Gluon-gluon contributions to the production of continuum diphoton pairs at hadron colliders

We compute the contributions to continuum photon pair production at hadron colliders from processes initiated by gluon-gluon and gluon-quark scattering into two photons through a four-leg virtual quark loop. Complete two-loop cross sections in perturbative quantum chromodynamics are combined with contributions from soft parton radiation resummed to all orders in the strong coupling strength. The structure of the resummed cross section is examined in detail, including a new type of unintegrated parton distribution function affecting azimuthal angle distributions of photons in the pair's rest frame. As a result of this analysis, we predict diphoton transverse momentum distributions in gluon-gluon scattering in wide ranges of kinematic parameters at the Fermilab Tevatron collider and the CERN Large Hadron Collider.Comment: 28 pages, 11 figures; published versio

A pQCD-based description of heavy and light flavor jet quenching

We present a successful description of the medium modification of light and heavy flavor jets within a perturbative QCD (pQCD) based approach. Only the couplings involving hard partons are assumed to be weak. The effect of the medium on a hard parton, per unit time, is encoded in terms of three non-perturbative, related transport coefficients which describe the transverse momentum squared gained, the elastic energy loss and diffusion in elastic energy transfer. A fit of the centrality dependence of the suppression and the azimuthal anisotropy of leading hadrons tends to favor somewhat larger transport coefficients for heavy quarks. Imposing additional constraints based on leading order (LO) Hard Thermal Loop (HTL) effective theory, leads to a worsening of the fit.Comment: v2, 4 pages, 3 figure

Modified Fragmentation Function from Quark Recombination

Within the framework of the constituent quark model, it is shown that the single hadron fragmentation function of a parton can be expressed as a convolution of shower diquark or triquark distribution function and quark recombination probability, if the interference between amplitudes of quark recombination with different momenta is neglected. The recombination probability is determined by the hadron's wavefunction in the constituent quark model. The shower diquark or triquark distribution functions of a fragmenting jet are defined in terms of overlapping matrices of constituent quarks and parton field operators. They are similar in form to dihadron or trihadron fragmentation functions in terms of parton operator and hadron states. Extending the formalism to the field theory at finite temperature, we automatically derive contributions to the effective single hadron fragmentation function from the recombination of shower and thermal constituent quarks. Such contributions involve single or diquark distribution functions which in turn can be related to diquark or triquark distribution functions via sum rules. We also derive QCD evolution equations for quark distribution functions that in turn determine the evolution of the effective jet fragmentation functions in a thermal medium.Comment: 23 pages in RevTex with 8 postscript figure

Gluon Structure Function of a Color Dipole in the Light-Cone Limit of Lattice QCD

We calculate the gluon structure function of a color dipole in near-light-cone SU(2) lattice QCD as a function of $x_B$. The quark and antiquark are external non-dynamical degrees of freedom which act as sources of the gluon string configuration defining the dipole. We compute the color dipole matrix element of transversal chromo-electric and chromo-magnetic field operators separated along a direction close to the light cone, the Fourier transform of which is the gluon structure function. As vacuum state in the pure glue sector, we use a variational ground state of the near-light-cone Hamiltonian. We derive a recursion relation for the gluon structure function on the lattice similar to the perturbative DGLAP equation. It depends on the number of transversal links assembling the Schwinger string of the dipole. Fixing the mean momentum fraction of the gluons to the "experimental value" in a proton, we compare our gluon structure function for a dipole state with four links with the NLO \emph{MRST} 2002 and the \emph{CTEQAB-0} parameterizations at $Q^2=1.5 \mathrm{GeV}^2$. Within the systematic uncertainty we find rather good agreement. We also discuss the low $x_B$ behavior of the gluon structure function in our model calculation.Comment: 44 pages, 10 figures, to be in accordance with the variant submitted to Phys. Rev.

