Recently the Event Horizon Telescope Collaboration, with very-long baseline interferometric observations, resolved structure at the scale of $\sim 5$ Schwarzschild radii about the center of $\mathrm{M}{87}^{*}$, the supermassive black hole resident at the center of Messier 87. This important observation has paved the way for testing what is known as the “no-hair” theorem, stating that isolated black holes are described by the Kerr metric, parametrized only by their mass and spin. Generic, parametrized spacetimes beyond Kerr allow one to arbitrarily test the no-hair theorem for deviations from the Kerr result with no prior theoretical knowledge or motivation. In this paper, we present such a new general, stationary, axisymmetric and asymptotically flat black hole solution with separable geodesic equations (thus preserving symmetries of a Kerr black hole), extending the previous work of Johannsen. In this new metric, five free nonlinear functions parameterically deviate from the Kerr result, allowing one to effectively transform to many alternative black hole solutions present in the literature. We then derive analytic expressions for the Keplerian and epicyclic frequencies, the orbital energy and angular momentum, and the location of the innermost stable orbit of circular equatorial particle orbits. We also compute the image of the photon rings in the new spacetime, which correspond to the boundary of the black hole shadow image taken by the Event Horizon Telescope. We finally compare each quantity for the Kerr result against various parametrizations of the metric, finding that, especially for highly rotating black holes, the two solutions disagree significantly. Such a metric parametrization allows one to perform the no-hair tests in a model-independent way, and finally map constraints to specific alternative theories of gravity.

We show that the energy density of the superhorizon modes released in a non-Bunch-Davies vacuum can be arbitrarily large during inflation and it decreases like $\ln (a)/a^4$ in the subsequent radiation-dominated era. This may constitute a dark radiation component which can sufficiently alter the early cosmological evolution to alleviate the Hubble tension.

Short baseline neutrino experiments, like Liquid Scintillator Neutrino Detector and MiniBooNE experiments, pointed toward the existence of eV mass scale sterile neutrinos. To reconcile sterile neutrinos with cosmology self-interaction between sterile neutrinos has been studied. We analyzed Planck cosmic microwave background (CMB) data with self-interacting sterile neutrino ($\mathrm{SI}\nu$) and studied their impact on inflation models. The fit to the CMB data in $\mathrm{SI}\nu$ model is as good as the fit to $\mathrm{\Lambda }\mathrm{CDM}$ model. We find that the spectral index ($n_s$) values shift to $0.9361\pm 0.0055$ in $\mathrm{SI}\nu$ model. This has a significant impact on the validity of different inflation models. For example, the Starobinsky and quartic hilltop model, which were allowed within $\mathrm{\Lambda }\mathrm{CDM}$ cosmology, are ruled out. On the other hand, some models like natural and Coleman-Weinberg inflation are now favored. Therefore, the existence of self-interacting sterile neutrinos with eV order of mass will play an important role in the selection of correct inflation model.

We argue that the global signal of neutral hydrogen 21cm line can be a powerful probe of primordial power spectrum on small scales. Since the amplitude of small scale primordial fluctuations is important to determine the early structure formation and the timing when the sources of Lyman $\alpha$ photons are produced, they in turn affect the neutral hydrogen 21cm line signal. We show that the information of the position of the absorption trough can severely constrain the small scale amplitude of primordial fluctuations once astrophysical parameters relevant to the 21cm line signal are fixed. We also discuss how the uncertainties of astrophysical parameters affect the constraints.

We analyze the thermal emission and resonant scattering of CMB radiation from the dark ages halos in the rotational lines of the helium hydride ion (${\mathrm{HeH}}^{+}$), one of the first chemical compounds in the Universe. Evaluating the optical depth, thermal, and resonant brightness temperatures and spectral fluxes of dark ages halos is based on computing the cross sections and rate coefficients of excitation/deexcitation of the lowest five rotational states of ${\mathrm{HeH}}^{+}$ by inelastic collisions with atomic hydrogen. It was shown that in the Dark Ages the collisional excitation/deexcitation by atoms of neutral hydrogen and electrons are competitive, whereas in the denser regions, e.g., in the halos, the contribution of collisions with atomic hydrogen is dominant. We demonstrate the peaklike time-dependence of halo luminosities in ${\mathrm{HeH}}^{+}$ lines.

