We calculate the von Neumann and Rényi bipartite entanglement entropy of the $O(2)$ model with a chemical potential on a $1+1$-dimensional Euclidean lattice with open and periodic boundary conditions. We show that the Calabrese-Cardy conformal field theory predictions for the leading logarithmic scaling of these entropies are consistent with a central charge $c=1$. This scaling survives the time-continuum limit and truncations of the microscopic degrees of freedom, modifications which allow us to connect the Lagrangian formulation to quantum Hamiltonians. At half-filling, the forms of the subleading corrections imposed by conformal field theory allow the determination of the central charge with an accuracy better than 2% for moderately sized lattices. We briefly discuss the possibility of estimating the central charge using quantum simulators.

In the present investigation we study the leading and subleading high-energy behavior of hadron-hadron total cross sections using a best-fit analysis of hadronic scattering data. The parametrization used for the hadron-hadron total cross sections at high energy is inspired by recent results obtained by Giordano and Meggiolaro [J. High Energy Phys. 03 (2014) 002] using a nonperturbative approach in the framework of QCD, and it reads ${\sigma }_{\mathrm{tot}}\sim B{\ln }^{2}s+C\ln s\ln \ln s$. We critically investigate if $B$ and $C$ can be obtained by means of best-fits to data for proton-proton and antiproton-proton scattering, including recent data obtained at the LHC, and also to data for other meson-baryon and baryon-baryon scattering processes. In particular, following the above-mentioned nonperturbative QCD approach, we also consider fits where the parameters $B$ and $C$ are set to $B=\kappa B_{\mathrm{th}}$ and $C=\kappa C_{\mathrm{th}}$, where $B_{\mathrm{th}}$ and $C_{\mathrm{th}}$ are universal quantities related to the QCD stable spectrum, while $\kappa$ (treated as an extra free parameter) is related to the asymptotic value of the ratio ${\sigma }_{\mathrm{el}}/{\sigma }_{\mathrm{tot}}$. Different possible scenarios are then considered and compared.

Quantum field theories with complex actions cannot be investigated using importance sampling due to the sign problem. One possible solution is to use the holomorphic gradient flow, a method we introduced related to the Lefschetz thimbles idea. In many cases the probability distribution generated by this method is multimodal and standard Monte Carlo sampling fails. We propose an algorithm that incorporates tempered proposals to solve this problem. We apply this algorithm to the Thirring $0+1$-dimensional model at finite density for a parameter set where standard sampling fails and show that tempered proposals cure this problem.

Sterile neutrino dark matter, a popular alternative to the WIMP paradigm, has generally been studied in non-supersymmetric setups. If the underlying theory is supersymmetric, we find that several interesting and novel dark matter features can arise. In particular, in scenarios of freeze-in production of sterile neutrino dark matter, its superpartner, the sterile sneutrino, can play a crucial role in early Universe cosmology as the dominant source of cold, warm, or hot dark matter, or of a subdominant relativistic population of sterile neutrinos that can contribute to the effective number of relativistic degrees of freedom $N_{\mathrm{eff}}$ during big bang nucleosynthesis.

We investigate the properties of triply periodic Skyrme crystals in the generalized Skyrme model ${\mathcal{L}}_6+{\mathcal{L}}_4+{\mathcal{L}}_2+{\mathcal{L}}_0$ with higher-derivative terms up to sixth order. Three different symmetry breaking potential terms ${\mathcal{L}}_0$ are considered, the generalized pion mass term, double-vacuum potential, and mixed potential. Various scenarios of phase transitions from the low-density phase to the high-density phase are examined for different choices of the parameters of the model. In particular, we investigated limiting behavior of the Skyrme crystals in the truncated submodel without the Skyrme term ${\mathcal{L}}_4$ and/or without the ${\mathcal{L}}_2$ term. We show that the Skyrme crystal may exist in the pure ${\mathcal{L}}_4$ and ${\mathcal{L}}_6$ models and investigated the phase structure of these solutions. Considering the near-Bogomolny-Prasad-Sommerfeld submodel, we found that there are indications of the phase transition from a low-density quasi-liquid phase to the high-density symmetric phase of the Skyrmionic matter.

