A recent extension of a variationally optimized perturbation, combined with renormalization group properties in a straightforward way, can provide approximations to nonperturbative quantities such as the chiral symmetry breaking order parameters typically. We apply this to evaluate, up to third order in this modified perturbation, the ratio $F_{\pi }/{\Lambda }_{\overline{\mathrm{MS}}}$, where $F_{\pi }$ is the pion decay constant and ${\Lambda }_{\overline{\mathrm{MS}}}$ the basic QCD scale in the $\overline{\mathrm{MS}}$ scheme. Using experimental $F_{\pi }$ input value we obtain ${\Lambda }_{\overline{\mathrm{MS}}}^{n_f=2}\simeq {255}_{-15}^{+40}\mathrm{MeV}$, where quoted errors are estimates of theoretical uncertainties of the method. This compares reasonably well with some recent lattice simulation results. We briefly discuss prospects (and obstacles) for extrapolation to ${\alpha }_S(\mu )$ at perturbative $\mu$ values.

We note that the theoretical prediction of neutrinos from gamma-ray bursts (GRBs) by IceCube overestimates the GRB neutrino flux, because they ignore both the energy dependence of the fraction of proton energy transferred to charged pions and the radiative energy loss of secondary pions and muons when calculating the normalization of the neutrino flux. After correction, GRB neutrino flux is reduced by a factor $\sim 5$ for typical GRB spectral parameters and may be consistent with the present zero event detected by IceCube. More observations are important to push the sensitivity below the prediction and test whether GRBs are the sources of ultrahigh energy cosmic rays.

We study the viscous damping of r-modes of compact stars and analyze in detail the regions where small amplitude modes are unstable to the emission of gravitational radiation. We present general expressions for the viscous damping times for arbitrary forms of interacting dense matter and derive general semi-analytic results for the boundary of the instability region. These results show that many aspects, such as the physically important minima of the instability boundary, are surprisingly insensitive to detailed microscopic properties of the considered form of matter. Our general expressions are applied to the cases of hadronic stars, strange stars, and hybrid stars, and we focus on equations of state that are compatible with the recent measurement of a heavy compact star. We find that hybrid stars with a sufficiently small core can “masquerade” as neutron stars and feature an instability region that is indistinguishable from that of a neutron star, whereas neutron stars with a core density high enough to allow direct Urca reactions feature a notch on the right side of the instability region.

We present current and future constraints on the Hu and Sawicki modified gravity scenario. This model can reproduce a late time accelerated universe and evade Solar System constraints. While current cosmological data still allows for distinctive deviations from the cosmological constant picture, future measurements of the growth of structure combined with supernova Ia luminosity distance data will greatly improve present constraints.

Bouncing models have been proposed by many authors as a completion of, or even as an alternative to, inflation for the description of the very early and dense Universe. However, most bouncing models contain a contracting phase from a very large and rarefied state, where dark energy might have had an important role as it has today in accelerating our large Universe. In that case, its presence can modify the initial conditions and evolution of cosmological perturbations, changing the known results already obtained in the literature concerning their amplitude and spectrum. In this paper, we assume the simplest and most appealing candidate for dark energy, the cosmological constant, and evaluate its influence on the evolution of cosmological perturbations during the contracting phase of a bouncing model, which also contains a scalar field with a potential allowing background solutions with pressure and energy density satisfying $p=wϵ$, $w$ being a constant. An initial adiabatic vacuum state can be set at the end of domination by the cosmological constant, and an almost scale-invariant spectrum of perturbations is obtained for $w\approx 0$, which is the usual result for bouncing models. However, the presence of the cosmological constant induces oscillations and a running towards a tiny red-tilted spectrum for long-wavelength perturbations.

