We show that in the SU(2) Georgi-Glashow model, ’t Hooft–Polyakov monopoles are produced by a classical instability in magnetic fields above the Ambjørn-Olesen critical field, which coincides approximately with the field at which Schwinger pair production becomes unsuppressed. Below it, monopoles can be produced thermally, and we show that the rate is higher than for pointlike monopoles by calculating the sphaleron energy as a function of the magnetic field. The results can be applied to production of monopoles in heavy-ion collisions or in the early Universe.

We show that heavy-ion collisions at the LHC provide a promising environment to search for signatures with displaced vertices in well-motivated new physics scenarios. Compared to proton collisions, they offer several advantages: (i) the number of parton level interactions per collision is larger, (ii) there is no pileup, (iii) the lower instantaneous luminosity compared to proton collisions allows one to operate the LHC experiments with very loose triggers, and (iv) there are new production mechanisms that are absent in proton collisions We focus on the third point and show that the modification of the triggers alone can increase the number of observable events by orders of magnitude if the long-lived particles are predominantly produced with low transverse momentum. Our results show that collisions of ions lighter than lead are well motivated from the viewpoint of searches for new physics. We illustrate this for the example of heavy neutrinos in the Neutrino Minimal Standard Model.

Inspired by BESIII’s measurement of the decay $Z_c(3900{)}^{\pm }\to {\rho }^{\pm }{\eta }_c$, we calculate the branching fraction ratio between the $\rho {\eta }_c$ and $\pi J/\psi$ decay modes for the charged states $Z_c(3900)$, $Z_c(4020)$, and $Z_c(4430)$ using a quark interchange model. Our results show that (i) the ratio $R_{Z_c(3900)}=\frac{\mathcal{B}(Z_c(3900{)}^{\pm }\to {\rho }^{\pm }{\eta }_c)}{\mathcal{B}(Z_c(3900{)}^{\pm }\to {\pi }^{\pm }J/\psi )}$ is 1.3 and 1.6 in the molecular and tetraquark scenarios, respectively, which is roughly consistent with the experimental data $R^{\exp }=2.2\pm 0.9$, and (ii) the ratios $\frac{\mathrm{\Gamma }[Z_c(3900)\to \rho {\eta }_c]}{\mathrm{\Gamma }[Z_c(4020)\to \rho {\eta }_c]}$ and $\frac{\mathrm{\Gamma }[Z_c(3900)\to \pi J/\psi ]}{\mathrm{\Gamma }[Z_c(4020)\to \pi J/\psi ]}$ are about 12.5 and 24.2, respectively, in the molecular scenario. In contrast, these ratios are about 1.2 in the tetraquark scenario. The nonobservation of the $Z_c(4020)$ signal in the $\pi J/\psi$ decay mode strongly indicates that $Z_c(3900)$ and $Z_c(4020)$ are moleculelike signals which arise from the $D^{(*)}{\overline{D}}^{(*)}$ hadronic interactions.

We develop a method for obtaining exact time-dependent solutions in Jackiw-Teitelboim gravity coupled to nonconformal matter and study consequences for $N{AdS}_2$ holography. We study holographic quenches in which we find that the black hole mass increases. A semiholographic model composed of an infrared $N{AdS}_2$ holographic sector representing the mutual strong interactions of trapped impurities confined at a spatial point is proposed. The holographic sector couples to the position of a displaced impurity acting as a self-consistent boundary source. This effective $0+1$-dimensional description has a total conserved energy. Irrespective of the initial velocity of the particle, the black hole mass initially increases, but after the horizon runs away to infinity in the physical patch, the mass vanishes in the long run. The total energy is completely transferred to the kinetic energy or the self-consistent confining potential energy of the impurity. For initial velocities below a critical value determined by the mutual coupling, the black hole mass changes sign in finite time. Above this critical velocity, the initial condition of the particle can be retrieved from the $SL(2,R)$ invariant exponent that governs the exponential growth of the bulk gravitational $SL(2,R)$ charges at late time.

