to the steady state. For $q>2$, we identify a potential gravity dual of the real-time dissipative SYK model: a double-trumpet configuration in a near-de Sitter space in two dimensions with matter. This configuration, which we term a Keldysh wormhole, is responsible for a finite decay rate even in the absence of coupling to the environment.

We construct holographic superconductors at zero density. The model enjoys a luxury property that the background geometry dual to the ground state is analytically available. It has a hyperscaling-violating geometry in the IR and is asymptotically anti–de Sitter in the UV. Classification by IR geometries gives new insights on supergravity solutions. We numerically construct the finite temperature solution of hairy black holes and verify the phase transition by tuning a double-trace deformation parameter. For a holographic superconductor from M theory, we obtain an analytic solution of the ac conductivity, which explicitly shows a superconducting delta function and a hard gap.

In violation of the generalized Lichnerowicz theorem advocated in the past, quadratic gravity admits vacua with nonconstant scalar curvature. In a recent publication, we revitalized a program—that Buchdahl originated but prematurely abandoned circa 1962—and uncovered a novel exhaustive class of static spherically symmetric vacua for pure ${\mathcal{R}}^2$ gravity. The Buchdahl-inspired metrics we obtained therein are exact solutions which exhibit nonconstant scalar curvature. A product of fourth-order gravity, the metrics entail a new (Buchdahl) parameter $k$ which allows the Ricci scalar to vary on the manifold. The metrics are able to defeat the generalized Lichnerowicz theorem by evading an overly strong restriction on the asymptotic falloff in the spatial derivatives of the Ricci scalar as assumed in the earlier theorem. The Buchdahl parameter $k$ is a new characteristic of pure ${\mathcal{R}}^2$ gravity, a higher-derivative theory. By venturing that the Buchdahl parameter should be a universal hallmark of higher-derivative gravity at large, in this paper we seek to extend the concept to the quadratic action ${\mathcal{R}}^2+\gamma (\mathcal{R}-2\mathrm{\Lambda })$. We determine that, up to the order $\mathcal{O}(k^2)$, the quadratic field equation admits the following vacuum solution $d{s}^2=e^{k\phi (r)}[-\mathrm{\Psi }(r)d{t}^2+\frac{d{r}^2}{\mathrm{\Psi }(r)}+r^{2}d{\mathrm{\Omega }}^2]$ in which $\mathrm{\Psi }(r)≔1-\frac{r_{\mathrm{s}}}{r}-\frac{\mathrm{\Lambda }}{3}{r}^2$ and the function $\phi (r)$ obeys a linear second-order ordinary differential equation, per $6{(r^2\mathrm{\Psi }(r){\phi }^{\prime }(r))}^{\prime }=\gamma r^2\phi (r)\text{subject to}\phi (r\to \infty )=0$. Conforming with our intuition, the Ricci scalar carries the footprint of a higher-derivative characteristic $k$, given by $\mathcal{R}(r)=4\mathrm{\Lambda }-k(4\mathrm{\Lambda }+\frac{\gamma }{2})\phi (r)+\mathcal{O}(k^2)$. The Ricci scalar is nonconstant, including the asymptotically flat case (i.e., $\mathrm{\Lambda }=0$) as long as $k\ne 0$ and $\gamma \ne 0$. The existence of such an asymptotically flat vacuo with nonconstant scalar curvature defeats the generalized Lichnerowicz theorem in its entirety. Our finding thus warrants restoring the ${\mathcal{R}}^2$ term in the full quadratic action, $\gamma \mathcal{R}+\beta {\mathcal{R}}^2-\alpha {\mathcal{C}}^{\mu \nu \rho \sigma }{\mathcal{C}}_{\mu \nu \rho \sigma }$, when applying the Lü-Perkins-Pope-Stelle ansatz. Implications to the Lü-Perkins-Pope-Stelle solution are discussed herein.

