A novel first-order action principle has been proposed as the possible foundation for a more fundamental theory of general relativity and the Standard Model. It is shown in this article that the proposal consistently incorporates gravity and matter fields, and guides one to a new and robust path toward unification of fundamental interactions.

We revisit the mathematics of exactly solvable Unruh-DeWitt detector models, interacting with massless scalar fields under instantaneous interactions, to construct a relativistic quantum Otto heat engine. By deriving the conditions under which the thermodynamic cycle is closed we study the effects of motion on the amount of work that can be extracted from the machine when the working medium is moving at a constant relativistic velocity through the heat baths. While there is a degrading effect with respect to speed in the hot bath, we demonstrate that in the case of the cold bath, genuine enhancing effects are sometimes present. For couplings the same order as the inverse frequency of the detector and a specific value for the temporal separation between the two instantaneous interactions—needed in order to be possible to cool the detector—a nonmonotonic dependence between speed and extracted work exists raising the intriguing possibility of exploiting relativistic effects for the enhancement of thermodynamic processes in tabletop experiments.

Astrophysical black holes (BHs) are universally expected to be described by the Kerr metric, a stationary, vacuum solution of general relativity (GR). Indeed, by imaging M87* and Sgr A* and measuring the size of their shadows, we have substantiated this hypothesis through successful null tests. Here we discuss the potential of upcoming improved imaging observations in constraining deviations of the spacetime geometry from that of a Schwarzschild BH (the nonspinning, vacuum GR solution), with a focus on the photon ring. The photon ring comprises a series of time-delayed, self-similarly nested higher-order images of the accretion flow, and is located close to the boundary of the shadow. In spherical spacetimes, these images are indexed by the number of half-loops executed around the BH by the photons that arrive in them. The delay time offers an independent shadow size estimate, enabling tests of shadow achromaticity, as predicted by GR. The image self-similarity relies on the lensing Lyapunov exponent, which is linked to photon orbit instability near the unstable circular orbit. Notably, this critical exponent, specific to the spacetime, is sensitive to the $rr$–component of the metric, and also offers insights into curvature, beyond the capabilities of currently available shadow size measurements. The Lyapunov time, a characteristic instability timescale, provides yet another probe of metric and curvature. The ratio of the Lyapunov and the delay times also yields the lensing Lyapunov exponent, providing alternative measurement pathways. Remarkably, the width of the first-order image can also serve as a discriminator of the spacetime. Each of these observables, potentially accessible in the near future, offers spacetime constraints that are orthogonal to those of the shadow size, enabling precision tests of GR.

We derive an expression for the local transverse polarization of a boost-invariant expanding system of massive particles, which involves a set of dynamical spin moments. Starting from spin kinetic theory, we obtain a closed set of equations of motion for these spin moments. These equations are valid during the full evolution of the system, from free streaming to local equilibrium, and can be used to study polarization phenomena in relativistic heavy-ion collisions.

We demonstrate that existing gravitational wave data from LIGO already places constraints on well-motivated Pati-Salam models that allow the Standard Model to be embedded within grand unified theories. For the first time in these models we also constrain the parameter space by requiring that the phase transition completes, with the resulting constraint being competitive with the limits from LIGO data. Both constraints are complementary to the LHC constraints and can exclude scenarios that are much heavier than can be probed in colliders. Finally, we show that results from future LIGO runs, and the planned Einstein Telescope, will substantially increase the limits we place on the parameter space.

General relativity predicts that gravitational waves propagate at the speed of light. Although ground-based gravitational-wave detectors have successfully constrained the velocity of gravitational waves in the high-frequency range, extending this constraint to the lower frequency range remains a challenge. In this work, we utilize the deviations in the overlap reduction function for a gravitational-wave background within pulsar timing arrays to investigate the velocity of gravitational waves in the nanohertz frequency band. By analyzing the NANOGrav 15-year data set, we obtain a well-constrained lower bound for the velocity of gravitational waves that $v\gtrsim 0.85c$ at the $3\sigma$ credible level, where $c$ is the speed of light.