The usual description of inflationary fluctuations uses the framework of quantum field theory (QFT) in curved spacetime, in which quantum fluctuations are superimposed on a classical background spacetime. Even for large fluctuations, such as those envisioned during a regime of eternal inflation, this framework is frequently used. In the present work we go one step beyond this description by quantizing both the scalar field and the scale factor of the universe. Employing the Lorentzian path integral formulation of semiclassical gravity we restrict to a simplified minisuperspace setting by considering homogeneous transitions. This approach allows us to determine the dominant geometry and inflaton evolution contributing to such amplitudes. We find that for precisely specified initial scale factor and inflaton values (and uncertain momenta), two distinct saddle point geometries contribute to the amplitude, leading to interference effects. However, when the momenta of both scale factor and inflaton are specified with sufficient certainty, only a single saddle point is relevant and QFT in curved spacetime is applicable. In particular we find that for inflaton transitions up the potential, meaningful results are only obtained when the initial uncertainty in the inflaton value is large enough, allowing the dominant evolution to be a complexified slow-roll solution down from a comparatively unlikely position higher up in the potential.

It has been speculated for a long time that neutrinos from a supernova (SN) may undergo fast flavor conversions near the collapsed stellar core. We perform a detailed study of this intriguing possibility, for the first time analyzing two time-dependent state-of-the-art three-dimensional (3D) SN models of $9M_{\odot }$ and $20M_{\odot }$ from recent papers of Glas et al. Both models were computed with multidimensional three-flavor neutrino transport based on a two-moment solver, and both exhibit the presence of the so-called lepton-number emission self-sustained asymmetry (LESA). The transport solution does not provide the angular distributions of the flavor-dependent neutrino fluxes, which are crucial to track the fast flavor instability. To overcome this limitation, we use a recently proposed approach based on the angular moments of the energy-integrated electron lepton-number distribution up to second order, i.e., angle-energy integrals of the difference between ${\nu }_e$ and ${\overline{\nu }}_e$ phase-space distributions multiplied by corresponding powers of the unit vector of the neutrino velocity. With this method we find the possibility of fast neutrino flavor instability at radii smaller than $\sim 20\mathrm{km}$, which is well interior to the neutrinosphere where neutrinos are still in the diffusive and near-equilibrium regime. Our results confirm recent observations in a two-dimensional (2D) (axisymmetric) SN model and in 2D and 3D models with a fixed matter background, which were computed with Boltzmann neutrino transport. However, the flavor unstable locations are not isolated points as discussed previously, but thin skins surrounding volumes where ${\overline{\nu }}_e$ are more abundant than ${\nu }_e$. These volumes grow with time and appear first in the convective layer of the proto-neutron star (PNS), where a decreasing electron fraction and high temperatures favor the occurrence of regions with negative neutrino chemical potential. Since the electron fraction remains higher in the LESA dipole direction, where convective lepton-number transport out from the nonconvective PNS core slows down the deleptonization, flavor unstable conditions become more widespread in the opposite hemisphere. This interesting phenomenon deserves further investigation, since its impact on SN modeling and possible consequences for SN dynamics and neutrino observations are presently unclear.

Electron-positron pair production from vacuum is studied in combined background fields, a binding electric potential well and a laser field. The production process is triggered by the interactions between the bound states in the potential well and the continuum states in the Dirac sea. By tuning the binding potential well, the pair production can be strongly affected by the locality of the bound states. The narrower bound states in position space are more efficient for pair production. This is in contrast to what is commonly expected that the wider extended bound states have larger region to interact with external fields and would thus create more particles. This surprise can be explained as the more localized bound states have a much wider extension in the momentum space, which can enhance the bound-continuum interactions in the creation process. This enhancement manifests itself in both perturbative and nonperturbative production regimes.

