58 research outputs found
One-loop radiative corrections to the helicity amplitudes of QCD processes involving four quarks and one gluon
We present analytic results for the one-loop corrections of the helicity
amplitudes of the QCD five-parton subprocesses involving four quarks and one
gluon obtained with a standard Feynman diagram calculation using dimensional
reduction.Comment: 9 pages LaTeX, ETH-TH/94-14, KLTE-DTP/94-
Nagy pontosságú Monte Carlo programok nagyenergiás részecskeütközések leírására = Precision Monte Carlo tools for collider experiments
Nem pontosan a terveknek megfelelően, mégis nemzetközileg elismert eredményeket értünk el a QCD két területén: a sugárzási korrekciók számításában és a Monte Carlo partonzápor programokban. Sugárzási korrekciók számítása Teljesen numerikus eljárást dolgoztunk ki egyhurok QCD amplitúdók számolására, és elsőként számítottuk ki az N-foton (N = 4,6,8) amplitúdókat. Fél-automatikus eljárást fejlesztettünk ki parton szintű súlyozatlan események generálásához, amelyekre közvetlenül lehet partonzápor programot alkalmazni. Ezzel a módszerrel a t tbar és a t tbar + jet keletkezésnek hadron szintű hatáskeresztmetszeteit számítottuk ki, amelyek az első sugárzási korrekciókat pontosan visszaadják az erős csatolás szerinti sorfejtésben. Kifejlesztettük a második QCD sugárzási korrekciók számításának általános elméletét. Duplán feloldatlan tagok kivételével az összes levonási tagot integráltuk a feloldatlan partonok faktorizált fázisterén. Monte Carlo partonzápor programok Rekurziós egyenleteket írtunk fel, amelyek segítségével vezető rendű partonzáport lehet generálni hadronütközésekben. Az eljárás újszerűsége abban áll, hogy figyelembe veszi a különböző amplitúdók közötti interferencia tagokat akkor, amikor azoknak vezető rendű lágy, vagy kollineáris szingularitása van. Figyelembe veszi az elsődleges kölcsönhatásban keletkező partonok szín és spin korrelációit is. | Although not precisely as planned, we made internationally recognized progress in two main areas of QCD: exact higher order computations and Shower Monte Carlo programs. Exact higher order computations We developed a completely numerical approach to computing one-loop QCD amplitudes and for the first time we computed N-photon (N=4,6,8) amplitudes numerically. We devised a semi-automatic approach to generate unweighted Monte Carlo events at the parton level that can be interfaced with shower programs. Using this method, we produced predictions for the hadron level cross section of t tbar and t tbar + jet hadroproduction which are correct up to next-to- leading order accuracy upon expansion in the strong coupling. We presented a general subtraction scheme for computing QCD jet cross sections at next-to-next-to-leading order accuracy. We integrated the subtraction terms over the factorised phase space of the unresolved partons except the doubly-unresolved subtractions. Shower Monte Carlo programs We specified recursive equations that can be used to generate a lowest order parton shower for hard scattering in hadron-hadron collisions. The novelty of the approach is that it incorporates quantum interference between different amplitudes in those cases in which the interference diagrams have leading soft or collinear singularities. It also incorporates the color and spin information carried by partons emerging from a hard interaction
Singular terms of helicity amplitudes at one-loop in QCD and the soft limit of the cross sections of multi-parton processes
We describe a general method that enables us to obtain all the singular terms
of helicity amplitudes of n-parton processes at one loop. The algorithm uses
helicity amplitudes at tree level and simple color algebra.
We illustrate the method by calculating the singular part of the one loop
helicity amplitudes of all parton subprocesses. The results are used
to derive the soft gluon limit of the cross sections of all parton
scattering subprocesses which provide a useful initial condition for the
angular ordering approximation to coherent multiple soft gluon emission,
incorporated in existing Monte Carlo simulation programs.Comment: Latex,13 pages, ETH-TH/94-
One-loop helicity amplitudes for all 2 -> 2 processes in QCD and N=1 supersymmetric Yang-Mills theory
One loop corrections to the helicity amplitudes of all 2 -> 2 subprocesses
are calculated in QCD and in N=1 supersymmetric Yang-Mills theory using two
versions of dimensional regularization: the `t~Hooft-Veltman scheme and
dimensional reduction. Studying the structure of the soft and collinear
singularities, we found universal transition rules for the squared matrix
element which can be used to translate the results obtained in these schemes to
the results valid in the conventional dimensional regularization scheme. With
explicit calculation it is demonstrated that the one loop helicity amplitudes
of the 2 -> 2 subprocesses calculated using dimensional reduction in the N=1
supersymmetric gauge theory respect the supersymmetry Ward identities.
Our transition rules can also be used to calculate the next-to-leading order
Altarelli-Parisi kernels in the dimensional reduction scheme when they satisfy
supersymmetry Ward identities as well.Comment: 40 pages + 1 table, uses LaTeX and table.tex, ETH-TH/93-1
Object Persistency for HEP data using an Object-Relational Database
We present an initial study of the object features of Oracle 9i - the first of the market-leading object-relational database systems that supports a true object model on the server side as well as an ODMG-style C++ language binding on the client side. We discuss how these features can be used to provide persistent object storage in the HEP environment
One-loop Singular Behaviour of QCD and SUSY QCD Amplitudes with Massive Partons
We discuss the structure of infrared and ultraviolet singularities in
on-shell QCD and supersymmetric QCD amplitudes at one-loop order. Previous
results, valid for massless partons, are extended to the case of massive
partons. Using dimensional regularization, we present a general factorization
formula that controls both the singular epsilon-poles and the logarithmic
contributions that become singular for vanishing masses. We introduce
generalized Altarelli--Parisi splitting functions and discuss their relations
with the singular terms in the amplitudes. The dependence on the regularization
scheme is also considered.Comment: 15 pages, 1 Latex fil
QCD Corrections to Photon Production in Association with Hadrons in Annihilation
A detailed investigation of the theoretical ambiguities present in the QCD
description of photon production in annihilation is given. It is
pointed out that in a well-defined perturbative analysis it is necessary to
subtract the quark-photon collinear singularities. This subtraction requires
the introduction of an unphysical parameter in the perturbative part of the
cross section. The subtracted term is factored into non-perturbative
fragmentation function. The dependence on the unphysical parameter cancels in
the sum of non-perturbative and perturbative parts. It is pointed out that for
the non-perturbative contributions
are suppressed. Using a general purpose next-to-leading order Monte Carlo
program, we calculate various physical quantities that were measured in LEP
experiments recently.Comment: 23 pages, 11 figures, 1 table (available by request), LaTeX,
ETH-TH/92-2
Three-jet cross sections in hadron-hadron collisions at next-to-leading order
We present a new QCD event generator for hadron collider which can calculate
one-, two- and three-jet cross sections at next-to-leading order accuracy. In
this letter we study the transverse energy spectrum of three-jet hadronic
events using the kT algorithm. We show that the next-to-leading order
correction significantly reduces the renormalization and factorization scale
dependence of the three-jet cross section.Comment: 4 pages, 4 figures, REVTEX
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