This work pioneers the quantization of primordial fermion perturbations in hybrid loop quantum cosmology (LQC). We consider a Dirac field coupled to a spatially flat, homogeneous, and isotropic cosmology, sourced by a scalar inflaton, and treat the Dirac field as a perturbation. We describe the inhomogeneities of this field in terms of creation and annihilation variables, chosen to admit a unitary evolution if the Dirac fermion were treated as a test field. Considering instead the full system, we truncate its action at quadratic perturbative order and construct a canonical formulation. In particular this implies that, in the global Hamiltonian constraint of the model, the contribution of the homogeneous sector is corrected with a quadratic perturbative term. We then adopt the hybrid LQC approach to quantize the full model, combining the loop representation of the homogeneous geometry with the Fock quantization of the inhomogeneities. We assume a Born-Oppenheimer ansatz for physical states and show how to obtain a Schrödinger equation for the quantum evolution of the perturbations, where the role of time is played by the homogeneous inflaton. We prove that the resulting quantum evolution of the Dirac field is indeed unitary, despite the fact that the underlying homogeneous geometry has been quantized as well. Remarkably, in such evolution, the fermion field couples to an infinite sequence of quantum moments of the homogeneous geometry. Moreover, the evolved Fock vacuum of our fermion perturbations is shown to be an exact solution of the Schrödinger equation. Finally, we discuss in detail the quantum backreaction that the fermion field introduces in the global Hamiltonian constraint. For completeness, our quantum study includes since the beginning (gauge-invariant) scalar and tensor perturbations, that were studied in previous works.

Existing xenon dark matter (DM) direct detection experiments can probe the DM-nucleon interaction of DM with a sub-GeV mass through a search for photon emission from the recoiling xenon atom. We show that LUX’s constraints on sub-GeV DM, which utilize the scintillation (S1) and ionization (S2) signals, are approximately 3 orders of magnitude more stringent than previous xenon constraints in this mass range, derived from the XENON10 and XENON100 S2-only searches. The new LUX constraints provide the most stringent direct detection constraints for DM particles with a mass below 0.5 GeV. In addition, the photon emission signal in LUX and its successor LZ maintain the discrimination between background and signal events so that an unambiguous discovery of sub-GeV DM is possible. We show that LZ has the potential to reconstruct the DM mass with $\simeq 20\%$ accuracy for particles lighter than 0.5 GeV.

The apparent breakdown of unitarity in low order perturbation theory is often is used to place bounds on the parameters of a theory. In this work we give an algorithm for approximately computing the next-to-leading order (NLO) perturbativity bounds on the quartic couplings of a renormalizable theory whose scalar sector is ${\varphi }^4$-like. By this we mean theories where either there are no cubic scalar interactions, or the cubic couplings are related to the quartic couplings through spontaneous symmetry breaking. The quantity that tests where perturbation theory breaks down itself can be written as a perturbative series, and having the NLO terms allows one to test how well the series converges. We also present a simple example to illustrate the effect of considering these bounds at different orders in perturbation theory. For example, there is a noticeable difference in the viable parameter when the square of the NLO piece is included versus when it is not.

We identify the parameter regions of the phenomenological minimal supersymmetric standard model (pMSSM) with the minimal possible fine-tuning. We show that the fine-tuning of the pMSSM is not large, nor under pressure by LHC searches. Low sbottom, stop and gluino masses turn out to be less relevant for low fine-tuning than commonly assumed. We show a link between low fine-tuning and the dark matter relic density. Fine-tuning arguments point to models with a dark matter candidate yielding the correct dark matter relic density: a bino-higgsino particle with a mass of 35–155 GeV. Some of these candidates are compatible with recent hints seen in astrophysics experiments such as Fermi-LAT and AMS-02. We argue that upcoming direct search experiments, such as XENON1T, will test all of the most natural solutions in the next few years due to the sensitivity of these experiments on the spin-dependent WIMP-nucleon cross section.

We comprehensively evaluate renormalized Higgs boson couplings at one-loop level in nonminimal Higgs models such as the Higgs singlet model (HSM) and the four types of two Higgs doublet models (THDMs) with a softly broken $Z_2$ symmetry. The renormalization calculation is performed in the on-shell scheme improved by using the pinch technique to eliminate the gauge dependence in the renormalized couplings. We first review the pinch technique for scalar boson two-point functions in the standard model (SM), the HSM and the THDMs. We then discuss the difference in the results of the renormalized Higgs boson couplings between the improved on-shell scheme and the ordinal one with a gauge dependence appearing in mixing parameters of scalar bosons. Finally, we widely investigate how we can identify the HSM and the THDMs focusing on the pattern of deviations in the renormalized Higgs boson couplings from predictions in the SM.

