Towards a fully massive five-flavor scheme

Krauss, Frank; Napoletano, Davide

doi:10.1103/physrevd.98.096002

Cited by 28 publications

(36 citation statements)

References 71 publications

(90 reference statements)

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“…At this time development is moving towards SHERPA 3.0.0, heralding a major new development effort with exciting improvements across the board. They will include the description of ever higher-orders in the perturbative parts of the simulation, for example incorporating advances in the resummation properties of parton showers [21,20,114], the inclusion of approximate high-energy EW effects as based on [193], a fully massive five-flavour scheme for heavy quarks in the initial state [107], the improvements in efficient phase-space sampling [194], and extensions such as the automation of QCD soft-gluon resummation at NLL accuracy following the CAESAR formalism [195]. This will be supplemented with continuous improvements in the non-perturbative modelling, such as an improved cluster hadronisation or a new model for inclusive QCD scattering.…”

Section: Discussionmentioning

confidence: 99%

“…Note that the CSSHOWER fully supports finite-mass effects. This is important in particular for the production and evolution of bquarks [17,105], thus allowing for systematic studies of b-quark associated/initiated processes in the fourand the five-flavour scheme [106,107]. Furthermore, general electroweak splittings are implemented in CSSHOWER [108,91].…”

Section: Parton Showersmentioning

confidence: 99%

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Event generation with Sherpa 2.2

et al. 2019

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SHERPA is a general-purpose Monte Carlo event generator for the simulation of particle collisions in high-energy collider experiments. We summarise essential features and improvements of the SHERPA 2.2 release series, which is heavily used for event generation in the analysis and interpretation of LHC Run 1 and Run 2 data. We highlight a decade of developments towards ever higher precision in the simulation of particle-collision events. Figure 1: Overview of the SHERPA 2.2 event generator framework. Interfaces/OutputsAMEGIC [6] and COMIX [7,8]. They are used for the simulation of parton-level events within the Standard Model and beyond, and for the decay of heavy resonances such as W , Z, or Higgs bosons or top quarks. Both include automated methods for efficient phase-space integration and algorithms for the subtraction of infrared divergences in calculations at next-to-leading order (NLO) in QCD [9, 10, 11] and the electroweak theory [12]. For the evaluation of virtual corrections at NLO accuracy SHERPA relies on interfaces to dedicated one-loop providers, e.g. BLACKHAT [13], OPENLOOPS [14] and RECOLA [15,16]. The default parton-showering algorithm of the SHERPA 2.2 series is the CSSHOWER [17], based on Catani-Seymour dipole factorisation [9,10,18]. As of version 2.2.0 SHERPA also features an independent second shower implementation, DIRE [19,20,21]. For the matching of NLO QCD matrix elements with parton showers SHERPA implements the MC@NLO method [22,23]. For NNLO QCD calculations the UN 2 LOPS method [24, 25] is used. The merging of multi-jet production processes at leading order [26,27,28] and next-to-leading order [29,30] is based on truncated parton showers. Multiple parton interactions are implemented via the Sjöstrand-van-Zijl model [31]. The hadronisation of partons into hadrons is modelled by a cluster fragmentation model [32]. Alternatively, in particular for uncertainty estimations, an interface to the Lund fragmentation model [33] of PYTHIA [34] is available. SHERPA provides a large library for the simulation of τ -lepton and hadron decays, including many form-factor models. Furthermore, a module for the simulation of QED final-state radiation in particle decays [35], which is accurate to first order in α for many channels is built-in. To account for spin correlations in production and subsequent decay processes the algorithm described in [36] is implemented. Events generated with SHERPA can be cast into various output formats for further processing, with the HEPMC [37] format being the most commonly used. In the specific case of parton-level events, at the leading and next-to-leading order in QCD, additional output formats are supported. They include Les Houches Event Files [38], NTUPLE files for NLO QCD events [39] and cross-section interpolation grids produced via MCGRID [40,41] in the APPLGRID [42] and FASTNLO [43,44] formats. To analyse events on-the-fly a runtime interface to the RIVET package [45] can be used conveniently.