The weakly constrained double field theory, in the sense of Hull and Zwiebach, captures the subsector of string theory on toroidal backgrounds that includes gravity, $B$-field, and dilaton together with all of their massive Kaluza-Klein and winding modes, which are encoded in doubled coordinates subject to the “weak constraint.” Due to the complications of the weak constraint, this theory was only known to cubic order. Here we construct the quartic interactions for the case that all dimensions are toroidal and doubled. Starting from the kinematic $C_{\infty }$ algebra $\mathcal{K}$ of pure Yang-Mills theory and its hidden Lie-type algebra, we construct the $L_{\infty }$ algebra of weakly constrained double field theory on a subspace of the “double copied” tensor product space $\mathcal{K}\otimes \overline{\mathcal{K}}$, by doing homotopy transfer to the weakly constrained subspace and performing a nonlocal shift that is well-defined on the torus. We test the resulting three-brackets and establish their uniqueness up to cohomologically trivial terms, by verifying the Jacobi identities up to homotopy for the gauge sector.

The impact of rotation on the deconfinement phase transition under the Einstein-Maxwell system of the soft and the hard wall models in holographic quantum chromodynamics is studied in this paper. The metric by cylindrical coordinates with rotation is introduced into the system to calculate the Hawking temperature. The first holographic study on the influence of the radius of a homogeneous rotating system on the phase diagram is proposed. It is found that the phase transition temperature hardly changes with the rotation angular velocity for a small rotation radius. Only with a larger rotation radius can the change in rotational angular velocity significantly alter the phase transition temperature. The phase transition temperature decreases rapidly with the increase of rotation angular velocity as the rotation radius increases.

Pointlike systems coupled to quantum fields are often employed as toy models for measurements in quantum field theory. In this paper, we identify the field observables recorded by such models. In the case of detector models that work in the strong coupling regime, we show that the detector’s pointer variable is correlated with time-extended smeared field amplitudes, and it is insensitive to the particle content of the field’s state. Then, we show that detector models that work in the weak coupling regime can account for particle phenomenology, such as the existence of a particlelike time-of-arrival and resonant behavior. Finally, we develop an improved field-detector interaction model, adapting the formalism of quantum Brownian motion, that is exactly solvable. This model confirms the association of field and particle properties in the strong and weak coupling regimes, respectively. Further, it can also describe the intermediate regime, in which the field-particle characteristics “merge.” In contrast to standard perturbation techniques, this model also recovers the relativistic Breit-Wigner resonant behavior in the weak coupling regime. The modulation of particle-field duality by a single tunable parameter is a novel feature that is, in principle, experimentally accessible.

In this paper, light propagation in a pressure-free nonmagnetized plasma on Kerr spacetime is considered, which is a continuation of our previous study [V. Perlick and O. Y. Tsupko, Light propagation in a plasma on Kerr spacetime: Separation of the Hamilton-Jacobi equation and calculation of the shadow, Phys. Rev. D 95, 104003 (2017).]. It is assumed throughout that the plasma density is of the form that allows for the separability of the Hamilton-Jacobi equation for light rays, i.e., for the existence of a Carter constant. Here we focus on the analysis of different types of orbits and find several peculiar phenomena which do not exist in the vacuum case. We start with studying spherical orbits, which are contained in a coordinate sphere $r=\text{constant}$, and conical orbits, which are contained in a coordinate cone $\vartheta =\text{constant}$. In particular, it is revealed that in the ergoregion in the presence of a plasma there can exist two different spherical light rays propagating through the same point. Then we study circular orbits and demonstrate that, contrary to the vacuum case, circular orbits can exist off the equatorial plane in the domain of outer communication of a Kerr black hole. Necessary and sufficient conditions for that are formulated. We also find a compact equation for circular orbits in the equatorial plane of the Kerr metric, with several examples developed. Considering the light deflection in the equatorial plane, we derive a new exact formula for the deflection angle which has the advantage of being directly applicable to light rays both inside and outside of the ergoregion. Remarkably, the possibility of a nonmonotonic behavior of the deflection angle as a function of the impact parameter is demonstrated in the presence of a nonhomogeneous plasma. Furthermore, in order to separate the effects of the black hole spin from the effects of the plasma, we investigate weak deflection gravitational lensing. We also add some further comments to our discussion of the black hole shadow which was the main topic of our previous paper.

