We discuss $Z$ mediated flavor changing neutral currents within a model where the hierarchical quark and lepton masses are explained via a fourth vectorlike family, together with a scalar sector consisting of two Higgs doublets augmented by a gauge singlet scalar field that spontaneously breaks an extra global $U(1{)}^{\prime }$ symmetry. The $Z$ mediated flavor violating interactions arise from the mixings between the SM fermions and the vectorlike fermions, where the mixing is discussed in an analytic approximation and also exactly numerically. We first discuss charged lepton flavor violating (CLFV) $\tau \to \mu \gamma$, $\tau \to 3\mu$ and $Z\to \mu \tau$ decays and find that they cannot significantly constrain the masses of charged vectorlike leptons. However, the 790 GeV mass bound arising from collider searches on vectorlike lepton doublets can set further constraints on the model parameter space. We also consider rare $t\to cZ$ decays as well as unitarity violation in the CKM mixing in order to constrain the quark sector of the model under consideration.

We point out kaon decays to multiple charged leptons as a novel probe of light dark particles $X$. Previously neglected channels, such as $K^{+}\to {\pi }^{+}(XX\to 2(e^{+}{e}^{-}))$, $K_L\to {\pi }^0(XX\to 2(e^{+}{e}^{-}))$, and $K_S\to (XX\to 2(e^{+}{e}^{-}))$ may have very large rates, exceeding not only the Standard Model expectations but also possible backgrounds, such as Dalitz decays of neutral pions. We apply this idea to dark sector models where the production of dark Higgses or heavy neutral leptons leads to final states with several visible dark photons. We also investigate a recently proposed model of an MeV-scale QCD axion, where the rates for kaon decays to multiple axion states are large due to the nonlinear interactions of the axion with the light mesons. In addition, we point out new probes of this axion in pion decays, such as the single production of $a$ in ${\pi }^{+}\to \nu ((e^{+}{)}^{*}\to e^{+}a\to e^{+}{e}^{+}{e}^{-})$, double production in pion capture ${\pi }^{-}+(p\text{or}\mathrm{D})\to aa+(n\mathrm{or}nn)\to 2(e^{+}{e}^{-})+(n\mathrm{or}nn)$, as well as ${\pi }^0\to aaa\to 3(e^{+}{e}^{-})$. The latter decay is fixed at $\mathcal{B}({\pi }^0\to aaa)=1.0\times {10}^{-3}$ for a 17 MeV axion.

We evaluate the light baryonium spectrum, viz. the baryon-antibaryon states, in the framework of QCD sum rules. The nonperturbative contributions up to dimension 12 are taken into account. Numerical results indicate that there might exist eight possible light baryonium states, i.e. $p-\overline{p}$, $\mathrm{\Lambda }-\overline{\mathrm{\Lambda }}$, $\mathrm{\Sigma }-\overline{\mathrm{\Sigma }}$, and $\mathrm{\Xi }-\overline{\mathrm{\Xi }}$ with quantum numbers of $0^{-+}$ and $1^{--}$. For the $\mathrm{\Lambda }-\overline{\mathrm{\Lambda }}$, $\mathrm{\Sigma }-\overline{\mathrm{\Sigma }}$, and $\mathrm{\Xi }-\overline{\mathrm{\Xi }}$ states, their masses are found above the corresponding dibaryon thresholds, while the masses of $p-\overline{p}$ states are not. The possible baryonium decay modes are analyzed, which are hopefully measurable in BESIII, BELLEII, and LHCb experiments.

Binary black hole spins are among the key observables for gravitational wave astronomy. Among the spin parameters, their orientations within the orbital plane, ${\varphi }_1$, ${\varphi }_2$ and $\mathrm{\Delta }\varphi ={\varphi }_1-{\varphi }_2$, are critical for understanding the prevalence of the spin-orbit resonances and merger recoils in binary black holes. Unfortunately, these angles are particularly hard to measure using current detectors, LIGO and Virgo. Because the spin directions are not constant for precessing binaries, the traditional approach is to measure the spin components at some reference stage in the waveform evolution, typically the point at which the frequency of the detected signal reaches 20 Hz. However, we find that this is a poor choice for the orbital-plane spin angle measurements. Instead, we propose measuring the spins at a fixed dimensionless time or frequency near the merger. This leads to significantly improved measurements for ${\varphi }_1$ and ${\varphi }_2$ for several gravitational wave events. Furthermore, using numerical relativity injections, we demonstrate that $\mathrm{\Delta }\varphi$ will also be better measured near the merger for louder signals expected in the future. Finally, we show that numerical relativity surrogate models are key for reliably measuring the orbital-plane spin orientations, even at moderate signal-to-noise ratios like $\sim 30–45$.