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Section: Discussionmentioning

confidence: 99%

Section: Parton Showersmentioning

confidence: 99%

Event generation with Sherpa 2.2

et al. 2019

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“…However, in a matched-b framework the massive computation is still beset by the need to start at high perturbative order. As a possible way out, the use of a "massive five flavor scheme" has been suggested recently [7,8], in which there is a b PDF (hence five flavors), yet b quark mass effects are included (possibly, at least in part, also in parton showering). The use of an independently parametrized b quark PDF within a framework in which massive and massless computations are combined offers a simpler way of dealing with the same problem.…”

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confidence: 99%

Fitting the b-quark PDF as a massive-b scheme: Higgs production in bottom fusion

2019

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We show that a simple and accurate approach to the computation of hadron collider processes involving initial-state b quarks can be obtained by introducing an independently parametrized b PDF. We use the so-called FONLL method for the matching of a scheme in which the b quark is treated as a massless parton to that in which it is treated as a massive state, and extend it to the case in which the b quark PDF is not necessarily determined by perturbative matching conditions. This generalizes to hadronic collisions analogous results previously obtained for deep-inelastic scattering. The results corresponds to a "massive b" scheme, in which b mass effects are retained, yet the b quark is endowed with a PDF. We specifically study Higgs production in bottom fusion, and show that our approach overcomes difficulties related to the fact that in a standard massive four-flavor scheme b-quark induced processes only start at high perturbative orders. arXiv:1905.02207v2 [hep-ph] 18 Jul 20191 The treatment of heavy quark PDFs It has been recently shown [1] that for accurate phenomenology at the LHC it is advantageous to treat the charm parton distribution (PDF) on the same footing as light-quark PDFs, namely, to parametrize it and extract it from data, rather than to take it as radiatively generated from the gluon using perturbative matching conditions. This is likely to be due to the fact that matching conditions are only known to the lowest nontrivial order, which may well be subject to large higher order corrections, as revealed by the strong dependence of results on the choice of matching scale. On top of this, of course, the starting low-scale heavy quark PDFs could in principle also have a non-perturbative "intrinsic" component [2,3]. It is important to note that whether or not the heavy quark PDF has a nonperturbative component, and whether it is advantageous to parametrize the heavy quark PDF are separate issues: in fact in Ref. [1] it was shown that the main phenomenological advantage in parametrizing and fitting the charm PDF comes from a region in which any nonperturbative contribution to charm is likely to be extremely small.The case of the bottom quark PDF is, in this respect, particularly interesting. On the one hand, one may think that that the problem of large higher order corrections to the matching conditions is alleviated in this case by the larger value of the mass. However, on the other hand, there is a more subtle consideration. Namely, there are b-initiated hadron collider processes -some of which are especially relevant for new physics searches -such as Higgs production in bottom fusion, for which b quark mass effects might be nonnegligible [4][5][6]. This suggests the use of a scheme in which the b quark is treated as a massive final-state parton -hence not endowed with a PDF. In such a scheme the b-induced process necessarily starts at a higher perturbative order than in a scheme in which there exists a b PDF, because the b production process is included in the hard matrix element. As a consequence, th...

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“…The default choice is α = 0 and may be set with Vincia:octetPartitioning 2 We note, however, that we could in principle use the above antennae also for massive initial state partons, should a set of massive PDFs become available, see e.g. recent developments in [41,42].…”

Section: B Massive Initial-final Antenna Functionsmentioning

confidence: 99%

Coherent showers in decays of colored resonances

Brooks

2019

Phys. Rev. D

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We present a new approach to coherent parton showers in the decays of coloured resonances, based on the notion of "resonance-final" (RF) QCD antennae. A full set of mass-and helicity-dependent 2 → 3 antenna functions are defined, with the additional requirement of positivity over the respective branching phase spaces. Their singularity structure is identical to that of initial-final (IF) antennae in 2 → N hard processes (once mass terms associated with the incoming legs are allowed for), but the phase-space factorisations are different. The consequent radiation patterns respect QCD coherence (at leading colour) and reduce to DGLAP and eikonal kernels in the respective collinear and soft limits. The main novelty in the phase-space factorisation is that branchings in RF antennae impart a collective recoil to the other partons within the same decay system. An explicit implementation of these ideas, based on the Sudakov veto algorithm, is provided in the Vincia antenna-shower plug-in to the Pythia 8 Monte Carlo event generator. We apply our formalism, matched to next-to-leading order accuracy using Powheg , to top quark production at the LHC, and investigate implications for direct measurement of the top quark mass. Finally, we make recommendations for assessing theoretical uncertainties arising from parton showers in this context.

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Towards a fully massive five-flavor scheme

Cited by 28 publications

References 71 publications

Event generation with Sherpa 2.2

Event generation with Sherpa 2.2

Fitting the b-quark PDF as a massive-b scheme: Higgs production in bottom fusion

Coherent showers in decays of colored resonances

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