We consider the 2D S-matrix bootstrap in the presence of supersymmetry, ${\mathbb{Z}}_2$, and ${\mathbb{Z}}_4$ symmetry. At the boundary of the allowed S-matrix space we encounter well-known integrable models such as the supersymmetric sine-Gordon and restricted sine-Gordon models, novel elliptic deformations thereof, as well as a two parameter family of ${\mathbb{Z}}_4$ elliptic S-matrices previously proposed by Zamolodchikov. We highlight an intricate web of relations between these various S-matrices.

We develop a new real-time approach to vacuum decay based on a reduction to a finite number of degrees of freedom. The dynamics is followed by solving a generalized Schrödinger equation. We first apply this method to a real scalar field in Minkowski space and compare the decay rate with that obtained by the instanton approach. The main difference is in the early time dynamics, where the decay is faster due to the tail of the wave function. We then apply it to a cold atom model recently proposed to simulate vacuum decay experimentally. This approach will be extended to include gravity in a future work.

Mass-varying massive gravity allows the graviton mass to vary according to different environments. We investigate neutron star and white dwarf solutions in this theory and find that the graviton mass can become very large near the compact stars and settle down quickly to small cosmological values away the stars, similar to that of black holes in the theory. It is found that there exists a tower of compact star solutions where the graviton mass decreases radially to zero nontrivially. We compute the massive graviton effects on the mass-radius relations of the compact stars and also compare the relative strengths between neutron stars and white dwarfs in constraining the parameter space of mass-varying massive gravity.

We consider static black holes in Einstein gravity and study parameters characterizing the black hole size, namely the radii of the horizon $R_{+}$, photon sphere $R_{\mathrm{ph}}$, and black hole shadow $R_{\mathrm{sh}}$. We find a sequence of inequalities $\frac{3}{2}{R}_{+}\le R_{\mathrm{ph}}\le \frac{1}{\sqrt{3}}{R}_{\mathrm{sh}}\le 3M$, where $M$ is the black hole mass. These are consistent with and beyond the previously known upper bounds in literature. The Schwarzschild black hole saturates all the inequalities, making it the biggest of all for a given mass. The inequalities include an upper bound of entropy for any quantum system with a given energy. We also point out that some black holes satisfying the dominant energy condition can trap photons to form a stable photon shield outside the event horizon, but the shadow hides it from an observer at infinity.

We analyze the early-time isotropic cosmology in the so-called energy-momentum squared gravity (EMSG). In this theory, a $T_{\mu \nu }{T}^{\mu \nu }$ term is added to the Einstein-Hilbert action, which has been shown to replace the initial singularity by a regular bounce. We show that this is not the case, and the bouncing solution obtained does not describe our Universe since it belongs to a different solution branch. The solution branch that corresponds to our Universe, while nonsingular, is geodesically incomplete. We analyze the conditions for having viable regular-bouncing solutions in a general class of theories that modify gravity by adding higher order matter terms. Applying these conditions on generalizations of EMSG that add a ${(T_{\mu \nu }{T}^{\mu \nu })}^n$ term to the action, we show that the case of $n=5/8$ is the only one that can give a viable bouncing solution, while the $n>5/8$ cases suffer from the same problem as EMSG, i.e., they give nonsingular, geodesically incomplete solutions. Furthermore, we show that the $1/2<n<5/8$ cases can provide a nonsingular initially de Sitter solution. Finally, the expanding, geodesically incomplete branch of EMSG or its generalizations can be combined with its contracting counterpart using junction conditions to provide a (weakly) singular bouncing solution. We outline the junction conditions needed for this extension and provide the extended solution explicitly for EMSG. In this sense, EMSG replaces the standard early-time singularity by a singular bounce instead of a regular one.

We show how a deformed composition law of four-momenta can be used to define, at the classical level, a modified notion of spacetime for a system of two particles through the crossing of worldlines in particle interactions. We present a derivation of a generic relativistic isotropic deformed kinematics and discuss the complementarity and relations with other derivations based on $\kappa$-Poincaré Hopf algebra or on the geometry of a maximally symmetric momentum space.

