We calculate the transport of a theory with two conserved currents by the holographic method and compare it with graphene data to determine its dynamical exponents $(z,\theta )$, which characterizes a “quantum critical point.” As a result, we find that the electric and the thermal conductivity data can be fit more naturally if we assume $(z,\theta )=(3/2,1)$ rather than (1,0). Furthermore, we find that thermoelectric power data at high temperature can be fit if we use ($3/2,1$) but not by (1,0). The $\theta =1$ result can be interpreted as taking into account the fermionic nature of the electrons, and $z=3/2$ can be interpreted as the flattened band by the strong interaction.

We consider spacetime endowed with a zero-point length, i.e., with an effective metric structure which allows for a (quantum-mechanically arising) finite distance $L_0$ between events in the limit of their coincidence. Restricting attention to null separated events, we find an expression for the Ricci (bi)scalar in this zero-point-length metric; this is done for when geometric circumstances are such that the collection of all null geodesics emerging from a point $P$ has all the information needed to fix the value of scalar curvature at $P$. Taking then the coincidence and further $L_0\to 0$ limits, we find that this expression does not reduce to the Ricci scalar $R$ of the ordinary metric but to $(D-1)R_{ab}{l}^a{l}^b$ in $D$-dimensional spacetime ($D\ge 4$), where $R_{ab}$ and $l^a$ are the ordinary Ricci tensor and tangent vector to the null geodesics. This adds nicely to the existing results for time and space separations. This finding seems to give further support to the view that the quantity $R_{ab}{l}^a{l}^b$, ubiquitous in horizon thermodynamics, embodies something which remains as a relic/remnant/memory of a quantum underlying structure for spacetime in the limit of (actual detectability of) this quantumness fading away, and which as such should enter the scene when aiming to derive/motivate the field equations. Further, it turns out to be the same quantity used in an existing derivation of field equations from a thermodynamic variational principle, thus adding further evidence of an origin as quantum-spacetime relic for the latter.

Scalar perturbations around the Kerr black hole in scalar-Einstein-Gauss-Bonnet (SEGB) theory are studied in the time domain. To overcome the “outer boundary problem” that usually encountered in traditional numerical calculations, we apply the hyperboloidal compactification technique to perform a ($2+1$)-dimensional simulation aiming to obtain a precise object picture of the wave propagation under the scalar field perturbation. We find that the big enough coupling constant between the scalar field and the Gauss-Bonnet curvature is responsible to destroy the original Kerr black hole. The breakdown of the Kerr spacetime happens earlier and the instability becomes more violent when the coupling becomes stronger. We further present object confirmations on the special case for the negative coupling where there exists a minimum rotation and below which the instability can never happen no matter how strong the coupling is. We also illustrate the fine structure property in the quasinormal ringing frequency once there is the coupling, and present the characteristic imprint of the SEGB theory. We expect that such a fine structure can be detected in the future gravitational wave observation to test the SEGB theory.

It was recently discovered that Killing horizons in the generic Kerr-NUT–(anti-) de Sitter spacetimes are projectively singular, i.e., their spaces of the null generators have singular geometry. Only if the cosmological constant takes the special value determined by the Kerr and NUT parameters, and the radius of the horizon, then the corresponding horizon does not suffer that problem. In the current paper, the projectively nonsingular horizons are investigated. They are found to be outermost and nonextremal. Every projectively nonsingular horizon can be used to define a global completion of the Kerr-NUT–de Sitter spacetime it is contained in. The resulting spacetime extends from ${\mathcal{I}}^{-}$ to ${\mathcal{I}}^{+}$, has the topology of $\mathbb{R}\times S_3$, and is smooth except for a possible Kerr-like singularity.

