We evaluate the kinematic distributions in phase space of 4-parton final-state subprocesses produced by double parton scattering, and we contrast these with the final-state distributions that originate from conventional single parton scattering. Our goal is to establish the distinct topologies of events that arise from these two sources and to provide a methodology for experimental determination of the relative magnitude of the double parton and single parton contributions at Large Hadron Collider energies. We examine two cases in detail, the $b\overline{b}$ jet-jet and the 4 jet final states. After full parton-level simulations, we identify a few variables that separate the two contributions remarkably well, and we suggest their use experimentally for an empirical measurement of the relative cross section. We show that the double parton contribution falls off significantly more rapidly with the transverse momentum $p_T^{j1}$ of the leading jet, but, up to issues of the relative normalization, may be dominant at modest values of $p_T^{j1}$.

We study the charge density, form factors and spin distributions of the electron induced by its $|e\gamma ⟩$ light-front Fock state in impact parameter space. Only transversally compact Fock states contribute to the leading behavior of the Dirac and Pauli form factors as the momentum transfer tends to infinity. Power suppressed contributions are not compact, and distributions weighted by the transverse size have endpoint contributions. The Fock state conserves the spin of the parent electron locally, but the separate contributions of the electron, photon, and orbital angular momentum depend on longitudinal momentum and impact parameter. The sign of the anomalous magnetic moment of the electron may be understood intuitively from the density distribution, addressing a challenge by Feynman.

It has been postulated that partonic orbital angular momentum can lead to a significant double-helicity dependence in the net transverse momentum of Drell-Yan dileptons produced in longitudinally polarized $p+p$ collisions. Analogous effects are also expected for dijet production. If confirmed by experiment, this hypothesis, which is based on semiclassical arguments, could lead to a new approach for studying the contributions of orbital angular momentum to the proton spin. We report the first measurement of the double-helicity dependence of the dijet transverse momentum in longitudinally polarized $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$ from data taken by the PHENIX experiment in 2005 and 2006. The analysis deduces the transverse momentum of the dijet from the widths of the near- and far-side peaks in the azimuthal correlation of the dihadrons. When averaged over the transverse momentum of the triggered particle, the difference of the root mean square of the dijet transverse momentum between like- and unlike-helicity collisions is found to be $-37\pm {88}^{\mathrm{stat}}\pm {14}^{\mathrm{syst}}\mathrm{MeV}/c$.

We show how to obtain a single chiral family of an $SO(10)$ grand unified theory (GUT), starting from a Majorana-Weyl representation of a unifying (GraviGUT) group $SO(3,11)$, which contains the gravitational Lorentz group $SO(3,1)$. An action is proposed, which reduces to the correct fermionic grand unified theory action in the broken phase.

We investigate the critical behavior of three-dimensional relativistic fermion models with a $\mathrm{U}(N_{\mathrm{L}}{)}_{\mathrm{L}}\otimes \mathrm{U}(1{)}_{\mathrm{R}}$ chiral symmetry reminiscent of the Higgs-Yukawa sector of the standard model of particle physics. We classify all possible four-fermion interaction terms and the corresponding discrete symmetries. For sufficiently strong correlations in a scalar parity-conserving channel, the system can undergo a second-order phase transition to a chiral-symmetry broken phase, which is a $3d$ analog of the electroweak phase transition. We determine the critical behavior of this phase transition in terms of the critical exponent $\nu$ and the fermion and scalar anomalous dimensions for $N_{\mathrm{L}}\ge 1$. Our models define new universality classes that can serve as prototypes for studies of strongly correlated chiral fermions.

We investigate constraints that the requirements of perturbativity and gauge coupling unification impose on extensions of the standard model and of the minimal supersymmetric standard model. In particular, we discuss the renormalization group running in several supersymmetric left-right symmetric and Pati-Salam models and show how the various scales appearing in these models have to be chosen in order to achieve unification. We find that unification in the considered models occurs typically at scales below $M_{\mathrm{B̸}}^{\min }={10}^{16}\mathrm{GeV}$, implying potential conflicts with the nonobservation of proton decay. We emphasize that extending the particle content of a model in order to push the grand unified theory scale higher or to achieve unification in the first place will very often lead to nonperturbative evolution. We generalize this observation to arbitrary extensions of the standard model and of the minimal supersymmetric standard model and show that the requirement of perturbativity up to $M_{\mathrm{B̸}}^{\min }$, if considered a valid guideline for model building, severely limits the particle content of any such model, especially in the supersymmetric case. However, we also discuss several mechanisms to circumvent perturbativity and proton decay issues, for example, in certain classes of extra dimensional models.

