A measurement of the top quark pair-production cross section in the $\mathrm{lepton}+\text{jets}$ decay channel is presented. It is based on $4.6{\mathrm{fb}}^{-1}$ of $\sqrt{s}=7\mathrm{TeV}$ $pp$ collision data collected during 2011 by the ATLAS experiment at the CERN Large Hadron Collider. A three-class, multidimensional event classifier based on support vector machines is used to differentiate $t\overline{t}$ events from backgrounds. The $t\overline{t}$ production cross section is found to be ${\sigma }_{t\overline{t}}=168.5\pm 0.7(\mathrm{stat}){}_{-5.9}^{+6.2}(\mathrm{syst}){}_{-3.2}^{+3.4}(\mathrm{lumi})\mathrm{pb}$. The result is consistent with the Standard Model prediction based on QCD calculations at next-to-next-to-leading order.

Following the nonperturbative light-front Hamiltonian formalism developed in our preceding work [Li. et al. Phys. Rev. D 104, 056014 (2021)], we investigate the momentum broadening of a quark jet inside a SU(3) colored medium. We perform the numerical simulation of the real-time jet evolution in Fock spaces of a single quark, a quark-gluon state, and coupled quark- and quark-gluon states at various jet momenta $p^{+}$ and medium densities. With the obtained jet light-front wave function, we extract the jet transverse momentum distribution, the quenching parameter, and the gluon emission rate. We analyze the dependence of momentum broadening on $p^{+}$, medium density, color configuration, spatial correlation, and medium-induced gluon emission. For comparison, we also derive analytically the expectation value of the transverse momentum of a quark-gluon state in any color configuration and in an arbitrary spatial distribution in the eikonal limit. This work can help understand jet momentum broadening in the noneikonal regime.

In recent years, several hints for new scalar particles have been observed by the Large Hadron Collider at CERN. In this context, we recast and combine the CMS and ATLAS analyses of the Standard Model Higgs boson decaying to a pair of $W$ bosons in order to search for low-mass resonances. We provide limits on the corresponding cross section assuming direct production via gluon fusion. For the whole range of masses, we consider (90–200 GeV); the observed limit on the cross section turns out to be weaker than the expected one. Furthermore, at $\approx 95\mathrm{GeV}$, the limit is weakest, and a new scalar decaying into a pair of $W$ bosons (which subsequently decay leptonically) with a cross section $\approx 0.5\mathrm{pb}$ is preferred over the Standard Model hypothesis by $\gtrsim 2.5\sigma$. In light of the previously existing excesses in other channels at similar masses, this strengthens the case for such a new Higgs boson and gives room for the scalar candidate at 95 GeV decaying into $W$ bosons.

We compute the axion-pion scattering $a\pi \to \pi \pi$, relevant for the axion thermalization rate in the early Universe, within unitarized next-to-leading-order chiral perturbation theory. The latter extends the range of validity of the chiral expansion of axion-pion scattering and thus provides a crucial ingredient for the reliable determination of the relic density of thermal axions, whenever the axion decoupling temperature is below that of the QCD phase transition. Implications for cosmological observables are briefly discussed.

The QCD axion has been postulated to exist because it solves the strong-$CP$ problem. Furthermore, if it exists axions should be created in the early Universe and could account for all the observed dark matter. In particular, axion masses of order ${10}^{-10}\mathrm{eV}$ to ${10}^{-7}\mathrm{eV}$ correspond to axions in the vicinity of the grand unified theory scale (GUT-scale). In this mass range many experiments have been proposed to search for the axion through the standard QED coupling parameter $g_{a\gamma \gamma }$. Recently axion electrodynamics has been expanded to include two more coupling parameters, $g_{aEM}$ and $g_{aMM}$, which could arise if heavy magnetic monopoles exist. In this work we show that both $g_{aMM}$ and $g_{aEM}$ may be searched for using a high-voltage capacitor. Since the experiment is not sensitive to $g_{a\gamma \gamma }$, it gives a new way to search for effects of heavy monopoles if the GUT-scale axion is shown to exist, or to simultaneously search for both the axion and the monopole at the same time.

In this work, we study higher $B_c$ mesons to the $L=S$, $P$, $D$, $F$, $G$ multiplets using the Cornell potential model, which takes account of the screening effect. The calculated mass spectra of $B_c$ states are in reasonable agreement with the present experimental data. Based on the spectroscopy, partial widths of all allowed radiative transitions and strong decays of each state are also evaluated by applying our numerical wave functions. Comparing our results with the former results, we point out the difference among various models and derive new conclusions obtained in this paper. Our theoretical results are valuable for searching more $B_c$ mesons in experiments.