We study the imprints of new ultralight particles on the gravitational-wave signals emitted by binary black holes. Superradiant instabilities may create large clouds of scalar or vector fields around rotating black holes. The presence of a binary companion then induces transitions between different states of the cloud, which become resonantly enhanced when the orbital frequency matches the energy gap between the states. We find that the time dependence of the orbit significantly impacts the cloud’s dynamics during a transition. Following an analogy with particle colliders, we introduce an $S$-matrix formalism to describe the evolution through multiple resonances. We show that the state of the cloud, as it approaches the merger, carries vital information about its spectrum via time-dependent finite-size effects. Moreover, due to the transfer of energy and angular momentum between the cloud and the orbit, a dephasing of the gravitational-wave signal can occur, which is correlated with the positions of the resonances. Notably, for intermediate and extreme mass ratio inspirals, long-lived floating orbits are possible, as well as kicks that yield large eccentricities. Observing these effects, through the precise reconstruction of waveforms, has the potential to unravel the internal structure of the boson clouds, ultimately probing the masses and spins of new particles.

High energy $\gamma$-rays from giant molecular clouds (GMCs) carry direct information about the spatial and energy distributions of galactic cosmic rays (CRs). The recently released catalogs of GMCs contain sufficiently massive clouds to be used as barometers for probing, through their $\gamma$-ray emission, the density of CRs throughout the galactic disk. Based on the data of Fermi-LAT, we report the discovery of $\gamma$-ray signals from nineteen GMCs located at distances up to 12.5 kpc. The galactocentric radial distribution of the CR density derived from the $\gamma$-ray and CO observations of these objects, as well as from some nearby clouds that belong to the Gould Belt complex, unveil a homogeneous “sea” of CRs with a constant density and spectral shape close to the flux of directly (locally) measured CRs. This concerns the galactocentric distances exceeding 8 kpc, as well as the Sagittarius B complex, in the region of the Galactic Center. On the other hand, for the galactocentric distances between 4 and 8 kpc, we found noticeable deviations from the CR sea level; in some locations, GMCs are characterized by enhanced CR density. This could be the result of a possible global increase of the level of the CR sea towards the Galactic Center and/or by the presence of recent CR accelerators close to some specific clouds.

We present a general strategy to solve the stationary Schrödinger-Poisson (SP) system of equations for multistates with axial symmetry. The approach allows us to obtain the well-known single and multistate solutions with spherical symmetry, Newtonian multistate $\ell$-boson stars and axially symmetric multistate configurations. For each case we construct particular examples that illustrate the method, whose stability properties are studied by numerically solving the time-dependent SP system. Among the stable configurations there are the mixed-two-state configurations including spherical and dipolar components, which might have an important value as potential anisotropic dark matter halos in the context of ultralight bosonic dark matter scenarios. This is the reason why we also present a possible process of formation of these mixed-two-state configurations that could open the door to the exploration of more general multistate structure formation scenarios.

We present a dark sector model addressing both the Hubble tension and the core-cusp problem. The model is based on a hidden Abelian gauge symmetry group with some chiral fermions required by the anomaly cancellation conditions, producing a candidate for the decaying fermion dark matter as a solution to the Hubble tension. Moreover, the sub-keV mass regime and the thermal history of the dark sector help the dark matter candidate resolve the core-cusp problem occurring in the standard $\mathrm{\Lambda }$ cold dark matter cosmology.

In a model with an extra $U(1)$ gauge to SM gauge group, we have shown the allowed region of masses of the extra gauge boson and the dark matter which is the lightest one among other right-handed Majorana fermions present in the model. To obtain this region, we have used bounds coming from constraints on active-sterile neutrino masses and mixing from various oscillation experiments, constraint on dark matter relic density obtained by PLANCK together with the constraint on the extra gauge boson mass and its gauge coupling recently obtained by ATLAS Collaboration at LHC. From the allowed regions, it is possible to get some lower bounds on the masses of the extra gauge boson and the dark matter and considering those values it is possible to infer what could be the spontaneous symmetry breaking scale of an extra $U(1)$ gauge symmetry.

We investigate the sensitivity of future searches for the top-Higgs boson flavor changing neutral current (FCNC) couplings $tqh$ ($q=u$, $c$) at the proposed High Energy Large Hadron Collider (HE-LHC) and Future Circular Collider in the hadron-hadron mode (FCC-hh). We perform a full simulation for the two processes in the $h\to \gamma \gamma$ decay channel (where $h$ is the discovered Higgs boson state): the single top quark FCNC production in association with the Higgs boson (plus a jet) and the top quark pair production with FCNC decays $t\to qh$. All the relevant backgrounds are considered in a cut based analysis to obtain the limits on the branching ratios (BRs) of $t\to uh$ and $t\to ch$. It is shown that, at the HE-LHC with an integrated luminosity of $15{\mathrm{ab}}^{-1}$ and at the FCC-hh with an integrated luminosity of $30{\mathrm{ab}}^{-1}$, the $\mathrm{BR}(t\to uh)$ [$\mathrm{BR}(t\to ch)$] can be probed, respectively, to $7.0(8.5)\times {10}^{-5}$ and $2.3(3.0)\times {10}^{-5}$ at the 95% confidence level (C.L.) (assuming a 10% systematic uncertainty on the background), which is almost 2 orders of magnitude better than the current 13 TeV LHC experimental results.

