Supermassive black hole binary mergers generate a stochastic gravitational wave background detectable by pulsar timing arrays. While the amplitude of this background is subject to significant uncertainties, the frequency dependence is a robust prediction of general relativity. We show that the effects of new forces beyond the Standard Model can modify this prediction and introduce unique features into the spectral shape. In particular, we consider the possibility that black holes in binaries are charged under a new long-range force, and we find that pulsar timing arrays are capable of robustly detecting such forces. Supermassive black holes and their environments can acquire charge due to high-energy particle production or dark sector interactions, making the measurement of the spectral shape a powerful test of fundamental physics.

We study the influence of relativity on the chaotic properties and dynamical outcomes of an unstable triple system; the Pythagorean three-body problem. To this end, we extend the brutus N-body code to include post-Newtonian pairwise terms up to 2.5 order, and the first order Taylor expansion to the Einstein-Infeld-Hoffmann equations of motion. The degree to which our system is relativistic depends on the scaling of the total mass (the unit size was 1 parsec). Using the brutus method of convergence, we test for time-reversibility in the conservative regime, and demonstrate that we are able to obtain definitive solutions to the relativistic three-body problem. It is also confirmed that the minimal required numerical accuracy for a successful time-reversibility test correlates with the amplification factor of an initial perturbation, as was found previously for the Newtonian case. When we take into account dissipative effects through gravitational wave emission, we find that the duration of the resonance, and the amount of exponential growth of small perturbations depend on the mass scaling. For a unit mass $\le 10{\mathrm{M}}_{\odot }$, the system behavior is indistinguishable from Newton’s equations of motion, and the resonance always ends in a binary and one escaping body. For a mass scaling up to ${10}^7{\mathrm{M}}_{\odot }$, relativity gradually becomes more prominent, but the majority of the systems still dissolve in a single body and an isolated binary. The first mergers start to appear for a mass of $\sim {10}^5{\mathrm{M}}_{\odot }$, and between ${10}^7{\mathrm{M}}_{\odot }$ and ${10}^9{\mathrm{M}}_{\odot }$ all systems end prematurely in a merger. These mergers are preceded by a gravitational wave driven in-spiral. For a mass scaling $\ge {10}^9{\mathrm{M}}_{\odot }$, all systems result in a gravitational wave merger upon the first close encounter. Relativistic three-body encounters thus provide an efficient pathway for resolving the final parsec problem. The onset of mergers at the characteristic mass scale of ${10}^7{\mathrm{M}}_{\odot }$ potentially leaves an imprint in the mass function of supermassive black holes.

What if the dark matter-nucleon scattering cross section is too small to be detected by direct detection experiments? It is well known in the literature that some interactions lead to dark matter-nucleon scattering cross sections that can be velocity and momentum suppressed. We show that in the case of bosonic dark matter, neutron star spectroscopy offers a possible detection. Firstly, we discuss the case of scalar dark matter with scalar, pseudoscalar, and vector mediators. Later, we do this exercise for vector dark matter. We show that, depending on the nature of dark matter and the interaction involved, neutron stars can improve the sensitivity on the dark matter-nucleon scattering cross section by orders of magnitude, representing a major step forward in the dark matter siege.

The widely used locally constant field approximation (LCFA) can be utilized in order to derive a simple closed-form expression for the total number of particles produced in the presence of a strong electromagnetic field of a general spatiotemporal configuration. A usual justification for this approximate approach is the requirement that the external field vary slowly in space and time. In this investigation, we examine the validity of the LCFA by comparing its predictions to the results obtained by means of exact nonperturbative numerical techniques. To benchmark the LCFA in the regime of small field amplitudes and low frequencies, we employ a semiclassical approach. As a reference, we consider a standing electromagnetic wave oscillating both in time and space as well as two spatially uniform field configurations: Sauter pulse and oscillating electric field. Performing a thorough numerical analysis, we identify the domain of the field parameters where the approximation is well justified. In particular, it is demonstrated that the Keldysh parameter itself is not a relevant quantity governing the accuracy of the LCFA. It is also found that for field amplitudes close to the Schwinger critical value, the LCFA may overestimate the particle yield. We discuss how this effect is related to the Pauli exclusion principle and quantum interference.

We consider searches for the inelastic scattering of low-mass dark matter at direct detection experiments, using the Migdal effect. We find that there are degeneracies between the dark matter mass and the mass splitting that are difficult to break. Using XENON1T data we set bounds on a previously unexplored region of the inelastic dark matter parameter space. For the case of exothermic scattering, we find that the Migdal effect allows xenon-based detectors to have sensitivity to dark matter with $\mathcal{O}(\mathrm{MeV})$ mass, far beyond what can be obtained with nuclear recoils alone.