We consider a scalar field interacting with a cosmic string configuration. The origin of the scalar field is given by a compactification mechanism in the context of a five-dimensional Brans-Dicke theory. We analyze the behavior of a charged cosmic string given by the Maxwell-Chern-Simons term on the 3-brane. The Cosmic Microwave Background Radiation constraint is used to analyze the possibility of optical activity effect in connection with the Brans-Dicke parameter $\omega$. We show that the dilatons produced by a cosmic string can decay into gauge bosons with masses given by the compactification modes. The Brans-Dicke parameter $\omega$ imposes stringent constraints on the mass of the dilaton and help us to understand the energy scales. In this scenario the lifetime of the dilaton which decays into light gauge bosons as well as the dependence of this phenomenon with the Brans-Dicke parameter are estimated.

In scalar-tensor theories the scalar fields generically couple nontrivially to gravity. We study the observable properties of inflationary models with nonminimally coupled inflaton and Dirac-Born-Infeld (DBI) kinetic term. Within the assumptions of the priors of our Monte-Carlo simulations we find these models can generate new interesting observable signatures. Our discussion focuses on string theory inspired phenomenological models of relativistic $D$-brane inflation. While successful string theory constructions of ultraviolet DBI brane inflation remain elusive, we show that in suitable regions of the parameter space it is possible to use cosmological observables to probe the nonminimal coupling. Fortunately, the most observationally promising range of parameters include models yielding intermediate levels of non-Gaussianity in the range consistent with WMAP 5-year data, and to be constrained further by the Planck satellite.

We investigate analytical gravitational lensing by charged, stationary, axially symmetric Kerr-Sen dilaton-axion black holes in the weak-deflection limit. Approximate solutions to the lightlike equations of motion are present up to and including third-order terms in $M/b$, $a/b$, and $r_{\alpha }/b$, where $M$ is the black hole mass, $a$ is the angular momentum, $r_{\alpha }=Q^2/M$, $Q$ being the charge and $b$ is the impact parameter of the light ray. We compute the positions of the two weak field images, the corresponding signed and absolute magnifications up to post-Newtonian order. It is shown that there are static post-Newtonian corrections to the signed magnification and their sum as well as to the critical curves, which are functions of the charge. The shift of the critical curves as a function of the lens angular momentum is found, and it is shown that they decrease slightly with the increase of the charge. The pointlike caustics drift away from the optical axis and do not depend on the charge. All of the lensing quantities are compared to particular cases as Schwarzschild and Kerr black holes as well as the Gibbons-Maeda-Garfinkle-Horowitz-Strominger black hole.

We explore the phase diagram and the critical behavior of QCD thermodynamic quantities in the context of the so-called Polyakov–Nambu–Jona-Lasinio model. We show that this improved field theoretical model is a successful candidate for studying the equation of state and the critical behavior around the critical endpoint. We argue that a convenient choice of the model parameters is crucial to get the correct description of isentropic trajectories. The effects of the regularization procedure in several thermodynamic quantities is also analyzed. The results are compared with simple thermodynamic expectations and lattice data.

We study SU(2) lattice gauge theory with two flavors of Dirac fermions in the adjoint representation. We measure the running of the coupling in the Schrödinger functional scheme and find it is consistent with existing results. We discuss how systematic errors affect the evidence for an infrared fixed point (IRFP). We present the first measurement of the running of the mass in the Schrödinger functional scheme. The anomalous dimension of the chiral condensate, which is relevant for phenomenological applications, can be easily extracted from the running of the mass, under the assumption that the theory has an IRFP. At the current level of accuracy, we can estimate $0.05<\gamma <0.56$ at the IRFP.

We have computed the $SU(2)$ low-energy constant $l_5$ and the mass splitting between charged and neutral pions from a lattice QCD simulation of $n_f=2+1$ flavors of domain wall fermions at a scale of $a^{-1}=2.33\mathrm{GeV}$. Relating $l_5$ to the $S$ parameter in QCD we obtain a value of $S(m_H=120\mathrm{GeV})=0.42(7)$, in agreement with previous determinations. Our result can be compared with the value of $S$ from electroweak precision data which constrains strongly interacting models of new physics like technicolor. This work in QCD serves as a test for the methods to compute the $S$ parameter with domain wall fermions in theories beyond the standard model. We also infer a value for the pion mass splitting in agreement with experiment.