Based on the results obtained in our previous study on gravity’s rainbow, we determine the quantum corrections to the space-time metric for the Schwarzschild and the de Sitter background, respectively. We analyze how quantum fluctuations alter these metrics, inducing modifications on the propagation of test particles. Significantly enough, we find that quantum corrections can become relevant not only for particles approaching the Planck energy but, due to the one-loop contribution, even for low-energy particles as far as Planckian length scales are considered. We briefly compare our results with others obtained in similar studies and with the recent experimental OPERA announcement of superluminal neutrino propagation.

We use the framework of a recently proposed model of reduced relativistic gas to obtain the bounds for $\Omega$’s of dark matter and dark energy (in the present case, a cosmological constant), taking into consideration an arbitrary warmness of dark matter. An equivalent equation of state has been used by Sakharov to predict the oscillations in the matter power spectrum. Two kinds of tests are accounted for in what follows, namely, the ones coming from the dynamics of the conformal factor of the homogeneous and isotropic metric and also the ones based on linear cosmic perturbations. The reduced relativistic gas model demonstrated its high effectiveness, permitting to explore a large volume in the space of mentioned parameters in a rather economic way. Taking together the results of such tests as supernova type Ia (Union2 sample), $H(z)$, cosmic microwave background ($R$ factor), baryon acoustic oscillation, and large-scale structure data (2dFGRS data), we confirm that $\Lambda \mathrm{CDM}$ is the most favored model. At the same time, for the 2dFGRS data alone we found that an alternative model with a very small quantity of dark matter is also viable. This output is potentially relevant in view of the fact that the large-scale structure data is the only test which cannot be affected by the possible quantum contributions to the low-energy gravitational action.

Observations of the CMB and large-scale structure provide a unique opportunity to explore the fundamental properties of the constituents that compose the cosmic dark radiation background (CDRB), of which the three standard neutrinos are thought to be the dominant component. We report on the first constraint to the CDRB rest-frame sound speed, $c_{\mathrm{eff}}^2$, using the most recent CMB and large-scale structure data. Additionally, we report improved constraints to the CDRB viscosity parameter, $c_{\mathrm{vis}}^2$. For a noninteracting species, these parameters both equal $1/3$. Using current data we find that a standard CDRB, composed entirely of three noninteracting neutrino species, is ruled out at the 99% confidence level (C.L.) with $c_{\mathrm{eff}}^2={0.30}_{-0.026}^{+0.027}$ and $c_{\mathrm{vis}}^2={0.44}_{-0.21}^{+0.27}$ (95% C.L.). We also discuss how constraints to these parameters from current and future observations (such as the Planck satellite) allow us to explore the fundamental properties of any anomalous radiative energy density beyond the standard three neutrinos.

In this paper, we discuss a formulation of relativistic quantum mechanics that uses model Euclidean Green functions or their generating functional as input. This formalism has a close relation to quantum field theory, but as a theory of linear operators on a Hilbert space, it has the advantages of quantum mechanics. One interesting feature of this approach is that matrix elements of operators in normalizable states on the physical Hilbert space can be calculated directly using the Euclidean Green functions without performing an analytic continuation. The formalism is summarized in this paper. We discuss the motivation, advantages, and difficulties in using this formalism. We discuss how to compute bound states, scattering cross sections, and finite Poincaré transformations without using analytic continuation. A toy model is used to demonstrate how matrix elements of $e^{-\beta H}$ in normalizable states can be used to construct sharp-momentum transition-matrix elements.

