Axions are one of the well-motivated candidates for dark matter, originally proposed to solve the strong $CP$ problem in particle physics. Dark matter Axion search with riNg Cavity Experiment (DANCE) is a new experimental project to broadly search for axion dark matter in the mass range of ${10}^{-17}\mathrm{eV}<m_a<{10}^{-11}\mathrm{eV}$. We aim to detect the rotational oscillation of linearly polarized light caused by the axion-photon coupling with a bow-tie cavity. The first results of the prototype experiment, DANCE Act-1, are reported from a 24-hour observation. We found no evidence for axions and set 95% confidence level upper limit on the axion-photon coupling $g_{a\gamma \gamma }\lesssim 8\times {10}^{-4}{\mathrm{GeV}}^{-1}$ in ${10}^{-14}\mathrm{eV}<m_a<{10}^{-13}\mathrm{eV}$. Although the bound did not exceed the current best limits, this optical cavity experiment is the first demonstration of polarization-based axion dark matter search without any external magnetic field.

A search for the lepton-flavor violating decay of the Higgs boson and potential additional Higgs bosons with a mass in the range 110–160 GeV to an $e^{\pm }{\mu }^{\mp }$ pair is presented. The search is performed with a proton-proton collision dataset at a center-of-mass energy of 13 TeV collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$. No excess is observed for the Higgs boson. The observed (expected) upper limit on the $e^{\pm }{\mu }^{\mp }$ branching fraction for it is determined to be $4.4(4.7)\times {10}^{-5}$ at 95% confidence level, the most stringent limit set thus far from direct searches. The largest excess of events over the expected background in the full mass range of the search is observed at an $e^{\pm }{\mu }^{\mp }$ invariant mass of approximately 146 GeV with a local (global) significance of 3.8 (2.8) standard deviations.

We devise a general setup to investigate the violation of the Bell-Clauser-Horne-Shimony-Holt (Bell-CHSH) inequality in the vacuum state in the context of quantum field theory. We test the method with massless spinor fields in ($1+1$)-dimensional Minkowski space-time. Alice’s and Bob’s test functions are explicitly constructed, first by employing Haar wavelets which are then bumpified into proper test functions via a smoothening procedure relying on the Planck-taper window function. Relativistic causality is implemented by requiring the support of Alice’s and Bob’s test functions to be located in the left and right Rindler wedges, respectively. Violations of the Bell-CHSH inequality as close as desired to Tsirelson’s bound are reported. We briefly comment on the extra portal, compared to earlier works; this opens the opportunity to scrutinize Bell-CHSH inequalities with generic, interacting quantum field theories.

We explicitly demonstrate that the nonlocal ghost-free ultraviolet modification of general relativity (GR) known as the infinite derivative gravity (IDG) resolves nonscalar curvature singularities in exact solutions of the full theory. We analyze exact pp-wave solutions of GR and IDG describing gravitational waves generated by null radiation. Curvature of GR and IDG solutions with the same energy-momentum tensor is compared in parallel-propagated frames along timelike and null geodesics at finite values of the affine parameter. While the GR pp-wave solution contains a physically problematic nonscalar curvature singularity at the location of the source, the curvature of its IDG counterpart is finite.

We investigate the survival probability of unstable states, the time-dependence of an initial state, in coupled channels. First, we extend the formulation of the survival probability from single channel to coupled channels (two channels). We derive an exact general expression of the two-channel survival probability using uniformization, a method which makes the coupled-channel $S$ matrix single valued, and the Mittag-Leffler expansion, i.e., a pole expansion. Second, we calculate the time dependence of the two-channel survival probability by employing the derived expression. It is the minimal distance between the pole and the physical region in the complex energy plane, not the imaginary part of the pole energy, which determines not only the energy spectrum of the Green’s function but also the survival probability. The survival probability of the “threshold cusp” caused by a pole on the unusual complex-energy Riemann sheet is shown to decay, not grow in time though the imaginary part of the pole energy is positive. We also show that the decay of the “threshold cusp” is nonexponential. Thus, the “threshold cusp” is shown to be a new type of unstable mode, which is found only in coupled channels.

We propose a general hydrodynamic framework for systems with spontaneously broken approximate symmetries. The second law of thermodynamics naturally results in relaxation in the hydrodynamic equations and enables us to derive a universal relation between damping and diffusion of pseudo-Goldstones. We discover entirely new physical effects sensitive to explicitly broken symmetries. We focus on systems with approximate U(1) and translation symmetries, with direct applications to pinned superfluids and charge density waves. We also comment on the implications for chiral perturbation theory.