Vector bosons, such as dark photon $A^{\prime }$ or $Z^{\prime }$, can couple to muons and be produced in the bremsstrahlung reaction ${\mu }^{-}+N\to {\mu }^{-}+N+A^{\prime }(Z^{\prime })$. Their possible subsequent invisible decay can be detected in fixed target experiments through missing energy/momentum signature. In such experiments, not only is the energy transfer to $A^{\prime }(Z^{\prime })$ important but also the recoil muon angle ${\psi }_{{\mu }^{\prime }}$. In this paper, we derive the total and the double differential cross sections involved in this process using the phase space Weizsäcker-Williams and improved Weizsäcker-Williams approximations, as well as using exact-tree-level calculations. As an example, we compare the derived cross sections and resulting signal yields in the $\mathrm{NA}64\mu$ experiment that uses a 160 GeV muon beam at the CERN Super Proton Synchrotron accelerator. We also discuss its impact on the $\mathrm{NA}64\mu$ expected sensitivity to explore the $(g-2{)}_{\mu }$ anomaly favored region with a $Z^{\prime }$ boson considering ${10}^{12}$ muons accumulated on target.

We investigate the phenomenology of an extension of the Standard Model (SM) by a non-Abelian gauge group $\mathrm{SU}(2{)}_{\mathrm{HS}}$ where all SM particles are singlets under this gauge group, and a new scalar representation $\varphi$ that is singlet under SM gauge group and doublet under $\mathrm{SU}(2{)}_{\mathrm{HS}}$. In this model, the dark matter (DM) candidates are the three mass-degenerate dark photons $A_i$ ($i=1$, 2, 3) of $\mathrm{SU}(2{)}_{\mathrm{HS}}$, and the hidden sector interacts with the (SM) particles through the Higgs portal interactions. Consequently, there will be a new $CP$-even scalar $\eta$ that could be either heavier or lighter than the SM-like Higgs. By taking into account all theoretical and experimental constraints such as perturbativity, unitarity, vacuum stability, non-SM Higgs decays, DM direct detection, and $M$ relic density, we found viable DM is possible in the range from GeV to TeV. Within the viable parameters space, both of the triple Higgs coupling and the di-Higgs production at LHC14 could be enhanced or reduced depending on the scalar mixing and the mass of the scalar particle $\eta$.

We point out that heavy new physics contributions in leptonic dipole moments and high-energy cross sections of lepton pairs into Higgs bosons and photons are connected model-independently. In particular, we demonstrate that a muon collider, running at center-of-mass energies of several TeV, can provide a unique test of new physics in the muon $g-2$ through the study of high-energy processes such as ${\mu }^{+}{\mu }^{-}\to h\gamma$. This high-energy test would be of the utmost importance to shed light on the longstanding muon $g-2$ anomaly as it is not affected by the hadronic and experimental uncertainties entering the current low-energy determination of the muon $g-2$. Furthermore, we show that the current bound on the muon electric dipole moment can be improved by three orders of magnitude, down to $\mathrm{few}\times {10}^{-22}e\mathrm{cm}$.

We minimally extend the Standard Model (SM) with a ${\mathrm{Z}}_2$ symmetric potential containing a single scalar field, serving as our inflaton with a quartic self-coupling. In the model we have symmetry breaking in both sectors, and with the addition of an inflaton-Higgs portal, the universe is able to efficiently reheat via 2-2 inflaton-Higgs scattering. Assuming that the universe with a positive cosmological constant should be metastable, only one particular symmetry breaking pattern in the vacuum is possible, without the need to finely tune the Higgs quartic self-coupling. Inflatons with masses in the range $O({10}^{-3})\le m_{\chi }\le m_h$ and mixing angles that span ${\theta }_m^2=O({10}^{-11}–{10}^{-2})$ evade all current cosmological, experimental, and stability constraints required for a metastable electroweak vacuum. Upgraded particle physics experiments may be able to probe the parameter space with ${\theta }_m^2\ge O({10}^{-4})$, where we would observe trilinear Higgs couplings suppressed by up to 2% compared to the SM value. However, to access the parameter space of very weakly coupled inflatons, we rely on the proposals to build experiments that target the hidden sector.