Future observations of continuous gravitational waves from single neutron stars, apart from their monumental astrophysical significance, could also shed light on fundamental physics and exotic particle states. One such avenue is based on the fact that magnetic fields cause deformations of a neutron star, which results in a magnetic-field-induced quadrupole ellipticity. If the magnetic and rotation axes are different, this quadrupole ellipticity may generate continuous gravitational waves which may last decades, and may be observable in current or future detectors. Light, milli-magnetic monopoles, if they exist, could be pair-produced nonperturbatively in the extreme magnetic fields of neutron stars, such as magnetars. This nonperturbative production furnishes a new, direct dissipative mechanism for the neutron star magnetic fields. Through their consequent effect on the magnetic-field-induced quadrupole ellipticity, they may then potentially leave imprints in the early stage continuous gravitational wave emissions. We speculate on this possibility in the present study, by considering some of the relevant physics and taking a very simplified toy model of a magnetar as the prototypical system. Preliminary indications are that new-born millisecond magnetars could be promising candidates to look for such imprints. Deviations from conventional evolution, and comparatively abrupt features in the early stage gravitational waveforms, distinct from other astrophysical contributions, could be distinguishable signatures for these exotic monopole states.

Many extensions of the Standard Model include a new $U(1)$ gauge group that is broken spontaneously at a scale much above TeV. If a $U(1)$-breaking phase transition occurs at nucleation temperature of $O(100)–O(1000)\mathrm{TeV}$, it can generate stochastic gravitational waves in $O(10)–O(100)\mathrm{Hz}$ range if ${\beta }_{\mathrm{n}}/H_{\mathrm{n}}=1000$, which can be detected by ground-based detectors. Meanwhile, supersymmetry (SUSY) may play a crucial role in the dynamics of such high-scale $U(1)$ gauge symmetry breaking, because SUSY breaking scale is expected to be at TeV to solve the hierarchy problem. In this paper, we study the phase transition of $U(1)$ gauge symmetry breaking in a SUSY model in the SUSY limit. We consider a particular example, the minimal SUSY $U(1{)}_{B-L}$ model. We derive the finite temperature effective potential of the model in the SUSY limit, study a $U(1{)}_{B-L}$-breaking phase transition, and estimate gravitational waves generated from it.

Axion is a promising candidate of dark matter. After the Peccei-Quinn symmetry breaking, axion strings are formed and attached by domain walls when the temperature of the universe becomes comparable to the QCD scale. Such objects can cause cosmological disasters if they are long lived. As a solution for it, the Lazarides-Shafi mechanism is often discussed through introduction of a new non-Abelian (gauge) symmetry. We study this mechanism in detail and show configuration of strings and walls. Even if Abelian axion strings with a domain wall number greater than 1 are formed in the early universe, each of them is split into multiple Alice axion strings due to a repulsive force between the Alice strings even without domain wall. When domain walls are formed as the universe cools down, a single Alice string can be attached by a single wall because a vacuum is connected by a non-Abelian rotation without changing energy. Even if an Abelian axion string attached by domain walls is created due to the Kibble Zurek mechanism at the chiral phase transition, such strings are also similarly split into multiple Alice strings attached by walls in the presence of the domain wall tension. Such walls do not form stable networks since they collapse by the tension of the walls, emitting axions.

We study an effective field theory (EFT) describing the interaction of an approximate dilaton with a set of pseudo-Nambu-Goldstone bosons (pNGBs). The EFT is inspired by, and employed to analyze, recent results from lattice calculations that reveal the presence of a remarkably light singlet scalar particle. We adopt a simple form for the scalar potential for the EFT, which interpolates among earlier proposals. It describes departures from conformal symmetry, by the insertion of a single operator at leading order in the EFT. To fit the lattice results, the global internal symmetry is explicitly broken, producing a common mass for the pNGBs, as well as a further, additive deformation of the scalar potential. We discuss subleading corrections arising in the EFT from quantum loops. From lattice measurements of the scalar and pNGB masses and of the pNGB decay constant, we extract model parameter values, including those that characterize the scalar potential. The result includes the possibility that the conformal deformation is clearly nonmarginal. The extrapolated values for the decay constants and the scalar mass would then be not far below the current lattice-determined values.

