The last two decades are marked by a renaissance in hadronic spectroscopy caused by the arrival of vast experimental information on exotic states in the spectrum of charmonium and bottomonium. Most of such states have properties at odds with the predictions of the quark model and reside very close to strong hadronic thresholds. Prominent examples are provided by the glorious $X(3872)$ charmoniumlike state and the doubly charmed tetraquark $T_{cc}^{+}$ with the masses within less than 1 MeV from the $D{\overline{D}}^{*}$ and $D{D}^{*}$ open-charm thresholds, respectively. The universality of this feature hints towards the existence of a general pattern for such exotic states. In this work we discuss a possible generic mechanism for the formation of near-threshold molecular states as a result of the strong coupling of compact quark states with a hadronic continuum channel. The compact states that survive the strong coupling limit decouple from the continuum channel and therefore also from the formed hadronic molecule—if realized this scenario would provide a justification to treat hadronic molecules isolated, ignoring the possible influence from surrounding, compact quark-model states. We confront the phenomenology of the $D_{s1}(2460)$ and $D_{s1}(2536)$ with this picture and find consistency, although other explanations remain possible for those states.

We investigate the $B^0\to {\pi }^{-}{K}^{+}X(3872)$ decay via the $D_{s1}(2536)\overline{D}{D}^{*}$ rescattering diagram. The line shape of the $K^{+}X(3872)$ distribution curve around $D_{s1}(2536)\overline{D}$ threshold is very sensitive to the $X(3872)$ mass because the triangle singularity (TS) can be generated from the loop. By means of this characteristic, we can determine whether the $X(3872)$ mass is below or above the $D^{*0}{\overline{D}}^0$ threshold with high precision. The narrowness of $D_{s1}(2536)$ in the loop is one of the key reasons why the TS mechanism of measuring the $X(3872)$ mass may work. The $X(3872)$ width impact on the $K^{+}X(3872)$ line shape is also crucial in the TS mechanism. If the width is as large as 1 MeV, the proposed method of measuring the $X(3872)$ mass would be ruined.

We investigate the effective potential of the Polyakov loop, which is the order parameter for the deconfinement phase transition in finite temperature QCD. Our work is based on the Hamiltonian approach in Coulomb gauge where finite temperature $T$ is introduced by compactifying one space direction. We briefly review this approach and extend earlier work in the Yang-Mills sector by including dynamical quarks. In a first approximation, we follow the usual functional approach and include only 1-loop contributions to the energy, with the finite temperature propagators replaced by their $T=0$ counterparts. It is found that this gives a poor description of the phase transition, in particular for the case of full QCD with $N_f=3$ light flavors. The physical reasons for this unexpected result are discussed, and pinned down to a relative weakness of gluon confinement compared to the deconfining tendency of the quarks. We attempt to overcome this issue by including the relevant gluon contributions from the 2-loop terms to the energy. We find that the 2-loop corrections have indeed a tendency to strengthen the gluon confinement and weaken the unphysical effects in the confining phase, while slightly increasing the (pseudo)critical temperature $T^{*}$ at the same time. To fully suppress artifacts in the confining phase, we must tune the parameters to rather large values, increasing the critical temperature to $T^{*}\approx 340\mathrm{MeV}$ for $G=SU(2)$.

The Unruh-DeWitt model of particle detector is widely used to probe quantum effects in noninertial frames and curved spacetimes. In this paper, we investigate the entanglement dynamics of a quantum system composed of two rotating Unruh-DeWitt detectors in interaction with a fluctuating massless scalar field in vacuum. We obtain the necessary and sufficient condition for the detectors to achieve steady-state entanglement, and systematically investigate the steady-state entanglement of two Unruh-DeWitt detectors rotating in coaxial orbits with the same orbital radius and angular velocity. When the separation between the detectors is vanishing, the detectors can obtain steady-state entanglement dependent on the initial state, as the detectors in uniform acceleration and static detectors in a thermal bath do. Remarkably, however, when the separation between the detectors is nonvanishing but small compared with the transition wavelength of detectors, the detectors can obtain steady-state entanglement irrespectively of what the initial state is. This is the unique phenomenon caused by the centripetal acceleration and is not present in the uniform acceleration case and the case of static detectors in a thermal bath.

We show that thick spectral walls exist in antikink-kink collisions in the ${\varphi }^6$ model. In this model, they are triggered by the so-called delocalized modes that do not exist in the single-soliton sector but emerge in antikink-kink collisions. Therefore, spectral walls are a rather common phenomenon that should occur in many solitonic collisions involving nonsymmetric kinks as, e.g., in the ${\varphi }^8$ or higher power models.