We evaluate by means of lattice QCD calculations the low-energy constant ${\ell }_7$ which parametrizes strong isospin effects at next-to-leading order (NLO) in SU(2) chiral perturbation theory. Among all low-energy constants at NLO, ${\ell }_7$ is the one known less precisely, and its uncertainty is currently larger than 50%. Our strategy is based on the RM123 approach in which the lattice path-integral is expanded in powers of the isospin breaking parameter $\mathrm{\Delta }m=(m_d-m_u)/2$. In order to evaluate the relevant lattice correlators we make use of the recently proposed rotated twisted-mass (RTM) scheme. Within the RM123 approach, it is possible to cleanly extract the value of ${\ell }_7$ from either the pion mass splitting $M_{{\pi }^{+}}-M_{{\pi }^0}$ induced by strong isospin breaking at order $\mathcal{O}((\mathrm{\Delta }m{)}^2)$ (mass method), or from the coupling of the neutral pion ${\pi }^0$ to the isoscalar operator $(\overline{u}{\gamma }_{5}u+\overline{d}{\gamma }_{5}d)/\sqrt{2}$ at order $\mathcal{O}(\mathrm{\Delta }m)$ (matrix element method). In this pilot study we limit the analysis to a single ensemble generated by the Extended Twisted Mass Collaboration (ETMC) with $N_f=2+1+1$ dynamical quark flavors, which corresponds to a lattice spacing $a\simeq 0.095\mathrm{fm}$ and to a pion mass $M_{\pi }\simeq 260\mathrm{MeV}$. We find that the matrix element method outperforms the mass method in terms of resulting statistical accuracy. Our determination, ${\ell }_7=2.5(1.4)\times {10}^{-3}$, is in agreement and improves previous calculations.

We update lattice QCD results for second order cumulants of conserved charge fluctuations and correlations at nonzero temperature and vanishing values of the conserved charge chemical potentials. We compare these results to hadron resonance gas calculations with and without excluded volume terms as well as S-matrix results in the hadronic phase of QCD and comment on their current limitations. We, furthermore, use these results to characterize thermal conditions in the vicinity of the pseudocritical line of the chiral transition in QCD. We argue that the ratio of strange to baryon chemical potentials is a robust observable that, on the one hand, deviates only little from hadron resonance gas results, but, on the other hand, is very sensitive to the spectrum of strange baryon resonances.

Parton distribution functions (PDFs) and light cone distribution amplitudes (LCDAs) are central nonperturbative objects of interest in high-energy inelastic and elastic scattering, respectively. As a result, an ab initio determination of these objects is highly desirable. In this paper we present theoretical details for the calculation of the PDFs and LCDAs using a heavy-quark operator product expansion method. This strategy was proposed in a previous paper [Phys. Rev. D 73, 014501 (2006)] for computing higher moments of the PDFs using lattice QCD. Its central feature is the introduction of a fictitious, valence heavy quark. In the current article, we show that the operator product expansion of the hadronic matrix element we study can also be expressed as the convolution of a perturbative matching kernel and the corresponding light cone distribution, which in principle can be inverted to determine the parton momentum fraction dependence. Regarding the extraction of higher moments, this work also provides the one-loop Wilson coefficients in the operator product expansion formulas for the unpolarized PDF, helicity PDF and pseudoscalar meson LCDAs. Although these Wilson coefficients for the PDFs can be inferred from existing results in the literature, those for the LCDAs are new.

Symmetry is among the most fundamental and powerful concepts in nature, whose existence is usually taken as given, without explanation. We explore whether symmetry can be derived from more fundamental principles from the perspective of quantum information. Starting with a two-qubit system, we show there are only two minimally entangling logic gates: the identity and the $\mathrm{SWAP}$, which interchanges the two states of the qubits. We further demonstrate that, when viewed as an entanglement operator in the spin-space, the $S$-matrix in the two-body scattering of fermions in the $s$-wave channel is uniquely determined by unitarity and rotational invariance to be a linear combination of the identity and the $\mathrm{SWAP}$. Realizing a minimally entangling $S$-matrix would give rise to global symmetries, as exemplified in Wigner’s spin-flavor symmetry and Schrödinger’s conformal invariance in low energy quantum chromodynamics. For $N_q$ species of qubit, the identity gate is associated with an $[SU(2){]}^{N_q}$ symmetry, which is enlarged to $SU(2N_q)$ when there is a species-universal coupling constant.

It is frequently supposed that quark-gluon plasma created in heavy-ion collisions undergoes free streaming at early times. We examine this issue based on the assumption that a universal attractor dominates the dynamics already at the earliest stages, which offers a way to connect the initial state with the start of the hydrodynamic expansion in an approximate but conceptually transparent fashion. We demonstrate that the centrality dependence of the measured particle multiplicities can be used to quantitatively constrain the pressure anisotropy and find that it strongly depends on the model of the initial energy deposition. As an illustration, we compare three initial state models and show that they predict rather different early-time values of the pressure anisotropy. This suggests that while assuming free streaming prior to hydrodynamization may be compatible with some initial state models, in general, features of the prehydrodynamic flow need to be matched with the model of the initial state.

