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George Sterman

Publications and source records attributed to George Sterman.

At least 55 records · Page 3Linked to original sources
arXiv2012

Three-particle templates for boosted Higgs

We explore the ability of three-particle templates to distinguish color neutral objects from QCD background. This method is particularly useful to identify the standard model Higgs, as well as other massive neutral particles. Simple cut-based analysis in the overlap distributions of the signal and background is shown to provide a significant rejection power. By combining with other discriminating variables, such as planar flow, and several variables that depend on the partonic template, three-particle templates are used to characterize the influence of gluon emission and color flow in collider events. The performance of the method is discussed for the case of a highly boosted Higgs in association with a leptonically-decaying W boson.

hep-ph↗
arXiv2011

Heavy quarkonium production and polarization

We present a perturbative QCD factorization formalism for the production of heavy quarkonia of large transverse momentum $p_T$ at collider energies, which includes both the leading power (LP) and next-to-leading power (NLP) contributions to the cross section in the $m_Q^2/p_T^2$ expansion for heavy quark mass $m_Q$. We estimate fragmentation functions in the non-relativistic QCD formalism, and reproduce the bulk of the large enhancement found in explicit NLO calculations in the color singlet model. Heavy quarkonia produced from NLP channels prefer longitudinal polarization.

hep-ph↗
arXiv2011

Collinear and Soft Divergences in Perturbative Quantum Gravity

Collinear and soft divergences in perturbative quantum gravity are investigated to arbitrary orders in amplitudes for wide-angle scattering, using methods developed for gauge theories. We show that collinear singularities cancel when all such divergent diagrams are summed over, by using the gravitational Ward identity that decouples the unphysical polarizations from the S-matrix. This analysis generalizes a result previously demonstrated in the eikonal approximation. We also confirm that the only virtual graviton corrections that give soft logarithmic divergences are of the ladder and crossed ladder type.

hep-th↗
arXiv2010

Diagrammatic Exponentiation for Products of Wilson Lines

We provide a recursive diagrammatic prescription for the exponentiation of gauge theory amplitudes involving products of Wilson lines and loops. This construction generalizes the concept of webs, originally developed for eikonal form factors and cross sections with two eikonal lines, to general soft functions in QCD and related gauge theories. Our coordinate space arguments apply to arbitrary paths for the lines.

hep-ph↗
arXiv2010

Template Overlap Method for Massive Jets

We introduce a new class of infrared safe jet observables, which we refer to as template overlaps, designed to filter targeted highly boosted particle decays from QCD jets and other background. Template overlaps are functional measures that quantify how well the energy flow of a physical jet matches the flow of a boosted partonic decay. Any region of the partonic phase space for the boosted decays defines a template. We will refer to the maximum functional overlap found this way as the template overlap. To illustrate the method, we test lowest-order templates designed to distinguish highly-boosted top and Higgs decays from backgrounds produced by event generators. For the functional overlap, we find good results with a simple construction based on a Gaussian in energy differences within angular regions surrounding the template partons. Although different event generators give different averages for our template overlaps, we find in each case excellent rejection power, especially when combined with cuts based on jet shapes. The template overlaps are capable of systematic improvement by including higher order corrections in the template phase space.

hep-ph↗
arXiv2010

Computation of the Soft Anomalous Dimension Matrix in Coordinate Space

We complete the coordinate space calculation of the three-parton correlation in the two-loop massive soft anomalous dimension matrix. The full answer agrees with the result found previously by a different approach. The coordinate space treatment of renormalized two-loop gluon exchange diagrams exhibits their color symmetries in a transparent fashion. We compare coordinate space calculations of the soft anomalous dimension matrix with massive and massless eikonal lines and examine its nonuniform limit at absolute threshold.

hep-ph↗
arXiv2009

Threshold Resummation for Di-hadron Production in Hadronic Collisions

We study the resummation of large logarithmic perturbative corrections to the partonic cross sections relevant for di-hadron production in hadronic collisions, H1 H2 -> h1 h2 X, at high invariant mass of the produced hadron pair. These corrections arise near the threshold for the partonic reaction and are associated with soft-gluon emission. We perform the resummation to next-to-leading logarithmic accuracy, and show how to incorporate consistently cuts in rapidity and transverse momentum of the observed particles. We present numerical results for fixed-target and ISR regimes and find enhancements over the next-to-leading order cross section, which significantly improve the agreement between theoretical predictions and data.

hep-ph↗
arXiv2009

The Massive Soft Anomalous Dimension Matrix at Two Loops

We study two-loop anomalous dimension matrices in QCD and related gauge theories for products of Wilson lines coupled at a point. We verify by an analysis in Euclidean space that the contributions to these matrices from diagrams that link three massive Wilson lines do not vanish in general. We show, however, that for two-to-two processes the two-loop anomalous dimension matrix is diagonal in the same color-exchange basis as the one-loop matrix for arbitrary masses at absolute threshold and for scattering at ninety degrees in the center of mass. This result is important for applications of threshold resummation in heavy quark production.