The two-Higgs-doublet model (2HDM) with spontaneously broken $Z_2$ symmetry predicts a production of domain walls at the electroweak scale. We derive cosmological constraints on model parameters for both type-I and type-II 2HDMs from the requirement that domain walls do not dominate the Universe by the present day. For type-I 2HDMs, we deduce the lower bound on the key parameter $\tan \beta >{10}^5$ for a wide range of Higgs-boson masses $\sim 100\mathrm{GeV}$ or greater, close to the Standard Model alignment limit. In addition, we perform numerical simulations of the 2HDM with an approximate as well as an exact $Z_2$ symmetry but biased initial conditions. In both cases, we find that domain wall networks are unstable and, hence, do not survive at late times. The domain walls experience an exponential suppression of scaling in these models, which can help ameliorate the stringent constraints found in the case of an exact discrete symmetry. For a 2HDM with softly broken $Z_2$ symmetry, we relate the size of this exponential suppression to the soft-breaking bilinear parameter $m_{12}$, allowing limits to be placed on this parameter of order $\mu \mathrm{eV}$, such that domain wall domination can be avoided. In particular, for type-II 2HDMs, we obtain a corresponding lower limit on the $CP$-odd phase $\theta$ generated by QCD instantons, $\theta \gtrsim {10}^{-11}/(\sin \beta \cos \beta )$, which is in some tension with the upper limit of $\theta \lesssim {10}^{-11}–{10}^{-10}$, as derived from the nonobservation of a nonzero neutron electric dipole moment. For a $Z_2$-symmetric 2HDM with biased initial conditions, we are able to relate the size of the exponential suppression to a biasing parameter $ϵ$ so as to avoid domain wall domination.

We study cosmological perturbation theory in the cosmology associated with conformal gravity, establish the validity of the decomposition theorem for it, and then use the theorem to provide an exact solution to the theory in the recombination era. Central to our approach is the use of a fully gauge invariant formulation of the cosmological fluctuation equations. In the recombination era, not only is perturbation theory applicable, because of its specific structure in the conformal case, the fluctuation equations are found to greatly simplify. Using a master equation for scalar, vector, and tensor fluctuation modes, we show that the radial equations for the three-dimensional vector and tensor modes are, respectively, the same as those of scalar modes in five and seven spatial dimensions. This enables us to construct normalization conditions for the three-dimensional modes.

Motivated by evidence for quark-gluon plasma signatures in small systems, we study a simple model of a massless, noninteracting scalar field confined with Dirichlet boundary conditions. We use this system to investigate the finite size corrections to thermal field–theoretically derived quantities compared to the usual Stefan-Boltzmann limit of an ideal gas not confined in any direction. Two equivalent expressions with different numerical convergence properties are found for the free energy in $D$ rectilinear spacetime dimensions with $c\le D-1$ spatial dimensions of finite extent. We find that the first law of thermodynamics generalizes such that the pressure depends on direction. For systems with finite dimension(s) but infinite volumes, such as a field constrained between two parallel plates or a rectangular tube, the relative fluctuations in energy are zero, and hence the canonical and microcanonical ensembles are equivalent. We present precise numerical results for the free energy, total internal energy, pressure, entropy, and heat capacity of our field between parallel plates, in a tube, and in finite volume boxes of various sizes in four spacetime dimensions. For temperatures and system sizes relevant for heavy ion phenomenology, we find large deviations from the Stefan-Boltzmann limit for these quantities, especially for the pressure. Our main result is the discovery that an isolated system of fields constrained between parallel plates reveals a divergent isoenergetic compressibility at a critical length $L_c\sim 1/T$. This divergence constitutes a novel phase transition, which, unlike the usual temperature-driven phase transition, is driven solely by the size of the system.

