Merging multi-leg NLO matrix elements with parton showers

By L. Lonnblad, Stefan Prestel. 2012

221 citations recorded by the source corpus.

A bstractWe discuss extensions of multi-jet matrix element and parton shower merging approaches, to also include next-to-leading order accuracy. Specifically, we generalise the so-called CKKW-L prescription and the recently developed unitarised matrix element + parton shower (UMEPS) scheme. Endowing tree-level merging methods with NLO corrections greatly enhances the perturbative accuracy of parton shower Monte Carlo programs.To generalise the CKKW-L approach, we augment the Nils-Lavesson-Leif-Lönnblad (NL3) scheme, which was previously developed for e+e−-annihilation, with a careful treatment of parton densities. This makes the application of the NL3 method to hadronic collisions possible. NL3 is further updated to use for more readily accessible next-to-leading order input calculations.We also extend the UMEPS scheme to NLO accuracy. The resulting approach, dubbed unitarised next-to-leading order + parton shower (UNLOPS) merging, does not inherit problematic unitarity-breaking features of CKKW-L, and thus allows for a theoretically more appealing definition of NLO order merging.Both schemes have been implemented in PYTHIA8. We 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. We find that NL3 and UNLOPS yield a very similar description for W production. For Higgs production however, UNLOPS produces more stable results.The implementation of the NLO merging procedures 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.

Subfields: Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/6c7bc9d8151a327291d26f0322d5c1053bc03f06

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Citations in the corpus

Search for a scalar partner of the top quark in the jets plus missing transverse momentum final state at √s=13 TeV with the ATLAS detector (2017, unknown): “The ME-PS matching was performed with the CKKW-L [43] prescription, with a matching scale set to one quarter of the mass of the t̃1, or g̃ for the gluino pair production model.”
Combining higher-order resummation with multiple NLO calculations and parton showers in GENEVA (2012, background): “[31, 32, 39], or alternatively by explicitly restricting the fixed NLO corrections in the spectrum to the fixed-order region at large τ , such that the induced O(α2 s) terms in the total cross section are not logarithmically enhanced and are formally O(α2 s).”
Tuning PYTHIA 8.1: the Monash 2013 tune (2014, methodology): “Especially in the latter context of matrix-element matching, we expect that in many cases PYTHIA 8 will be used together with codes such as ALPGEN [127], MADGRAPH [128], aMCatNLO [129], or POWHEG [130], either using the matching algorithms of those programs themselves, or via any of PYTHIA’s several”
Search for electroweak production of charginos and sleptons decaying into final states with two leptons and missing transverse momentum in $\sqrt{s}=13$ TeV $pp$ collisions using the ATLAS detector (2019, methodology): “Jet–parton matching was performed following the CKKW-L prescription [52], with a matching scale set to one quarter of the mass of the pair-produced SUSY particles.”
Systematics of quark/gluon tagging (2017, background): “It would therefore be interesting to determine the extent to which multi-leg NLO merging techniques (such as UNLOPS [120]) would reduce it, and/or whether second-order corrections to the shower kernels are required (for which only a proof of concept currently exists [121]).”
Event generation with Sherpa 2.2 (2019, methodology): “ements and parton showers is taken, and any overlaps are carefully removed to fully maintain the respective accuracies throughout. Other formulations and approximations thereof have been presented in [134, 135, 136, 137]. NLO EW Corrections in Matching and Merging In [138, 139], an approach to incor”
Measurement of the forward Z boson production cross-section in pp collisions at s=7$$ \sqrt{s}=7 $$ TeV (2015, methodology): “The MLM [28] sample has leading-order accuracy for the emission of zero, one or two jets; the FxFx [29] sample has next-to-leading-order (NLO) accuracy for zero- or one-jet emission; and the UNLOPS [30] sample is accurate at NLO for zero- or one-jet emission and accurate at LO for two-jet emission.”
The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations (2014, background): “NLO (and beyond) mergings are thus inherently more complicated than LO ones, because LO-matching is basically trivial; they have attracted a significant amount of attention lately [184,191,198–207].”
The midpoint between dipole and parton showers (2015, unknown): “In the past decade, the matching of parton showers to NLO calculations [32–43] and the merging of LO [44– 53] and NLO matched results for different jet multiplicity [54–59] was in the focus of interest of the majority of”
String formation beyond leading colour (2015, methodology): “ones based on QCD dipoles [13–19]), and better techniques for how to combine them (matching and merging, see [3,12,20–22] and references therein).”
Merging H/W/Z + 0 and 1 jet at NLO with no merging scale: a path to parton shower + NNLO matching (2012, methodology): “[13] proposes to tackle the problem by forcing unitarity on the approach, using suitable subtractions in order to restore NLO accuracy.”
Les Houches 2015: Physics at TeV Colliders Standard Model Working Group Report (2016, methodology): “The computation is performed in the uN2loPs method [154, 347], which is based on the uNloPs algorithm for merging inclusive NLO matched calculations of varying jet multiplicity, while leaving their total cross sections unchanged [370,371].”
Les Houches 2013: Physics at TeV Colliders: Standard Model Working Group Report (2014, methodology): “Other ideas have been floated for matching NNLO parton-level calculations with a parton shower [35] and one hopes that in the next few years the precision of event generators for collider physics can be improved to NNLO accuracy.”