We present a method for comparing the classical and quantum calculations of the electric quadrupolar Love number $k_2$ and show that our previous derivation of the quantum Love number of a quantum black hole matches exactly the classical calculation of $k_2$ when quantum expectation values are replaced by the corresponding classical quantities, as dictated by the Bohr correspondence principle. The standard derivation of $k_2$ for classical relativistic stars relies on fixing boundary conditions on the surface of the star for the Einstein equations in the presence of an external perturbing field. An alternative method for calculating $k_2$ uses properties of the spectrum of the nonrelativistic fluid modes of the star. We adopt this alternative method and use it to derive an effective description of the interior modes in terms of a collection of driven harmonic oscillators characterized by different frequencies and amplitudes. We compare these two classical methods and find that most of the interior information can be integrated out, reducing the problem of calculating $k_2$ to fixing a single boundary condition for the perturbed Einstein equations on the surface of the deformed star. We then determine this single boundary condition in terms of the spectrum of the object and proceed to identify the relationship between classical quantities and quantum expectation values for the case of a quantum black hole and to verify the agreement between the results of the effective classical calculation and the quantum calculation.

The response of a gravitating object to an external tidal field is encoded in its Love numbers, which identically vanish for classical black holes (BHs). Here we show, using standard time-independent quantum perturbation theory, that for a quantum BH, generically, the Love numbers are nonvanishing and negative, and that their magnitude depends on the lowest-lying levels of the quantum spectrum of the BH. We calculate the quadrupolar electric quantum Love number of slowly rotating BHs and show that it depends most strongly on the first excited level of the quantum BH. We then compare our results to the same Love number of exotic ultra compact objects and to that of classical compact stars and highlight their different parametric dependence. Finally, we discuss the detectability of the quadrupolar quantum Love number in future precision gravitational-wave observations and show that, under favorable circumstances, its magnitude is large enough to imprint an observable signature on the gravitational waves emitted during the inspiral phase of two moderately spinning BHs.

We calculate the production of ultralight axionlike particles (ALPs) in a nearby supernova progenitor. Once produced, ALPs escape from the star and a part of them is converted into photons during propagation in the Galactic magnetic field. It is found that the MeV photon flux that reaches Earth may be detectable by $\gamma$-ray telescopes for ALPs lighter than $\sim 1\mathrm{neV}$ when Betelgeuse undergoes oxygen and silicon burning. The dependence of the $\gamma$-ray flux on the stellar mass is much smaller than the uncertainty that originates from the Galactic magnetic field. If ALPs are lighter than $\sim 0.1\mathrm{neV}$ and the supernova progenitor is close enough to the Solar System, the $\gamma$-ray flux is insensitive to the distance $d$ because the ALP-photon conversion probability is proportional to $d^2$. (Non)detection of $\gamma$ rays from a supernova progenitor with next-generation $\gamma$-ray telescopes just after presupernova neutrino alerts would lead to an independent constraint on ALP parameters as stringent as a SN 1987A limit.

The detection of GW190521 gains huge attention because it is the most massive binary that LIGO and Virgo ever confidently detected until the release of GWTC-3 (GW190426_190642 is more massive), and it is the first black hole merger whose remnant is believed to be an intermediate mass black hole. Furthermore, the primary black hole mass falls in the black hole mass gap, where the pair-instability supernova prevents the formation of astrophysical black holes in this range. In this paper, we systematically explore the prospect of TianQin on detecting GW190521-like sources. For sources with small orbital eccentricities, (i) TianQin could resolve up to a dozen of sources with signal-to-noise ratio larger than 8. Even if the signal-to-noise ratio threshold increases to 12, TianQin could still detect GW190521-like binaries. (ii) The parameters of sources merging within several years would be precisely recovered. The precision of coalescence time and sky localization closes to 1 s and $1{\mathrm{deg}}^2$, respectively. This indicates that TianQin could provide early warnings for ground-based gravitational waves detectors and electromagnetic telescopes for these sources. Furthermore, TianQin could distinguish the formation channels of these sources by measuring the orbital eccentricities with a relative precision of ${10}^{-4}$. (iii) TianQin could constrain the Hubble constant with a 10% precision with GW190521-like sources. Finally, for very eccentric GW190521-like sources, although their gravitational wave signal might be too weak for TianQin to detect, even the null detection of TianQin could still present a significant contribution to the understanding of the underlying science.