A study of the $B^{+}\to K_{\mathrm{S}}^0{K}^{+}{K}^{-}{\pi }^{+}$ and $B^{+}\to K_{\mathrm{S}}^0{K}^{+}{K}^{+}{\pi }^{-}$ decays is performed using proton-proton collisions at center-of-mass energies of 7, 8 and 13 TeV at the LHCb experiment. The $K_{\mathrm{S}}^{0}K\pi$ invariant mass spectra from both decay modes reveal a rich content of charmonium resonances. New precise measurements of the ${\eta }_c$ and ${\eta }_c(2S)$ resonance parameters are performed and branching fraction measurements are obtained for $B^{+}$ decays to ${\eta }_c$, $J/\psi$, ${\eta }_c(2S)$ and ${\chi }_{c1}$ resonances. In particular, the first observation and branching fraction measurement of $B^{+}\to {\chi }_{c0}{K}^0{\pi }^{+}$ is reported as well as first measurements of the $B^{+}\to K^0{K}^{+}{K}^{-}{\pi }^{+}$ and $B^{+}\to K^0{K}^{+}{K}^{+}{\pi }^{-}$ branching fractions. Dalitz plot analyses of ${\eta }_c\to K_{\mathrm{S}}^{0}K\pi$ and ${\eta }_c(2S)\to K_{\mathrm{S}}^{0}K\pi$ decays are performed. A new measurement of the amplitude and phase of the $K\pi$ $S$-wave as functions of the $K\pi$ mass is performed, together with measurements of the $K_0^{*}(1430)$, $K_0^{*}(1950)$ and $a_0(1700)$ parameters. Finally, the branching fractions of ${\chi }_{c1}$ decays to $K^{*}$ resonances are also measured.

We observe a narrow peaking structure in the $p{K}^{-}$ invariant-mass spectrum near the $\mathrm{\Lambda }\eta$ threshold. The peak is clearly seen in 1.5 million events of ${\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+}$ decay using the $980{\mathrm{fb}}^{-1}$ data sample collected by the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We try two approaches to explain this structure: as a new resonance and as a cusp at the $\mathrm{\Lambda }\eta$ threshold. The best fit is obtained with a coherent sum of a Flatté function and a constant background amplitude with the reduced ${\chi }^2$ of $257/243$ ($p=0.25$), while the fits to Breit-Wigner functions are unfavored by more than $7\sigma$. The best fit explains the structure as a cusp at the $\mathrm{\Lambda }\eta$ threshold, and the obtained parameters are consistent with the known properties of $\mathrm{\Lambda }(1670)$. The observation gives the first identification of a threshold cusp in hadrons from the spectrum shape.

We consider positivity constraints applicable to the effective field theory (EFT) of gravity in arbitrary dimensions. By considering scattering of indefinite initial and final states, we highlight the existence of a gravitational scattering amplitude for which full crossing symmetry is manifest and utilize the recently developed crossing symmetric dispersion relations to derive compact nonlinear bounds. We show that the null constraints built into these dispersion relations lead to a finite energy sum rule for gravity which may be extended to a one-parameter family of continuous moment sum rules. These sum rules enforce a UV-IR relation which imposes constraints on both the Regge trajectory and residue. We also highlight a situation where the Regge trajectory is uniquely determined in terms of the sub-Regge scale amplitude. Generically the Regge behavior may be split into an IR sensitive part calculable within a given EFT, which mainly depends on the lightest fields in nature, and an IR independent part, which is subject to universal positivity constraints following from unitarity and analyticity.

We investigate multipartite entanglement in quantum geometry states described by loop quantum gravity spin networks. We focus on states corresponding to bounded regions of 3D spacelike slices of spacetime with nonvanishing intrinsic curvature, realized via spin networks defined on a graph with nontrivial $SU(2)$ holonomies. The presence of intrinsic curvature in the region is encoded—via coarse graining—into tag spins attached to the vertices of the graph. The resulting states are generalized bulk-to-boundary maps defined in an extended boundary Hilbert space comprised by the spin representations carried by the tags and by the uncontracted edges at the boundary. We consider a tripartition of a random quantum geometry state in the extended-boundary space consisting of a bipartite boundary subsystem and a bulk-tags complement, and we propose a measure of logarithmic negativity to study the change in the entanglement phases of the boundary marginal mixed state, while varying the dimension of the bulk curvature environment. In the large spin regime, we find that the typical entanglement negativity is well described by the generalized Page curve of a tripartite random state. In particular, we find area scaling behavior of negativity for small curvature, while for large curvature the negativity vanishes, suggesting an effective thermalization of the boundary. Remarkably, the positive partial transpose character of the mixed boundary state corresponds to a change in the effective topology of the network, with the two boundary subregions becoming disconnected.