hep-ph↗
arXiv2008

Soft-Gluon Cancellation, Phases and Factorization with Initial-State Partons

We outline arguments for the cancellation of soft singularities in transition probabilities and parton distributions with incoming partons and Wilson lines, and with observed jets or heavy colored particles in the final states. We show the cancellation of Glauber gluons and divergent phases, relating finite remainders to corrections that arise from restrictions on final states in factorized cross sections.

hep-ph↗
arXiv2008

Factorization of e+e- Event Shape Distributions with Hadronic Final States in Soft Collinear Effective Theory

We present a new analysis of two-jet event shape distributions in soft collinear effective theory. Extending previous results, we observe that a large class of such distributions can be expressed in terms of vacuum matrix elements of operators in the effective theory. We match these matrix elements to the full theory in the two-jet limit without assuming factorization of the complete set of hadronic final states into independent sums over partonic collinear and soft states. We also briefly discuss the relationship of this approach to diagrammatic factorization in the full theory.

hep-ph↗
arXiv2008

Universal structure of subleading infrared poles in gauge theory amplitudes

We study the origin of subleading soft and collinear poles of form factors and amplitudes in dimensionally-regulated massless gauge theories. In the case of form factors of fundamental fields, these poles originate from a single function of the coupling, denoted G(alpha_s), depending on both the spin and gauge quantum numbers of the field. We relate G(alpha_s) to gauge-theory matrix elements involving the gluon field strength. We then show that G(alpha_s) is the sum of three terms: a universal eikonal anomalous dimension, a universal non-eikonal contribution, given by the coefficient B_delta (alpha_s) of delta(1 - z) in the collinear evolution kernel, and a process-dependent short-distance coefficient function, which does not contribute to infrared poles. Using general results on the factorization of soft and collinear singularities in fixed-angle massless gauge theory amplitudes, we conclude that all such singularities are captured by the eikonal approximation, supplemented only by the knowledge of B_delta (alpha_s). We explore the consequences of our results for conformal gauge theories, where in particular we find a simple exact relation between the form factor and the cusp anomalous dimension.

hep-ph↗
arXiv2008

Some Basic Concepts of Perturbative QCD

This is a brief review of some of the basic concepts of perturbative QCD, including infrared safety and factorization, relating them to more familiar ideas from quantum mechanics and relativity. It is intended to offer perspective on methods and terms whose use is commonplace, but whose physical origins are sometimes obscure.

hep-ph↗
arXiv2008

Threshold Resummation for the Top Quark Charge Asymmetry

We study the QCD charge asymmetry in t\bar{t} production at the Tevatron. We investigate the role of higher orders in perturbation theory by considering the resummation of potentially large logarithmic corrections that arise near partonic threshold. This requires us to employ the rapidity-dependent anomalous dimension matrices that describe color mixing due to soft gluon emission in both quark- and gluon-initiated processes. The charge asymmetry appears directly in the resummed cross section at next-to-leading logarithm (NLL), and we find that the first-order expansion of the NLL resummed charge asymmetry reproduces the known fixed-order result for the asymmetry well. Beyond its lowest order, the asymmetric component of the cross section is enhanced by the same leading-logarithmic threshold corrections as the total cross section. As a result, the charge asymmetry is robust with respect to the higher-order perturbative corrections generated by threshold resummation. We observe that the asymmetry increases with pair mass and with scattering angle.

hep-ph↗
arXiv2008

Substructure of high-p_T Jets at the LHC

We study high-pt jets from QCD and from highly-boosted massive particles such as tops, W, Z and Higgs, and argue that infrared-safe observables can help reduce QCD backgrounds. Jets from QCD are characterized by different patterns of energy flow compared to the products of highly-boosted heavy particle decays, and we employ a variety of jet shapes, observables restricted to energy flow within a jet, to explore this difference. Results from Monte Carlo generators and arguments based on perturbation theory support the discriminating power of the shapes we refer to as planar flow and angularities. We emphasize that for massive jets, these and other observables can be analyzed perturbatively.

hep-ph↗
arXiv2008

Color Transfer Enhancement for Heavy Quarkonium Production

We study the transfer of color between a heavy quark pair and an unpaired heavy quark or antiquark moving at a nonrelativistic velocity with respect to the pair. We find that the open heavy quark or antiquark can catalyze the transformation of the pair from octet representation at short distances to singlet at long distances. This process is infrared sensitive in general, and we exhibit double poles in dimensional regularization at next-to-next-to-leading order in the transition probability. Because of their dependence on kinematic variables, these poles cannot be matched to the non-perturbative matrix elements of effective field theories based on a single heavy quark pair.

hep-ph↗
arXiv2007

Color Transfer in Associated Heavy-Quarkonium Production

We study the production of heavy quarkonium in association with an additional heavy pair. We argue that important contributions may come from phase space regions where three heavy fermions are separated by relative velocities much lower than the speed of light, and to which standard effective field theories do not apply. In this region, infrared sensitive color exchange is specific to the presence of the unpaired (anti)quark. This effect vanishes as the motion of the additional particle becomes relativistic with respect to the pair, and is completely absent for massless quarks and gluons in the final state.

hep-ph↗