We report the first observation of the Dalitz decay ${\eta }^{\prime }\to \gamma e^{+}{e}^{-}$, based on a data sample of 1.31 billion $J/\psi$ events collected with the BESIII detector. The ${\eta }^{\prime }$ mesons are produced via the $J/\psi \to \gamma {\eta }^{\prime }$ decay process. The ratio $\mathrm{\Gamma }({\eta }^{\prime }\to \gamma e^{+}{e}^{-})/\mathrm{\Gamma }({\eta }^{\prime }\to \gamma \gamma )$ is measured to be $(2.13\pm 0.09(\text{stat})\pm 0.07(\text{sys}))\times 10^{-2}$. This corresponds to a branching fraction $\mathcal{B}({\eta }^{\prime }\to \gamma e^{+}{e}^{-})=(4.69\pm 0.20(\text{stat})\pm 0.23(\text{sys}))\times 10^{-4}$. The transition form factor is extracted and different expressions are compared to the measured dependence on the $e^{+}{e}^{-}$ invariant mass. The results are consistent with the prediction of the vector meson dominance model.

The complex Langevin method aims at performing path integral with a complex action numerically based on complexification of the original real dynamical variables. One of the poorly understood issues concerns occasional failure in the presence of logarithmic singularities in the action, which appear, for instance, from the fermion determinant in finite density QCD. We point out that the failure should be attributed to the breakdown of the relation between the complex weight that satisfies the Fokker-Planck equation and the probability distribution associated with the stochastic process. In fact, this problem can occur, in general, when the stochastic process involves a singular drift term. We show, however, in a simple example that there exists a parameter region in which the method works, although the standard reweighting method is hardly applicable.

We investigate the inflation model with a massive vector multiplet in a case that the action of the vector multiplet is extended to the Dirac–Born–Infeld (DBI) type one. We show the massive DBI action in four-dimensional $\mathcal{N}=1$ supergravity and find that the higher-order corrections associated with the DBI extension make the scalar potential flat with a simple choice of the matter couplings. We also discuss the DBI extension of the new minimal Starobinsky model and find that it is dual to a special class of the massive DBI action.

Conventionally, one calculates a zero in a beta function by computing this function to a given loop order and solving for the zero. Here we discuss a different method which is applicable in theories where one can perform a partial diagrammatic summation to infinite-loop order. We show that this method, compared with the conventional method, yields much better agreement with exact results in the case of an asymptotically free gauge theory with $\mathcal{N}=1$ supersymmetry. Applications to other field theories are also discussed.

In this paper we will discuss the derivation of the so-called vanishing beta function curves which can be used to explore the fixed point structure of the theory under consideration. This can be applied to the $\mathrm{O}(N)$ symmetric theories, essentially, for arbitrary dimensions ($D$) and field component ($N$). We will show the restoration of the Mermin-Wagner theorem for theories defined in $D\le 2$ and the presence of the Wilson-Fisher fixed point in $2<D<4$. Triviality is found in $D>4$. Interestingly, one needs to make an excursion to the complex plane to see the triviality of the four-dimensional $\mathrm{O}(N)$ theories. The large-$N$ analysis shows a new fixed point candidate in $4<D<6$ dimensions which turns out to define an unbounded fixed point potential supporting the recent results by Percacci and Vacca [Phys. Rev. D 90, 107702 (2014)].

Within the framework of adiabatic regularization, we present a simple formalism to calculate number density and renormalized energy-momentum density of spin $1/2$ particles in spatially flat FLRW spacetimes using an appropriate WKB ansatz for the adiabatic expansion for the field modes. The conformal and axial anomalies thus found are in exact agreement with those obtained from other renormalization methods. This formalism can be considered as an appropriate extension of the techniques originally introduced for scalar fields, applicable to fermions in curved space.

We investigate the cosmological effects of neutrino lumps in growing neutrino quintessence. The strongly nonlinear effects are resolved by means of numerical N-body simulations which include relativistic particles, nonlinear scalar field equations, and backreaction effects. For the investigated models with a constant coupling between the scalar field and the neutrinos, the backreaction effects are so strong that a realistic cosmology is hard to realize. This points toward the necessity of a field-dependent coupling in growing neutrino quintessence. In this case realistic models of dynamical dark energy exist which are testable by the observation or nonobservation of large neutrino lumps.