Recently, we proposed that the one-loop tadpole diagram in perturbative nonsupersymmetric heterotic string vacua that contain an anomalous $U(1)$ symmetry, leads to an analog of the Fayet-Iliopoulos $D$-term in $\mathcal{N}=1$ supersymmetric models, and may uplift the vacuum energy from negative to positive value. In this paper, we extend this analysis to new types of vacua, including those with stringy Scherk-Schwarz (SSS) spontaneous supersymmetry breaking versus those with explicit breaking. We develop a criteria that facilitates the extraction of vacua with Scherk-Schwarz breaking. We develop systematic tools to analyze the T-duality property of some of the vacua and demonstrate them in several examples. The extraction of the anomalous $U(1)$ $D$-terms is obtained in two ways. The first utilizes the calculation of the $U(1)$-charges from the partition function, whereas the second utilizes the free fermionic classification methodology to classify large spaces of vacua and analyze the properties of the massless spectrum. The systematic classification method also ensures that the models are free from physical tachyons. We provide a systematic tool to relate the free fermionic basis vectors and one-loop generalized GSO phases that define the string models, to the one-loop partition function in the orbifold representation. We argue that a $D$-term uplift, while rare, is possible for both the SSS class of models, as well as in those with explicit breaking. We discuss the steps needed to further develop the arguments presented here.

A recently proposed effective action for the trace anomaly describes a tensor-scalar theory that is weakly coupled up to a certain high energy scale, where it becomes strongly interacting. Its ultraviolet completion is obtained by coupling to gravity a quantum field theory in which conformal invariance is spontaneously broken. In this paper, we show that if the field theory that gives rise to the trace anomaly is a large $N_c$ conformal field theory, then the trace anomaly action has a completion above the strong scale in a holographic Randall-Sundrum two-brane theory, with the radion as a low energy remnant of the spontaneously broken conformal symmetry. Furthermore, we note that the subleading $N_c$ terms can be derived by adding localized fields to the UV brane, so that the theory remains weakly coupled. The subleading terms are also obtained by introducing the Weyl squared terms in the 5D bulk. These, however, exhibit strongly coupled behavior at the respective sub-Planckian energy scales.

We investigate the dynamical transition processes of an Einstein-Maxwell-scalar gravitational system between two local ground states and an excited state in the anti–de Sitter spacetime. From the linear perturbation theory, only the excited state possesses a single unstable mode, indicating the dynamical instability. Such an instability is associated with the tachyonic instability due to the presence of an effective potential well near the event horizon. From the nonlinear dynamics simulation, through the scalar field accretion mechanism, the critical phenomena in the transition process of the gravitational system between the two local ground states are revealed. The threshold of the accretion strength indicates the existence of a dynamical barrier in this transition process, which depends on the coupling strength between the scalar and Maxwell fields. On the other hand, for the unstable excited state, there exists a special kind of critical dynamics with a zero threshold for the perturbation strength. The perturbations of different signs push the gravitational system to fall into different local ground states. Interestingly, in an extended parameter space, there exist specific parameters such that the perturbations of nonzero amplitude fail to trigger the single unstable mode of the excited state.

Numerical-relativity surrogate models for both black-hole merger waveforms and remnants have emerged as important tools in gravitational-wave astronomy. While producing very accurate predictions, their applicability is limited to the region of the parameter space where numerical-relativity simulations are available and computationally feasible. Notably, this excludes extreme mass ratios. We present a machine-learning approach to extend the validity of existing and future numerical-relativity surrogate models toward the test-particle limit, targeting in particular the mass and spin of postmerger black-hole remnants. Our model is trained on both numerical-relativity simulations at comparable masses and analytical predictions at extreme mass ratios. We extend the gaussian-process-regression model NRSur7dq4Remnant, validate its performance via cross validation, and test its accuracy against additional numerical-relativity runs. Our fit, which we dub NRSur7dq4EmriRemnant, reaches an accuracy that is comparable to or higher than that of existing remnant models while providing robust predictions for arbitrary mass ratios.

We provide a mean curvature flow method for numerical cosmology and test it on cases of inhomogenous inflation. The results show (in a proof-of-concept way) that the method can handle even large inhomogeneities that result from different regions exiting inflation at different times.