A flavor symmetry based on ${\mathsf{Z}}_4$ is analyzed in the context of $\mathsf{S}\mathsf{U}(\mathsf{5})$ Grand Unification with the standard fermionic content plus three right-handed neutrinos. The role of ${\mathsf{Z}}_4$ is to forbid some Yukawa couplings of up- and down-quarks to Higgs scalars such that the quark mass matrices $M_u$, $M_d$ have Nearest-Neighbor-Interaction (NNI) structure, once they are generated through the electroweak symmetry breaking. It turns out in this framework that ${\mathsf{Z}}_4$ is indeed the minimal discrete symmetry and its implementation requires the introduction of at least two Higgs quintets, which leads to a two Higgs doublet model at low energy scale. Because of the $\mathsf{S}\mathsf{U}(\mathsf{5})$ unification, it is shown that the charged lepton mass matrix develops also NNI form. However, the effective neutrino mass matrix exhibits a nonparallel pattern, in the framework of the type-I seesaw mechanism. Analyzing all possible zero textures allowed by gauge-horizontal symmetry $\mathsf{S}\mathsf{U}(\mathsf{5})\times {\mathsf{Z}}_4$, it is seen that only two patterns are in agreement with the leptonic experimental data and they could be further distinguished by the light neutrino mass spectrum hierarchy. It is also demonstrated that ${\mathsf{Z}}_4$ freezes out the possibility of proton decay through exchange of color Higgs triplets at tree-level.

In extensions of the standard model with compactified extra dimensions, perturbative unitarity of the longitudinal gauge bosons is maintained through the contribution of heavy Kaluza-Klein excitations of the gauge fields, without the necessity of introducing a Higgs field. The three-site Higgsless model represents a minimal approach in this respect, containing just one extra set of heavy gauge bosons $Z^{\prime }/W^{\prime \pm }$ in the spectrum. While the $Z^{\prime }$ can have robust couplings to SM fermions and, hence, may be detected within the first $1–20{\mathrm{fb}}^{-1}$ of LHC data ($\sqrt{s}=14\mathrm{TeV}$), the coupling of the $W^{\prime }$ to light fermions is suppressed and depends on the model parameters. Expanding on previous parton-level studies, we determine discovery thresholds of the $W^{\prime }$ in $s$-channel Drell-Yan production at the LHC for masses $m_{W^{\prime }}=380$, 500, and 600 GeV, combining analyses of the semileptonic final states $\ell \ell jj$, $\ell \nu jj$ and the leptonic final state $\ell \nu \ell \ell$ ($\ell =e,\mu$) including fast detector simulation.

Recent lattice QCD calculations, at physical pion masses and small lattice spacings that approach the continuum limit, have revealed that nondiagonal quark correlators above the critical temperature are finite up to about 2 $T_c$. Since the transition from hadronic to free partonic degrees of freedom is merely an analytic crossover, it is likely that, in the temperature regime between 1–2 $T_c$, quark and gluon quasiparticles and prehadronic bound states can coexist. The correlator values, in comparison to Polyakov loop-extended Nambu–Jona-Lasinio model calculations beyond mean field, indicate that at least part of the mixed phase resides in color-neutral bound states. A similar effect was postulated for the in-medium fragmentation process, i.e. for partons which do not thermalize with the system and thus constitute the nonequilibrium component of the particle emission spectrum from a deconfined plasma phase. Here, for the first time we investigate the likelihood of forming bound states also in the equilibrated, parton-dominated phase above $T_c$ which is described by lattice QCD.

Soft-collinear effective theory (SCET) is an effective field theory which describes the interactions of low invariant mass jets which are highly boosted with respect to one another. In the standard formulation of SCET, the effective Lagrangian for collinear fields is expanded in inverse powers of the energy. At leading order this leads to manifest decoupling of soft and collinear degrees of freedom; however, subleading terms in the effective Lagrangian violate this manifest decoupling. In this paper we point out that the collinear expansion in the SCET Lagrangian is unnecessary, and that the SCET Lagrangian may instead be written as multiple decoupled copies of QCD. The interactions between the sectors in full QCD are reproduced in the effective theory by an external current consisting of QCD fields coupled to Wilson lines. We illustrate this picture with two examples: dijet production and $B\to X_s\gamma$.

Recently, a new mechanism has been proposed to cure the problem of fermion mass hierarchy in the standard model (SM) model. In this scenario, all SM charged fermions other than top quark arise from higher dimensional operators involving the SM Higgs field. This model also predicted some interesting phenomenology of the Higgs boson. We generalize this model to accommodate neutrino masses (Dirac and Majorana) and also obtain the mixing pattern in the leptonic sector. To generate neutrino masses, we add extra three right-handed neutrinos $(N_{iR})$ in this model.

