Transition generalized parton distributions have emerged as a novel tool for studying the quantum chromodynamics (QCD) structure of resonances. They provide an integrated picture of the transition form factors and the transition parton distribution functions. In this study, we delve into the angular momentum (AM) properties for the $N\to \mathrm{\Delta }$ transition and its decomposition into the orbital angular momentum and the intrinsic spin in the context of the quark distribution functions. First, we explore the spin-flavor structures within the framework of both the overlap representations of the three-quark light-cone wave functions and the large $N_c$ limit of QCD. We then estimate the AM quark distribution functions for the $N\to \mathrm{\Delta }$ transition. Our analysis reveals a substantial flavor asymmetry present in both the orbital angular momentum and intrinsic spin components.

In this paper we consider the collective modes of a momentum-space anisotropic quark-gluon plasma taking into account the effect of collisions between the plasma constituents. Our analysis is carried out using a collisional kernel of Bhatnagar-Gross–Krook form and extends prior analyses in the literature by considering all possible angles of propagation of the gluonic modes relative to the momentum-anisotropy axis. We extract both the stable and unstable modes as a function of the collision rate and confirm prior findings that gluonic unstable modes can be eliminated from the spectrum if the collision rate is sufficiently large. In addition, we discuss the conditions necessary for the existence of unstable modes and present evidence that unstable mode growth rates are maximal for modes with momentum along the anisotropy direction. Finally, we demonstrate that when there is a finite collisional rate, gluonic unstable modes are absent from the spectrum at both small and large momentum anisotropy. These results pave the way for understanding the impact of collisions on a variety of nonequilibrium quark-gluon plasma observables.

A search for the charged-lepton-flavor-violating process $Z\to e\mu$ is presented, using $139{\mathrm{fb}}^{-1}$ of $\sqrt{s}=13\mathrm{TeV}$ $pp$ collision data collected by the ATLAS experiment at the LHC. An excess in the $e\mu$ invariant mass spectrum near the $Z$ boson mass would be a striking signature of new physics. No excess is observed, and an upper limit $\mathcal{B}(Z\to e\mu )<2.62\times {10}^{-7}$ is placed on the branching fraction at 95% confidence level, which is the most stringent limit to date.

In this paper we analyze the problem of “apparent” superluminal signalling and retrocausation that can appear for particle detector models when considering noncompactly supported field-detector interactions in quantum field theory in curved spacetimes and in relativistic quantum information protocols. For this purpose, we define a signalling estimator based on an adapted version of the quantum Fisher information to perturbative regimes. This allows us to study how the internal dynamics of the detectors (for example the gap between the detector energy levels) have an impact on the ability of particle detectors to communicate with one another. Moreover, we show that, very generally, even for detectors with infinite tails in space and time, if the tails decay exponentially, one can define an effective light cone, outside of which signalling is negligible. This provides concrete evidence supporting the use of noncompact (but exponentially decaying) detector smearings in protocols of relativistic quantum information.

We demonstrate that it is possible to test models of gravity, such as Palatini $f(R)$ and Eddington-inspired Born-Infeld models, using seismic data from Earth. By incorporating additional limitations on Earth’s moment of inertia and mass given from observational data, the models’ parameters can be restricted to a $2\sigma$ level of accuracy. Our novel tool provides that the parameter $\beta$ parametrizing the quadratic curvature term in the gravitational Lagrangian of Palatini $f(R)$ gravity is constrained to $\beta \lesssim {10}^9{\mathrm{m}}^2$, while the Eddington-inspired Born-Infeld gravity parameter $\epsilon$ is restricted to $\epsilon \lesssim 4\times {10}^9{\mathrm{m}}^2$. We also discuss further enhancements to the proposed method, aimed at imposing even more stringent constraints on modified gravity proposals.