We consider the polarized Sunyaev-Zel’dovich (pSZ) effect for a tomographic probe of cosmic birefringence, including all relevant terms of the pSZ effect in the cosmic microwave background (CMB) observables, some of which were ignored in the previous works. The pSZ effect produces late-time polarization signals from the scattering of the local temperature quadrupole seen by an electron. We forecast the expected constraints on cosmic birefringence at the late time of the universe with the pSZ effect. We find that the birefringence angles at $2\lesssim z\lesssim 5$ are constrained at a subdegree level by the cross-correlations between CMB $E$ and $B$ modes or between CMB $B$ modes and remote quadrupole $E$ modes using data from LiteBIRD, CMB-S4, and LSST. In particular, the cross-correlation between large-scale CMB $B$ modes and remote-quadrupole $E$ modes has a much smaller bias from the Galactic foregrounds and is useful to cross-check the results from the $EB$ power spectrum.

The axion is well motivated in physics. It solves the strong charge conjugation-parity reversal problem $CP$ in fundamental physics and the dark matter problem in astronomy. Its interaction with the electromagnetic field has been expected but never detected experimentally. Such particles may convert to radio waves in the environment with a strong magnetic field. Inspired by the idea, various research groups have been working on theoretical modeling and radio data analysis to search for the signature of radio signals generated by the axion conversion in the magnetosphere of compact stars, where the surface magnetic field as strong as ${10}^{13}–{10}^{14}\mathrm{G}$ is expected. In this work, we calculate the observational properties of the axion-induced radio signals (AIRSs) in the neutron star magnetosphere, where both the total intensity and polarization properties of radio emission are derived. Based on the ray tracing method, assuming 100% linear polarization of radio waves generated in each conversion, we compute the polarization emission profile concerning different viewing angles. We note that plasma and general relativistic effects are important for the polarization properties of AIRSs. Our work suggests that AIRSs can be identified by the narrow bandwidth and distinct polarization features.

The nonradial oscillations of the neutron stars (NSs) have been suggested as an useful tool to probe the composition of neutron star matter (NSM). With this scope in mind, we consider a large number of equations of states (EOSs) that are consistent with nuclear matter properties and pure neutron matter EOS based on a chiral effective field theory (chEFT) calculation for the low densities and perturbative QCD EOS at very high densities. This ensemble of EOSs is also consistent with astronomical observations, gravitational waves in GW170817, mass and radius measurements from Neutron star Interior Composition Explorer (NICER). We analyze the robustness of known universal relations (URs) among the quadrupolar $f$ mode frequencies, masses and radii with such a large number of EOSs and we find a new UR that results from a strong correlation between the $f$ mode frequencies and the radii of NSs. Such a correlation is very useful in accurately determining the radius from a measurement of $f$ mode frequencies in the near future. We also show that the quadrupolar $f$ mode frequencies of NS of masses $2.0M_{\odot }$ and above lie in the range $\sim 2–3\mathrm{kHz}$ in this ensemble of physically realistic EOSs. A NS of mass $2M_{\odot }$ with a low $f$ mode frequency may indicate the existence of non-nucleonic degrees of freedom.

The Taiji program is a space mission aiming to detect gravitational waves in the low-frequency band. Taiji-1 is the first technology demonstration satellite of the Taiji Program in Space, with the gravitational reference sensor (GRS) serving as one of its key scientific payloads. For accurate accelerometer measurements, the test-mass center of the GRS must be positioned precisely at the center of gravity of the satellite to avoid measurement disturbances caused by angular acceleration and gradients. Due to installation and measurement errors, fuel consumption during the in-flight phase, and other factors, the offset between the test-mass center and the center of mass (c.m.) of the satellite can be significant, degrading the measurement accuracy of the accelerometer. Therefore, the offset needs to be estimated and controlled within the required range by the c.m. adjustment mechanism during the satellite’s lifetime. In this paper, we present a novel method—the extended Kalman filter combined with the Rauch-Tung-Striebel smoother—to estimate the offset while utilizing the chi-square test to eliminate outliers. Additionally, the nonlinear least squares estimation algorithm is employed as a cross-check to estimate the offset of the c.m. The two methods are shown to give consistent results, with the offset estimated to be $dx\approx -0.19\mathrm{mm}$, $dy\approx 0.64\mathrm{mm}$, and $dz\approx -0.82\mathrm{mm}$. The results indicate a significant improvement on the noise level of GRS after the c.m. calibration, which will be of great help for the future Taiji program.