In a recent publication [Phys. Rev. D 106, 104004 (2022)], we advanced a program that Buchdahl originated but prematurely abandoned circa 1962 [Nuovo Cimento 23, 141 (1962)]. Therein we obtained an exhaustive class of metrics that constitute the branch of nontrivial solutions to the pure ${\mathcal{R}}^2$ field equation in vacuo. The Buchdahl-inspired metrics in general possess nonconstant scalar curvature, thereby defeating the generalized Lichnerowicz theorem advocated in recent literature. We found that the said theorem makes an overly strong assumption on the asymptotic falloff in the spatial derivatives of the Ricci scalar, rendering it violable against the Buchdahl-inspired metrics. In this paper, we shall further extend our work by showing that, within the class of Buchdahl-inspired metrics, the asymptotically flat member takes on the following exact closed analytical expression $d{s}^2={|1-\frac{r_{\mathrm{s}}}{r}|}^{\frac{k}{r_{\mathrm{s}}}}\{-(1-\frac{r_{\mathrm{s}}}{r})d{t}^2+{(1-\frac{r_{\mathrm{s}}}{r})}^{-1}\frac{{\rho }^4(r)}{r^4}d{r}^2+{\rho }^2(r)d{\mathrm{\Omega }}^2\}$, in which the areal coordinate $\rho$ is related to the radial coordinate $r$ per $\rho (r)=\frac{\zeta r_{\mathrm{s}}{|1-\frac{r_{\mathrm{s}}}{r}|}^{\frac{1}{2}(\zeta -1)}}{|1-\mathrm{sgn}(1-\frac{r_{\mathrm{s}}}{r}){|1-\frac{r_{\mathrm{s}}}{r}|}^{\zeta }|}$; $\zeta ≔\sqrt{1+3k^2/r_{\mathrm{s}}^2}$. The special Buchdahl-inspired metric, as we shall call it, is characterized by a “Schwarzschild” radius $r_{\mathrm{s}}$ and the Buchdahl parameter $k$, the latter of which arises via the higher-derivative nature of ${\mathcal{R}}^2$ gravity. The case $k=0$ corresponds precisely to the classic Schwarzschild metric. Equipped with this exact expression, we shall investigate pure ${\mathcal{R}}^2$ spacetime structures. The asymptotically flat spacetime is split into an interior region and an exterior region, with the boundary situated at $\rho =r_{\mathrm{s}}$. We find that, except for $k=0$ and $k=-r_{\mathrm{s}}$, the Kretschmann invariant of this metric exhibits an additional singularity at the interior-exterior boundary. Accordingly, the surface area of the interior-exterior boundary is found to vanish for $k\in (-\infty ,-r_{\mathrm{s}})\cup (0,+\infty )$, diverge for $k\in (-r_{\mathrm{s}},0)$, equal $4\pi r_{\mathrm{s}}^2$ for $k=0$, and equal $16\pi r_{\mathrm{s}}^2$ for $k=-r_{\mathrm{s}}$. This behavior signals a naked singularity or a wormhole. We shall also analytically construct the Kruskal-Szekeres (KS) diagram for pure ${\mathcal{R}}^2$ spacetime. The Buchdahl parameter $k$ is found to modify the KS diagram in some fundamental way. A striking result is that the (modified) KS diagram develops a “gulf” that sandwiches between the four established quadrants. The gulf resides strictly on the interior-exterior boundary and does not correspond to any domain in the physical spacetime, specified by $(t,r,\theta ,\varphi )$. The nature of this novel “virtual” region in the KS diagram is an open question, related to which we make a conjecture on a possible path forward.

Many extensions of the Standard Model of particle physics and in particular superstring and superbrane theories predict the existence of axionlike particles (ALPs). ALPs are very elusive, extremely light and interact primarily with photons. In the presence of an external magnetic field two effects show up: (i) photon-ALP oscillations and (ii) a change of the photon polarization state. The astrophysical context represents the best opportunity to get indirect evidence for the ALP existence thanks to various effects that the photon-ALP interaction produces in the sky. Great attention has been paid so far to photon-ALP oscillations, since they modify the transparency of the crossed media at very high energies and so the final spectra of faraway sources exhibit a flux excess and a characteristic oscillatory behavior. Two hints at the ALP existence have hitherto been discovered. But less interest has been attracted by the modification of the photon polarization. In this paper we address it in the x-ray and in the high energy (HE) bands. Specifically, we analyze the photon degree of linear polarization and the polarization angle induced by the photon-ALP interaction for photons generated in the central region of two galaxy clusters: Perseus and Coma. We find a substantial departure from conventional physics in both considered bands. We conclude that the ALP-induced polarization effects are more likely detectable with the proposed missions like COSI (approved to launch), e-ASTROGAM and AMEGO in the HE range. Still, possible ALP-induced effects on photon polarization could also be detected by IXPE (already operative) and by the proposed eXTP, XL-Calibur, NGXP, and XPP in the x-ray band.