We present the results of our QCD analysis for nonsinglet unpolarized quark distributions and structure function $F_2(x,Q^2)$ up to next-to-next-to-next-to leading order (${\mathrm{N}}^3\mathrm{LO}$). In this regards 4-loop anomalous dimension can be obtained from the Padé approximations. The analysis is based on the Jacobi polynomials expansion of the structure function. New parameterizations are derived for the nonsinglet quark distributions for the kinematic wide range of $x$ and $Q^2$. Our calculations for nonsinglet unpolarized quark distribution functions up to ${\mathrm{N}}^3\mathrm{LO}$ are in good agreement with available theoretical models. The higher twist contributions of $F_2^{p,d}(x,Q^2)$ are extracted in the large $x$ region in ${\mathrm{N}}^3\mathrm{LO}$ analysis. The values of ${\Lambda }_{\mathrm{QCD}}$ and ${\alpha }_s(M_z^2)$ are determined.

The decays of pseudoscalar bottomonium ${\eta }_b$ into a pair of vector charmonia, $J/\psi J/\psi$, $J/\psi {\psi }^{\prime }$, ${\psi }^{\prime }{\psi }^{\prime }$, are considered in the light cone formalism. Relativistic and leading logarithmic radiative corrections to the amplitudes of these processes are resummed. It is shown that the small value for the branching ratio of the decay ${\eta }_b\to J/\psi J/\psi$ obtained within the leading-order nonrelativistic QCD is a consequence of a fine-tuning between certain parameters, which is broken when relativistic and leading logarithmic radiative corrections are taken into account. As a result, the branching ratio obtained in this paper is enhanced by an order of magnitude.

A large approximate degeneracy appears in the light meson spectrum measured at the CERN low energy antiproton ring, suggesting a novel principal quantum number $n+j$ in QCD spectra. We recently showed that the large degeneracy could not be understood with state-of-the-art confining and chiral invariant quark models, derived in a truncated Coulomb gauge. To search for a solution to this problem, here we add the gluon or string degrees of freedom. Although independently the quarks or the gluons would lead to a $2n+j$ or $2n+l$ spectrum, adding them together may lead to the desired $n+j$ pattern.

We make a theoretical study of the charge and forward-backward pion asymmetries in the $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}\gamma$ process on and off the $\varphi$ resonance energy. These observables are rather sensitive to the inner details of the theoretical models to describe the reaction. In addition to the standard implementation of the initial state radiation and the bremsstrahlung contribution to the final state radiation, we use the techniques of the chiral unitary approach to evaluate the contribution from the mechanisms of $\varphi$ decay into ${\pi }^{+}{\pi }^{-}\gamma$. This contribution involves the implementation of final state interaction from direct chiral loops, the exchange of vector and axial-vector resonances and the final state interaction through the consideration of the meson-meson unitarized amplitudes, which were found important in a previous work describing the $\varphi \to \pi \pi \gamma$. We find a good reproduction of the experimental data from KLOE for the forward-backward asymmetry, both at the $\varphi$ peak and away from it. We also make predictions for the angular distributions of the charge asymmetry and show that this observable is very sensitive to the chiral loops involved in $\varphi$ radiative decay.

We analyze diffractive electroweak vector boson production in hadronic collisions and show that the single diffractive $W$ boson production asymmetry in rapidity is a particularly good observable at the LHC to test the concept of the flavor symmetric Pomeron parton distributions. It may also provide an additional constraint for the parton distribution functions in the proton.

We consider the radiation of photons from quarks scattering on color-magnetic monopoles in the quark-gluon plasma. We consider a temperature regime $T\gtrsim 2T_c$, where monopoles can be considered as static, rare objects embedded into matter consisting mostly of the usual “electric” quasiparticles, quarks and gluons. The calculation is performed in the classical, nonrelativistic approximation and results are compared to photon emission from Coulomb scattering of quarks, known to provide a significant contribution to the photon emission rates from quark-gluon plasma. The present study is a first step towards understanding whether this scattering process can give a sizeable contribution to dilepton production in heavy-ion collisions. Our results are encouraging: by comparing the magnitudes of the photon emission rate for the two processes, we find a dominance in the case of quark-monopole scattering. Our results display strong sensitivity to finite densities of quarks and monopoles.

In this paper, we show how to improve and extend the integration by fractional expansion technique (IBFE) by applying it to certain families of scalar massive Feynman diagrams. The strategy is based on combining this method together with the integration by parts technique. In particular, we want to calculate certain Feynman diagrams which have a triangle loop as a subgraph. The main idea is to use the integration by parts technique in this subgraph in order to simplify the topology of the original diagram in which it is immersed, using then, in a second step, the IBFE technique. The result we have obtained, after the application of both techniques, represents a simplification in the complexity of the solution, compared with having used only the IBFE technique.