It was recently discovered that scalarized neutron stars in scalar-tensor theories can undergo a gravitational phase transition to a nonscalarized (GR) state. Surprisingly, even though the driving mechanism is totally different, the process resembles closely the first-order matter phase transition from confined nuclear matter to deconfined quark matter in neutron star cores. The studies until now were limited, though, to only one theory of gravity and a limited range of parameters. With the present paper, we aim at demonstrating that gravitational phase transitions are more common than expected. More specifically, we show that the phenomenon of nonlinear scalarization is present for neutron stars in Gauss-Bonnet gravity leading to the possibility of gravitational phase transition. Moreover, it can be observed for a wide range of parameters so no fine-tuning is needed. This solidifies the conjecture that gravitational phase transitions are an important phenomenon for compact objects and their astrophysical implications deserve an in-depth study.

Supermassive black hole binaries (SMBHBs) are expected to be detected by the future space-based gravitational-wave detector LISA with a large signal-to-noise ratio (SNR). This prospect enhances the possibility of differentiating higher harmonics in the inspiral-merger-ringdown (IMR) waveform. In this study, we test the ability of LISA to identify the presence of different modes in the IMR waveform from a SMBHB. We analyze the contribution of each mode to the total SNR for different sources. Higher modes, in particular the mode (3, 3) and (4, 4), can dominate the signal observed through the LISA detector for SMBHB of the order of ${10}^8{M}_{\odot }$. With Bayesian analysis, we can discriminate models with different harmonics. While spherical harmonics are often considered orthogonal, we observe it is not the case in the merger-ringdown phase observed by LISA. Omitting harmonics not only diminishes the SNR but can also lead to biased parameter estimates. We analyze the bias for each model in a source example and quantify the threshold SNR where we can expect the parameter bias to be comparable to the statistical error. By computing the waveform model error with the Fisher approximation and comparing it with the posterior distribution from our sampler results, we can evaluate the veracity of the analytical bias, which converges with the sampler results as more harmonics are introduced. To conclude, SMBHB events with SNR of a few hundred, as expected in LISA, are required to use templates with at least modes (2, 2), (2, 1), (3, 3), (3, 2), (4, 4), (4, 3) to estimate all intrinsic parameters correctly. Our work highlights the importance of higher modes to describe the gravitational waveform of events detected by LISA.

We show that binary black hole mergers with precessing evolution can potentially excite photons from the quantum vacuum in such a way that total helicity is not preserved in the process. Helicity violation is allowed by quantum fluctuations that spoil the electric-magnetic duality symmetry of the classical Maxwell theory without charges. We show here that precessing binary black hole systems in astrophysics generate a flux of circularly polarized gravitational waves which, in turn, provides the required helical background that triggers this quantum effect. Solving the fully nonlinear Einstein’s equations with numerical relativity we explore the parameter space of binary systems and extract the detailed dependence of the quantum effect with the spins of the two black holes. We also introduce a set of diagrammatic techniques that allows us to predict when a binary black hole merger can or cannot emit circularly polarized gravitational radiation, based on mirror-symmetry considerations. This framework allows to understand and to interpret correctly the numerical results, and to predict the outcomes in potentially interesting astrophysical systems.

