We revisit the old-standing problem of the electromagnetic interaction for particles of arbitrary spin. Based on the assumption that light-cone helicity at tree level and $Q^2=0$ should be conserved nontrivially by the electromagnetic interaction, we are able to derive all the natural electromagnetic moments for a pointlike particle of any spin. We provide here a transparent decomposition of the electromagnetic current in terms of covariant vertex functions. We also define in a general way the electromagnetic multipole form factors, and show their relation with the electromagnetic moments and covariant vertex functions. The light-cone helicity conservation argument determines uniquely the values of all electromagnetic moments, which we refer to as the “natural” ones. These specific values are in accordance with the standard model, and the prediction of universal $g=2$ gyromagnetic factor is naturally recovered. We provide a very simple and compact formula for these natural moments. As an application of our results, we generalize the discussion of quark transverse charge densities to particles with arbitrary spin, giving more physical support to the light-cone helicity conservation argument.

The cross section of the process $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}$ was measured in the spherical neutral detector experiment at the VEPP-2M $e^{+}{e}^{-}$ collider in the energy region $\sqrt{s}=980$, 1040–1380 MeV. The event numbers of the process $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}$ were normalized to the integrated luminosity measured using $e^{+}{e}^{-}\to e^{+}{e}^{-}$ and $e^{+}{e}^{-}\to \gamma \gamma$ processes. The ratio of the measured cross section to the theoretically predicted value is $1.006\pm 0.007\pm 0.016$ and $1.005\pm 0.007\pm 0.018$ in the first and second case, respectively. Using results of the measurements, the electromagnetic running coupling constant $\alpha$ in the energy region $\sqrt{s}=1040–1380\mathrm{MeV}$ was obtained $⟨1/\alpha ⟩=134.1\pm 0.5\pm 1.2$ and this is in agreement with theoretical expectation.

We present a search for $WW$ and $WZ$ production in final states that contain a charged lepton (electron or muon) and at least two jets, produced in $\sqrt{s}=1.96\mathrm{TeV}$ $p\overline{p}$ collisions at the Fermilab Tevatron, using data corresponding to $1.2{\mathrm{fb}}^{-1}$ of integrated luminosity collected with the CDF II detector. Diboson production in this decay channel has yet to be observed at hadron colliders due to the large single $W$ plus jets background. An artificial neural network has been developed to increase signal sensitivity, as compared with an event selection based on conventional cuts. We set a 95% confidence level upper limit of ${\sigma }_{WW}\times \mathrm{BR}(W\to \ell {\nu }_{\ell },W\to \mathrm{\text{jets}})+{\sigma }_{WZ}\times \mathrm{BR}(W\to \ell {\nu }_{\ell },Z\to \mathrm{\text{jets}})<2.88\mathrm{pb}$, which is consistent with the standard model next-to-leading-order cross section calculation for this decay channel of $2.09\pm 0.12\mathrm{pb}$.

Previous analysis about thin-shell wormholes associated to cosmic strings are extended. More evidence is found supporting the conjecture that, under reasonable assumptions about the equations of state of matter on the shell, the configurations are not stable under radial velocity perturbations.

The classical counterpart of noncommutative quantum mechanics is a constrained system containing only second-class constraints. The embedding procedure formulated by Batalin, Fradkin and Tyutin (BFT) enables one to transform this system into an Abelian gauge theory exhibiting only first class constraints. The appropriateness of the BFT embedding, as implemented in this work, is verified by showing that there exists a one to one mapping linking the second-class model with the gauge invariant sector of the gauge theory. As is known, the functional quantization of a gauge theory calls for the elimination of its gauge freedom. Then, we have at our disposal an infinite set of alternative descriptions for noncommutative quantum mechanics, one for each gauge. We study the relevant features of this infinite set of correspondences. The functional quantization of the gauge theory is explicitly performed for two gauges and the results compared with that corresponding to the second-class system. Within the operator framework the gauge theory is quantized by using Dirac’s method.

Theories beyond the standard model include a number of new particles, some of which might be light and weakly coupled to ordinary matter. Such particles affect the equation of state of nuclear matter and can shift admissible masses of neutron stars to higher values. The internal structure of neutron stars is modified provided the ratio between coupling strength and mass squared of a weakly interacting light boson is above $g^2/{\mu }^2\sim 25{\mathrm{GeV}}^{-2}$. We provide limits on the couplings with the strange sector, which cannot be achieved from laboratory experiments analysis. When the couplings to the first family of quarks is considered, the limits imposed by the neutron stars are not more stringent than the existing laboratory ones. The observations on neutron stars give evidence that the equation of state of the $\beta$-equilibrated nuclear matter is stiffer than expected from many-body theory of nuclei and nuclear matter. A weakly interacting light vector boson coupled predominantly to the second family of the quarks can produce the required stiffening.