It is well established that the magnetic Penrose process (MPP) could be highly efficient (efficiency can even exceed 100%) for extracting the energy from a Kerr black hole, if it is immersed in a mG order magnetic field. Considering the exact solution of the magnetized Kerr spacetime, here we derive the exact expression of efficiency (${\eta }_{\mathrm{MPP}}$) for MPP, which is valid for both the Kerr black hole (BH) as well as Kerr superspinar (SS), and also from the weak magnetic field to an ultrastrong magnetic field ($B$) which can even distort the original Kerr geometry. We show that although the value of ${\eta }_{\mathrm{MPP}}$ increases up to a certain value of ultrastrong magnetic field ($B_p$), it decreases to zero for $B>B_p$, in case of the Kerr BHs. On the other hand, ${\eta }_{\mathrm{MPP}}$ shows the opposite behavior in case of the Kerr SSs. One intriguing feature that emerges is, ${\eta }_{\mathrm{MPP}}$ acquires the maximum value for the Kerr parameter $a_{*}\approx 0.786$ (unlike $a_{*}=1$ for the ordinary Penrose process), decreases for the range $0.786<a_{*}\le 1$, and reaches to 20.7% for $a_{*}=1$ with a few limitations. This indicates that the BH starts to expel the effect of magnetic field for $a_{*}>0.786$, and is fully expelled from the extremal Kerr BH due to the gravitational Meissner effect. As a special case of MPP, we also study the ordinary Penrose process (PP) for the magnetized Kerr spacetime. We show that the efficiency of PP decreases with increasing the magnetic field for the Kerr BH. In case of the Kerr SS, the efficiency of PP decreases from ${10}^3\%$ to 0 for increasing the value of magnetic field from 0 to a specific value of ultrastrong magnetic field. Thus, the MPP for Kerr BHs, Kerr SSs and the ordinary PP for Kerr SSs can be superefficient for the astrophysical applications to powering engines in the high-energy sources like active galactic nuclei and quasars, in the weak magnetic fields. Our strong magnetic field result of MPP could be important to the primordial BHs in the early Universe immersed in the primordial magnetic fields, and to the transmuted BHs which are formed by collapsing and/or by merging of the magnetized progenitors. It is almost impossible to extract the energy from a BH (SS) through MPP (PP) in the ultrastrong magnetic fields.

Beyond-two-point statistics contain additional information on cosmological as well as astrophysical and observational (systematics) parameters. In this methodology paper we provide an end-to-end simulation-based analysis of a set of Gaussian and non-Gaussian weak lensing statistics using detailed mock catalogs of the Dark Energy Survey (DES). We implement: 1) second and third moments; 2) wavelet phase harmonics (WPH); 3) the scattering transform (ST). Our analysis is fully based on simulations, it spans a space of seven $\nu w\mathrm{CDM}$ cosmological parameters, and it forward models the most relevant sources of systematics of the data (masks, noise variations, clustering of the sources, intrinsic alignments, and shear and redshift calibration). We implement a neural network compression of the summary statistics, and we estimate the parameter posteriors using a likelihood-free-inference approach. We validate the pipeline extensively, and we find that WPH exhibits the strongest performance when combined with second moments, followed by ST, and then by third moments. The combination of all the different statistics further enhances constraints with respect to second moments, up to 25 percent, 15 percent, and 90 percent for $S_8$, ${\mathrm{\Omega }}_{\mathrm{m}}$, and the figure-of-merit ${\mathrm{FoM}}_{{\mathrm{S}}_8,{\mathrm{\Omega }}_{\mathrm{m}}}$, respectively. We further find that non-Gaussian statistics improve constraints on $w$ and on the amplitude of intrinsic alignment with respect to second moments constraints. The methodological advances presented here are suitable for application to Stage IV surveys from Euclid, Rubin-LSST, and Roman with additional validation on mock catalogs for each survey. In a companion paper we present an application to DES Year 3 data.