Pulsar timing arrays (PTAs) detect gravitational waves (GWs) via the correlations they induce in the arrival times of pulses from different pulsars. We assume that the GWs are described by a Gaussian ensemble, which models the confusion noise produced by expected PTA sources. The mean correlation $h^2{\mu }_{\mathrm{u}}(\gamma )$ as a function of the angle $\gamma$ between the directions to two pulsars was predicted by Hellings and Downs in 1983. The variance ${\sigma }_{\mathrm{tot}}^2(\gamma )$ in this correlation was recently calculated [B. Allen, Variance of the Hellings-Downs correlation, Phys. Rev. D 107, 043018 (2023)] for a single noise-free pulsar pair at angle $\gamma$, which shows that after averaging over many pairs, the variance reduces to an intrinsic cosmic variance ${\sigma }_{\cos }^2(\gamma )$. Here, we extend this to an arbitrary set of pulsars at specific sky locations, with pulsar pairs binned by $\gamma$. We derive the linear combination of pulsar-pair correlations which is the optimal estimator of the Hellings and Downs correlation for each bin, illustrating our methods with plots of the expected range of variation away from the Hellings and Downs curve, for the sets of pulsars monitored by three active PTA collaborations. We compute the variance of and the covariance between these binned estimates, and show that these reduce to the cosmic variance and covariance $s(\gamma ,{\gamma }^{\prime })$ respectively, in the many-pulsar limit. The likely fluctuations away from the Hellings and Downs curve ${\mu }_{\mathrm{u}}(\gamma )$ are strongly correlated/anticorrelated in the three angular regions where ${\mu }_{\mathrm{u}}(\gamma )$ is successively positive, negative, and positive. We also construct the optimal estimator of the squared strain $h^2$ from pulsar-pair correlation data. Remarkably, when there are very many pulsar pairs, this determines $h^2$ with arbitrary precision because (in contrast to LIGO-like GW detectors) PTAs probe an infinite set of GW modes. To assess if observed deviations away from the Hellings and Downs curve are consistent with predictions, we propose and characterize several ${\chi }^2$ goodness-of-fit statistics. While our main focus is ideal noise-free data, we also show how pulsar noise and measurement noise can be included. Our methods can also be applied to future PTAs, where the improved telescopes will provide larger pulsar populations and higher-precision timing.

The current situation with the recently discovered $a_0(1700/1800)$ resonance is very paradoxical: it is believed that $a_0(1700/1800)$ must be strongly coupled with two vector mesons, but there is no direct experimental confirmation of this yet. Based on the assumption that the $a_0(1700/1800)$ is a state similar to the four-quark state from the MIT bag, belonging to either the ${\underset{\_}{9}}^{*}$ or the ${\underset{\_}{36}}^{*}$ $q^2{\overline{q}}^2$ multiplet, we analyze the influence of the strong $a_0(1700/1800)$ coupling to the vector channels $K^{*}{\overline{K}}^{*}$, $\rho \varphi$, and $\rho \omega$ on its line shape in the decay channels into pseudoscalar mesons $K\overline{K}$, $\pi \eta$, and $\pi {\eta }^{\prime }$. This effect depends on the location of the resonance mass $m_{a_0}$ relative to the nominal thresholds of vector channels. For example, if $m_{a_0}\approx 1700\mathrm{MeV}$, then the influence turns out to be hidden in a fairly wide range of coupling constants. On the whole, our analysis shows that, to confirm the presence of the strong $a_0(1700/1800)$ coupling to the vector channels, utterly required is the direct detection of the decays $a_0(1700/1800)\to K^{*}{\overline{K}}^{*},\rho \varphi ,\rho \omega$. The appearance of even certain hints at the existence of these decays would make it possible to fundamentally advance in understanding the nature of the new $a_0$ state.

We explore the possibility of probing freeze-in dark matter (DM) produced via the right-handed neutrino (RHN) portal using the RHN search experiments. We focus on a simplified framework of minimally-extended type-I seesaw model consisting of only four free parameters, namely the RHN mass, the fermionic DM mass, the Yukawa coupling between the DM and the RHN, and a real singlet scalar mass. We consider two cases for the DM production either via decay of the thermal RHN or via scattering of the bath particles mediated by the RHN. In both cases, we show that for sub-TeV scale DM masses, the allowed model parameter space satisfying the observed DM relic density for freeze-in scenario falls within the reach of current and future collider, beam dump and forward physics facilities looking for feebly coupled heavy neutrinos.

The concept of diquark is important for understanding hadron structure and high-energy particle reactions. We attempt to apply the Regge trajectory approach to the doubly heavy diquarks. We present a method for determining the parameters in the diquark Regge trajectory. The spectra of diquarks $(cc)$, $(bb)$, and $(bc)$ are obtained by using the Regge trajectory approach and are found to agree with other theoretical results. The diquark Regge trajectory becomes a new and very simple approach for estimating the spectra of diquarks.