The Use of Thin Disc Samples for the Determination of the Tear Resistance of Brittle Materials

Technical requirements of complex systems the nuclear industry is obliged to develop new types of materials and methods mechanical tests to determine their mechanical properties. The development of known and development of new methods of mechanical testing is an important task for nuclear power. It is proposed to use the test of bending small thin disk specimens simply supported along the contour, to determine the resistance of material to tensile strain. The results of computer analysis of stress-strain state and test a thin disk specimens made of brittle materials are cast iron and graphite as a possible model, and directly samples made by electro-impuls methods are presented. It is shown the effect of size of specimens on the resistance to their destruction and different character of deformation and destruction of samples of cast iron and graphite. The possibility of application of thin disc samples for the determination the resistance to tensile strain of the composite ceramics based on SiAlON with various additives Y2O3, SiC, TiN, and boron carbide B4C is confirmed

Coherent Vector Meson Photo-Production from Deuterium at Intermediate Energies

We analyze the cross section for vector meson photo-production off a deuteron for the intermediate range of photon energies starting at a few GeVs above the threshold and higher. We reproduce the steps in the derivation of the conventional non-relativistic Glauber expression based on an effective diagrammatic method while making corrections for Fermi motion and intermediate energy kinematic effects. We show that, for intermediate energy vector meson production, the usual Glauber factorization breaks down and we derive corrections to the usual Glauber method to linear order in longitudinal nucleon momentum. The purpose of our analysis is to establish methods for probing interesting physics in the production mechanism for phi-mesons and heavier vector mesons. We demonstrate how neglecting the breakdown of Glauber factorization can lead to errors in measurements of basic cross sections extracted from nuclear data.Comment: 41 pages, 13 figures, figure 9 is compressed from previous version, typos fixe

QCD evolution of naive-time-reversal-odd fragmentation functions

We study QCD evolution equations of the first transverse-momentum-moment of the naive-time-reversal-odd fragmentation functions - the Collins function and the polarizing fragmentation function. We find for the Collins function case that the evolution kernel has a diagonal piece same as that for the transversity fragmentation function, while for the polarizing fragmentation function case this piece is the same as that for the unpolarized fragmentation function. Our results might have important implications in the current global analysis of spin asymmetries.Comment: 8 pages,4 figure

Evolution Equation for Generalized Parton Distributions

The extension of the method [arXiv:hep-ph/0503109] for solving the leading order evolution equation for Generalized Parton Distributions (GPDs) is presented. We obtain the solution of the evolution equation both for the flavor nonsinglet quark GPD and singlet quark and gluon GPDs. The properties of the solution and, in particular, the asymptotic form of GPDs in the small x and \xi region are discussed.Comment: REVTeX4, 34 pages, 3 figure

Method of characteristics and solution of DGLAP evolution equation in leading order (LO) and next to leading order (NLO) at small-x

In this paper the singlet and non-singlet structure functions have been obtained by solving Dokshitzer, Gribove, Lipatov, Alterelli, Parisi (DGLAP) evolution equations in leading order (LO) and next to leading order (NLO) at the small x limit. Here we have used a Taylor Series expansion and then the method of characteristics to solve the evolution equations. We have also calculated t and x-evolutions of deuteron structure function and the results are compared with the New Muon Collaboration (NMC) data.Comment: 16 pages including 7 figure

R-Current DIS on a Shock Wave: Beyond the Eikonal Approximation

We find the DIS structure functions at strong coupling by calculating R-current correlators on a finite-size shock wave using AdS/CFT correspondence. We improve on the existing results in the literature by going beyond the eikonal approximation for the two lowest orders in graviton exchanges. We argue that since the eikonal approximation at strong coupling resums integer powers of 1/x (with x the Bjorken-x variable), the non-eikonal corrections bringing in positive integer powers of x can not be neglected in the small-x limit, as the non-eikonal order-x correction to the (n+1)st term in the eikonal series is of the same order in x as the nth eikonal term in that series. We demonstrate that, in qualitative agreement with the earlier DIS analysis based on calculation of the expectation value of the Wilson loop in the shock wave background using AdS/CFT, after inclusion of non-eikonal corrections DIS structure functions are described by two momentum scales: Q_1^2 ~ \Lambda^2 \, A^{1/3}/x and Q_2^2 ~ \Lambda^2 \, A^{2/3}, where \Lambda is the typical transverse momentum in the shock wave and A is the atomic number if the shock wave represents a nucleus. We discuss possible physical meanings of the scales Q_1 and Q_2.Comment: 44 pages, 3 figures; v2: typos corrected, refs added, discussion extende