Neutrinos propagating in a dense neutrino gas, such as those expected in core-collapse supernovae (CCSNe) and neutron star mergers (NSMs), can experience fast flavor conversions on relatively short scales. This can happen if the neutrino electron lepton number ($\nu \mathrm{ELN}$) angular distribution crosses zero in a certain direction. Despite this, most of the state-of-the-art CCSN and NSM simulations do not provide such detailed angular information and instead, supply only a few moments of the neutrino angular distributions. In this study we employ, for the first time, a machine learning (ML) approach to this problem and show that it can be extremely successful in detecting $\nu \mathrm{ELN}$ crossings on the basis of its zeroth and first moments. We observe that an accuracy of $\sim 95\%$ can be achieved by the ML algorithms, which almost corresponds to the Bayes error rate of our problem. Considering its remarkable efficiency and agility, the ML approach provides one with an unprecedented opportunity to evaluate the occurrence of fast flavor conversions in CCSN and NSM simulations on the fly. We also provide our ML methodologies on GitHub.

We describe a new axion search method based on measuring the variance in the interference of the axion signal using injected photons with a power detector. The need for a linear amplifier is eliminated by putting a strong signal into the microwave cavity, to acquire not only the power excess but also measure the variance of the output power. The interference of the external photons with the axion to photon converted signal greatly enhances the variance at the particular axion frequency, providing evidence of its existence. This method has an advantage in that it can always obtain sensitivity near the quantum noise limit even for a power detector with high dark count rate. We describe the basic concept of this method both analytically and numerically, and we show experimental results using a simple demonstration circuit.

The advent of space-based gravitational-wave detectors like the Laser Interferometer Space Antenna will allow us to observe signals, most of which are expected to be emitted by white-dwarf binaries. Among these systems another kind of compact objects could be hidden, postulated by Glendenning, Kettner, and Weber in 1995, containing a small core made of strange quark matter surrounded by layers of hadronic matter reaching densities much higher than in white dwarfs. These so-called strange dwarfs cannot be easily distinguished from white dwarfs through electromagnetic observations alone; their outermost envelopes are expected to have the same composition and their radii are quite similar (except for low masses). However, future measurements of the tidal deformability through gravitational-wave observations could provide a new way to reveal their existence. In this paper, we revisit the structure and the tidal deformability of strange dwarfs in full general relativity taking into account the possible crystallization of their envelope. Unlike most previous studies, we do not describe the hadronic layers using the equation of state of Baym, Pethick, and Sutherland, which was originally designed for the outer crust of a neutron star. The conditions leading to such heavy elements in gravitational-core collapse supernova explosions are not expected to be met during the stellar evolution leading to the formation of strange dwarfs. Instead, we consider the same light elements as found in white dwarfs but we take into account electron captures by nuclei and possibly pycnonuclear fusions that may be triggered by compression. We find that the radius of strange dwarfs is systematically smaller than that of their white dwarf relatives. These deviations can be large for very low-mass stars but mostly lie within the current observational uncertainties for typical white dwarf masses. On the other hand, the observable tidal deformability coefficient ${\mathrm{\Lambda }}_2$ is strongly reduced in comparison to white dwarfs and the deviation will be potentially measurable by future space-based gravitational-wave detectors.

In certain situations, such as one-particle inclusive processes, it is possible to model the hadronization through fragmentation functions (FFs), which are universal nonperturbative functions extracted from experimental data through advanced fitting techniques. Constraining the parameters of such fits is crucial to reduce the uncertainties, and provide reliable and accurate FFs. In this article, we explore strategies to relate pion and FFs for other hadrons (in particular, kaons), comparing cross-section ratios imposing proper kinematical cuts. We exploit the phenomenology of photon-hadron production at colliders, including up to Next-to-Leading Order (NLO) in Quantum Chromodynamics (QCD) and Leading Order (LO) in Quantum Electrodynamics(QED) corrections, and make use of accurate formulas to reconstruct the partonic momentum fractions. By studying different cuts, we manage to isolate the contribution of $u$-started FFs. Then, we relate the ratios of the $z$-spectrum for pion and kaon production, with the corresponding FFs ratios. The methodology described in this article can be used to relate FFs for any pair of hadrons, and could be further explored to keep track of the flavor of the partons undergoing the hadronization.