Recently, a new nonlinear mechanism for black hole scalarization, different from the standard spontaneous scalarization, was demonstrated to exist for scalar Gauss-Bonnet theories in which no tachyonic instabilities can occur. Thus the Schwarzschild black hole is linearly stable but instead nonlinear instability can kick in. In the present paper we expand on this idea in the case of multiscalar Gauss-Bonnet gravity with exponential coupling functions of third and fourth leading order in the scalar field. The main motivation comes from the fact that these theories admit hairy compact objects with zero scalar charge, thus zero scalar-dipole radiation, which automatically evades the binary pulsar constraints on the theory parameters. We demonstrate numerically the existence of scalarized black holes for both coupling functions and for all possible maximally symmetric scalar field target spaces. The thermodynamics and the stability of the obtained solution branches is also discussed.

Searches for continuous gravitational waves target nearly monochromatic gravitational wave emission from, e.g., nonaxisymmetric fast-spinning neutron stars. Broad surveys often require us to explicitly search for a very large number of different waveforms, easily exceeding $\sim {10}^{17}$ templates. In such cases, for practical reasons, only the top, say $\sim {10}^{10}$, results are saved and followed up through a hierarchy of stages. Most of these candidates are not completely independent of neighboring ones, but arise due to some common cause: a fluctuation, a signal, or a disturbance. By judiciously clustering together candidates stemming from the same root cause, the subsequent follow-ups become more effective. A number of clustering algorithms have been employed in past searches based on iteratively finding symmetric and compact overdensities around candidates with high detection statistic values. The new clustering method presented in this paper is a significant improvement over previous methods: it is agnostic about the shape of the overdensities, is very efficient and it is effective: at a very high detection efficiency, it has a noise rejection of $99.99\%$, is capable of clustering two orders of magnitude more candidates than attainable before and, at fixed sensitivity it enables more than a factor of 30 faster follow-ups. We also demonstrate how to optimally choose the clustering parameters.

Peculiar phenomena appear in the discretization of a system invariant under reparametrization. The structure of the continuum limit is markedly different from the usual one, as in lattice QCD. First, the continuum limit does not require tuning a parameter in the action to a critical value. Rather, there is a regime where the system approaches a sort of asymptotic topological invariance (“Ditt-invariance”). Second, in this regime the expansion in the number of discretization points provides a good approximation to the transition amplitudes. These phenomena are relevant for understanding the continuum limit of quantum gravity. I illustrate them here in the context of a simple system.

We combine the ideas of a Weyl scaling invariant dark energy action, which eliminates black hole horizons, with the “gravastar” idea of a jump in the hole interior from a normal matter equation of state to an equation of state where pressure plus density approximately sum to zero. Using the Tolman-Oppenheimer-Volkoff equation, which requires continuous pressure, we present Mathematica notebooks in which the structure of the gravastar is entirely governed by the action and the equation of state, with the radii where structural changes occur emerging from the dynamics, rather than being specified in advance. The notebooks work even with zero cosmological constant, but when the cosmological constant is nonzero, there is a very small black hole “wind” that we calculate by a relativistic extension of standard pressure driven isothermal stellar wind theory.

We present a rotating regular black hole whose inner horizon has zero surface gravity for any value of the spin parameter and is, therefore, stable against mass inflation. Our metric is built by combining two successful strategies for regularizing singularities, i.e., by replacing the mass parameter with a function of $r$ and by introducing a conformal factor. The mass function controls the properties of the inner horizon, whose displacement away from the Kerr geometry’s inner horizon is quantified in terms of a parameter $e$; while the conformal factor regularizes the singularity in a way that is parametrized by the dimensionful quantity $b$. The resulting line element not only avoids the stability issues that are common to regular black hole models endowed with inner horizons, but is also free of problematic properties of the Kerr geometry, such as the existence of closed timelike curves. While the proposed metric has all the phenomenological relevant features of singular rotating black holes—such as ergospheres, light ring and innermost stable circular orbit—showing a remarkable similarity to a Kerr black hole in its exterior, it allows nonetheless sizable deviations, especially for large values of the spin parameter $a$. In this sense, the proposed rotating “inner-degenerate” regular black hole metric is not only amenable to further theoretical investigations but most of all can represent a viable geometry to contrast to the Kerr one in future phenomenological tests.

We investigate the leading-order behavior of matter fields in the preinflationary era using the semiclassical approximation. Many inflationary models assume without supporting arguments that the Universe was radiation dominated prior to inflation, leading to modifications of cosmological observables, such as the cosmic microwave background power spectrum. In previous work, we demonstrated that conformally coupled scalar fields do have a radiationlike contribution to the stress-energy tensor at sufficiently early times. In this work, we extend these arguments to apply to massless spin-1 fields and massive or massless spin-$\frac{1}{2}$ fields. We find massless spin-1 fields always have a radiationlike contribution. For spin-$\frac{1}{2}$ fields, we find the contribution at early times is radiationlike assuming this is the dominant contribution to the stress-energy tensor.

