45,069 research outputs found
Hadronic final states in deep-inelastic scattering with Sherpa
We extend the multi-purpose Monte-Carlo event generator Sherpa to include
processes in deeply inelastic lepton-nucleon scattering. Hadronic final states
in this kinematical setting are characterised by the presence of multiple
kinematical scales, which were up to now accounted for only by specific
resummations in individual kinematical regions. Using an extension of the
recently introduced method for merging truncated parton showers with
higher-order tree-level matrix elements, it is possible to obtain predictions
which are reliable in all kinematical limits. Different hadronic final states,
defined by jets or individual hadrons, in deep-inelastic scattering are
analysed and the corresponding results are compared to HERA data. The various
sources of theoretical uncertainties of the approach are discussed and
quantified. The extension to deeply inelastic processes provides the
opportunity to validate the merging of matrix elements and parton showers in
multi-scale kinematics inaccessible in other collider environments. It also
allows to use HERA data on hadronic final states in the tuning of hadronisation
models.Comment: 32 pages, 22 figure
Introduction to parton-shower event generators
This lecture discusses the physics implemented by Monte Carlo event
generators for hadron colliders. It details the construction of parton showers
and the matching of parton showers to fixed-order calculations at higher orders
in perturbative QCD. It also discusses approaches to merge calculations for a
varying number of jets, the interface to the underlying event and
hadronization.Comment: 40 pages, 12 figures. Lectures presented at TASI 201
Combining states without scale hierarchies with ordered parton showers
We present a parameter-free scheme to combine fixed-order multi-jet results
with parton-shower evolution. The scheme produces jet cross sections with
leading-order accuracy in the complete phase space of multiple emissions,
resumming large logarithms when appropriate, while not arbitrarily enforcing
ordering on momentum configurations beyond the reach of the parton-shower
evolution equation. This requires the development of a matrix-element
correction scheme for complex phase-spaces including ordering conditions as
well as a systematic scale-setting procedure for unordered phase-space points.
The resulting algorithm does not require a merging-scale parameter. We
implement the new method in the Vincia framework and compare to LHC data.Comment: updated to version published in EPJ
The midpoint between dipole and parton showers
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 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 Pythia and Sherpa.Comment: 23 pages, 9 figure
Hard photon production and matrix-element parton-shower merging
We present a Monte-Carlo approach to prompt-photon production, where photons
and QCD partons are treated democratically. The photon fragmentation function
is modelled by an interleaved QCD+QED parton shower. This known technique is
improved by including higher-order real-emission matrix elements. To this end,
we extend a recently proposed algorithm for merging matrix elements and
truncated parton showers. We exemplify the quality of the Monte-Carlo
predictions by comparing them to measurements of the photon fragmentation
function at LEP and to measurements of prompt photon and diphoton production
from the Tevatron experiments.Comment: 18 pages, 5 figures, revised version with minor update
GenEvA (II): A phase space generator from a reweighted parton shower
We introduce a new efficient algorithm for phase space generation. A parton
shower is used to distribute events across all of multiplicity, flavor, and
phase space, and these events can then be reweighted to any desired analytic
distribution. To verify this method, we reproduce the e+e- -> n jets tree-level
result of traditional matrix element tools. We also show how to improve
tree-level matrix elements automatically with leading-logarithmic resummation.
This algorithm is particularly useful in the context of a new framework for
event generation called GenEvA. In a companion paper [arXiv:0801.4026], we show
how the GenEvA framework can address contemporary issues in event generation.Comment: 54 pages, 20 figures, v2: corrected typos, added reference
Report of the QCD Working Group
The activities of the QCD working group concentrated on improving the
understanding and Monte Carlo simulation of multi-jet final states due to hard
QCD processes at LEP, i.e. quark-antiquark plus multi-gluon and/or secondary
quark production, with particular emphasis on four-jet final states and b-quark
mass effects. Specific topics covered are: relevant developments in the main
event generators PYTHIA, HERWIG and ARIADNE; the new multi-jet generator
APACIC++; description and tuning of inclusive (all-flavour) jet rates; quark
mass effects in the three- and four-jet rates; mass, higher-order and
hadronization effects in four-jet angular and shape distributions; b-quark
fragmentation and gluon splitting into b-quarks.Comment: 95 pages, 48 figures, contribution to Proceedings of the LEP2 Monte
Carlo Workshop. References for NLO 4-jet matrix elements adde
Matching Tree-Level Matrix Elements with Interleaved Showers
We present an implementation of the so-called CKKW-L merging scheme for
combining multi-jet tree-level matrix elements with parton showers. The
implementation uses the transverse-momentum-ordered shower with interleaved
multiple interactions as implemented in PYTHIA8. We validate our procedure
using e+e--annihilation into jets and vector boson production in hadronic
collisions, with special attention to details in the algorithm which are
formally sub-leading in character, but may have visible effects in some
observables. We find substantial merging scale dependencies induced by the
enforced rapidity ordering in the default PYTHIA8 shower. If this rapidity
ordering is removed the merging scale dependence is almost negligible. We then
also find that the shower does a surprisingly good job of describing the
hardness of multi-jet events, as long as the hardest couple of jets are given
by the matrix elements. The effects of using interleaved multiple interactions
as compared to more simplistic ways of adding underlying-event effects in
vector boson production are shown to be negligible except in a few sensitive
observables. To illustrate the generality of our implementation, we also give
some example results from di-boson production and pure QCD jet production in
hadronic collisions.Comment: 44 pages, 23 figures, as published in JHEP, including all changes
recommended by the refere
Angular-ordered parton showers with medium-modified splitting functions
Modified Altarelli-Parisi splitting functions were recenty proposed to model
multi-parton radiation in a dense medium and describe jet quenching, one of
most striking features of heavy-ion collisions. We implement medium-modified
splitting functions in the HERWIG parton shower algorithm, which satisfies the
angular ordering prescription, and present a few parton-level results, such as
transverse momentum, angle and energy-fraction distributions, which exhibit
remarkable medium-induced effects. We also comment on the comparison with
respect to the results yielded by other implementations of medium-modified
splitting functions in the framework of virtuality-ordered parton cascades.Comment: 19 pages, 8 figures, 1 table. Minor changes after referee repor
Adaptive multiresolution computations applied to detonations
A space-time adaptive method is presented for the reactive Euler equations
describing chemically reacting gas flow where a two species model is used for
the chemistry. The governing equations are discretized with a finite volume
method and dynamic space adaptivity is introduced using multiresolution
analysis. A time splitting method of Strang is applied to be able to consider
stiff problems while keeping the method explicit. For time adaptivity an
improved Runge--Kutta--Fehlberg scheme is used. Applications deal with
detonation problems in one and two space dimensions. A comparison of the
adaptive scheme with reference computations on a regular grid allow to assess
the accuracy and the computational efficiency, in terms of CPU time and memory
requirements.Comment: Zeitschrift f\"ur Physicalische Chemie, accepte
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