Based on the properties of uniform nuclear matter at the nuclear saturation density and basic thermodynamic relations, we first restudy the composition of matter on the surface of normal neutron stars and hybrid stars. It is found that hybrid stars are composed of uniform hadronic matter on the surface rather than heavy nuclei. Then we use the Walecka model and the self-consistent NJL model to describe the equation of state of low-density hadrons and quark matter at high densities respectively. The P-interpolation method is employed to connect the equation of state at the extreme densities to study hybrid stars. As a result, we find that the obtained hybrid star mass-radius relation and tidal deformability meet the requirements of the latest astronomical data. More importantly, we find that the hybrid stars we obtained can be self-bound rather than gravitationally bound, which is completely different from previous related studies.

The anomalies recently reported in lepton pair transitions of ${{}^8\mathrm{Be}}^{*}$ and ${}^4\mathrm{He}$ nuclei may be attributed to the existence of a feebly interacting light vector boson $X17$. We study the effects of this hypothetic particle in the semileptonic $H^{*}\to H{e}^{+}{e}^{-}$ decays ($H$ a $Q\overline{q}$ meson) in the framework of the $\mathrm{HQET}+\mathrm{VMD}$ model. Using current bounds and the universality assumption of the $X17$ boson to quarks, we find that decays of $D^{*+}$ and $D_s^{*+}$ mesons can be importantly enhanced relative to the dominant photon-mediated contributions. Dedicated experimental searches at current heavy meson factories may confirm the existence of this light boson or set stronger bounds of their couplings to ordinary matter.

The STAR Collaboration reports measurements of the transverse single-spin asymmetry (TSSA) of inclusive ${\pi }^0$ at center-of-mass energies ($\sqrt{s}$) of 200 GeV and 500 GeV in transversely polarized proton-proton collisions in the pseudo-rapidity region 2.7 to 4.0. The results at the two different energies show a continuous increase of the TSSA with Feynman-$x$, and, when compared to previous measurements, no dependence on $\sqrt{s}$ from 19.4 GeV to 500 GeV is found. To investigate the underlying physics leading to this large TSSA, different topologies have been studied. ${\pi }^0$ with no nearby particles tend to have a higher TSSA than inclusive ${\pi }^0$. The TSSA for inclusive electromagnetic jets, sensitive to the Sivers effect in the initial state, is substantially smaller, but shows the same behavior as the inclusive ${\pi }^0$ asymmetry as a function of Feynman-$x$. To investigate final-state effects, the Collins asymmetry of ${\pi }^0$ inside electromagnetic jets has been measured. The Collins asymmetry is analyzed for its dependence on the ${\pi }^0$ momentum transverse to the jet thrust axis and its dependence on the fraction of jet energy carried by the ${\pi }^0$. The asymmetry was found to be small in each case for both center-of-mass energies. All the measurements are compared to QCD-based theoretical calculations for transverse-momentum-dependent parton distribution functions and fragmentation functions. Some discrepancies are found, which indicates new mechanisms might be involved.

The main focus of this paper is to introduce a new method to control perturbative calculations of $CP$ asymmetric reaction rates in Boltzmann equation. $CP$ asymmetries in particle reactions are traditionally calculated in terms of complex couplings, Feynman integrals, and Cutkosky rules. We use an expansion of the $S$-matrix unitarity condition instead, obtaining a general expression for the asymmetries without reference to the imaginary part of the loops. Asymmetry cancelations implied by $CPT$ and unitarity are manifested in a diagrammatic way and easy to track at any order of perturbation theory. We demonstrate the power of this general framework within the right-handed neutrino and top-quark scattering asymmetries in seesaw type-I leptogenesis.

The $\mathrm{SL}(2,\mathbb{R})$ Wess-Zumino-Novikov-Witten model realizes bosonic-string theory in ${\mathrm{AdS}}_3$ with pure Neveu-Schwarz-Neveu-Schwarz flux. We construct an effective action in the semiclassical limit of the model, which corresponds to a $\mathrm{SL}(2,\mathbb{R})$ spin-chain $\sigma$-model. We adopt two complementary points of view. First, we consider the classical action. We identify fast and slow target-space coordinates. We impose a gauge-fixing condition to the former. By expanding the gauge-fixed action in an effective coupling, we obtain the effective action for the slow coordinates. Second, we consider the spin chain of the model. We postulate a set of coherent states to express a transition amplitude in the spin chain as a path integral. We observe that the temporal interval is discretized in terms of the step length of the spatial interval. This relationship implies that the Landau-Lifshitz limit of the spin chain involves both intervals. The limit yields a semiclassical path integral over coherent states, wherein we identify the effective action again.