The current standard model of cosmology, $\mathrm{\Lambda }\mathrm{CDM}$, requires dark matter to make up around 25% of the total energy budget of the Universe. Yet, quite puzzlingly, there appears to be no candidate particle in the current Standard Model of particle physics. Assuming the validity of the cold dark matter (CDM) paradigm, dark matter has evaded detection thus far either because it is intrinsically a weakly interacting substance or because its interactions are suppressed by its high constituent mass and low number density. Most approaches to explain dark matter to date assume the former and therefore require beyond-the-Standard-Model particles that have yet to be observed directly or indirectly. Given the dearth of evidence for this class of candidates it is timely to consider the latter possibility, which allows for candidates that may or may not arise from the Standard Model. In this work we extend a recent study of this general class of so-called macro dark matter—candidates with characteristic masses of grams and geometric cross sections of ${\mathrm{cm}}^2$. We consider new bounds that can be set using existing data from the resonant bar gravitational wave detectors NAUTILUS and EXPLORER.

We present a new class of statistical error reduction techniques for Monte Carlo simulations. Using covariant symmetries, we show that correlation functions can be constructed from inexpensive approximations without introducing any systematic bias in the final result. We introduce a new class of covariant approximation averaging techniques, known as all-mode averaging (AMA), in which the approximation takes account of contributions of all eigenmodes through the inverse of the Dirac operator computed from the conjugate gradient method with a relaxed stopping condition. In this paper we compare the performance and computational cost of our new method with traditional methods using correlation functions and masses of the pion, nucleon, and vector meson in $N_f=2+1$ lattice QCD using domain-wall fermions. This comparison indicates that AMA significantly reduces statistical errors in Monte Carlo calculations over conventional methods for the same cost.

We present the first upper-limit measurement of the branching fractions of the isospin-violating transitions ${\chi }_{c0,2}\to {\pi }^0{\eta }_c$. The measurements are performed using $106\times 10^6$ $\psi (3686)$ events accumulated with the BESIII detector at the BEPCII $e^{+}{e}^{-}$ collider at a center-of-mass energy corresponding to the $\psi (3686)$ mass. We obtained upper limits on the branching fractions at a 90% confidence level of $B({\chi }_{c0}\to {\pi }^0{\eta }_c)<1.6\times 10^{-3}$ and $B({\chi }_{c2}\to {\pi }^0{\eta }_c)<3.2\times 10^{-3}$.

The ARGO–YBJ experiment is a full-coverage air shower detector located at the Yangbajing Cosmic Ray Observatory (Tibet, People’s Republic of China, 4300 m a.s.l.). The high altitude, combined with the full-coverage technique, allows the detection of extensive air showers in a wide energy range and offer the possibility of measuring the cosmic ray proton plus helium spectrum down to the TeV region, where direct balloon/space-borne measurements are available. The detector has been in stable data taking in its full configuration from November 2007 to February 2013. In this paper the measurement of the cosmic ray proton plus helium energy spectrum is presented in the region 3–300 TeV by analyzing the full collected data sample. The resulting spectral index is $\gamma =-2.64\pm 0.01$, the error is dominated by systematic uncertainties. The accurate measurement of the spectrum of light elements with a ground based air shower detector demonstrates the possibility of extending these measurements at larger energies, where galactic cosmic ray sources should run out of power in accelerating light elements.

We calculate fermionic Green’s functions for states of the three-dimensional Aharony-Bergman-Jafferis-Maldacena M2-brane theory at large N using the gauge-gravity correspondence. We embed extremal black brane solutions in four-dimensional maximally supersymmetric gauged supergravity, obtain the linearized Dirac equations for each spin-$1/2$ mode that cannot mix with a gravitino, and solve these equations with infalling boundary conditions to calculate retarded Green’s functions. For generic values of the chemical potentials, we find Fermi surfaces with universally non-Fermi liquid behavior, matching the situation for four-dimensional $\mathcal{N}=4$ super-Yang-Mills. Fermi surface singularities appear and disappear discontinuously at the point where all chemical potentials are equal, reminiscent of a quantum critical point. One limit of parameter space has zero entropy at zero temperature, and fermionic fluctuations are perfectly stable inside an energy region around the Fermi surface. An ambiguity in the quantization of the fermions is resolved by supersymmetry.

The integrable ansatz for pure Einstein gravity is extended to supergravity, and the procedure for constructing exact supergravitational solitonic solutions is described.

