75 research outputs found
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
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
Merging Multi-leg NLO Matrix Elements with Parton Showers
We discuss extensions the CKKW-L and UMEPS tree-level matrix element and
parton shower merging approaches to next-to-leading order accuracy.
The generalisation of CKKW-L is based on the NL3 scheme previously developed
for e+e- -annihilation, which is extended to also handle hadronic collisions by
a careful treatment of parton densities. NL3 is further augmented to allow for
more readily accessible NLO input.
To allow for a more careful handling of merging scale dependencies we
introduce an extension of the UMEPS method. This approach, dubbed UNLOPS, does
not inherit problematic features of CKKW-L, and thus allows for a theoretically
more appealing definition of NLO merging.
We have implemented both schemes in Pythia8, and present results for the
merging of W- and Higgs-production events, where the zero- and one-jet
contribution are corrected to next-to-leading order simultaneously, and higher
jet multiplicities are described by tree-level matrix elements. The
implementation of the procedure is completely general and can be used for
higher jet multiplicities and other processes, subject to the availability of
programs able to correctly generate the corresponding partonic states to
leading and next-to-leading order accuracy.Comment: updated references, submitted to JHE
Combining parton showers and NNLO matrix elements
In this talk, we discuss recent developments in combining parton showers and
fixed-order calculations. We focus on the UNNLOPS method for matching
next-to-next-to-leading order computations to the parton shower, and we present
results from Sherpa for Drell-Yan lepton-pair and Higgs-boson production at the
LHC.Comment: 4 pages, 2 figures, proceedings for Moriond QCD 201
Simulation of vector boson plus many jet final states at the high luminosity LHC
We present a novel event generation framework for the efficient simulation of
vector boson plus multi-jet backgrounds at the high-luminosity LHC and at
possible future hadron colliders. MPI parallelization of parton-level and
particle-level event generation and storage of parton-level event information
using the HDF5 data format allow us to obtain leading-order merged Monte-Carlo
predictions with up to nine jets in the final state. The parton-level event
samples generated in this manner correspond to an integrated luminosity of
3ab-1 and are made publicly available for future phenomenological studies.Comment: 10 pages, 8 figures, 5 table
Higgs-boson production through gluon fusion at NNLO QCD with parton showers
We discuss how the UN2LOPS scheme for matching 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.Comment: 9 pages, 3 figures, 1 tabl
Unitarising Matrix Element + Parton Shower merging
We revisit the CKKW-L method for merging tree-level matrix elements with
parton showers, and amend it with an add/subtract scheme to minimise
dependencies on the merging scale. The scheme is constructed to, as far as
possible, recover the unitary nature of the underlying parton shower, so that
the inclusive cross section is retained for each jet multiplicity separately.Comment: 30 pages, 13 figures, updated references, submitted to JHE
Drell-Yan lepton pair production at NNLO QCD with parton showers
We present a simple approach to combine NNLO QCD calculations and parton
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.Comment: 9 pages, 1 table, 3 figure
Improving Parton Showers with Fixed Order Matrix Elements
This thesis considers numerical simulations of high energy particle collisions, and improvements in the description of multiple collimated jets of hadrons in collision events in particular. In the event simulation software called event generators, the structure of multi-jet events is determined through an interplay of the input calculation, parton showers and phenomenological hadronisation models. Prescriptions to imbue existing event generator simulations with more complex input calculations are presented, and public implementations of the proposed methods provided. Paper I describes the implementation of an earlier algorithm to include regularised tree-level multi-jet configurations into event generators. The implementation is applied to collisions at LEP, Tevatron and LHC. Paper II proposes a method to amend cross section changes in traditional multi-jet merging schemes. The implementation of the new scheme is tested for W-boson + jets and QCD Dijet production at the LHC. Paper III presents extensions of the algorithms of Papers I and II to allow the inclusion of virtual orrections to the scattering processes. The resulting NLO merging methods are applied to W-boson + jets and Higgs-boson + jets at the LHC
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