We explore the possibility of a blue-tilted gravitational wave spectrum from potential-driven slow-roll inflation in the Horndeski theory. In Kamada et al. [Phys. Rev. D 86, 023504 (2012)], it was claimed that a blue gravitational wave spectrum cannot be obtained from stable potential-driven slow-roll inflation within the Horndeski framework. However, it has been demonstrated that the spectrum of primordial gravitational waves can be blue in inflation with the Gauss-Bonnet term, where the potential term is dominant and slow-roll conditions as well as the stability conditions are satisfied. To fill in this gap, we clarify where the discrepancy is coming from. We extend the formulation of Kamada et al. and show that a blue gravitational wave spectrum can certainly be generated from stable slow-roll inflation if some of the conditions previously imposed on the form of the free functions in the Lagrangian are relaxed.

In this article we perform dynamical analysis of a broad class of barotropic fluids in the spatially curved Friedmann-Robertson-Walker (FRW) spacetime background without considering the cosmological constant. The first part of our study concerns the dynamics of a fluid with an unspecified barotropic equation of state (EoS) having as the only assumption the non-negativity of the fluid’s energy density. After defining a new set of dimensionless variables and a new evolution parameter, we introduce the function $\mathrm{\Gamma }$ that encodes the EoS. In this general setup several features of the system are identified: critical points, invariant subsets and the characteristics of the function $\mathrm{\Gamma }$, along with their cosmological interpretations. The second part of our work provides two examples with specific $\mathrm{\Gamma }$ functions. In the first example we provide a $\mathrm{\Gamma }$ function and then we exhibit how it can be trimmed down to a specific class of EoS through physical arguments, while in the second example we discuss the quadratic EoS studied in Phys. Rev. D 74, 023523 (2006) by comparing our approach with their analysis.

We examine natural inflation without using the standard slow-roll approximation by considering the number of physical $e$-foldings, $\ln (a_e{H}_e/aH)$. We show that $\tilde{H}=aH\propto \cos (A\varphi /2{)}^{2/A^2}\sin (A\varphi /2)$ produces a natural inflationary scenario. This model may be solved exactly, showing that the slow-roll approximation overestimates the tensor-to-scalar ratio by about 13%–19% for $n_s\approx 0.96$ and 50–60 $e$-foldings.

The observation of maximally rotating neutron stars (in comparison to nonrotating ones) may provide more information on the behavior of nuclear matter at high densities. We provide a theoretical treatment concerning the effects of the upper bound of the sound speed in dense matter on the bulk properties of maximally rotating (at the mass-shedding limit) neutron stars. In particular, we consider two upper bounds for the speed of sound, $v_s=c$ and $v_s=c/\sqrt{3}$, and the one provided by relativistic kinetic theory. We investigate to what extent the possible predicted upper bounds (from various theories and conjectures) on the speed of sound constrain the ones of various key quantities, including the maximum mass and the corresponding radius, Keplerian frequency, Kerr parameter, and moment of inertia. We mainly focus on the lower proposed limit, $v_s=c/\sqrt{3}$, and we explore in which mass region a rotating neutron star collapses to a black hole. In any case, useful relations of the mentioned bulk properties with the transition density are derived and compared with the corresponding nonrotating cases. We conclude that the proposed limit $v_s=c/\sqrt{3}$ leads to a dramatic decrease on the values of the maximum mass, Kerr parameter, and moment of inertia, preventing a neutron star to reach values which are derived with the consideration of realistic equations of state or from other constraints. Possible measurements of the Kerr parameter and moment of inertia shed light on these issues and help to reveal the speed of sound bound in dense matter.

We present a plausible counterexample to the weak cosmic censorship conjecture in the four-dimensional Einstein-Scalar theory with asymptotically flat boundary conditions. Our setup stems from the analysis of the massive Klein-Gordon equation on a fixed Kerr black hole background. In particular, we construct the quasinormal spectrum numerically, and analytically in the WKB approximation, then go on to compute its backreaction on the Kerr geometry. In the regime of parameters where the analytic and numerical techniques overlap we find perfect agreement. We give strong evidence for the existence of a nonlinear instability at late times.