The AMS-02 experiment has observed new properties of primary cosmic rays (CRs) categorized into two groups: He-C-O-Fe and Ne-Mg-Si-S, which are independent of CR propagation. In this study, we investigate the unexpected properties of these nuclei using a spatial propagation model. All nuclei spectra are accurately reproduced and separated into primary and secondary contributions. Our findings include: 1. Primary CR spectra are identical. 2. Our calculations align with AMS-02 results for primary-dominated nuclei within a 10% difference, but show significant discrepancies for the secondary-dominated nuclei. 3. The primary element abundance at around 200 GeV is presented and compared with previous solar and Galactic results. We hope that future DAMPE experiments can provide more experimental observational evidence to validate our model calculations.

We study graviton-photon conversion in the magnetosphere of a pulsar and explore the possibility of detecting high frequency gravitational waves with pulsar observations. It is shown that conversion of one polarization mode of photons to that of gravitons can be enhanced significantly due to strong magnetic fields around a pulsar. We also derive the conservative upper bound on stochastic gravitational waves in a frequency range of ${10}^8–{10}^9\mathrm{Hz}$ and ${10}^{13}–{10}^{27}\mathrm{Hz}$ by using data of observations of the Crab pulsar and the Geminga pulsar, respectively. Our method widely fills the gap among existing high frequency gravitational wave experiments and boosts the frequency frontier in gravitational wave observations.

In this paper we discuss the analytical properties of the binary collision integral for a gas of ultrarelativistic particles interacting via a constant cross section. Starting from a near-equilibrium expansion over a complete basis of irreducible tensors in momentum space we compute the linearized collision matrices analytically. Using these results we then numerically compute all transport coefficients of relativistic fluid dynamics with various power-counting schemes that are second order in Knudsen and/or inverse Reynolds numbers. Furthermore, we also exactly compute the leading-order contribution with respect to the particle mass to the coefficient of bulk viscosity, the relaxation time, and other second-order transport coefficients of the bulk viscous pressure.

Kinetic mixing of the dark photon, the gauge boson of a hidden $U(1{)}_D$, with the Standard Model (SM) gauge fields to induce an interaction between ordinary matter and dark matter (DM) at 1-loop requires the existence of portal matter (PM) fields having both dark and SM charges. As discussed in earlier work, these same PM fields can also lead to other loop-level mechanisms besides kinetic mixing that can generate significant interactions between SM fermions and the dark photon in a manner analogous to those that can be generated between a Dirac neutrino and a SM photon, i.e., dark moments. In either case, there are reasons to believe, e.g., due to the renormalization group equation running of the $U(1{)}_D$ gauge coupling, that PM fields may have $\sim \mathrm{TeV}$-scale masses that lie at or above those directly accessible to the HL-LHC. If they lie above the reach of the HL-LHC, then the only way to possibly explore the physics at this high scale in the short term is via indirect measurements made at lower energies, e.g., at lepton colliders operating in the $m_Z$ to 1 TeV range. In particular, processes such as $e^{+}{e}^{-}\to \gamma +\mathrm{DM}$ or $e^{+}{e}^{-}\to \overline{f}f$, where $f$ is a SM fermion, may be most useful in this regard. Here we explore these possibilities within the framework of a simple toy PM model, introduced in earlier work, based on a non-Abelian dark gauge group completion operating at the PM scale. In the kinetic mixing setup, we show these efforts fail due to the inherently tiny cross sections in the face of substantial SM backgrounds. However, in the case of interactions via induced dark moments, since they necessarily take the form of higher dimensional operators whose influence grows with energy, we show that access to PM-scale information may become possible for certain ranges of the toy model parameters for both of these $e^{+}{e}^{-}$ processes at a 1 TeV collider.

