QCD@LHC2017

Session

Resummation and Monte Carlo generators

29 Aug 2017, 11:00

Other Institutes

18. What is a parton shower?

Zoltan Nagy

29/08/2017, 11:00

Resummation and Monte Carlo generators

We present a formulation for a parton shower at an arbitrary order of perturbation theory, with the aim of indicating how a parton shower can be understood as a systematically improvable approximation, similarly to ordinary perturbative calculations of cross sections. The formalism requires input functions that represent the soft and collinear singularities of QCD at the requisite perturbative...

29. Analytical and numerical approaches to (N)LL resummation

Daniel Reichelt

29/08/2017, 11:30

Resummation and Monte Carlo generators

While large logarithms arising from the IR structure of QCD can be
resummed to higher logarithmic accuracy for specific observables,
numerical calculations based on parton showers are formally correct to
LL only. However, they incorporate additional effects like local
momentum conservation, which are beyond this formal accuracy. As a
first step towards a quantitative study of these effects, a...

37. Precision QCD simulations for the LHC with Sherpa

Enrico Bothmann (University of Edinburgh)

29/08/2017, 12:00

Resummation and Monte Carlo generators

The improvements of Monte-Carlo event generators in the last decade enable quantitative predictions for the LHC at an unprecedented accuracy.

Combining matrix elements at next-to-leading order in QCD with parton-shower emissions ("matching") and using (N)LO matrix elements for hard emissions at different final-state multiplicities ("merging") led to considerably reduced theoretical...

40. Parton densities from a parton branching solution of QCD evolution equations

Aleksandra Anna Lelek (Deutsches Elektronen-Synchrotron (DE))

29/08/2017, 14:00

Resummation and Monte Carlo generators

QCD evolution equations can be recast in terms of parton branching processes. We present a new numerical solution of the equations and show, that this method reproduces the semi-analytical solutions.
We discuss numerical effects of the kinematic boundary of resolvable branchings on the resulting parton distribution functions. We show, how this method can be used to determine Transverse...

64. Heavy quark radiation in NLO+PS POWHEG generators

Luca Buonocore (INFN - National Institute for Nuclear Physics)

29/08/2017, 14:30

Resummation and Monte Carlo generators

We discuss about radiation from heavy
quarks in the context of next-to-leading order calculations
matched to parton shower generators. A new algorithm for
radiation from massive quarks is presented that offers
advantages over the one previously employed. We implement
the algorithm in the framework of the POWHEG-BOX,
and compare it with the previous one in the case of the hvq
generator for...

36. Bottom mass effects on the Z boson transverse momentum and their implications for the W mass measurements

Emanuele Angelo Bagnaschi (DESY Hamburg)

29/08/2017, 15:00

Hard QCD and EW

The inclusive transverse momentum spectrum (pt) of the Z boson is measured with high precision at the LHC and for this reason it is used a standard candle to tune Monte Carlo event generators. In turn, this implies that a change in theoretical description of the Z pt affects MC predictions used in other SM precision measurements, such as the W mass measurement. One of the uncertainties that...

41. Simpler WW production with a jet veto

Luke Arpino (University of Sussex)

Resummation and Monte Carlo generators

I present a new method to compute the cross section for WW production with a jet veto in QCD, fully exclusive in the decay products of the WW pair. The method is independent of the physics underlying the production of the WW pair, and allows for the interface with any NLO event generator.