Results are presented of searches for continuous gravitational waves from 20 accreting millisecond x-ray pulsars with accurately measured spin frequencies and orbital parameters, using data from the third observing run of the Advanced LIGO and Advanced Virgo detectors. The search algorithm uses a hidden Markov model, where the transition probabilities allow the frequency to wander according to an unbiased random walk, while the $\mathcal{J}$-statistic maximum-likelihood matched filter tracks the binary orbital phase. Three narrow subbands are searched for each target, centered on harmonics of the measured spin frequency. The search yields 16 candidates, consistent with a false alarm probability of 30% per subband and target searched. These candidates, along with one candidate from an additional target-of-opportunity search done for SAX $\mathrm{J}1808.4-3658$, which was in outburst during one month of the observing run, cannot be confidently associated with a known noise source. Additional follow-up does not provide convincing evidence that any are a true astrophysical signal. When all candidates are assumed nonastrophysical, upper limits are set on the maximum wave strain detectable at 95% confidence, $h_0^{95\%}$. The strictest constraint is $h_0^{95\%}=4.7\times {10}^{-26}$ from IGR $\mathrm{J}17062-6143$. Constraints on the detectable wave strain from each target lead to constraints on neutron star ellipticity and $r$-mode amplitude, the strictest of which are ${\epsilon }^{95\%}=3.1\times {10}^{-7}$ and ${\alpha }^{95\%}=1.8\times {10}^{-5}$ respectively. This analysis is the most comprehensive and sensitive search of continuous gravitational waves from accreting millisecond x-ray pulsars to date.

An algorithm is developed for efficiently constructing the Lorentz covariant effective three-point vertices of the decay of a particle into two daughter particles in which all the masses and spins of the three particles can be arbitrary. The closely related one-to-one correspondence between the helicity formalism and the covariant formulation is exploited for counting the number of independent terms and identifying the basic covariant three-point vertices. Assembling the basic operators according to the developed algorithm enables us to construct all the covariant three-point vertices systematically.

The Tera-$Z$ phase of future $e^{+}{e}^{-}$ colliders, FCC-ee and CEPC, is a gold mine for exploring $Z$ portal physics. We focus on axionlike particles (ALPs) that can be produced via $Z$ decays with a monochromatic photon. As a template model, we consider composite Higgs models with a light pseudoscalar that couples through the Wess-Zumino-Witten term to the electroweak gauge bosons. For both photophilic and photophobic cases, we show that the Tera-$Z$ can probe composite scales up to 100s of TeV, well beyond the capability of the LHC and current precision physics. Our results also apply to generic ALPs and, in particular, severely constrain models that explain the muon $g-2$ anomaly.

We study the spectra of two-gluon glueballs and three-gluon oddballs and corresponding equation of state in 5-dimensional deformed holographic QCD models in the gravity-dilaton system, where the metric, the dilaton field, and the dilaton potential are self-consistently solved from each other through the Einstein field equations and the equation of motion of the dilaton field. We compare the models by inputting the dilaton field, inputting the deformed metric, and inputting the dilaton potential, and find that with only 2 parameters, the 5-dimensional holographic QCD model predictions on glueballs/oddballs spectra, in general, are in good agreement with lattice results except three oddballs states $0^{+-}$, $2^{+-}$ and $3^{--}$. From the results of glueballs/oddballs spectra at zero temperature and the equation of state at finite temperature, we observe that the model with quadratic dilaton field can simultaneously describe glueballs/oddballs spectra as well as the equation of state of pure gluon system. The model with quadratic $A_E(z)$ can describe glueballs/oddballs spectra, but its corresponding equation of state behaves more like $N_f=2+1$ quark matter, which is consistent with the dimension analysis at ultraviolet (UV) boundary. Our results suggest that the Einstein-Maxwell-dilaton model with the profile $\varphi (z)=z^2$ can be regarded as a candidate of dual theory of pure gluodynamics. Though it is still difficult to find the dual theory of full QCD, the existence of dual theory of pure gluodynamics would be quite encouraging.

Some issues of recoil effects in $\mathrm{AdS}/\mathrm{CFT}$ are studied from the point of view of operator product expansions expansions for generalized free fields. We show that the conformal group structure encodes the relative energies and momenta at a collision center. This is done by being careful with the analysis of Clebsch-Gordan coefficients for an $SL(2)$ subalgebra of the conformal group. The collision fraction of kinetic energy carried by the particles is derived from a probability distribution that arises from these coefficients. We specifically identify a precise statement of when recoil of a heavy particle in AdS can be ignored: the maximum probability is for the heavy particle to be in its ground state. We also argue how a notion of reduced mass appears in these collisions, in the limit where the particles are moving slowly with respect to each other. This controls the notion of the impact parameter of the collision.

We introduce a concept of ridgelines to investigate the semiclassical predictions from wave packets with arbitrary widths in conventional quantum mechanics and the Wheeler–DeWitt quantum cosmology. Two primary approaches are applied to the exact calculation of the ridgelines, namely the contour and the stream approaches. Moreover, aspects of these are discussed and compared to other scenarios and approaches, i.e., the narrow WKB wave packets and the first-derivative test. As the main result, we show that the semiclassical predictions in toy models have more abundant solutions than the ones in the classical theory, and most interestingly they may deviate from classical solutions due to the quantum corrections.

