Parton

About: Parton is a research topic. Over the lifetime, 13484 publications have been published within this topic receiving 368877 citations.

Deep inelastic scattering off a N=4 SYM plasma at strong coupling

Abstract: By using the AdS/CFT correspondence we study the deep inelastic scattering of an R-current off a N=4 supersymmetric Yang-Mills (SYM) plasma at finite temperature and strong coupling. Within the supergravity approximation valid when the number of colors is large, we compute the structure functions by solving Maxwell equations in the space-time geometry of the AdS_5 black three-brane. We find a rather sharp transition between a low energy regime where the scattering is weak and quasi-elastic, and a high-energy regime where the current is completely absorbed. The critical energy for this transition determines the plasma saturation momentum in terms of its temperature T and the Bjorken x variable: Q_s=T/x. These results suggest a partonic picture for the plasma where all the partons have transverse momenta below the saturation momentum and occupation numbers of order one.

Four jet event shapes in electron-positron annihilation

Abstract: We report next-to-leading order perturbative QCD predictions of 4 jet event shape variables for the process $e^+e^-\to\rm{4\,\,jets}$ obtained using the general purpose Monte Carlo EERAD2. This program is based on the known `squared' one loop matrix elements for the virtual $\gamma^* \to 4$ parton contribution and squared matrix elements for 5 parton production. To combine the two distinct final states numerically we present a hybrid of the commonly used subtraction and slicing schemes based on the colour antenna structure of the final state which can be readily applied to other processes. We have checked that the numerical results obtained with EERAD2 are consistent with next-to-leading order estimates of the distributions of previously determined four jet-like event variables. We also report the next-to-leading order scale independent coefficients for some previously uncalculated observables; the light hemisphere mass, narrow jet broadening and the 4 jet transition variables with respect to the JADE and Geneva jet finding algorithms. For each of these observables, the next-to-leading order corrections calculated at the physical scale significantly increase the rate compared to leading order (the K factor is approximately 1.5–2). With the exception of the 4 jet transition variables, the published DELPHI data lies well above the ${\cal O}(\alpha_s^3)$ predictions. The renormalisation scale uncertainty is still large and in most cases the data prefers a scale significantly smaller than the physical scale. This situation is reminiscent of that for three jet shape variables, and should be improved by the inclusion of power corrections and resummation of large infrared logarithms.

Valence parton distribution function of pion from fine lattice

Abstract: We present a lattice QCD study of the valence parton distribution inside the pion within the framework of Large Momentum Effective Theory. We use a mixed action approach with 1-HYP smeared valence Wilson clover quarks on $2+1$ flavor HISQ sea with the valence quark mass tuned to 300 MeV pion mass. We use $4{8}^{3}\ifmmode\times\else\texttimes\fi{}64$ lattice at a fine lattice spacing $a=0.06\text{ }\text{ }\mathrm{fm}$ for this computation. We renormalize the quasi parton distribution function matrix element in the nonperturbative regularization independent momentum subtraction (RI-MOM) scheme. As a byproduct, we test the validity of a 1-loop matching procedure by comparing the RI-MOM renormalized quasi parton distribution function matrix element with off-shell quark external states as computed in the continuum 1-loop perturbation theory with the lattice results at $a=0.04$ and 0.06 fm. By applying the RI-MOM to $\overline{\mathrm{MS}}$ one-loop matching, implemented through a fit to phenomenologically motivated parton distribution functions, we obtain the valence parton distribution function of pion.

Soft function for exclusive N -jet production at hadron colliders

Abstract: The $N$-jettiness event shape divides phase space into $N+2$ regions, each containing one jet or beam. Using a geometric measure, these regions correspond to jets with circular boundaries. We give a factorization theorem for the cross section fully differential in the mass of each jet, and compute the corresponding soft function at next-to-leading order (NLO). The ultraviolet divergences are analytically extracted by exploiting hemispheres for interactions between each pair of hard partons, leaving only convergent integrals that are sensitive to the precise boundaries. This hemisphere decomposition can also be applied to other $N$-jet soft functions, including other observables. For $N$-jettiness, the final result for the soft function involves stable one-dimensional numerical integrals, and all ingredients are now available to extend NLO cross sections to resummed predictions at next-to-next-to-leading logarithmic order.

Spin densities in the transverse plane and generalized transversity distributions

Abstract: We show how generalized quark distributions in the nucleon describe the density of polarized quarks in the impact parameter plane, both for longitudinal and transverse polarization of the quark and the nucleon. This density representation entails positivity bounds including chiral-odd distributions, which tighten the known bounds in the chiral-even sector. Using the quark equations of motion, we derive relations between the moments of chiral-odd generalized parton distributions of twist two and twist three. We exhibit the analogy between polarized quark distributions in impact parameter space and transverse momentum dependent distribution functions.