The analytic continuation of the gluon propagator is revised in the light of recent findings on the possible existence of complex conjugated poles. The contribution of the anomalous pole must be added when Wick rotating, leading to an effective Minkowskian propagator which is not given by the trivial analytic continuation of the Euclidean function. The effective propagator has an integral representation in terms of a spectral function which is naturally related to a set of elementary (complex) eigenvalues of the Hamiltonian, thus generalizing the usual Källén-Lehmann description. A simple toy model shows how the elementary eigenvalues might be related to actual physical quasiparticles of the nonperturbative vacuum.

The bubble wall velocity is essential for the spectra of phase-transition gravitational waves, electroweak baryogenesis and the dynamical dark matter mechanism. We perform detailed calculations of the bubble wall velocity in the well-motivated inert doublet model using the microphysical approach with some recent new methods.

Monte Carlo calculations of fermionic systems with continuous auxiliary fields frequently suffer from a diverging variance for fermionic observables. If the simulations have an infinite variance problem, one cannot estimate observables reliably even with an arbitrarily large number of samples. In this paper, we explore a method to solve this problem using sampling based on the distribution of a system with an extra time slice. The necessary reweighting factor is computed both perturbatively and through a secondary Monte Carlo. We show that the Monte Carlo reweighting coupled to the use of an unbiased estimator of the reweighting factor leads to a method that eliminates the infinite variance problem at a very small extra cost. We compute the double occupancy in the Hubbard model at half-filling to demonstrate the method and compare the results to well-established results obtained by other methods.

This work presents a determination of potential short-distance contributions to the unphysical ${\pi }^{-}\to {\pi }^{+}{e}^{-}{e}^{-}$ decay through lattice QCD calculations. The hadronic contributions to the transition amplitude are described by the pion matrix elements of five Standard Model effective field theory operators, which are computed on five ensembles of domain-wall fermions with $N_f=2+1$ quark flavors with a range of heavier-than-physical values of the light quark masses. The matrix elements are extrapolated to the continuum, physical light-quark mass, and infinite volume limit using a functional form derived in chiral effective field theory ($\chi \mathrm{EFT}$). This extrapolation also yields the relevant low-energy constants of $\chi \mathrm{EFT}$, which are a necessary input for $\chi \mathrm{EFT}$ calculations of neutrinoless double beta decay of nuclei.

We discuss the generalized parton distributions (GPDs) for unpolarized hadrons, as a continuation of our recent work on hadronic structure on the light front. We analyze the unpolarized GPDs for the light nucleon and delta, as well as generic mesons, using the lowest Fock states. We use these GPDs to reconstruct the charge and gravitational form factors, and discuss their relative sizes. The results are also compared to reported QCD lattice results.

We compute the azimuthal eccentricities arising from initial stage fluctuations in high energy proton-nucleus collisions at proper times $\tau \ge 0^{+}$. We consider two sources of fluctuations, namely the geometrical structure of the proton and the fluctuation of color fields carried by both proton and nucleus. Describing these effects with Gaussian models allows us to analytically calculate the one- and two-point correlators of energy density, from which the eccentricities are obtained. We compute the proper time evolution of these quantities by approximating the Glasma dynamics in terms of linearized Yang-Mills equations, which we solve by assuming free field propagation and adopting the dilute-dense limit.

Energetic jets that traverse the quark-gluon plasma created in heavy-ion collisions serve as excellent probes to study this new state of deconfined QCD matter. Presently, however, our ability to achieve a crisp theoretical interpretation of the crescent number of jet observables measured in experiments is hampered by the presence of selection biases. The aim of this work is to minimize those selection biases associated to the modification of the quark- versus gluon-initiated jet fraction in order to assess the presence of other medium-induced effects, namely, color decoherence, by exploring the rapidity dependence of jet substructure observables. So far, all jet substructure measurements at midrapidity have shown that heavy-ion jets are narrower than vacuum jets. We show both analytically and with Monte Carlo simulations that if the narrowing effect persists at forward rapidities, where the quark-initiated jet fraction is greatly increased, this could serve as an unambiguous experimental observation of color decoherence dynamics in heavy-ion collisions.