We systematically study the field equations of $f(\mathbb{Q})$ gravity for spherically symmetric and stationary metric-affine spacetimes. Such spacetimes are described by a metric as well as a flat and torsionless affine connection. In the symmetric teleparallel equivalent of general relativity (STEGR), the connection is pure gauge and hence unphysical. However, in the nonlinear extension $f(\mathbb{Q})$, it is promoted to a dynamical field which changes the physics. Starting from a general metric-affine geometry, we construct the most general static and spherically symmetric forms of the metric and the affine connection. We then use these symmetry reduced geometric objects to prove that the field equations of $f(\mathbb{Q})$ gravity admit general relativity (GR) solutions as well as beyond-GR solutions, contrary to what has been claimed in the literature. We formulate precise criteria, under which conditions it is possible to obtain GR solutions and under which conditions it is possible to obtain beyond-GR solutions. We subsequently construct several perturbative corrections to the Schwarzschild solution for different choices of $f(\mathbb{Q})$, which in particular include a hair stemming from the now dynamical affine connection. We also present an exact beyond-GR vacuum solution. Lastly, we apply this method of constructing spherically symmetric and stationary solutions to $f(\mathbb{T})$ gravity, which reproduces similar solutions but without a dynamical connection.

In this paper we describe a model of a four-dimensional spherically symmetric black hole in a limiting curvature theory of gravity. In this theory the Einstein-Hilbert action is modified by adding to the action terms providing inequality constraints on chosen curvature invariants. We demonstrated that in such a model, instead of formation of the curvature singularity, the spacetime remains regular in the black hole interior. For black holes with gravitational radius much larger than the radius $\ell$ of the critical curvature the obtained solutions describe a space exponentially expanding in one direction and periodically oscillating in the other two (spherical) directions.

The speed of sound of the matter within neutron stars may contain nonsmooth structure related to first- or higher-order phase transitions. Here we investigate what are the observable consequences of structure in the speed of sound, such as bumps, spikes, step functions, plateaus, and kinks. One of the main consequences is the possibility of ultraheavy neutron stars (with masses larger than 2.5 solar masses), mass twins in heavy (with masses larger than 2 solar masses) and ultraheavy neutron stars. These stars pass all observational and theoretical constraints, including those imposed by recent LIGO/Virgo gravitational-wave observations and NICER x-ray observations. We thoroughly investigate other consequences of this structure in the speed of sound to develop an understanding of how nonsmooth features affect astrophysical observables, such as stellar radii, tidal deformability, moment of inertia, and Love number. Our results have important implications for future gravitational wave and x-ray observations of neutron stars and their impact in nuclear astrophysics.

Axionlike particles (ALPs) could mix with photons in the presence of astrophysical magnetic fields. Searching for this effect in gamma-ray observations of blazars has provided some of the strongest constraints on ALP parameter space so far. Previously, photon-photon dispersion of gamma rays off of the cosmic microwave background has been shown to be important for these calculations and is universally included in ALP-photon mixing models. Here, we assess the effects of dispersion off of other photon fields within the blazar (produced by the accretion disk, the broad line region, the dust torus, starlight, and the synchrotron field) by modeling the jet and fields of the flat spectrum radio quasar 3C454.3 and propagating ALPs through the model both with and without the full dispersion calculation. We find that the full dispersion calculation can strongly affect the mixing, particularly at energies above 100 GeV—often reducing the ALP-photon conversion probability. This could have implications for future searches planned with, e.g., the Cherenkov Telescope Array, particularly those looking for a reduced opacity of the Universe at the highest energies.

Using the helicity amplitudes formalism, we study deeply virtual exclusive electron photoproduction off an unpolarized nucleon target, $ep\to e^{\prime }{p}^{\prime }\gamma$, through a range of kinematics both in the fixed target setting with initial electron energies of 6, 11, and 24 GeV and for an electron ion collider. We reformulate the cross section bringing to the forefront the defining features of the $ep\to e^{\prime }{p}^{\prime }\gamma$ process, where the observables are expressed as bilinear products of the independent helicity amplitudes which completely describe it in terms of the electric, magnetic, and axial currents of the nucleon. These contributions are checked against the Fourier harmonics-based formalism which has provided so far the underlying mathematical framework to study deeply virtual Compton scattering (DVCS) and related experiments. Using theoretical model calculations of the twist-two generalized parton distributions, $H$, $E$, $\tilde{H}$, and $\tilde{E}$, we uncover large discrepancies between the harmonic series and our proposed framework. Most importantly, these numerical differences appear in the intermediate $Q^2$ range which represents a sweet spot for extracting generalized parton distributions from data. We provide a framework that is ideal, on one side, to study and compare the different conventions that can be used to describe the leading order contribution to DVCS in QCD, while on the other it facilitates a quantitative extraction of physically meaningful information from experiment through traceable and controllable approximations in the intermediate $Q^2$ region.