Hydrodynamic and magnetohydrodynamic turbulence in the early Universe can drive gravitational waves (GWs) and imprint their spectrum onto that of GWs, which might still be observable today. We study the production of the GW background from freely decaying magnetohydrodynamic turbulence from helical and nonhelical initial magnetic fields. To understand the produced GW spectra, we develop a simple model on the basis of the evolution of the magnetic stress tensor. We find that the GW spectra obtained in this model reproduce those obtained in numerical simulations if we consider the detailed time evolution of the low frequency tail of the stress spectrum from numerical simulations. We also show that the shapes of the produced GW frequency spectra are different for helical and nonhelical cases for the same initial magnetic energy spectra. Such differences can help distinguish helical and nonhelical initial magnetic fields from a polarized background of GWs—especially when the expected circular polarization cannot be detected directly.

Extreme-mass-ratio inspirals are important sources for space-borne gravitational-wave detectors. Such a source normally consists of a stellar-mass black hole (BH) and a Kerr supermassive BH (SMBH), but recent astrophysical models predict that the small body could also be a stellar-mass binary BH (BBH). A BBH reaching several gravitational radii of a SMBH will induce rich observable signatures in the waveform, but the current numerical tools are insufficient to simulate such a triple system while capturing the essential relativistic effects. Here we solve the problem by studying the dynamics in a frame freely falling alongside the BBH. Since the BBH is normally nonrelativistic and much smaller than the curvature radius of the Kerr background, the evolution in the free-fall frame reduces essentially to Newtonian dynamics, except for a perturbative gravitoelectromagnetic force induced by the curved background. We use this method to study the BBHs on near-circular orbits around a SMBH and track their evolution down to a distance of 2–3 gravitational radii from the SMBH. Our simulations reveal a series of dynamical effects that are not shown in the previous studies using conventional methods. The most notable one is a radial oscillation and azimuthal drift of the BBH relative to the SMBH. These results provide new insight into the evolution and detection of the extreme-mass-ratio inspirals containing BBHs.

Dense clouds of neutrinos and antineutrinos can exhibit fast collective flavor oscillations. Previously, in [S. Bhattacharyya and B. Dasgupta, Phys. Rev. Lett. 126, 061302 (2021).], we proposed that such flavor oscillations lead to depolarization—i.e., an irreversible mixing of the flavors—whose extent depends on the initial momentum distributions of the different flavors. In this paper, we elaborate and extend this proposal, and compare it with related results in the literature. We present an accurate analytical estimate for the lower resting point of the fast flavor pendulum and underline the relaxation mechanisms—i.e., transverse relaxation, multipole cascade, and mixing of flavor waves—that cause it to settle down. We estimate the extent of depolarization, its dependence on momentum and net lepton asymmetry, and its generalization to three flavors. Finally, we prescribe approximate analytical recipes for the depolarized distributions and fluxes that can be used in supernova/nucleosynthesis simulations and supernova neutrino phenomenology.

The dynamical tide can play an important role in the orbital motion of close eccentric double white dwarf binaries. As the launching of the space-based gravitational-wave detector, the Laser Interferometer Space Antenna (LISA), is just around the corner, detection of gravitational wave signals from such systems is anticipated. In this paper, we discuss the influence of the dynamical tide on eccentric orbits, focusing on the effect on periastron precession. We show that, in orbits with a high eccentricity, resonance can cause a large precession when a harmonic of the orbital frequency matches the natural frequencies of the normal modes of the star. In contrast to the case with circular orbits, each mode can encounter multiple resonances with different harmonics and these resonant regions can cover about 10% of the frequency space for orbits with close separations. In this case, the tidal precession effect is distinct from the other contributions and can be identified with LISA if the signal-to-noise ratio is high enough. However, within the highly eccentric-small separation region, the dynamical tide causes chaotic motion and the gravitational wave signal becomes unpredictable. Even not at resonance, the dynamical tide can contribute up to 20% of the precession for orbits close to Roche-lobe filling separation with low eccentricities and LISA can resolve these off-resonant dynamical tide effects within the low eccentricity-small orbital separation region of the parameter space. For lower mass systems, the dynamical tide effect can degenerate with the uncertainties of the eccentricity, making it unmeasurable from the precession rate alone. For higher mass systems, the radiation reaction effect becomes significant enough to constrain the eccentricity, allowing the measurement of the dynamical tide.

We present the analytical results of the two-loop QCD amplitudes for hadronic $tW$ production, focusing on the leading color and light fermion-loop contributions. The calculation of the two-loop integrals is performed using the method of canonical differential equations. The results have been expressed in terms of multiple polylogarithms and checked by comparing the infrared divergences with the predictions from anomalous dimensions. Combining these with the one-loop squared amplitudes we have computed previously, we obtain the hard function relevant to a next-to-next-to-leading order Monte Carlo calculation. We find that the hard function varies slowly in the region with small top-quark velocity but increases dramatically in the region with large top-quark velocity. After performing phase space integration and convolution with the parton distribution function, the leading color hard function gives a correction of about 5.4% to the leading order cross section, while the light fermion-loop diagrams contribute about $-1.4\%$.