A $Z^{\prime }$ gauge boson with sub-GeV mass has acquired a significant interest in phenomenology, particularly in view of the muon $g-2$ anomaly and coherent elastic neutrino-nucleon scattering. The latter is challenged by the nuclear recoil energy of a few tens of keV but has been observed by the COHERENT experiment. To further reconcile the observed excesses in $R(D^{(*)})$ from semileptonic charmful $B$ decays and in the $W$ boson mass, we investigate a model with a gauged $U(1{)}_{L_{\mu }-L_{\tau }}$ symmetry and a scalar leptoquark. In contrast to the mechanism that involves kinetic mixing between the gauge bosons of $U(1{)}_{\mathrm{em}}$ and $U(1{)}_{L_{\mu }-L_{\tau }}$, we adopt a dynamical symmetry breaking of $U(1{)}_{L_{\mu }-L_{\tau }}$ by incorporating an additional Higgs doublet. Through mixing with the $U(1{)}_{L_{\mu }-L_{\tau }}$-charged Higgs doublet, new Higgs decay channels $h\to Z_1{Z}_1/Z_1{Z}_2$ occur at percent-level branching ratios, which could be accessible at the LHC. The $W$-mass anomaly observed by CDF II can be potentially resolved through the enhancement in the oblique parameter $T$. Due to the flavored gauge symmetry, the introduced scalar leptoquark $S^{\frac{1}{3}}=(\overline{3},1,2/3)$ exhibits a unique coupling to the $\tau$ lepton, offering an explanation for the excesses observed in $R(D^{(*)})$. Moreover, $\tau \to \mu (Z_1\to )e^{-}{e}^{+}$ via the resonant light gauge boson decay can reach the sensitivity of Belle II at an integrated luminosity of $50{\mathrm{ab}}^{-1}$.

Physics beyond the Standard Model (BSM) can be described in a consistent and general way through the Higgs effective field theory (HEFT). Measurements of model-independent HEFT coefficients allow one to constrain the parameter space of BSM models via a matching procedure. In this work, we show that this procedure is not unique and depends on the scalings of the parameters of the Lagrangian. As examples, we consider three BSM models: the real singlet extension of the SM with a $Z_2$ symmetry, the complex singlet extension of the SM and the two Higgs doublet model. We discuss several physical observables, and show that different scalings of the model parameters with the UV scale in the matching to the HEFT can yield quite different results. This complicates the interpretation of HEFT measurements in terms of parameters of BSM models. Additionally, as a byproduct, we report the first matching of the complex singlet extension to the HEFT.

The properties of a phase diagram of strange quark matter in equilibrium with hadronic matter at finite temperature are studied, where the quark phase and hadron phase are treated by a baryon density-dependent quark mass model and hadron resonance gas model with a hard core repulsion factor, respectively. The thermodynamic conditions for the formation of metastable strange quark droplets (“strangelets”) in relativistic nuclear collisions are discussed. We obtained a rich structure of the phase diagram at finite temperature, and study the dynamical trajectories of an expanding strange fireball. Our results indicate that the strangeness fraction $f_{\mathrm{s}}$, perturbation parameter $C$, and confinement parameter $D$ have a strong influence on the properties of the phase diagram and the formation of strangelets. Considering the isentropic expansion process, we found that the initial entropy per baryon is less than or equal to 5, which gives a large probability for the formation of strangelets. Furthermore, a sufficiently large strangeness fraction $f_{\mathrm{s}}$ and one-gluon-exchange interaction and sufficiently small confinement interaction create possibilities for the formation of strangelets. On the contrary, the fireball will always complete the hadronization process when $f_{\mathrm{s}}=0$ or $C\ge 0$ or $D^{1/2}\ge 170\mathrm{MeV}$.