The Unruh effect is the prediction that particle detectors accelerated through the vacuum get excited by the apparent presence of radiation quanta—a fundamental quantum phenomenon in the presence of acceleration. Prior treatments of the Unruh effect, that presume a classically prescribed trajectory, do not account for the quantum dynamics of the center of mass. Here, we study more realistic detectors whose center of mass is a quantized degree of freedom being accelerated by an external classical field. We investigate the detector’s recoil due to the emission of Unruh quanta. Vice versa, we also study the recoil’s impact on the emission of Unruh quanta and the excitation of the detector. We find that the recoil due to the emission of Unruh quanta may be a relevant experimental signature of the Unruh effect.

We revise the cosmological bounds on Hořava gravity, taking into account the stringent constraint on the speed of propagation of gravitational waves from GW170817 and GRB170817A. In light of this, we also investigate the degeneracy between massive neutrinos and Hořava gravity. We show that a luminal propagation of gravitational waves suppresses the large-scale cosmic microwave background (CMB) radiation temperature anisotropies, and the presence of massive neutrinos increases this effect. On the contrary, large neutrinos mass can compensate the modifications induced by Hořava gravity in the lensing, matter, and primordial B-mode power spectra. Another degeneracy is found, at a theoretical level, between the tensor-to-scalar ratio $r$ and massive neutrinos, as well as with the model’s parameters. We analyze these effects using CMB, supernovae type Ia (SNIa), galaxy clustering, and weak gravitational lensing measurements, and we show how such degeneracies are removed. We find that the model’s parameters are constrained to be very close to their general relativity limits, and we get a 2 orders of magnitude improved upper bound, with respect to the big bang nucleosynthesis constraint, on the deviation of the effective gravitational constant from the Newtonian one. The deviance information criterion suggests that in Hořava gravity, $\mathrm{\Sigma }{m}_{\nu }>0$ is favored when CMB data only are considered, while the joint analysis of all datasets prefers zero neutrinos mass.

Modified gravity (MG) theories predict, in general, that the ratio of gravitational wave (GW) to electromagnetic (EM) luminosity distances, $\mathrm{\Xi }$, differs from its general relativity (GR) value of unity at cosmological scales, thus providing another perturbative probe to MG. In this paper, we introduce new phenomenological parametrizations for both the Friedmann-Lemaître-Robertson-Walker (FLRW) background evolution of $f(R)$ models, via the dark energy equation of state parameter, $w_{\mathrm{DE}}$, and for $\mathrm{\Xi }$ in this class of theories. We simulate a mock dataset for the Einstein Telescope (ET) of 1,000 GW signals from binary neutron star (BNS) mergers and redshift information from their EM counterpart, exploring the consequent constraints on the relevant gravitational, cosmological, and phenomenological parameters. As a model of particular interest, we take $\gamma$-gravity theory and investigate whether it could be distinguished from GR’s $\mathrm{\Lambda }\mathrm{CDM}$ model. We then combine our results with actual data from type Ia supernovae (SNIa) and combined baryon acoustic oscillations (BAO) and cosmic microwave background (CMB) observations. Additionally, we also investigate the potential bounds to $f_{R0}$ for any viable $f(R)$ whose background evolution is indistinguishable from the standard model of cosmology above a certain redshift, showing that, for a $\mathrm{\Lambda }\mathrm{CDM}$ fiducial model, ET data would provide $|f_{R0}|<{10}^{-2}$ at a 95% level. We conclude altogether that probing the redshift evolution of the GW luminosity distance from detections of the ET in its first running decade will not substantially help constraining $f(R)$ theories of gravity.

We consider a five-dimensional Einstein-Chern-Simons action which is composed of a gravitational sector and a sector of matter, where the gravitational sector is given by a Chern-Simons gravity action instead of the Einstein-Hilbert action, and where the matter sector is given by a perfect fluid. The gravitational Lagrangian is obtained gauging some Lie-algebras, which in turn, were obtained by S-expansion procedure of anti–de Sitter and de Sitter algebras. On the cosmological plane, we discuss the field equations resulting from the anti–de Sitter and de Sitter frameworks and we show analogies with four-dimensional cosmological schemes.