The probabilities of various elementary laser-photon-electron/positron interactions display in selected phase space and parameter regions typical nonperturbative dependencies such as $\propto \mathcal{P}\exp \{-a{E}_{\mathrm{crit}}/E\}$, where $\mathcal{P}$ is a preexponential factor, $E_{\mathrm{crit}}$ denotes the critical Sauter-Schwinger field strength, and $E$ characterizes the (laser) field strength. While the Schwinger process with $a=a_S\equiv \pi$ and the nonlinear Breit-Wheeler process in the tunneling regime with $a=a_{n\ell \mathrm{BW}}\equiv 4m/3{\omega }^{\prime }$ (with ${\omega }^{\prime }$ the probe photon energy and $m$ the electron/positron mass) are famous results, we point out here that also the nonlinear Compton scattering exhibits a similar behavior when focusing on high harmonics. Using a suitable cutoff $c>0$, the factor $a$ becomes $a=a_{n\ell C}\equiv \frac{2}{3}cm/(p_0+\sqrt{p_0^2-m^2)}$. This opens the avenue toward a new signature of the boiling point of the vacuum even for field strengths $E$ below $E_{\mathrm{crit}}$ by employing a high electron beam-energy $p_0$ to counter balance the large ratio $E_{\mathrm{crit}}/E$ by a small factor $a$ to achieve $E/a\to E_{\mathrm{crit}}$. In the weak-field regime, the cutoff facilitates a threshold leading to multiphoton signatures showing up in the total cross section at subthreshold energies.

A number of proposed and ongoing experiments search for axion dark matter with a mass nearing the limit set by small scale structure [$\mathcal{O}({10}^{-21}\mathrm{eV})$]. We consider the late universe cosmology of these models, showing that requiring the axion to have a matter-power spectrum that matches that of cold dark matter constrains the magnitude of the axion couplings to the visible sector. Comparing these limits to current and future experimental efforts, we find that many searches require axions with an abnormally large coupling to Standard Model fields, independently of how the axion was populated in the early Universe. We survey mechanisms that can alleviate the bounds, namely, the introduction of large charges, various forms of kinetic mixing, a clockwork structure, and imposing a discrete symmetry. We provide an explicit model for each case and explore their phenomenology and viability to produce detectable ultralight axion dark matter.

A massless dark photon could affect standard-model particles only via higher-dimensional operators and would therefore have eluded recent searches for its massive counterpart, which were based on the assumption that the latter had renormalizable interactions with known fermions due to gauge kinetic mixing. In this study, we entertain the possibility that the massless dark photon has non-negligible flavor-changing dipole-type couplings with the $u$ and $c$ quarks, giving rise to the decays of charmed hadrons into a lighter hadron plus missing energy carried away by the dark photon. We propose to investigate decays of this kind, especially those in which the parents are the charmed pseudoscalar mesons $D^{+}$, $D^0$, and $D_s^{+}$ and singly charmed baryons ${\mathrm{\Lambda }}_c^{+}$, ${\mathrm{\Xi }}_c^{+}$, and ${\mathrm{\Xi }}_c^0$. Employing a simplified new-physics model satisfying the relevant constraints, we find that the branching fractions of these processes could be as large as several times ${10}^{-5}$. This suggests that one or more of them might in the near future fall within reach of the ongoing Belle II and BESIII experiments. Since the same underlying operators are responsible for all of these transitions, detecting one of them automatically implies particular predictions for the others, allowing for additional experimental checks on the massless-dark-photon scenario.

Starting from the lattice Landau gauge gluon and ghost propagator data we use a sequence of Padé approximants, identify the poles and zeros for each approximant and map them into the analytic structure of the propagators. For the Landau gauge gluon propagator the Padé analysis identifies a pair of complex conjugate poles and a branch cut along the negative real axis of the Euclidean $p^2$ momenta. For the Landau gauge ghost propagator the Padé analysis shows a single pole at $p^2=0$ and a branch cut also along the negative real axis of the Euclidean $p^2$ momenta. The method gives precise estimates for the gluon complex poles that agree well with other estimates found in the literature. For the branch cut the Padé analysis gives, at least, a rough estimate of the corresponding branch point.

We introduce a way to express compact quantum electrodynamics with dynamical matter on two- and three-dimensional spatial lattices in a gauge redundancy-free manner while preserving translational invariance. By transforming to a rotating frame, where the matter is decoupled from the gauge constraints, we can express the gauge field operators in terms of dual operators. In two space dimensions, the dual representation is completely free of any local constraints. In three space dimensions, local constraints among the dual operators remain but involve only the gauge field degrees of freedom (and not the matter degrees of freedom). These formulations, which reduce the required Hilbert space dimension, could be useful for both numerical (classical) Hamiltonian computations and quantum simulation or computation.