We consider (4, 1)-dimensional branes constructed with two scalar fields $\varphi$ and $\chi$ coupled to a Dirac spinor field by means of a general Yukawa coupling. The equation of motion for the coefficients of the chiral decomposition of the spinor in curved spacetime leads to a Schrödinger-like equation whose solutions allow to obtain the masses of the fermionic modes. The simplest Yukawa coupling $\overline{\Psi }\varphi \chi \Psi$ is considered for the Bloch brane model and fermion localization is studied. We found resonances for both chiralities and related their appearance to branes with internal structure.

Classical models for dark energy can exhibit a variety of singularities, many of which occur for scale factors much bigger than the Planck length. We address here the issue of whether some of these singularities, the big freeze and the big démarrage, can be avoided in quantum cosmology. We use the framework of quantum geometrodynamics. We restrict our attention to a class of models whose matter content can be described by a generalized Chaplygin gas and be represented by a scalar field with an appropriate potential. Employing the DeWitt criterion that the wave function be zero at the classical singularity, we show that a class of solutions to the Wheeler-DeWitt equation fulfilling this condition can be found. These solutions thus avoid the classical singularity. We discuss the reasons for the remaining ambiguity in fixing the solution.

In this paper we place new constraints on a $f(R)$ modified gravity model recently proposed by Hu and Sawicki (HS). After checking that the HS model produces a viable cosmology, i.e. a matter dominated epoch followed by a late-time acceleration, we constrain some of its parameters by using recent observations from the UNION compilation of luminosity distances of Supernovae type Ia, including complementary information from baryonic acoustic oscillations, Hubble expansion, and age data. We found that the data considered is unable to place significant constraints on the model parameters and we discuss the impact of a different assumption of the background model in cosmic parameters inference.

In this paper, we analyze the dynamical evolution of quintessence dark energy induced by the collapse of dark matter halos. Different from other previous studies, we develop a numerical strategy which allows us to calculate the dark energy evolution for the entire history of the spherical collapse of dark matter halos, without the need of separate treatments for linear, quasilinear, and nonlinear stages of the halo formation. It is found that the dark energy perturbations evolve with redshifts, and their specific behaviors depend on the quintessence potential as well as the collapsing process. The overall energy density perturbation is at the level of ${10}^{-6}$ for cluster-sized halos. The perturbation amplitude decreases with the decrease of the halo mass. At a given redshift, the dark energy perturbation changes with the radius to the halo center, and can be either positive or negative depending on the contrast of ${\partial }_t\varphi$, ${\partial }_r\varphi$, and $\varphi$ with respect to the background, where $\varphi$ is the quintessence field. For shells where the contrast of ${\partial }_r\varphi$ is dominant, the dark energy perturbation is positive and can be as high as about ${10}^{-5}$.

The $\mu$-$\tau$ symmetry imposed on the neutrino mass matrix in the flavor basis is known to be quite predictive. We integrate this very specific neutrino symmetry into a more general framework based on the supersymmetric $SO(10)$ grand unified theory. As in several other models, the fermion mass spectrum is determined by Hermitian mass matrices resulting from the renormalizable Yukawa couplings of the 16-plet of fermions with the Higgs fields transforming as 10, $\overline{126}$, and 120 representations of the $SO(10)$ group. The $\mu$-$\tau$ symmetry is spontaneously broken through the 120-plet. Consequences of this scheme are considered for fermion masses using both a type-I and a type-II seesaw mechanism. This scenario is shown to lead to a generalized $CP$ invariance of the mass matrices and vanishing $CP$ violating phases if the Yukawa couplings are invariant under the $\mu$-$\tau$ symmetry. Small explicit breaking of the $\mu$-$\tau$ symmetry is then shown to provide a very good understanding of all of the fermion masses and mixing. Detailed fits to the fermion spectrum are presented in several scenarios. One obtains a very good fit to all observables in the context of the type-I seesaw mechanism, but the type-II seesaw model also provides a good description except for the overall scale of the neutrino masses. Three major predictions on the leptonic mixing parameters in the type-I seesaw case are (i) the atmospheric mixing angle ${\theta }_{23}^l$ close to maximal, (ii) ${\theta }_{13}^l$ close to the present upper bound, and (iii) a negative but very small Dirac $CP$ violating phase in the neutrino oscillations.