We present a comprehensive study of the modifications of Higgs couplings in the $SO(5)/SO(4)$ minimal composite model. We focus on three couplings of central importance to Higgs phenomenology at the LHC: the couplings to top and bottom quarks and the coupling to two gluons. We consider three possible embeddings of the fermionic partners in $\mathbf{5}$, $\mathbf{10}$ and $\mathbf{14}$ of $SO(5)$ and find $t\overline{t}h$ and $b\overline{b}h$ couplings to be always suppressed in $\mathbf{5}$ and $\mathbf{10}$, while in $\mathbf{14}$ they can be either enhanced or suppressed. Assuming partial compositeness, we analyze the interplay between the $t\overline{t}h$ coupling and the top sector contribution to the Coleman-Weinberg potential for the Higgs boson, and the correlation between $t\overline{t}h$ and $ggh$ couplings. In particular, if the electroweak symmetry breaking is triggered radiatively by the top sector, we demonstrate that the ratio of the $t\overline{t}h$ coupling in composite Higgs models over the Standard Model expectation is preferred to be less than the corresponding ratio of the $ggh$ coupling.

We systematically study the $S$-wave doubly charmed baryons using the method of QCD sum rules. Our results suggest that the ${\mathrm{\Xi }}_{cc}^{++}$ recently observed by LHCb can be well identified as the $S$-wave ${\mathrm{\Xi }}_{cc}$ state of $J^P=1/2^{+}$. We study its relevant ${\mathrm{\Omega }}_{cc}$ state, the mass of which is predicted to be around 3.7 GeV. We also systematically study the $P$-wave doubly charmed baryons, the masses of which are predicted to be around 4.1 GeV. Especially, there can be several excited doubly charmed baryons in this energy region, and we suggest searching for them in order to study the fine structure of the strong interaction.

A time evolving fluid system is constructed on a timelike boundary hypersurface at finite cutoff in Vaidya spacetime. The approach used to construct the fluid equations is a direct extension of the ordinary gravity/fluid correspondence under the constrained fluctuation obeying Petrov type I conditions. The explicit relationships between the time dependent fluctuation modes and the fluid quantities such as density, velocity field and kinematic viscosity parameters are established, and the resulting fluid system is governed by a system of a sourced continuity equation and a compressible Navier-Stokes equation with nontrivial time evolution.

As was shown recently, non-Abelian vortex strings supported in four-dimensional $\mathcal{N}=2$ supersymmetric QCD with the U(2) gauge group and $N_f=4$ quark multiplets (flavors) become critical superstrings. In addition to the translational moduli, non-Abelian strings under consideration carry six orientational and size moduli. Together, they form a ten-dimensional target space required for a superstring to be critical. The target space of the string sigma model is a product of the flat four-dimensional space and a Calabi-Yau noncompact threefold, namely, the conifold. We study closed string states which emerge in four dimensions and identify them with hadrons of four-dimensional $\mathcal{N}=2$ QCD. One massless state was found previously; it emerges as a massless hypermultiplet associated with the deformation of the complex structure of the conifold. In this paper, we find a number of massive states. To this end, we exploit the approach used in LST little string theory, namely, the equivalence between the critical string on the conifold and noncritical $c=1$ string with the Liouville field and a compact scalar at the self-dual radius. The states we find carry “baryonic” charge (its definition differs from standard). We interpret them as “monopole necklaces” formed (at strong coupling) by the closed string with confined monopoles attached.

We show that measuring commuting observables can be sufficient to assess that a bipartite state is entangled according to either nonseparability or the stronger criterion of “steerability.” Indeed, the measurement of a single observable might reveal the strength of the interferences between the two subsystems, as if an interferometer were used. For definiteness, we focus on the two-point correlation function of density fluctuations obtained by in situ measurements in homogeneous one-dimensional cold atomic Bose gases. We then compare this situation to that found in transonic stationary flows mimicking a black hole geometry where correlated phonon pairs are emitted on either side of the sonic horizon by the analogue Hawking effect. We briefly apply our considerations to two recent experiments.