The $DDK$ three-body system is supposed to be bound due to the strongly attractive interaction between the $D$ meson and the $K$ meson in the isospin zero channel. The minimum quark content of this three-body bound state is $cc\overline{q}\overline{s}$ with $q=u$, $d$. It will be an explicitly exotic tetraquark state once discovered. In order to confirm the phenomenological study of the $DDK$ system, we can refer to lattice QCD as a powerful theoretical tool parallel to the experiment measurement. In this paper, a three-body quantization condition scheme is derived via the nonrelativistic effective theory and the particle-dimer picture in a finite volume. The lattice spectrum of this three-body system is calculated within the existing model inputs. The spectrum shows various interesting properties of the $DDK$ system, and it may reveal the nature of the $D^{*}(2317)$. This predicated spectrum is expected to be tested in future lattice simulations.

The effect of quark anomalous magnetic moment (AMM) to chiral restoration and deconfinement phase transitions under magnetic fields is investigated in a Pauli-Villars regularized Polyakov-extended Nambu–Jona-Lasinio model. A linear-in-$B$ term for quark anomalous magnetic moment is introduced to the Lagrangian density of our model, and it plays the role of inverse catalysis to the phase transitions. With fixed magnetic field, the critical temperature decreases with quark AMM. When fixing quark AMM, the critical temperature increases with magnetic field for a small quark AMM, but decreases with magnetic field for a large quark AMM. The critical temperature of chiral restoration and deconfinement phase transitions is determined by the two competing factors, the catalysis effect of magnetic field and inverse catalysis of quark anomalous magnetic moment.

Inspired by the event accumulation around 2.6 GeV in the ${\eta }^{\prime }{\pi }^{+}{\pi }^{-}$ invariant mass spectrum of $J/\psi \to \gamma {\eta }^{\prime }{\pi }^{+}{\pi }^{-}$, which was reported by the BESIII Collaboration, we carry out the study of the mass spectrum and decay behavior of four radial excitations in the pseudoscalar meson family, which include ${\eta }^{(\prime )}(6S)$ and ${\eta }^{(\prime )}(7S)$. Combining with these analysis, we present the calculation of the reactions induced by a pion or kaon on the proton target which are relevant to these four discussed states. According to this information, we give concrete experimental suggestion of searching for them, which will become a new task for future experiments.

The rare radiative flavor changing top decays $t\to c\gamma$ and $t\to cg$ (and the even rarer $t\to u\gamma$ and $t\to ug$) have been processes of interest for decades as they offer a key probe for studying top quark properties. However, an explicit analytical study of the branching ratios and $CP$ asymmetries resulting from these loop level processes has thus far evaded attention. In this work, we provide the formulation for the $CP$ asymmetry resulting from the total kinetic contribution of the loop integrals and their imaginary parts, as well as an updated numerical computation of the predicted Standard Model (SM) branching fractions. These rare processes are suppressed in the SM by the Glashow-Iliopoulos-Maiani mechanism. However, the results presented here can easily be exported for use in minimal extensions of the SM including vectorlike quarks or in two-Higgs-doublet models where radiative fermionic decay processes can be enhanced relative to the SM by several orders of magnitude. Such processes provide an experimentally clean signature for new fundamental physics and can potentially be tested by current collider experiments. These topical beyond the SM theories are an elegant means to provide improved global fits to the latest results emerging from flavor physics, Cabibbo-Kobayashi-Maskawa, and precision electroweak measurements.