The detection rate for compact binary mergers has grown as the sensitivity of the global network of ground based gravitational wave detectors has improved, now reaching the stage where robust automation of the analyses is essential. Automated low-latency algorithms have been developed that send out alerts when candidate signals are detected. The alerts include sky maps to facilitate electromagnetic follow-up observations, along with probabilities that the system might contain a neutron star, and hence be more likely to generate an electromagnetic counterpart. Data quality issues, such as loud noise transients (glitches), can adversely affect the low-latency algorithms, causing false alarms and throwing off parameter estimation. Here a new analysis method is presented that is robust against glitches, and capable of producing fully Bayesian parameter inference, including sky maps and mass estimates, in a matter of minutes. Key elements of the method are wavelet-based de-noising, penalized maximization of the likelihood during the initial search, rapid sky localization using precomputed inner products, and heterodyned likelihoods for full Bayesian inference.

We study the approach to scaling in axion string networks in the radiation era, through measuring the root-mean-square velocity $v$ as well as the scaled mean string separation $x$. We find good evidence for a fixed point in the phase-space analysis in the variables $(x,v)$, providing a strong indication that standard scaling is taking place. We show that the approach to scaling can be well described by a two parameter velocity-one-scale (VOS) model, and show that the values of the parameters are insensitive to the initial state of the network. The string length has also been commonly expressed in terms of a dimensionless string length density $\zeta$, proportional to the number of Hubble lengths of string per Hubble volume. In simulations with initial conditions far from the fixed point $\zeta$ is still evolving after half a light-crossing time, which has been interpreted in the literature as a long-term logarithmic growth. We show that all our simulations, even those starting far from the fixed point, are accounted for by a VOS model with an asymptote of ${\zeta }_{*}=1.20\pm 0.09$ (calculated from the string length in the cosmic rest frame) and $v_{*}=0.609\pm 0.014$.

We complete a detailed study of the gamma-ray sources eHWC $\mathrm{J}1907+063$ and eHWC $\mathrm{J}2019+368$. These are two out of only three sources detected by HAWC above 100 TeV. We also consider the source 2HWC $\mathrm{J}1857+027$, which is coincident with the location of an IceCube neutrino excess. For these sources, we show the prediction for neutrinos at the IceCube detector. Moreover, we present a calculation of the statistical significance, considering 10 and 20 years of running time, and we comment on the current results reported by the collaboration. While some leptonic models have currently been produced to describe this emission, we note that for the two brightest HAWC sources, IceCube observations are needed to conclusively differentiate between leptonic and hadronic models. We found that a detection at $3\sigma$ or more should be within reach of the next decade for the sources eHWC $\mathrm{J}1907+063$ and eHWC $\mathrm{J}2019+368$. However, a $3\sigma$ detection of the 2HWC $\mathrm{J}1857+027$ source will depend on the specific value of the flux, the extension of the source, and the cutoff energy.

A correlation between the angle of multiple scattering and the ionization energy loss for relativistic electrons in an amorphous medium is computed by solving the combined transport equation. The correlation is found to be the most pronounced at deflection angles larger than typical, reflecting the underlying single-scattering kinematical correlation, but is also sizable at typical deflection angles, where the width of the angular distribution increases with the increase of the energy loss. The mean energy loss as a function of the deflection angle is calculated. It grows quadratically both at small and at large angles, but the proportionality coefficient at large angles is greater than at small ones.

We study the effect of a small fermion mass in the formulation of the on-shell effective field theory (OSEFT). This is our starting point to derive small mass corrections to the chiral kinetic theory. In the massless case, only four Wigner functions are needed to describe positive and negative energy fermions of left and right chirality, corresponding to the vectorial components of a fermionic two-point Green’s function. As soon as mass corrections are introduced, tensorial components are also needed, while the scalar components strictly vanish in the OSEFT. The tensorial components are conveniently parametrized in the so-called spin coherent functions, which describe quantum coherent mixtures of left-right and right-left chiral fermions, of either positive or negative energy. We show that, up to second order in the energy expansion, vectorial and tensorial components are decoupled, and obey the same dispersion law and transport equation, depending on their respective chirality. We study the mass modifications of the reparametrization invariance of the OSEFT, and check that vector and tensorial components are related by the associated symmetry transformations. We study how the macroscopic properties of the system are described in terms of the whole set of Wigner functions, and check that our framework allows to account for the mass modifications to the chiral anomaly equation.