We consider a self-interacting, massive rank-1 field coupled to quantum gravity. The theory is renormalizable by power counting and contains a massive spin-1 field and a massive scalar field. The latter has a propagator with negative residue and it is quantized as a purely virtual particle, namely it cannot appear as external on-shell state but contributes to renormalization. In this way the resulting theory is also unitary, both in flat spacetime and when it is coupled to renormalizable quantum gravity (where purely virtual particles are required as well). We compute the full set of one-loop beta functions, including those of the nonminimal couplings. We show that the gravitational couplings cannot be made asymptotically free in absence of tachyons, even with the addition of the two nonminimal terms. Various aspects of renormalizability are discussed by studying the beta functions. Finally, we compare our model with Proca theory, where renormalizability is spoiled if self-interactions are present.

We perform the Batalin-Fradkin-Vilkovisky quantization of the anisotropic conformal Hořava theory in $d$ spatial dimensions. We introduce a model with a conformal potential suitable for any dimension. We define an anisotropic and local gauge-fixing condition that accounts for the spatial diffeomorphisms and the anisotropic Weyl transformations. We show that the BRST transformations can be expressed mainly in terms of a spatial diffeomorphism along a ghost field plus a conformal transformation with another ghost field as argument. We study the quantum Lagrangian in the $d=2$ case, obtaining that all propagators are regular, except for the fields associated with the measure of the second-class constraints. This behavior is qualitatively equal to the nonconformal case.

Understanding the dynamics of soft hair might shine some light on the information loss paradox. In this paper, we introduce a new coordinate system, dressed coordinates, in order to analyze the quantum states of Hawking radiation as a first step toward understanding the connection between soft hair and the information paradox. Dressed coordinates can be introduced by an operator-dependent coordinate transformation that makes the soft hair degrees of freedom apparently disappear from the metric. We show that some results of previous studies, such as the angle-dependent Hawking temperature and the soft graviton theorem, can be easily reproduced using these dressed coordinates. Finally, we discuss future possible applications of dressed coordinates toward understanding the information loss paradox.

The retarded Green function of a wave equation on a 4-dimensional curved background spacetime is a (generalized) function of two spacetime points and diverges when these are connected by a null geodesic. The Hadamard form makes explicit the form of this divergence but only when one of the points is in a normal neighborhood of the other point. In this paper we derive a representation for the retarded Green function for a scalar field in Schwarzschild spacetime which makes explicit its complete singularity structure beyond the normal neighborhood. We interpret this representation as a sum of Hadamard forms, the summation being taken over the number of times the null wavefront has passed through a caustic point; the sum of Hadamard forms applies to the nonsmooth contribution to the full Green function, not only the singular contribution. (The term nonsmooth applies modulo the causality-generating step functions that must appear in the retarded Green function.) The singularity structure is determined using two independent approaches, one based on a Bessel function expansion of the Green function, and another that exploits a link between the Green functions of Schwarzschild spacetime and Plebański-Hacyan spacetime (the latter approach also yields another representation for the full Schwarzschild Green function, not just for its nonsmooth part). Our representation is not valid in a neighborhood of caustic points. We deal with these points by providing a separate representation for the Green function in Schwarzschild spacetime which makes explicit its (different) singularity structure at caustics of this spacetime.

