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Davison E. Soper

Publications and source records attributed to Davison E. Soper.

At least 55 records · Page 3Linked to original sources
arXiv2007

Parton showers with quantum interference

We specify recursive equations that could be used to generate a lowest order parton shower for hard scattering in hadron-hadron collisions. The formalism is based on the factorization soft and collinear interactions from relatively harder interactions in QCD amplitudes. It incorporates quantum interference between different amplitudes in those cases in which the interference diagrams have leading soft or collinear singularities. It incorporates the color and spin information carried by partons emerging from a hard interaction. One motivation for this work is to have a method that can naturally cooperate with next-to-leading order calculations.

hep-ph↗
arXiv2007

Numerical integration of one-loop Feynman diagrams for N-photon amplitudes

In the calculation of cross sections for infrared-safe observables in high energy collisions at next-to-leading order, one approach is to perform all of the integrations, including the virtual loop integration numerically. One would use a subtraction scheme that removes infrared and collinear divergences from the integrand in a style similar to that used for real emission graphs. Then one would perform the loop integration by Monte Carlo integration along with the integrations over final state momenta. In this paper, we have explored how one can perform the numerical integration. We have studied the N-photon scattering amplitude with a massless electron loop in order to have a case with a singular integrand that is not, however, so singular as to require the subtractions. We report results for N = 4, N = 5 with left-handed couplings, and N=6.

hep-ph↗
arXiv2006

QCD and Monte Carlo event generators

Shower Monte Carlo event generators have played an important role in particle physics. Modern experiments would hardly be possible without them. In this talk I discuss how QCD physics is incorporated into the mathematical structure of these programs and I outline recent developments including matching between events with different numbers of hard jets and the inclusion of next-to-leading order effects.

hep-ph↗
arXiv2005

Matching parton showers to NLO computations

We give a prescription for attaching parton showers to next-to-leading order (NLO) partonic jet cross sections in electron-positron annihilation. Our method effectively extends to NLO the scheme of Catani, Krauss, Kuhn, and Webber for matching between m hard jets and (m+1) hard jets. The matching between parton splitting as part of a shower and parton splitting as part of NLO matrix elements is based on the Catani-Seymour dipole subtraction method that is commonly used for removing the singularities from the NLO matrix elements.}

hep-ph↗
arXiv2005

Next-to-leading order QCD jet production with parton showers and hadronization

We report on a method for matching the next-to-leading order calculation of QCD jet production in e+e- annihilation with a Monte Carlo parton shower event generator (MC) to produce realistic final states. The final result is accurate to next-to-leading order (NLO) for infrared-safe one-scale quantities, such as the Durham 3-jet fraction y_3, and agrees well with parton shower results for multi-scale quantities, such as the jet mass distribution in 3-jet events. For our numerical results, the NLO calculation is matched to the event generator Pythia, though the method is more general. We compare one scale and multi-scale quantities from pure NLO, pure MC, and matched NLO-MC calculations.

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arXiv2005

QCD Theory at the XL Rencontre de Moriond: Fish Eyes and Physics

I review a selection of the talks at the QCD Session of the XL Rencontre de Moriond, talks either by theorists or else of special theoretical interest. I use the talks to provide some assessment of where we stand with respect to the problems and opportunities facing QCD theory.

hep-ph↗
arXiv2004

Next-to-leading order QCD calculations with parton showers I: collinear singularities

Programs that calculate observables in quantum chromodynamics at next-to-leading order typically suffer from the problem that, when considered as event generators, the events generated consist of partons rather than hadrons and just a few partons at that. Thus the events generated are completely unrealistic. These programs would be much more useful if the few partons were turned into parton showers, in the style of standard Monte Carlo event generators. Then the parton showers could be given to one of the Monte Carlo event generators to produce hadron showers. We show how to generate parton showers related to the final state collinear singularities of the next-to-leading order perturbative calculation for the example of e+ + e- --> 3 jets . The soft singularities are left for a separate publication.

hep-ph↗
arXiv2004

Next-to-leading order QCD calculations with parton showers II: soft singularities

Programs that calculate observables in quantum chromodynamics at next-to-leading order typically generate events that consist of partons rather than hadrons -- and just a few partons at that. These programs would be much more useful if the few partons were turned into parton showers, which could be given to one of the Monte Carlo event generators to produce hadron showers. In a previous paper, we have seen how to generate parton showers related to the final state collinear singularities of the perturbative calculation for the example of e+ + e- --> 3 jets. This paper discusses the treatment of the soft singularities.

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arXiv2003

Partons and Jets at the LHC

I review some issues related to short distance QCD and its relation to the experimental program of the Large Hadron Collider (LHC) now under construction in Geneva.

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arXiv2003

General subtraction method for numerical calculation of one-loop QCD matrix elements

We present a subtraction scheme for eliminating the ultraviolet, soft, and collinear divergences in the numerical calculation of an arbitrary one-loop QCD amplitude with an arbitrary number of external legs. The subtractions consist of local counter terms in the space of the four-dimensional loop momentum. The ultraviolet subtraction terms reproduce MSbar renormalization. The key point in the method for the soft and collinear subtractions is that, although the subtraction terms are defined graph-by-graph and the matrix element is also calculated graph-by-graph, the sum over graphs of the integral of each the subtraction term can be evaluated analytically and provides the well known simple pole structure that arises from subtractions from real emission graphs, but with the opposite sign.

hep-ph↗
arXiv2002

Next-to-leading order numerical calculations in Coulomb gauge

Calculations of observables in quantum chromodynamics can be performed using a method in which all of the integrations, including integrations over virtual loop momenta, are performed numerically. We use the flexibility inherent in this method in order to perform next-to-leading order calculations for event shape variables in electron-positron annihilation in Coulomb gauge. The use of Coulomb gauge provides the potential to go beyond a purely order alpha_s^2 calculation by including, for instance, renormalon or parton showering effects. We expect that the approximations needed to include such effects at all orders in alpha_s will be simplest in a gauge in which unphysically polarized gluons do not propagate over long distances.

hep-ph↗
arXiv2001

Letting Real-Virtual Cancellations Happen by Themselves in QCD Calculations

Calculations of observables in quantum chromodynamics are typically performed using a method that combines numerical integrations over the momenta of final state particles with analytical integrations over the momenta of virtual particles. I review a method for performing all of the integrations numerically. In this method, the real-virtual cancellations happen inside the integrals -- simply because they are built into the Feynman rules. I indicate promising topics for further research on this subject.

hep-ph↗
arXiv2000

Basics of QCD Perturbation Theory: TASI 2000

This is an introduction to the use of QCD perturbation theory, emphasizing generic features of the theory that enable one to separate short-time and long-time effects. I also cover some important classes of applications: electron-positron annihilation to hadrons, deeply inelastic scattering, and hard processes in hadron-hadron collisions.

hep-ph↗
arXiv1999

Factorization scheme dependence of the NLO inclusive jet cross section

We study the factorization scheme dependence of the next-to-leading order inclusive one jet cross section d sigma/dE_T. The scheme is varied parametrically along the direction that transforms the MSbar scheme to the DIS scheme: we introduce a parameter lambda such that lambda = 0 is MSbar and lambda = 1 is DIS. The factorization scale mu is also varied. We observe a change of +/- 9% in the cross section for E_T = 50 GeV when mu and lambda are varied in the range -2 < lambda < 2, E_T/8 < mu < 2 E_T. This grows to +/- 32% for E_T = 400 GeV.

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