It has been shown recently that a neutron placed in an external quasistatic electric field develops an induced electric dipole moment ${\mathbf{p}}_{\mathrm{IND}}$ due to quantum fluctuations in the QED vacuum. A feasible experiment which could detect such an effect is proposed and described here. It is shown that the peculiar angular dependence of ${\mathbf{p}}_{\mathrm{IND}}$ on the orientation of the neutron spin leads to a characteristic asymmetry in polarized neutron scattering by heavy nuclei. This asymmetry can be of the order of ${10}^{-3}$ for neutrons with epithermal energies. For thermalized neutrons from a hot moderator, one still expects experimentally accessible values of the order of ${10}^{-4}$. The contribution of the induced effect to the neutron scattering length is expected to be only 1 order of magnitude smaller than that due to the neutron polarizability from its quark substructure. The experimental observation of this scattering asymmetry would be the first ever signal of nonlinearity in electrodynamics due to quantum fluctuations in the QED vacuum.

Diboson production ($WW+WZ+ZZ$) has been observed at the Tevatron in hadronic decay modes dominated by the $WW$ process. This paper describes the measurement of the cross section of $WZ$ and $ZZ$ events in final states with large ${\mathrm{E̸}}_T$ and using $b$-jet identification as a tool to suppress $WW$ contributions. Because of the limited energy resolution, we cannot distinguish between partially hadronic decays of $WZ$ and $ZZ$, and we measure the sum of these processes. The number of signal events is extracted using a simultaneous fit to the invariant mass distribution of the two jets for events with two $b$-jet candidates and events with fewer than two $b$-jet candidates. We measure a cross section $\sigma (p\overline{p}\to WZ,ZZ)={5.8}_{-3.0}^{+3.6}\mathrm{pb}$, in agreement with the standard model.

The production of $K_S^0$ and $\Lambda$ hadrons is studied in $pp$ collision data at $\sqrt{s}=0.9$ and 7 TeV collected with the ATLAS detector at the LHC using a minimum-bias trigger. The observed distributions of transverse momentum, rapidity, and multiplicity are corrected to hadron level in a model-independent way within well-defined phase-space regions. The distribution of the production ratio of $\overline{\Lambda }$ to $\Lambda$ baryons is also measured. The results are compared with various Monte Carlo simulation models. Although most of these models agree with data to within 15% in the $K_S^0$ distributions, substantial disagreements are found in the $\Lambda$ distributions of transverse momentum.

We reanalyze the two- and three-pion mass distributions in the decays $X(3872)\to \rho J/\psi$ and $X(3872)\to \omega J/\psi$ and argue that the present data favor the $1^{++}$ assignment for the quantum numbers of the $X$.

Current results of the MiniBooNE experiment show excess events that indicate neutrino oscillations, but only if one goes beyond the standard 3 family scenario. Recently a different explanation of the events has been given, not in terms of oscillations but by the production and decay of a massive sterile neutrino with large transition magnetic moment. We study the effect of such a sterile neutrino in the rare decays ${\tau }^{-}\to {\mu }^{-}{\mu }^{+}{\pi }^{-}\nu$ and ${\tau }^{-}\to {\mu }^{-}{\mu }^{+}{e}^{-}\nu \nu$. We find that searches for these decays, featuring displaced vertices between the ${\mu }^{-}$ and the other charged particles, constitute reliable tests for the existence of the sterile neutrino proposed to explain the MiniBooNE anomaly. These searches could be done with already existing experimental data.

We find polarization modes of gravitational waves in topologically massive and new massive gravities by using the Newman-Penrose formalism where the null real tetrad is necessary to specify gravitational waves. The number of polarization modes is two for the new massive gravity and one for the topologically massive gravity, which is consistent with the metric-perturbation approach.