The full one-loop corrections, both the weak and QED corrections, to the process $e^{+}{e}^{-}\to Z\varphi$ ($\varphi =h^0,H^0$) in the two-Higgs-doublet model (2HDM) at the Higgs factories are presented. Up to the $O({\alpha }_{\mathrm{em}})$ level, the virtual corrections are evaluated by using the feynarts/formcalc packages. The real emission corrections are computed using the Feynman Diagram Calculation (FDC) package, and the collinear divergences are regularized by the electron structure functions. Using the feynarts/formcalc and FDC packages, we study the corrections in the Standard Model (SM) and the 2HDM, respectively. Gauge dependence arising in the renormalization of mixing angels is removed by using the pinch technique. After taking into account experimental constraints from the current LHC data, we propose four interesting benchmark scenarios for future colliders. By using these benchmark scenarios, we evaluate the deviation of $\mathrm{\Delta }\sigma (e^{+}{e}^{-}\to Z\varphi )$ from their SM values. We also examine Higgs boson decays $\varphi \to b\overline{b}$ and $\varphi \to {\tau }^{+}{\tau }^{-}$, which can have large electroweak (EW) contributions from triple Higgs couplings which are absent in the SM. It is found that for these benchmark scenarios, both EW and real emission corrections are sizeable and could be measured at future $e^{+}{e}^{-}$ colliders such as the ILC, CLIC, and CEPC.

We present new results for classical-particle propagation subject to Lorentz violation. Our analysis is dedicated to spin-nondegenerate operators of arbitrary mass dimension provided by the fermion sector of the Standard-Model extension. In particular, classical Lagrangians are obtained for the operators ${\widehat{b}}_{\mu }$ and ${\widehat{H}}_{\mu \nu }$ as perturbative expansions in Lorentz violation. The functional dependence of the higher-order contributions in the background fields is found to be quite peculiar, which is probably attributed to particle spin playing an essential role for these cases. This paper closes one of the last gaps in understanding classical-particle propagation in the presence of Lorentz violation. Lagrangians of the kind presented will turn out to be valuable for describing particle propagation in curved backgrounds with diffeomorphism invariance and/or local Lorentz symmetry explicitly violated.

Ultralight dark photons predicted in several Standard Model extensions can trigger the superradiant instability around rotating black holes (BHs) if their Compton wavelength is comparable to the BH radius. Consequently, the angular momentum of the BH is reduced to a value which depends upon the mass and spin of the BH as well as the mass of the dark photon. We use the mass and spin measurements of the primary BHs in two recently observed blackhole binary (BBH) systems, GW190517_055101 and GW190426_152155, to constrain dark photon mass in the ranges $1.7\times {10}^{-14}\mathrm{eV}<m_{A^{\prime }}<7.6\times {10}^{-13}\mathrm{eV}$ and $1.3\times {10}^{-13}\mathrm{eV}<m_{A^{\prime }}<4.2\times {10}^{-12}\mathrm{eV}$, respectively, assuming a timescale of a few million years from the time of formation of the BBH system to the time of their merger. We also discuss an interesting x-ray binary system, MAXI J1820_070, albeit with a relatively small value of the spin parameter.

We demonstrate how probes of $CP$-violating observables in Higgs boson ditau decays at prospective future lepton colliders could provide a test of weak scale baryogenesis with significant discovery potential. Measurements at the Circular Electron Positron Collider, for example, could exclude a $CP$ phase larger than 2.9° (5.6°) at 68% (95%) C.L. assuming the Standard Model value for magnitude of the tau lepton Yukawa coupling. Conversely, this sensitivity would allow for a $5\sigma$ discovery for 82% of the $CP$ phase range $[0,2\pi )$. The reaches of the Future Circular Collider for electrons and positrons (FCC-ee) and International Linear Collider are comparable. As a consequence, future lepton colliders could establish the presence of $CP$ violation required by lepton flavored electroweak baryogenesis with at least $3\sigma$ sensitivity. Our results illustrate that Higgs factories are not just precision machines, but can also make $\mathcal{O}(1)$ measurement of the new physics beyond the Standard Model.