We consider a simple class of models in which the relic density of dark matter is determined by the baryon asymmetry of the Universe. In these models a $B-L$ asymmetry generated at high temperatures is transferred to the dark matter, which is charged under $B-L$. The interactions that transfer the asymmetry decouple at temperatures above the dark matter mass, freezing in a dark matter asymmetry of order the baryon asymmetry. This explains the observed relation between the baryon and dark matter densities for the dark matter mass in the range 5–15 GeV. The symmetric component of the dark matter can annihilate efficiently to light pseudoscalar Higgs particles $a$ or via $t$-channel exchange of new scalar doublets. The first possibility allows for $h^0\to aa$ decays, while the second predicts a light charged Higgs-like scalar decaying to $\tau \nu$. Direct detection can arise from Higgs exchange in the first model or a nonzero magnetic moment in the second. In supersymmetric models, the would-be lightest supersymmetric partner can decay into pairs of dark matter particles plus standard model particles, possibly with displaced vertices.

We study low energy hadronic interaction models based on BESS observed cosmic ray proton and antiproton spectra at medium high altitude. Among the three popular low energy interaction models, we find that FLUKA reproduces the results of BESS observations on the secondary proton spectrum reasonably well over the whole observed energy range, the model UrQMD works well at relatively higher energies, whereas the spectrum obtained with GHEISHA differs significantly from the measured spectrum. The simulated antiproton spectrum with FLUKA, however, exhibits significant deviation from the BESS observation whereas UrQMD and GHEISHA reproduce the BESS observations within experimental error.

We estimate the energy of the lowest charmonium and upsilon states with hybrid admixtures using the method of QCD sum rules. Our results show that the ${\Psi }^{\prime }(2S)$ and ${\rm Y}(3S)$ states both have about a 50% admixture of hybrid and meson components. From this we find explanations of both the famous $\rho -\pi$ puzzle for charmonium and the unusual pattern of $\sigma$ decays that have been found in ${\rm Y}$ decays. Moreover, this picture can be used for predictions of heavy quark production with the octet model for RHIC.

Rotational states (planar uniform rotations) of various string hadron models are tested for stability with respect to small disturbances. These models include an open or closed string carrying $n$ massive points (quarks), and their rotational states result in a set of quasilinear Regge trajectories. It is shown that rotations of the linear string baryon model $q\mathrm{\text{−}}q\mathrm{\text{−}}q$ and the similar states of the closed string are unstable, because spectra of small disturbances for these states contain complex frequencies, corresponding to exponentially growing modes of disturbances. Rotations of the linear model are unstable for any values of points’ masses, but for the closed string we have the threshold effect. This instability is important for describing excited hadrons; in particular, it increases predictions for their width $\Gamma$. Predicted large values $\Gamma$ for $N$, $\Delta$ and strange baryons in comparison with experimental data result in unacceptability of the linear string model $q\mathrm{\text{−}}q\mathrm{\text{−}}q$ for describing these baryon states.

We study the production of a large-$p_T$ photon in association with a jet in proton-proton collisions. We examine the sensitivity of the jet rapidity distribution to the gluon distribution function in the proton. We then assess the sensitivity of various $\mathrm{\text{photon}}+\mathrm{\text{jet}}$ correlation observables to the photon fragmentation functions. We argue that RHIC data on photon-jet correlations can be used to constrain the photon fragmentation functions in a region which was barely accessible in LEP experiments.

Singly and doubly charged Higgs bosons in the Higgs triplet model mediate the lepton flavor violating (LFV) decays $\tau \to \overline{l}ll$ and $\mu \to e\gamma$. The lepton flavor violating decay rates are proportional to products of two triplet Yukawa couplings ($h_{ij}$) which can be expressed in terms of the parameters of the neutrino mass matrix and an unknown triplet vacuum expectation value. We determine the parameter space of the neutrino mass matrix in which a signal for $\tau \to \overline{l}ll$ and/or $\mu \to e\gamma$ is possible at ongoing and planned experiments. The conditions for respecting the stringent upper limit for $\mu \to \overline{e}ee$ are studied in detail, with emphasis given to the possibility of $|h_{ee}|\simeq 0$, which can only be realized if Majorana phases are present.

We explore the physics of supersymmetric Janus gauge theories in four dimensions with spatial dependent coupling constants $e^2$ and $\theta$. For the 8 supersymmetric case, we study the vacuum and Bogomol’nyi-Prasad-Sommerfield spectrum, and the physics of a sharp interface where the couple constants jump. We also find less supersymmetric cases either due to additional expressions in the Lagrangian or to the fact that coupling constants depend on additional spatial coordinates.

In gauge theories in the limit of a large number $N_c$ of colors baryons are usually described as heavy solitonic objects with mass of order $N_c$. We discuss an alternative large $N_c$ description both directly in the field theory as well as using holography. In this alternative large $N_c$ limit at least some of the baryons behave like mesons, that is they stay light even at large $N_c$ and their interactions vanish in that limit. For $N_c=3$ these alternative large $N_c$ baryons are equivalent to the standard baryons. In the holographic description it is manifest that the Regge slopes of mesons and alternative baryons are degenerate.