Extracting quasinormal modes from compact binary mergers to perform black hole spectroscopy is one of the fundamental pillars in current and future strong-gravity tests. Among the most remarkable findings of recent works is that including a large number of overtones not only reduces the mismatch of the fitted ringdown but also allows one to extract black hole parameters from a ringdown analysis that goes well within the nonlinear merger part. At the same time, it is well understood that several details of the ringdown analysis have important consequences for the question of whether overtones are present or not, and subsequently, to what extent one can claim to perform black hole spectroscopy. To clarify and tackle some aspects of overtone fitting, we revisit the clearer problem of wave propagation in the scalar Regge-Wheeler and Pöschl-Teller potentials. This setup, which is to some extent qualitatively very similar to the nonlinear merger-ringdown regime, indicates that using even an approximate model for the overtones yields an improved extraction of the black hole mass at early ringdown times. We find that the relevant parameter is the number of included modes rather than using the correct model for the overtones themselves. These results show that some standard tests for verifying the physical contribution of an overtone to a waveform can be misleading, and that even in the linear case it can be difficult to distinguish the presence of an excited mode from the fitting of non-quasinormal mode effects.

Three theoretical criteria for gravitational theories beyond general relativity are considered: obtaining the cosmological constant as an integration constant, deriving the energy conservation law as a consequence of the field equations, rather than assuming it, and not necessarily considering conformally flat metrics as vacuum solutions. Existing theories, including general relativity, do not simultaneously fulfill all three criteria. To address this, a new gravitational field equation is proposed that satisfies these criteria. From this equation, a spherically symmetric exact solution is derived, which is a generalization of the Schwarzschild solution. It incorporates three terms: the Schwarzschild term, the de Sitter term, and a newly discovered term, which is proportional to $r^4$ in a radial coordinate, that becomes significant only at large distances. The equation is further applied to cosmology, deriving an equation for the scale factor. It then presents a solution that describes the transition from decelerating to accelerating expansion in a matter-dominated universe. This is achieved without the need for negative pressure as dark energy or the positive cosmological constant. This provides a novel explanation for the current accelerating expansion of the universe.

A way to test electromagnetic field and spacetime properties around black holes is by considering the dynamics of test particles. In fact, in real astrophysical scenarios, it is hard to determine spacetime geometry which is dominating due to degeneracy gravitational effects in parameters of gravity theories. In this work, we study for the first time the dynamics of spinning particles that have magnetic dipole moments around Schwarzschild black holes immersed in an external asymptotically uniform magnetic field using the Mathisson-Papapetrou-Dixon (MPD) equation. There are two combined interactions: gravitational interaction between the spin of the particle and (electro)magnetic interaction between the external magnetic field and the magnetic dipole moment of the particle to be taken into account. First, we derive the effective potential of the test spinning magnetized particles in motion around the black hole. We also study the combined effects of spin and magnetic interactions on innermost stable circular orbits (ISCOs), the energy, and angular momentum of the particles at ISCO together with superluminal bounds. We investigated the collision of the particles and evaluated the center-of-mass energy in the collisions. Finally, we consider various cases in which neutron stars and rotating stellar mass black holes can be treated as spinning magnetized particles, evaluating the effects of the spin and magnetic moment of objects around supermassive and intermediate-mass black holes. It is also found that magnetic interaction effects are much larger than spin ones in the case of a neutron star orbiting a supermassive mass black hole, while for the case of a neutron star and intermediate-mass black hole system, the effects are comparable where the magnetic field value is larger than 20 G for typical neutron stars and this value for the system with rotating stellar mass black holes is about 280 G.

We analyze the ringdown phase of the first detected black-hole merger, GW150914, using a simulation-based inference pipeline based on masked autoregressive flows. We obtain approximate marginal posterior distributions for the ringdown parameters, namely the mass, spin, and the amplitude and phases of the dominant mode and its first overtone. Thanks to the locally amortized nature of our method, we are able to calibrate our posteriors with injected simulations, producing posterior regions with guaranteed (i.e., exact) frequentist coverage of the true values. For GW150914, our calibrated posteriors provide only mild evidence ($\sim 2\sigma$) for the presence of an overtone, even if the ringdown is assumed to start at the peak of the amplitude.

We study the momentum-space entanglement between the sub- and super-Hubble modes of a spectator scalar field, with a cubic $\lambda {\varphi }^3$ interaction, in de Sitter space. Momentum-space entanglement has some universal properties for any interacting quantum field theory, and we examine them for this specific curved background using the Hubble scale as a natural delimiter to define UV/IR separation. We show that there are several new subtleties when generalizing flat space results due to having a time-dependent interaction term and a nontrivial vacuum state. Our main finding is that the momentum-space entanglement entropy in de Sitter space grows very rapidly, supporting previous similar results for cosmological perturbations [ , Entanglement entropy of cosmological perturbations, Phys. Rev. D 102, 043529 (2020)], which leads to interesting new questions.