An important aspect of Weyl anomalies is that they encode information on the irreversibility of the renormalization group flow. We consider, $\mathrm{\Delta }\overline{b}={\overline{b}}^{\mathrm{UV}}-{\overline{b}}^{\mathrm{IR}}$, the difference of the ultraviolet and infrared value of the $\square R$-term of the Weyl anomaly. The quantity is related to the fourth moment of the trace of the energy momentum tensor correlator for theories which are conformal at both ends. Subtleties arise for nonconformal fixed points as might be the case for infrared fixed points with broken chiral symmetry. Provided that the moment converges, $\mathrm{\Delta }\overline{b}$ is then automatically positive by unitarity. Written as an integral over the renormalization scale, flow-independence follows since its integrand is a total derivative. Furthermore, using a momentum subtraction scheme (MOM) the 4D Zamolodchikov-metric is shown to be strictly positive beyond perturbation theory and equivalent to the metric of a conformal manifold at both ends of the flow. In this scheme $\overline{b}(\mu )$ can be extended outside the fixed point to a monotonically decreasing function. The ultraviolet finiteness of the fourth moment enables us to define a scheme for the $\delta \mathcal{L}\sim b_0{R}^2$-term, for which the $R^2$-anomaly vanishes along the flow. In the MOM- and the $R^2$-scheme, $\overline{b}(\mu )$ is shown to satisfy a gradient flow type equation. We verify our findings in free field theories, higher derivative theories and extend $\mathrm{\Delta }\overline{b}$ and the Euler flow $\mathrm{\Delta }{\beta }_a$ for a Caswell-Banks-Zaks fixed point for QCD-like theories to next-to-next-to leading order using a recent $⟨G^2{G}^2⟩$-correlator computation.

We study in detail the phase space of a Friedmann-Robertson-Walker universe filled with various cosmological fluids that may or may not interact. We use various expressions for the equation of state, and we analyze the physical significance of the resulting fixed points. In addition, we discuss the effects of the stability or an instability of some fixed points. Moreover, we study an interesting phenomenological scenario for which there is an oscillating interaction between the dark energy and dark matter fluid. As we demonstrate, in the context of the model we use, at early times the interaction is negligible, and it starts to grow as the cosmic time approaches the late-time era. Also the cosmological dynamical system is split into two distinct dynamical systems that have two distinct de Sitter fixed points, with the early-time de Sitter point being unstable. This framework gives an explicit example of the unification of the early-time with late-time acceleration. Finally, we discuss in some detail the physical interpretation of the various models we present in this work.

Exact electromagnetic wave solutions to Maxwell equations on anisotropic Bianchi I cosmological spacetime backgrounds are studied. The waves evolving on Bianchi I spacetimes exhibit birefringence (associated with linear polarization) and dispersion. The particular case of a vacuum-dominated anisotropic Universe, which reproduces a Friedmann-Robertson-Walker Universe (for late times)—while, for earlier times, it matches a Kasner Universe—is studied. The electromagnetic waves do not, in general, follow null geodesics. This produces a modification of the cosmological redshift, which is then dependent on light polarization, its dispersion, and its non-null geodesic behavior. New results presented here may help to tackle some issues related to the “horizon” problem.

Testing general relativity in the nonlinear, dynamical, strong-field regime of gravity is one of the major goals of gravitational wave astrophysics. Performing precision tests of general relativity (GR) requires numerical inspiral, merger, and ringdown waveforms for binary black hole (BBH) systems in theories beyond GR. Currently, GR and scalar-tensor gravity are the only theories amenable to numerical simulations. In this article, we present a well-posed perturbation scheme for numerically integrating beyond-GR theories that have a continuous limit to GR. We demonstrate this scheme by simulating BBH mergers in dynamical Chern-Simons gravity (dCS), to linear order in the perturbation parameter. We present mode waveforms and energy fluxes of the dCS pseudoscalar field from our numerical simulations. We find good agreement with analytic predictions at early times, including the absence of pseudoscalar dipole radiation. We discover new phenomenology only accessible through numerics: a burst of dipole radiation during merger. We also quantify the self-consistency of the perturbation scheme. Finally, we estimate bounds that GR-consistent LIGO detections could place on the new dCS length scale, approximately $\ell \lesssim \mathcal{O}(10)\mathrm{km}$.

We investigate local well-posedness of the initial value problem for Lovelock and Horndeski theories of gravity. A necessary condition for local well-posedness is strong hyperbolicity of the equations of motion. Even weak hyperbolicity can fail for strong fields so we restrict to weak fields. The Einstein equation is known to be strongly hyperbolic in harmonic gauge so we study Lovelock theories in harmonic gauge. We show that the equation of motion is always weakly hyperbolic for weak fields but, in a generic weak-field background, it is not strongly hyperbolic. For Horndeski theories, we prove that, for weak fields, the equation of motion is always weakly hyperbolic in any generalized harmonic gauge. For some Horndeski theories there exists a generalized harmonic gauge for which the equation of motion is strongly hyperbolic in a weak-field background. This includes “k-essence” like theories. However, for more general Horndeski theories, there is no generalized harmonic gauge for which the equation of motion is strongly hyperbolic in a generic weak-field background. Our results show that the standard method used to establish local well-posedness of the Einstein equation does not extend to Lovelock or general Horndeski theories. This raises the possibility that these theories may not admit a well-posed initial value problem even for weak fields.