We explore a reduced phase space quantization of loop quantum cosmology (LQC) for a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) universe filled with reference fields and an inflaton field in a Starobinsky inflationary potential. We consider three separate cases in which the reference fields are taken to be the Gaussian dust, the Brown-Kuchař dust, and massless Klein-Gordon scalar reference fields respectively. This is a “two-fluid” model in which reference fields act as global clocks providing a physical time in an inflationary spacetime, and allow bypassing various technical hurdles in conventional quantum cosmological models. The reduced phase space is obtained in terms of the Dirac observables of the gravitational as well as the inflaton degrees of freedom. The physical Hamiltonians of the two dust models take the same form but turn out to be quite different from that of the Klein-Gordon reference field which reflects an aspect of the multiple choice problem of time. Loop quantization is implemented using the so-called $\overline{\mu }$ scheme and the Schrödinger equations involving the physical Hamiltonian operators generating the evolution in the physical time in the dust and massless Klein-Gordon models are obtained. These turn out to be quantum difference equations with the same nonsingular structure as for other models in LQC. We study some phenomenological implications of the quantization using the effective dynamics resulting from the reduced phase space quantization including the resolution of the big bang singularity via a quantum bounce, and effects of the different reference fields on e-foldings in both the preinflationary and the slow-roll inflationary phases. We find that different clocks, even when starting with a small but same energy density, can leave tiny but different imprints on the inflationary dynamics. In addition, for Brown-Kuchař dust, the choice of a negative energy density can result in a cyclic evolution before the onset of inflation constraining certain values for ideal clocks.

There are many reasons to believe that there is a fundamental minimum length scale below which distances cannot be reliably resolved. One method of constructing a quantum field with a finite minimum length scale is to use bandlimited quantum field theory, where the spacetime is mathematically both continuous and discrete. This is a modification to the field, which has been shown to have many consequences at the level of the field. We consider an operational approach and use a pair of particle detectors (two-level qubits) as a local probe of the field, which are coupled to the vacuum of the bandlimited massless scalar field in a time-dependent way through a switching function. We show that, mathematically, the bandlimit modifies the spatial profile of the detectors so that they are only quasilocal. We explore two different types of switching functions, Gaussian and Dirac delta. We find that, with Gaussian switching, the bandlimit exponentially suppresses the deexcitation of the detectors when the energy gap between the two levels is larger than the bandlimit. If the detectors are prepared in ground state, in certain regions of the parameter space they are able to extract more entanglement from the field than if there was no bandlimit. When the detectors couple with Dirac-delta switching, we show that a particle detector is most sensitive to the bandlimit when it couples to a small but finite region of spacetime. We find that the effects of a bandlimit are detectable using local probes. This work is important because it illustrates the possible observable consequences of a fundamental bandlimit in a quantum field.

We establish a one-to-one mapping between entanglement energy, entropy, and temperature (quantum entanglement mechanics) with Komar energy, black hole entropy, and Hawking temperature, respectively. We show this explicitly for 4D spherically symmetric asymptotically flat and nonflat space-times with single and multiple horizons. We exploit an inherent scaling symmetry of entanglement entropy and identify scaling transformations that generate an infinite number of systems with the same entanglement entropy, distinguished only by their respective energies and temperatures. We show that this scaling symmetry is present in most well-known systems starting from the two-coupled harmonic oscillator to quantum scalar fields in spherically symmetric space-time. The scaling symmetry allows us to identify the cause of divergence of entanglement entropy to the generation of (near) zero-modes in the systems. We systematically isolate the zero-mode contributions using suitable boundary conditions. We show that the entanglement entropy and energy of quantum scalar field scale differently in space-times with horizons and flat space-time. The relation $E=2TS$, in analogy with the horizon’s thermodynamic structure, is also found to be universally satisfied in the entanglement picture. We then show that there exists a one-to-one correspondence leading to the Smarr-formula of black hole thermodynamics for asymptotically flat and nonflat space-times.

We investigate a perturbatively renormalizable $S_q$ invariant model with $N=q-1$ scalar field components below the upper critical dimension $d_c=10/3$. Our results hint at the existence of multicritical generalizations of the critical models of spanning random clusters and percolations in three dimensions. We also discuss the role of our multicritical model in a conjecture that involves the separation of first and second order phases in the $(d,q)$ diagram of the Potts model.