We derive the gravitational field and the spacetime metric generated by sources in quantum superposition of different locations. We start by working in a Newtonian approximation, in which the effective gravitational potential is computed as the expectation value of the gravitational potential operator in a Gaussian distribution of width $R$ for the position of the source. The effective gravitational potential is then covariantly uplifted to a fully relativistic metric in general relativity, describing the spacetime generated by averaging over the state of such sources. These results are then rederived and extended by adopting an independent construction in terms of quantum reference frames. We find three classes of quantum effective metrics which are all asymptotically flat and reproduce the Schwarzschild metric at great distances. The solutions differ, however, in the inner core. The quantum uncertainty $\mathrm{\Delta }r\sim R$ in the position of the source prevents the radius of the transverse two-sphere to shrink to zero. Depending on the strength of the quantum superposition effects, we have either a nonsingular black hole with a “quantum hair” and an event horizon, a one-way wormhole with a critical null throat or a traversable wormhole. We also provide a detailed study of the geometric and thermodynamic properties of the spacetime structure for each of these three families of models, as well as their phenomenology.

We study the accretion of a Schwarzschild black hole due to spherically symmetric perturbations sourced by a minimally coupled massless scalar field. The backreaction of the black hole to low-frequency ingoing scalar waves is computed analytically as a second-order perturbative effect, using matched asymptotic expansions to relate the behavior of the scalar field in the vicinity of the horizon and at null infinity. As an application of our results, we compute the mass increase due to (i) ingoing wave packets with an arbitrary profile and (ii) incoherent radiation. Our results could serve as a model for the backreaction of environmental scalar fields on black holes.

We study here the unhindered gravitational collapse of spatially homogeneous (SH) scalar fields $\varphi$ with a potential $V_s(\varphi )$, as well as vector fields $\tilde{A}$ with a potential $V_v(B)$ where $B=g(\tilde{A},\tilde{A})$ and $g$ is the metric tensor. We show that in both cases, classes of potentials exist that give rise to black holes or naked singularities. The strength of the naked singularity when it occurs is examined, and these are found to be gravitationally strong in the sense of Tipler, for a wide class of respective potentials (for both scalar and vector fields). We match the collapsing scalar or vector field with a generalized Vaidya spacetime outside. We highlight that full generality is maintained within the domain of SH scalar or vector field collapse.

Joint analysis of cosmic microwave background (CMB) and large-scale structure at high redshifts provide new and unique windows into unexplored epochs of early structure formation. Here, we demonstrate how cosmic infrared background and high-redshift galaxies can be jointly analyzed with CMB to probe the epoch of helium reionization ($2<z<4$) on the light cone using kinetic Sunyaev Zel’dovich tomography. Characterizing this epoch has great potential significance for understanding astrophysics of galaxy formation, quasar activity and formation of the supermassive black holes. We find a detection at $8–10\sigma$ can be expected from combinations of data from CCAT prime, Vera Rubin Observatory, and CMB-S4 in the upcoming years.

Gravitational-wave (GW) astronomy offers the potential to probe the wave-optics regime of gravitational lensing. Wave-optics (WO) effects are relevant at low frequencies, when the wavelength is comparable to the characteristic lensing time delay multiplied by the speed of light, and are thus often negligible for electromagnetic signals. Accurate predictions require computing the conditionally convergent diffraction integral, which involves highly oscillatory integrands and is numerically difficult. We develop and implement several methods to compute lensing predictions in the WO regime valid for general gravitational lenses. First, we derive approximations for high and low frequencies, obtaining explicit expressions for several analytic lens models. Next, we discuss two numerical methods suitable in the intermediate frequency range: (1) regularized contour flow yields accurate answers in a fraction of a second for a broad range of frequencies; and (2) complex deformation is slower, but requires no knowledge of solutions to the geometric lens equation. Both methods are independent and complement each other. We verify subpercent accuracy for several lens models, which should be sufficient for applications to GW astronomy in the near future. Apart from modeling lensed GWs, our method will also be applicable to the study of plasma lensing of radio waves and tests of gravity.