We revisit the $D{D}^{*}$ interactions in chiral effective field theory up to the third order for the first time. We deal with the pion-exchanged interactions via local momentum-space regularization, in which we focus on their long-range behaviors through demanding their contributions vanish at the origin in the coordinate space. The short-range contact interactions and subleading pion-charmed meson couplings are estimated with the phenomenological resonance saturation model. The subleading pion-charmed meson couplings are much weaker than those in the pion-nucleon system, thus the $D{D}^{*}$ binding mechanism is very different with that of the $NN$ system. We also obtain the analytic structure of the two-pion exchange interactions in the coordinate space, and we find that its asymptotic behavior at long distance is similar to but slightly different than the $NN$ interactions. We get the same asymptotic behavior of the two-pion exchange interaction with that from the HAL QCD method, but appearing in the longer distance rather than $1<r<2\mathrm{fm}$. The binding solution only exists in the isoscalar channel. Our calculation supports the molecular interpretation of $T_{cc}^{+}$.

$CP$-odd asymmetries in the processes $e^{+}{e}^{-}\to {\tau }^{+}{\pi }^{-}{\nu }_{\tau }$, $e^{+}{e}^{-}\to {\pi }^{+}{\tau }^{-}{\overline{\nu }}_{\tau }$, $e^{+}{e}^{-}\to {\tau }^{+}{\rho }^{-}{\nu }_{\tau }$, $e^{+}{e}^{-}\to {\rho }^{+}{\tau }^{-}{\overline{\nu }}_{\tau }$, $e^{+}{e}^{-}\to {\tau }^{+}{e}^{-}{\nu }_{\tau }{\overline{\nu }}_e,$ and $e^{+}{e}^{-}\to {\tau }^{-}{e}^{+}{\nu }_e{\overline{\nu }}_{\tau }$ are investigated with account for longitudinal polarization of electron (or positron) beam. These asymmetries are a manifestation of electric dipole form factor $F_3^{\tau }\equiv b$ in the $\gamma {\tau }^{+}{\tau }^{-}$ vertex. It is shown that to measure $\mathrm{Im}b$ in the specified processes, polarization is not needed, while to measure $\mathrm{Re}b$ it is required. The processes $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}{\nu }_{\tau }{\overline{\nu }}_{\tau }$, $e^{+}{e}^{-}\to e^{+}{e}^{-}{\nu }_{\tau }{\overline{\nu }}_{\tau }{\nu }_e{\overline{\nu }}_e$, $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}{\nu }_{\tau }{\overline{\nu }}_{\tau }{\nu }_{\mu }{\overline{\nu }}_{\mu }$, $e^{+}{e}^{-}\to {\mu }^{+}{e}^{-}{\nu }_{\tau }{\overline{\nu }}_{\tau }{\nu }_{\mu }{\overline{\nu }}_e$, and $e^{+}{e}^{-}\to {\mu }^{-}{e}^{+}{\nu }_{\tau }{\overline{\nu }}_{\tau }{\nu }_e{\overline{\nu }}_{\mu }$ are also discussed for the case of unpolarized electron and positron beams. In the latter cases it is possible to measure $\mathrm{Re}b$ using the differential cross section over momenta of both registered particles.

The MicroBooNE experiment searched for an excess of electron-neutrinos in the Booster Neutrino Beam (BNB), providing direct constraints on ${\nu }_e$-interpretations of the MiniBooNE low-energy excess (LEE). In this article, we show that if the MiniBooNE LEE is caused instead by an excess of ${\overline{\nu }}_e$, then liquid argon detectors, such as MicroBooNE, SBND, and ICARUS, would have poor sensitivity to it. This is due to a strong suppression of ${\overline{\nu }}_e–{}^{40}\mathrm{Ar}$ cross sections in the low-energy region of the excess. The MicroBooNE results are consistent at the $2\sigma$ CL with a scenario in which the MiniBooNE excess is sourced entirely by ${\overline{\nu }}_e$ interactions. The opportune location of ANNIE, a Gd-loaded water Cherenkov detector, allows for a direct search for a ${\overline{\nu }}_e$ flux excess in the BNB using inverse beta-decay events.