Comix, a new matrix element generator Gleisberg, Tanju; Höche, Stefan
The journal of high energy physics,
12/2008, Letnik:
2008, Številka:
12
Journal Article
We present a new parton-shower algorithm. Borrowing from the basic ideas of dipole cascades, the evolution variable is judiciously chosen as the transverse momentum in the soft limit. This leads to a ...very simple analytic structure of the evolution. A weighting algorithm is implemented that allows one to consistently treat potentially negative values of the splitting functions and the parton distributions. We provide two independent, publicly available implementations for the two event generators P
ythia
and S
herpa
.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We present a simple approach to combine NNLO QCD calculations and parten showers, based on the UNLOPS technique. We apply the method to the computation of Drell-Yan lepton-pair production at the ...Large Hadron Collider. We comment on possible improvements and intrinsic uncertainties.
QCD matrix elements and truncated showers Höche, Stefan; Krauss, Frank; Schumann, Steffen ...
The journal of high energy physics,
05/2009, Letnik:
2009, Številka:
5
Journal Article
We discuss how the UN super(2) LOPS scheme for matching next-to-next-to-leading order (NNLO) calculations to parton showers can be applied to processes with large higher-order perturbative QCD ...corrections. We focus on Higgs-boson production through gluon fusion as an example. We also present an NNLO fixed-order event generator for this reaction.
A
bstract
We present a process-independent technique to consistently combine next-to-leading order parton-level calculations of varying jet multiplicity and parton showers. Double counting is avoided ...by means of a modified truncated shower scheme. This method preserves both the fixed-order accuracy of the parton-level result and the logarithmic accuracy of the parton shower. We discuss the renormalisation and factorisation scale dependence of the approach and present results from an automated implementation in the SHERPA event generator using the test case of
W
-boson production at the Large Hadron Collider. We observe a dramatic reduction of theoretical uncertainties compared to existing methods which underlines the predictive power of our novel technique.
A
bstract
We present a systematic study of differences between NLL resummation and parton showers. We first construct a Markovian Monte-Carlo algorithm for resummation of additive observables in ...electron-positron annihilation. Approximations intrinsic to the pure NLL result are then removed, in order to obtain a traditional, momentum and probability conserving parton shower based on the coherent branching formalism. The impact of each approximation is studied, and an overall comparison is made between the parton shower and pure NLL resummation. Differences compared to modern parton-shower algorithms formulated in terms of color dipoles are analyzed.
Beyond standard model calculations with Sherpa Höche, Stefan; Kuttimalai, Silvan; Schumann, Steffen ...
The European physical journal. C, Particles and fields,
2015/3, Letnik:
75, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We present a fully automated framework as part of the Sherpa event generator for the computation of tree-level cross sections in Beyond Standard Model scenarios, making use of model information given ...in the Universal FeynRules Output format. Elementary vertices are implemented into C++ code automatically and provided to the matrix-element generator Comix at runtime. Widths and branching ratios for unstable particles are computed from the same building blocks. The corresponding decays are simulated with spin correlations. Parton showers, QED radiation and hadronization are added by Sherpa, providing a full simulation of arbitrary BSM processes at the hadron level.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
In this publication, uncertainties in and differences between the M
C@NLO
and P
OWHEG
methods for matching next-to-leading order QCD calculations with parton showers are discussed. ...Implementations of both algorithms within the event generator S
HERPA
and based on Catani-Seymour subtraction are employed to assess the impact on a representative selection of observables. In the case of M
C@NLO
a substantial simplification is achieved by using dipole subtraction terms to generate the first emission. A phase space restriction is employed, which allows to vary in a transparent way the amount of non-singular radiative corrections that are exponentiated. Effects on various observables are investigated, using the production of a Higgs boson in gluon fusion, with or without an associated jet, as a benchmark process. The case of
H
+jet production is presented for the first time in an NLO+PS matched simulation. Uncertainties due to scale choices and non-perturbative effects are explored in the production of
W
±
and
Z
bosons in association with a jet. Corresponding results are compared to data from the Tevatron and LHC experiments.
Soft evolution of multi-jet final states Gerwick, Erik; Schumann, Steffen; Höche, Stefan ...
The journal of high energy physics,
02/2015, Letnik:
2015, Številka:
2
Journal Article
Recenzirano
Odprti dostop
A
bstract
We present a new framework for computing resummed and matched distributions in processes with many hard QCD jets. The intricate color structure of soft gluon emission at large angles ...renders resummed calculations highly non-trivial in this case. We automate all ingredients necessary for the color evolution of the soft function at next-to-leading-logarithmic accuracy, namely the selection of the color bases and the projections of color operators and Born amplitudes onto those bases. Explicit results for all QCD processes with up to 2 → 5 partons are given. We also devise a new tree-level matching scheme for resummed calculations which exploits a quasi-local subtraction based on the Catani-Seymour dipole formalism. We implement both resummation and matching in the Sherpa event generator. As a proof of concept, we compute the resummed and matched transverse-thrust distribution for hadronic collisions.
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