Factorization and Sudakov Resummation in B -> gamma l nu

We apply Soft-Collinear Effective Theory to prove at leading power in Lambda_QCD/m_b a factorization formula for the radiative leptonic decay B -> gamma l nu. Large logarithms entering the hard-scattering kernel are systematically resummed by a two-step perturbative matching procedure.Comment: 3 pages, 1 figure; contribution to proceedings of "International Europhysics Conference on High Energy Physics" EPS (July 17th-23rd 2003) in Aachen; to appear in European Physical Journal C direc

Refactorizing NRQCD short-distance coefficients in exclusive quarkonium production

In a typical exclusive quarkonium production process, when the center-of-mass energy, $\sqrt{s}$, is much greater than the heavy quark mass $m$, large kinematic logarithms of $s/m^2$ will unavoidably arise at each order of perturbative expansion in the short-distance coefficients of the nonrelativistic QCD (NRQCD) factorization formalism, which may potentially harm the perturbative expansion. This symptom reflects that the hard regime in NRQCD factorization is too coarse and should be further factorized. We suggest that this regime can be further separated into "hard" and "collinear" degrees of freedom, so that the familiar light-cone approach can be employed to reproduce the NRQCD matching coefficients at the zeroth order of $m^2/s$ and order by order in $\alpha_s$. Taking two simple processes, exclusive $\eta_b+\gamma$ production in $e^+ e^-$ annihilation and Higgs boson radiative decay into $\Upsilon$, as examples, we illustrate how the leading logarithms of $s/m^2$ in the NRQCD matching coefficients are identified and summed to all orders in $\alpha_s$ with the aid of Brodsky-Lepage evolution equation.Comment: v2, 17 pages, 2 figures; presentation improved, one important reference added, and Note adde

Threshold Resummation of Soft Gluons in Hadronic Reactions -- An Introduction

I discuss the motivation for resummation of the effects of initial-state soft gluon radiation, to all orders in the strong coupling strength, for processes in which the near-threshold region in the partonic subenergy is important. I summarize the method of "perturbative resummation" and its application to the calculation of the total cross section for top quark production at hadron colliders. Comments are included on the differences between the treatment of subleading logarithmic terms in this method and in other approaches.Comment: 15 pages. latex, one figure. Invited paper to be published in the Proceedings of the Symposium on QCD Corrections and New Physics, Hiroshima, October 27 - 29, 199

Photon-Neutrino Interactions

We discuss the interaction of photons with neutrinos including two lepton loops. The parity violation in the gamma-nu to gamma-nu channel due to two lepton loops is substantially enhanced relative to the one lepton loop contribution. However there is no corresponding enhancement in the parity conserving amplitude in either the direct or the cross channel.Comment: 12 pages, 5 figure

Helicity Parton Distributions from Spin Asymmetries in W-Boson Production at RHIC

We present a next-to-leading order QCD calculation of the cross section and longitudinal spin asymmetry in single-inclusive charged-lepton production, pp -> l X, at RHIC, where the lepton is produced in the decay of an electroweak gauge boson. Our calculation is presented in terms of a multi-purpose Monte-Carlo integration program that may be readily used to include experimental spin asymmetry data in a global analysis of helicity parton densities. We perform a toy global analysis, studying the impact of anticipated RHIC data on our knowledge about the polarized anti-quark distributions.Comment: 22 pages, 13 figures included. Typos in Figs 2, 6, 8 and scales correcte

Looking for the Logarithms in Four-Dimensional Nambu-Jona-Lasinio Models

We study the problem of triviality in the four dimensional Nambu-Jona-Lasinio model with discrete chiral symmetry using both large-N expansions and lattice simulations. We find that logarithmic corrections to scaling appear in the equation of state as predicted by the large-N expansion. The data from $16^4$ lattice simulations is sufficiently accurate to distinguish logarithmically trivial scaling from power law scaling. Simulations on different lattice sizes reveal an interesting interplay of finite size effects and triviality. We argue that such effects are qualitatively different for theories based on fundamental scalar rather than fermion fields. Several lessons learned here can be applied to simulations and analyses of more challenging field theories.Comment: 25 pages, 14 ps figure