We study the phase diagram for staggered quarks using chiral perturbation theory. In beyond-the-standard-model simulations using a large number ($>8$) of staggered fermions, unphysical phases appear for coarse enough lattice spacing. We argue that chiral perturbation theory can be used to interpret one of these phases. In addition, we show that only three broken phases for staggered quarks exist, at least for lattice spacings in the regime $a^2\ll {\mathrm{\Lambda }}_{\mathrm{QCD}}^2$.

We apply worldline methods to the study of vacuum polarization effects in plane wave backgrounds, in both scalar and spinor QED. We calculate helicity-flip probabilities to one loop order and treated exactly in the background field, and provide a toolkit of methods for use in investigations of higher-order processes. We also discuss the connections between the worldline, S-matrix, and lightfront approaches to vacuum polarization effects.

We consider the role of quantum effects, mainly, Weyl anomaly in modifying Friedmann-Lemaitre-Robertson-Walker (FLRW) model singular behavior at early times. Weyl anomaly corrections to FLRW models have been considered in the past, here we reconsider this model and show the following: The singularity of this model is weak according to Tipler and Krolak, therefore, the spacetime might admit a geodesic extension. Weyl anomaly corrections change the nature of the initial singularity from a big bang singularity to a sudden singularity. The two branches of solutions consistent with the semiclassical treatment form a disconnected manifold. Joining these two parts at the singularity provides us with a $C^1$ extension to nonspacelike geodesics and leaves the spacetime geodesically complete. Using Gauss-Codazzi equations one can derive generalized junction conditions for this higher-derivative gravity. The extended spacetime obeys Friedmann and Raychaudhuri equations and the junction conditions. The junction does not generate Dirac delta functions in matter sources which keeps the equation of state unchanged.

A loop quantization of the diagonal class A Bianchi models starting from the complex-valued self-dual connection variables is presented in this paper. The basic operators in the quantum theory correspond to areas and generalized holonomies of the Ashtekar connection, and the reality conditions are implemented via the choice of the inner product on the kinematical Hilbert space. The action of the Hamiltonian constraint operator is given explicitly for the case when the matter content is a massless scalar field (in which case the scalar field can be used as a relational clock), and it is shown that the big bang and big crunch singularities are resolved in the sense that singular and nonsingular states decouple under the action of the Hamiltonian constraint operator.

We consider the problem for the classification of static and asymptotically flat Einstein-Maxwell-dilaton spacetimes with a photon sphere. It is first proven that the photon spheres in Einstein-Maxwell-dilaton gravity have constant mean and constant scalar curvature. Then we derive some relations between the mean curvature and the physical characteristics of the photon spheres. Using further the symmetries of the dimensionally reduced Einstein-Maxwell-dilaton field equations we show that the lapse function, the electrostatic potential, and the dilaton field are functionally dependent in the presence of a photon sphere. Using all this we prove the main classification theorem by explicitly constructing all Einstein-Maxwell-dilaton solutions possessing a nonextremal photon sphere.

The dispersive representation of the virtual Compton forward scattering amplitude has been recently reexamined in connection with the evaluation of the Cottingham formula for the proton-neutron electromagnetic mass difference and the proton radius puzzle. The most difficult part of the analysis is related to one of the invariant amplitudes, denoted as $T_1(\nu ,Q^2)$, which requires a subtraction in the standard dispersion relation with respect to the energy $\nu$ at fixed photon momentum squared $q^2=-Q^2$. We propose an alternative dispersive framework, which implements analyticity and unitarity by combining the Cauchy integral relation at low and moderate energies with the modulus representation of the amplitude at high energies. Using techniques of functional analysis, we derive a necessary and sufficient condition for the consistency with analyticity of the subtraction function $S_1(Q^2)=T_1(0,Q^2)$, the cross sections measured at low and moderate energies and the Regge model assumed to be valid at high energies. From this condition we obtain model-independent constraints on the subtraction function, confronting them with the available information on nucleon magnetic polarizabilities and results reported recently in the literature. The formalism can be used also for testing the existence of a fixed pole at $J=0$ in the angular momentum plane, but more accurate data are necessary for a definite answer.