The equation of state (EOS) for cold dense matter inside neutron stars is investigated by using holographic QCD models that consist of the Einstein-Maxwell-dilaton system and the improved Karch-Katz-Son-Stephanov action for the flavor part. This method of describing holographic nuclear matter in the $\mathrm{Einstein}\text{−}\mathrm{Maxwell}\text{−}\mathrm{dilaton}+\mathrm{Karch}\text{−}\mathrm{Katz}\text{−}\mathrm{Son}\text{−}\mathrm{Stephanov}$ framework is different from that by using the Dirac-Born-Infeld action and the Chern-Simons terms. Combining with the Hebeler-Lattimer-Pethick-Schwenk intermediate EOS, the hybrid EOS inside the neutron stars is constructed. The obtained hybrid EOS is located in the range that is defined by the low-density chiral effective theory, the high-density perturbative QCD, and the polytropic interpolations between them, and is constrained by the astrophysical observations. The square of the sound velocity reaches a maximum value larger than 0.8 in the region of 2–5 times the saturation baryon number density and approaches the conformal limit at the high baryon density range. The mass-radius relation and the tidal deformability of the neutron stars are in agreement with astrophysical measurements. The possible maximum mass for the neutron star is about $2.5M_{\odot }$ and the radius is about 12 km then. It is noticed that the holographic quark matter branch in the mass-radius relation is always unstable and the holographic nuclear matter can produce a stable branch. These results indicate that even in the core of the NS, the matter is still in the confinement phase and the quark matter is not favored.

The partons model reveals the dynamical structure of nucleons (protons and neutrons). Studies related to thermodynamic quantities of nucleons present interesting and topical questions. In this work, for the first time we apply Page’s theory of the studies of black holes to investigate the entropy of a proton system. Inspired by the quantum entanglement entropy in black hole information theories, we establish the proton entanglement with the similar way. Based our calculations, the proton entanglement entropy has the approximate form $S=\ln m-1/2$, where $m$ represents the partons density of proton (mainly gluon and sea quark contributions) at small Bjorken $x$. Our calculations using Page’s theory are well in agreement with the recent deep inelastic scattering measurements from the H1 Collaboration.

Neutrinos might interact among themselves through forces that have so far remained hidden. Throughout the history of the Universe, such secret interactions could lead to scatterings between the neutrinos from supernova explosions and the nonrelativistic relic neutrinos left over from the big bang. Such scatterings can boost the cosmic neutrino background ($\mathrm{C}\nu \mathrm{B}$) to energies of $\mathcal{O}(\mathrm{MeV})$, making it, in principle, observable in experiments searching for the diffuse supernova neutrino background. Assuming a model-independent, but flavor universal, four-Fermi interaction, we determine the upscattered cosmic neutrino flux, and derive constraints on such secret interactions from the latest results from Super-Kamiokande. Furthermore, we also study prospects for detection of the boosted flux in future lead-based coherent elastic neutrino-nucleus scattering experiments. Nevertheless, given current constraints on flavor universal self-interactions, we find that the upscattered $\mathrm{C}\nu \mathrm{B}$ contribution to the total DSNB flux is negligible, making a possible measurement of the boosted $\mathrm{C}\nu \mathrm{B}$ insurmountable.

Direct detection experiments for dark matter are increasingly ruling out large parameter spaces. However, light dark matter models with particle masses $<\mathrm{GeV}$ are still largely unconstrained. Here we examine a proposal to use atom interferometers to detect a light dark matter subcomponent at sub-GeV masses. We describe the decoherence and phase shifts caused by dark matter scattering off of one “arm” of an atom interferometer using a generalized dark matter direct detection framework. This allows us to consider multiple channels: nuclear recoils, hidden photon processes, and axion interactions. We apply this framework to several proposed atom interferometer experiments. Because atom interferometers are sensitive to extremely low momentum deposition and their coherent atoms give them a boost in sensitivity, these experiments will be highly competitive and complementary to other direct detection methods. In particular, atom interferometers are uniquely able to probe a dark matter subcomponent with $m_{\chi }\lesssim 10\mathrm{keV}$. We find that, for a mediator mass $m_{\varphi }={10}^{-10}{m}_{\chi }$, future atom interferometers could close a gap in the existing constraints on nuclear recoils down to ${\overline{\sigma }}_n\sim {10}^{-50}{\mathrm{cm}}^2$ for $m_{\chi }\sim {10}^{-5}–{10}^{-1}\mathrm{MeV}$ dark matter masses.