We have performed the study of the phase transition of an effective model of fermions describing interactions between quarks in $D$ dimensions, taking into account effects induced by the finite size of the system, with periodic, quasiperiodic, and antiperiodic boundary conditions (PBC, QBC, ABC, respectively), chemical potential, and an external electromagnetic field. This has been done by generalized Matsubara formalism. Through the proper-time representation, the electric, magnetic, temperature, chemical potential, and finite size effects are written in terms of well-knowing functions and Jacobi theta functions. The gap equation of the system is numerically solved and we show the behavior of the system when the thermodynamic parameters of the model are changed. For fixed finite sizes, one of the findings is that the broken chiral symmetry is enhanced by the magnetic field increasing, but for the increasing electric field, we have the restoration of chiral symmetry. In fixed external fields, the influence of $L$ on the phase transition is not unique: ABC conditions tend to restore chiral symmetry, while PBC conditions improve the broken phase and the QBC conditions have intermediate behavior, considering the same range of temperatures and $L$ decreasing. We also observed little influence of finite chemical potential on the phase structure of the system and a minimal critical size of the system for ABC but not for QBC or PBC cases.

Recent studies reveal that more than a dozen of white dwarfs displaying near-perfect blackbody spectra in the optical range have been lurking in the Sloan Digital Sky Survey catalog. We point out that, in a way analogous to the cosmic microwave background, these stars serve as excellent test beds for new physics. Specifically, we show how their observed lack of spectral distortions translates into limits on the parameter space of axions with electromagnetic coupling. The prospects for future improvements are also discussed.

The two-Higgs-doublet model with a $U(1{)}_H$ gauge symmetry (N2HDM-$U(1)$) has several advantages compared to the “standard” $Z_2$ version (N2HDM-$Z_2$): It is purely based on gauge symmetries, involves only spontaneous symmetry breaking, and is more predictive because it contains one parameter less in the Higgs potential, which further ensures $CP$ conservation, i.e., avoiding the stringent bounds from electric dipole moments. After pointing out that a second, so far unknown version of the N2HDM-$U(1)$ exists, we examine the phenomenological consequences for the Large Hadron Collider (LHC) of the differences in the scalar potentials. In particular, we find that while the N2HDM-$Z_2$ predicts suppressed branching ratios for decays into different Higgs bosons for the case of the small scalar mixing (as suggested by Higgs coupling measurements), both versions of the N2HDM-$U(1)$ allow for sizable rates. This is particularly relevant in light of the CMS excess in resonant Higgs-pair production at around 650 GeV of a Standard Model Higgs boson subsequently decaying to photons and a new scalar with a mass of $\approx 90\mathrm{GeV}$ subsequently decaying to bottom quarks (i.e., compatible with the CMS and ATLAS $\gamma \gamma$ excesses at 95 GeV and $\approx 670\mathrm{GeV}$). As we will show, this excess can be addressed within the N2HDM-$U(1)$ in case of a nonminimal Yukawa sector, predicting an interesting and unavoidable $Z+b\overline{b}$ signal and motivating further asymmetric di-Higgs searches at the LHC.

In this paper we develop the theory of synchrotron radiation in $P$-odd two-dimensional electrodynamics based on the quantum theory of radiation and the solution of the Dirac equation in a constant magnetic field in ($2+1$)-dimensional space-time. Exact analytical expressions for the partial probabilities of the process obtained for the first time both in the case of a massive and massless charged fermion. Asymptotic formulas obtained for the spectral distribution and total radiation power of a relativistic massive fermion in the quasiquantum and ultraquantum limits. The spectral distribution and total power of the synchrotron radiation of a massless charged fermion calculated in the limiting case of large values of the principal quantum number of the initial and final states of the fermion.

It is well known that a magnetic monopole-electric charge system carries an angular momentum in its electromagnetic fields. Here we show that in the Dirac string formulation of magnetic charge, the monopole-electric charge system also carries a momentum in its electromagnetic fields. This overlooked field momentum arises from the Coulomb electric field of the electric charge and the solenoidal magnetic field of the Dirac string. This implies that the monopole-charge system must either (i) carry a “hidden momentum” in the string, indicating that the string is real, or (ii) that the monopole-charge system violates the center-of-energy theorem. The overall conclusion is that the Dirac monopole with a regulated Dirac string is not a true monopole, and it is not even a good effective description for topological monopoles (e.g., the ’t Hooft-Polyakov monopole outside the core).