Motivated by the diphoton resonance recently reported by the ATLAS and CMS collaborations at $\sqrt{s}=13\mathrm{TeV}$, we interpret the resonance as a scalar boson $X(750)$ in hidden-valley-like models. The scalar boson $X$ can mix with the standard model Higgs boson and thus can be produced via gluon fusion. It then decays into a pair of very light hidden particles $Y$ of $\mathcal{O}(1\mathrm{GeV})$, each of which in turn decays to a pair of collimated ${\pi }^0$’s, and these two ${\pi }^0$’s decay into photons which then form photon jets. A photon jet ($\gamma$ jet) is a special feature that consists of a cluster of collinear photons from the decay of a fast moving light particle [$\mathcal{O}(1\mathrm{GeV})$]. Because these photons inside the photon jet are so collimated that it cannot be distinguished from a single photon, in the final state of the decay of $X(750)$ a pair of photon jets looks like a pair of single photons, which the experimentalists observed and reconstructed the 750 GeV diphoton resonance. Prospects for LHC Run-2 about other new and testable features are also discussed.

We provide a possible explanation of a 750 GeV diphoton excess recently reported by both the ATLAS and CMS collaborations in the context of phenomenological spin-2 resonance scenarios, where the independent effective couplings of the resonance with gluons, quarks and photons are considered. We find a parameter region where the excess can be accounted for without conflicting with dijet constraints. We also show that the kinematical distributions might help to determine the couplings to gluons and quarks.

We analyze a recently proposed extension of the Standard Model based on the $SU(4)\times SU(2{)}_L\times U(1{)}_X$ gauge group, in which baryon number is interpreted as the fourth color and dark matter emerges as a neutral partner of the ordinary quarks under $SU(4)$. We show that under well-motivated minimal flavor-violating assumptions the particle spectrum contains a heavy dark matter candidate which is dominantly the partner of the right-handed top quark. Assuming a standard cosmology, the correct thermal relic density through freeze-out is obtained for dark matter masses around 2–3 TeV. We examine the constraints and future prospects for direct and indirect searches for dark matter. We also briefly discuss the LHC phenomenology, which is rich in top quark signatures, and investigate the prospects for discovery at a 100 TeV hadron collider.

LHC searches for fermionic top partners $T$ focus on three decay topologies: $T\to bW$, $T\to tZ$, and $T\to th$. However, top partners may carry new conserved quantum numbers that forbid these decays. The simplest possibility is a conserved parity, under which the top partner is odd and all SM states are even. In this case, decays of top partners may involve new particle-odd scalars, leading to signal topologies more commonly associated with supersymmetry, either with or without $R$-parity conservation. We study a simplified model in which this possibility is realized, and estimate the bounds on the top partner mass in this model implied by LHC searches for supersymmetry. We find that the bounds can be significantly weaker than in the conventional top partner decay scenario. For example, if the new parity is exact, a 500 GeV top partner is allowed as long as the lightest parity-odd scalar mass is between 325 and 500 GeV. The lower allowed top partner mass reduces the need for fine-tuning in the Higgs mass parameter, compared to the conventional decay scenario. We also present an explicit model, the oddest little Higgs, which exhibits this phenomenology.

We investigate the phenomenology of top squarks at the Large Hadron Collider (LHC) in a supersymmetric model where lepton number is identified with an approximate $U(1{)}_R$ symmetry in such a way that one of the left-chiral sneutrinos can acquire a large vacuum expectation value and can play the role of the down-type Higgs. This $R$ symmetry allows a subset of trilinear $R$-parity violating interactions, which determine the collider phenomenology of this model in a significant way. The gauginos are Dirac particles and gluinos are relatively heavy in this class of models. The model contains a right handed neutrino superfield, which gives a tree level mass to one of the active neutrinos. An order one neutrino Yukawa coupling also helps enhance the Higgs boson mass at the tree level and results in a very light bino-like neutralino (${\tilde{\chi }}_2^0$) with mass around a few hundred MeV, which is a carrier of missing (transverse) energy (${\cancel{E}}_T$). The model can accommodate two rather light top squarks, compatible with the observed mass of the Higgs boson. The lighter top squark (${\tilde{t}}_1$) can decay into $t{\tilde{\chi }}_2^0$, and thus the signal would be similar to the signal of top quark pair production at the LHC. In addition, fully visible decays such as ${\tilde{t}}_2\to b{e}^{+}$ can give rise to interesting final states. Such signals at the LHC combined with other features like a heavy gluino could provide strong evidence for this kind of a model. Our analysis shows that $m_{{\tilde{t}}_1}\lesssim 575(750)\mathrm{GeV}$ and $m_{{\tilde{t}}_2}\lesssim 1.2(1.4)\mathrm{TeV}$ can be probed with $5\sigma$ statistical significance at the 13 TeV LHC with $300(3000){\mathrm{fb}}^{-1}$ of integrated luminosity. Finally, we observe that in the presence of superlight carriers of ${\cancel{E}}_T$, the so-called “stealth” top squark scenario may naturally appear in our model.