A quark-meson-diquark plasma is considered within the Polyakov-loop extended Nambu–Jona-Lasinio model for dynamical chiral symmetry breaking and restoration in quark matter. Based on a generalized Beth-Uhlenbeck approach to mesons and diquarks we present the thermodynamics of this system including the Mott dissociation of mesons and diquarks at finite temperature. A striking result is the suppression of the diquark abundance below the chiral restoration temperature by the coupling to the Polyakov loop, because of their color degree of freedom. This is understood in close analogy to the suppression of quark distributions by the same mechanism. Mesons as color singlets are unaffected by the Polyakov-loop suppression. At temperatures above the chiral restoration mesons and diquarks are both suppressed due to the Mott effect, whereby the positive resonance contribution to the pressure is largely compensated by the negative scattering contribution in accordance with the Levinson theorem.

We continue the investigation of thermodynamical properties of the Bogomol’nyi-Prasad-Sommerfield (BPS) Skyrme model. In particular, we analytically compute the baryon chemical potential both in the full field theory and in a mean-field approximation. In the full field theory case, we find that the baryon chemical potential is always exactly proportional to the baryon density, for arbitrary solutions. We further find that, in the MF approximation, the BPS Skyrme model approaches the Walecka model in the limit of high density—their thermodynamical functions as well as the equation of state agree in this limit. This fact allows one to read off some properties of the $\omega$ meson from the BPS Skyrme action, even though the latter model is entirely based on the (pionic) $SU(2)$ Skyrme field. On the other hand, at low densities, at the order of the usual nuclear matter density, the equations of state of the two models are no longer universal, such that a comparison depends on some model details. Still, also the BPS Skyrme model gives rise to nuclear saturation in this regime, leading, in fact, to an exact balance between repulsive and attractive forces. The perfect fluid aspects of the BPS Skyrme model, which, together with its BPS properties, form the base of our results, are shown to be in close formal analogy with the Eulerian formulation of relativistic fluid dynamics. Within this analogy, the BPS Skyrme model, in general, corresponds to a nonbarotropic perfect fluid.

The multiscale entanglement renormalization ansatz (MERA) is a tensor network that provides an efficient way of variationally estimating the ground state of a critical quantum system. The network geometry resembles a discretization of spatial slices of an anti–de Sitter (AdS) spacetime and “geodesics” in the MERA reproduce the Ryu-Takayanagi formula for the entanglement entropy of a boundary region in terms of bulk properties. It has therefore been suggested that there could be an AdS/MERA correspondence, relating states in the Hilbert space of the boundary quantum system to ones defined on the bulk lattice. Here we investigate this proposal and derive necessary conditions for it to apply, using geometric features and entropy inequalities that we expect to hold in the bulk. We show that, perhaps unsurprisingly, the MERA lattice can only describe physics on length scales larger than the AdS radius. Further, using the covariant entropy bound in the bulk, we show that there are no conventional MERA parameters that completely reproduce bulk physics even on super-AdS scales. We suggest modifications or generalizations of this kind of tensor network that may be able to provide a more robust correspondence.

We study the Einstein gravity and dust system in three spacetime dimensions as an example of a nonperturbative quantum gravity model with local degrees of freedom. We derive the Hamiltonian theory in the dust time gauge and show that it has a rich class of exact solutions. These include the Bañados–Teitelboim–Zanelli black hole, static solutions with naked singularities, and traveling wave solutions with dynamical horizons. We give a complete quantization of the wave sector of the theory, including a definition of a self-adjoint spacetime metric operator. This operator is used to demonstrate the quantization of deficit angle and the fluctuation of dynamical horizons.

In this paper, we investigate the thermodynamics of hyperscaling-violating Lifshitz black branes in the presence of a nonlinear massless electromagnetic field. We, first, obtain analytic nonlinear charged black brane solutions with a hyperscaling-violating factor in dilaton gravity and give the condition on the parameters of the metric for having black brane solutions. Second, we introduce the appropriate finite action in the grand canonical and canonical ensembles for a nonlinear electromagnetic field. Next, by generalizing the counterterm method for the asymptotic Lifshitz spacetimes with a hyperscaling-violating factor, we calculate the energy density of our solutions. Furthermore, we present a relation between the energy density and the thermodynamic quantities, electric potential, charge density, temperature and entropy density. This relation is the generalization of the Smarr formula for anti–de Sitter black branes and charged Lifshitz solutions. Finally, we perform a stability analysis in both the canonical and grand canonical ensembles. We show that the nonlinearity of the electromagnetic field can make the solutions unstable in the grand canonical ensemble.

The D1D5 conformal field theory has provided a useful microscopic model for studying black holes. The coupling in this theory is a twist deformation whose action on the vacuum generates a squeezed state. We give a new derivation of the expression for this squeezed state using the conformal Ward identity; this derivation provides an insight into several features of the state. We also examine the squeezed state in a continuum limit where we describe it in terms of position space correlations created by the twist.