A preponderance of astrophysical and cosmological evidence indicates that the universe contains not only visible matter but also dark matter. In order to suppress the couplings between the dark and visible sectors, a standard assumption is that these two sectors communicate only through a mediator. In this paper we make a simple but important observation: if the dark sector contains multiple components with similar quantum numbers, then this mediator also generically gives rise to dark-sector decays, with heavier dark components decaying to lighter components. This in turn can even give rise to relatively long dark decay chains, with each step of the decay chain also producing visible matter. The visible byproducts of such mediator-induced decay chains can therefore serve as a unique signature of such scenarios. In order to investigate this possibility more concretely, we examine a scenario in which a multicomponent dark sector is connected through a mediator to Standard-Model quarks. We then demonstrate that such a scenario gives rise to multijet collider signatures, and we examine the properties of such jets at both the parton and detector levels. Within relatively large regions of parameter space, we find that such multijet signatures are not excluded by existing monojet and multijet searches. Such decay cascades therefore represent a potential discovery route for multicomponent dark sectors at current and future colliders.

Although the Higgs boson has been discovered, its self-couplings are poorly constrained. This leaves the nature of the Higgs boson undetermined. Motivated by different Higgs potential scenarios other than the Landau-Ginzburg type in the standard model, we systematically organize various new physics scenarios—elementary Higgs, Nambu-Goldstone Higgs, Coleman-Weinberg Higgs, and tadpole-induced Higgs, etc. We find that double-Higgs production at the 27 TeV high-energy LHC can be used to discriminate different Higgs potential scenarios, while it is necessary to use triple-Higgs production at a future 100 TeV proton-proton collider to fully determine the shape of the Higgs potential.

Nonlocal quark bilinear operators connected by link paths are used for studying parton distribution functions (PDFs) and transverse momentum-dependent PDFs of hadrons using lattice QCD. The nonlocality makes it difficult to understand the renormalization and improvement of these operators using standard methods. In previous work, we showed that by introducing an auxiliary field on the lattice, one can understand an on-axis Wilson-line operator as the product of two local operators in an extended theory. In this paper, we provide details about the calculation in perturbation theory of the factor for conversion from our lattice-suitable renormalization scheme to the $\overline{\mathrm{MS}}$ scheme. Extending our work, we study Symanzik improvement of the extended theory to understand the pattern of discretization effects linear in the lattice spacing, $a$, which are present even if the lattice fermion action exactly preserves chiral symmetry. This provides a prospect for an eventual $O(a)$ improvement of lattice calculations of PDFs. We also generalize our approach to apply to Wilson lines along lattice diagonals and to piecewise-straight link paths.

In this paper, we study the single spin asymmetry $A_{UT}^{\sin ({\varphi }_h-{\varphi }_S)}$ of a single hadron production in semi-inclusive deep inelastic scattering within the framework of transverse momentum dependent factorization up to the next-to-leading logarithmic order of QCD. The asymmetry is a contribution of the convolution of the Sivers function and the unpolarized fragmentation function. Specifically, the Sivers function in the coordinate space and perturbative region can represent the convolution of the $C$ coefficients and the corresponding collinear correlation functions, among which the Qiu-Sterman function is the most relevant one. We perform a detailed phenomenological analysis of the Sivers asymmetry at the kinematics of the HERMES and the COMPASS measurements. It is found that the obtained $x_B$-, $z_h$-, and $P_{h\perp }$-dependent asymmetries are basically consistent with the HERMES and the COMPASS measurements.

We describe here measurements with a new device, the “dichroicon,” a Winston-style light concentrator built out of dichroic reflectors, which could allow large-scale neutrino detectors to sort photons by wavelength with small overall light loss. Photon sorting would benefit large-scale water or ice Cherenkov detectors such as Hyper-Kamiokande or IceCube by providing a measure of dispersion, which, in turn, could allow improved position reconstruction and timing. For scintillator detectors like JUNO and upgrades to $\mathrm{SNO}+$ or KamLAND-ZEN or to water-based liquid scintillator detectors like Theia, dichroicons would provide effective discrimination between Cherenkov and scintillation light, allowing them to operate as true hybrid detectors. We include measurements with a prototype dichroicon using first a Cherenkov source to show that spectral photon sorting works as expected. We then present measurements of two different linear-alkyl-benzene-based liquid scintillator sources and demonstrate discrimination between Cherenkov and scintillation light. On the benchtop, we can identify Cherenkov light with better than 90% purity while maintaining a high collection efficiency for the scintillation light. First results from simulations of a large-scale detector are also presented.