In the context of the minimal supersymmetric standard model (MSSM), we discuss the possibility of the lightest Higgs boson with mass $M_h=98\mathrm{GeV}$ to be consistent with the $2.3\sigma$ excess observed at the LEP in the decay mode $e^{+}{e}^{-}\to Zh$, with $h\to b\overline{b}$. In the same region of the MSSM parameter space, the heavier Higgs boson $(H)$ with mass $M_H\sim 125\mathrm{GeV}$ is required to be consistent with the latest data on Higgs coupling measurements at the end of the $7+8\mathrm{TeV}$ LHC run with $25{\mathrm{fb}}^{-1}$ of data. While scanning the MSSM parameter space, we impose constraints coming from flavor physics, relic density of the cold dark matter as well as direct dark matter searches. We study the possibility of observing this light Higgs boson in vector boson fusion process and associated production with $W/Z$-boson at the high luminosity ($3000{\mathrm{fb}}^{-1}$) run of the 14 TeV LHC. Our analysis shows that this scenario can hardly be ruled out even at the high luminosity run of the LHC. However, the precise measurement of the Higgs signal strength ratios can play a major role to distinguish this scenario from the canonical MSSM one.

We study the phase diagram of $N_f=2+1$ QCD in the $T-{\mu }_B$ plane and investigate the critical point corresponding to the onset of the Roberge-Weiss transition, which is found for imaginary values of ${\mu }_B$. We make use of stout improved staggered fermions and of the tree level Symanzik gauge action and explore four different sets of lattice spacings, corresponding to $N_t=4$, 6, 8, 10, and different spatial sizes, in order to assess the universality class of the critical point. The continuum extrapolated value of the endpoint temperature is found to be $T_{\mathrm{RW}}=208(5)\mathrm{MeV}$, i.e. $T_{\mathrm{RW}}/T_c\sim 1.34(7)$, where $T_c$ is the chiral pseudocritical temperature at zero chemical potential, while our finite size scaling analysis, performed on $N_t=4$ and $N_t=6$ lattices, provides evidence for a critical point in the 3D Ising universality class.

We evaluate the neutron electric dipole moment $|{\overrightarrow{d}}_N|$ using lattice QCD techniques. The gauge configurations analyzed are produced by the European Twisted Mass Collaboration using $N_f=2+1+1$ twisted mass fermions at one value of the lattice spacing of $a\simeq 0.082\mathrm{fm}$ and a light quark mass corresponding to $m_{\pi }\simeq 373\mathrm{MeV}$. Our approach to extract the neutron electric dipole moment is based on the calculation of the $CP$-odd electromagnetic form factor $F_3(Q^2)$ for small values of the vacuum angle $\theta$ in the limit of zero Euclidean momentum transfer $Q^2$. The limit $Q^2\to 0$ is realized either by adopting a parametrization of the momentum dependence of $F_3(Q^2)$ and performing a fit or by employing new position space methods, which involve the elimination of the kinematical momentum factor in front of $F_3(Q^2)$. The computation in the presence of a $CP$-violating term requires the evaluation of the topological charge $\mathcal{Q}$. This is computed by applying the cooling technique and the gradient flow with three different actions, namely the Wilson, the Symanzik tree-level improved and the Iwasaki action. We demonstrate that cooling and gradient flow give equivalent results for the neutron electric dipole moment. Our analysis yields a value of $|{\overrightarrow{d}}_N|=0.045(6)(1)\overline{\theta }e·\mathrm{fm}$ for the ensemble with $m_{\pi }=373\mathrm{MeV}$ considered.

We study the possibility of using multilevel algorithms for the computation of correlation functions of gradient flow observables. For each point in the correlation function, an approximate flow is defined which depends only on links in a subset of the lattice. Together with a local action, this allows for independent updates and consequently a convergence of the Monte Carlo process faster than the inverse square root of the number of measurements. We demonstrate the feasibility of this idea in the correlation functions of the topological charge and the energy density.