Using $(10.087\pm 0.044)\times {10}^{9}J/\psi$ events collected by the Beijing Spectrum III (BESIII) detector at the Beijing Electron Positron Collider II (BEPCII) collider, we search for the hyperon semileptonic decay ${\mathrm{\Xi }}^{-}\to {\mathrm{\Xi }}^0{e}^{-}{\overline{\nu }}_e$. No significant signal is observed and the upper limit on the branching fraction $\mathcal{B}({\mathrm{\Xi }}^{-}\to {\mathrm{\Xi }}^0{e}^{-}{\overline{\nu }}_e)$ is set to be $2.59\times {10}^{-4}$ at 90% confidence level. This result is one order of magnitude more strict than the previous best limit.

In a topological description of elementary matter proposed by Bilson-Thompson, the leptons and quarks of a single generation, together with the electroweak gauge bosons, are represented as elements of the framed braid group of three ribbons. By identifying these braids with emergent topological excitations of ribbon networks, it has been possible to encode this braid model into the framework of quantum geometry provided by loop quantum gravity. One major hurdle in this promising approach to unifying matter and space-time has been the difficulty in implementing any dynamics that reflects observed particle interactions. In the case of trivalent networks, it has not been possible to generate particle interactions, because the braids correspond to noiseless subsystems, meaning they commute with the evolution algebra generated by the local Pachner moves. In the case of tetravalent networks, interactions are only possible when the model’s original simplicity, in which interactions take place via the composition of braids, is sacrificed. The main result of the present paper is to demonstrate that it is possible to preserve both the original classification of fermions, as well as their interaction via the braid product, if we embed the braid in a trivalent scheme and supplement the local Pachner moves, with a nonlocal and graph-changing one-handle attachment. Moreover, we use Kauffman-Lins recoupling theory to obtain invariants of braided networks that distinguish topological configurations associated to particles in the Bilson-Thompson model.

We explore conformal primary wave functions for all half integer spins up to the graviton. Half steps are related by supersymmetry, integer steps by the classical double copy. The main results are as follows: we 1) introduce a convenient spin frame and null tetrad to organize all radiative modes of varying spin; 2) identify the massless spin-$\frac{3}{2}$ conformal primary wave function as well as the conformally soft Goldstone mode corresponding to large supersymmetry transformations; 3) indicate how to express a conformal primary of arbitrary spin in terms of differential operators acting on a scalar primary; 4) demonstrate that conformal primary metrics satisfy the double copy in a variety of forms—operator, Weyl, and Kerr-Schild—and are exact, albeit complex, solutions to the fully nonlinear Einstein equations of Petrov type $\mathbf{N}$; 5) propose a novel generalization of conformal primary wave functions; and 6) show that this generalization includes a large class of physically interesting metrics corresponding to ultra-boosted black holes, shockwaves and more.

We consider the operator product expansion (OPE) of correlation functions in the supersymmetric $O(N)$ nonlinear sigma model at subleading order in the large $N$ limit in order to study the cancellation between ambiguities coming from infrared renormalons and those coming from various operators in the OPE. As has been discussed in the context of supersymmetric Yang-Mills theory in four dimensions, supersymmetry presents a challenge to this cancellation. In a bid to solve this problem we consider two-dimensional $O(N)$ as a toy model. A background field method inspired by Polyakov’s treatment of the renormalization of the bosonic $O(N)$ model is used to identify explicit operators in the OPE of the two-point functions of bosonic and fermionic fields in the model. In order to identify the coefficient functions in the OPE, the exact two-point functions at subleading order in large $N$ are expanded in powers of the natural infrared length scale. The ambiguities arising from renormalons in the coefficient functions and vacuum expectation values of operators in the OPE are shown to cancel to all orders. The question of supersymmetric Yang-Mills theory without matter remains open.

We study the shadow produced by a class of rotating black holes surrounded by plasma. The metric for these black holes arises by applying the Newman-Janis algorithm to a family of spherically symmetric spacetimes, which includes several well-known geometries as special cases. We derive a general expression for the shape of the shadow in the case that the plasma frequency leads to a separable Hamilton-Jacobi equation for light. We present two examples in which we obtain the shadow contours and the observables resulting from them. In one, we analyze Kerr-Newman-like geometries, including braneworld and Horndeski gravity black holes, while in the other, we consider scalar-tensor 4D Einstein-Gauss-Bonnet gravity spacetimes. In both cases, we find that the presence of plasma leads to a smaller and less deformed shadow.

The greybody factor of the massive Dirac field around the black hole in the dRGT massive gravity theory is investigated using the rigorous bound and the WKB approximation methods. In both methods, the greybody factor significantly depends on the shape of the potential. If the potential is smaller, there is more probability for the Dirac field to transmit through the black hole, therefore, the greybody factor is higher. Moreover, there exists a critical mass of the Dirac field such that the greybody factor is maximum. By comparing the results from these two methods, we argue that it is useful to use the rigorous bound method for the low potential cases, while using the WKB approximation method for the high potential cases.