We present a comprehensive new global QCD analysis of polarized inclusive deep-inelastic scattering, including the latest high-precision data on longitudinal and transverse polarization asymmetries from Jefferson Lab and elsewhere. The analysis is performed using a new iterative Monte Carlo fitting technique which generates stable fits to polarized parton distribution functions (PDFs) with statistically rigorous uncertainties. Inclusion of the Jefferson Lab data leads to a reduction in the PDF errors for the valence and sea quarks, as well as in the gluon polarization uncertainty at $x\gtrsim 0.1$. The study also provides the first determination of the flavor-separated twist-3 PDFs and the $d_2$ moment of the nucleon within a global PDF analysis.

We investigate the effect of the resummation of collinear double logarithms in the Balitsky-Fadin-Kuraev-Lipatov (BFKL) gluon Green function using the Monte Carlo event generator BFKLex. The resummed collinear terms in transverse momentum space were calculated by Sabio Vera [Nucl. Phys. B722, 65 (2005)] and correspond to the addition to the next-to-leading order BFKL kernel of a Bessel function of the first kind whose argument contains the strong coupling and a double logarithm of the ratio of the squared transverse momenta of the Reggeized gluons. We discuss how these additional terms improve the collinear convergence of the whole approach and reduce the asymptotic growth with energy of the cross sections. Taking advantage of the Monte Carlo implementation, we show how the new results reduce the diffusion of the gluon ladder into infrared and ultraviolet transverse momentum scales, while strongly affecting final state configurations by reducing the minijet multiplicity.

In the third fermion family and gauge symmetry of the Standard Model, we study the quark-quark, lepton-lepton and quark-lepton four-fermion operators in an effective theory at high energies. These operators have nontrivial contributions to the Schwinger-Dyson equations for fermion self-energy functions and the $W^{\pm }$-boson coupling vertex. As a result, the top-quark mass is generated via the spontaneous symmetry breaking of $⟨\overline{t}t⟩$-condensate and the $W^{\pm }$-boson coupling becomes approximately vectorlike at TeV scale. The bottom-quark, tau-lepton and tau-neutrino masses are generated via the explicit symmetry breaking of $W^{\pm }$-contributions and quark-lepton interactions. Their masses and Yukawa couplings are functions of the top-quark mass and Yukawa coupling. We qualitatively show the hierarchy of fermion masses and Yukawa couplings of the third fermion family. We also discuss the possible collider signatures due to the vectorlike (parity-restoration) feature of $W^{\pm }$-boson coupling at high energies.

We report a measurement of the ${\nu }_{\mu }$-nucleus inclusive charged-current cross section ($={\sigma }^{cc}$) on iron using data from the INGRID detector exposed to the J-PARC neutrino beam. The detector consists of 14 modules in total, which are spread over a range of off-axis angles from 0° to 1.1°. The variation in the neutrino energy spectrum as a function of the off-axis angle, combined with event topology information, is used to calculate this cross section as a function of neutrino energy. The cross section is measured to be ${\sigma }^{cc}(1.1\mathrm{GeV})=1.10\pm 0.15$ $(10^{-38}{\mathrm{cm}}^2/\text{nucleon})$, ${\sigma }^{cc}(2.0\mathrm{GeV})=2.07\pm 0.27$ $(10^{-38}{\mathrm{cm}}^2/\text{nucleon})$, and ${\sigma }^{cc}(3.3\mathrm{GeV})=2.29\pm 0.45$ $(10^{-38}{\mathrm{cm}}^2/\text{nucleon})$, at energies of 1.1, 2.0, and 3.3 GeV, respectively. These results are consistent with the cross section calculated by the neutrino interaction generators currently used by T2K. More importantly, the method described here opens up a new way to determine the energy dependence of neutrino-nucleus cross sections.

It has recently been understood that the hydrodynamic series generated by the Müller-Israel-Stewart theory is divergent and that this large-order behavior is consistent with the theory of resurgence. Furthermore, it was observed that the physical origin of this is the presence of a purely damped nonhydrodynamic mode. It is very interesting to ask whether this picture persists in cases where the spectrum of nonhydrodynamic modes is richer. We take the first step in this direction by considering the simplest hydrodynamic theory which, instead of the purely damped mode, contains a pair of nonhydrodynamic modes of complex conjugate frequencies. This mimics the pattern of black brane quasinormal modes which appear on the gravity side of the AdS/CFT description of $\mathcal{N}=4$ supersymmetric Yang-Mills plasma. We find that the resulting hydrodynamic series is divergent in a way consistent with resurgence and precisely encodes information about the nonhydrodynamic modes of the theory.