36 research outputs found
State of the art POWHEG generators for top mass measurements at the LHC
We study the theoretical uncertainties in the determination of the top-quark
mass using next-to-leading-order (NLO) generators, that describe the top-quark
decay at different levels of accuracy, interfaced to parton showers (PS).
Specifically we consider one generator that implements NLO corrections only in
the production dynamics, one that also takes them into account in the top-quark
decay in the narrow width approximation (NWA) and one that implements them
exactly, including finite-width and interference effects. We aim at assessing
the errors in top-mass determinations of purely theoretical origin. We do so by
measuring relative peak position shifts of -jet mass distributions. Besides
the theoretical errors due to the use of less accurate NLO+PS generators, we
also explore uncertainties related to shower and modelling of non-perturbative
effects by comparing the results obtained by interfacing our generators to both
Pythia and Herwig shower Monte Carlos (SMCs).Comment: Proceedings of the XXVI International Workshop on Deep-Inelastic
Scattering and Related Subjects (DIS2018), 16-20 April 2018, Kobe, Japa
All-orders behaviour and renormalons in top-mass observables
We study a simplified model of top production and decay, consisting in a
virtual vector boson decaying into a massive-massless -
quark-antiquark pair. The top has a finite width and further decays into a
stable vector boson and a quark. We then consider the emission or the
virtual exchange of one gluon, with all possible light-quark loop insertions.
These are the dominant diagrams in the limit of an infinite number of light
flavours. We devise a procedure to compute this process fully, by analytic and
numerical methods, and for any infrared-safe final-state observables. We
examine the results at arbitrary orders in perturbation theory, and assess the
factorial growth associated with renormalons. We look for renormalon effects
leading to corrections of order , that we dub `linear'
renormalons, in the inclusive cross section (with and without selection cuts),
in the mass of the reconstructed-top system, and in the average energy of the
final-state boson, considering both the pole and the
scheme for the top mass. We find that the total cross section without cuts, if
expressed in terms of the mass, does not exhibit linear
renormalons, but, as soon as selection cuts are introduced, jets-related linear
renormalons arise in any mass scheme. In addition, we show that the
reconstructed mass is affected by linear renormalons in any scheme and that the
average energy of the boson (that we consider as a simplified example of
leptonic observable), in any mass scheme, has a renormalon in the narrow-width
limit, that is however screened at large orders for finite top widths, provided
the top mass is in the scheme.Comment: 40 pages, 17 figure
Four-lepton production in gluon fusion at NLO matched to parton showers
We present a calculation of the next-to-leading order (NLO) QCD corrections
to gluon-induced electroweak gauge boson pair production, and , matched to the PYTHIA8 parton shower in the POWHEG approach. The
calculation consistently incorporates the continuum background, the
Higgs-mediated process, and their interference. We consider
leptonic decay modes of the massive vector bosons and retain offshell and
non-resonant contributions. The processes considered are loop-induced at
leading order and thus contain two-loop virtual contributions as well as
loop-squared real contributions. Parton-shower effects are found to be marginal
in inclusive observables and quite sizeable in observables that are exclusive
in additional jet radiation. The Monte Carlo generator presented here allows
for realistic experimental effects to be incorporated in state-of-the-art
precision analyses of diboson production and of the Higgs boson in the offshell
regime.Comment: 15 pages, 13 figures, 1 tabl
On linear power corrections in certain collider observables
We study linear power corrections O(Λ/Q) to certain collider observables. We present arguments that prove that such corrections cannot appear in observables that are inclusive with respect to QCD radiation, such as total cross sections as well as rapidity and transverse momentum distributions of color-neutral particles. Although our calculations are carried out in a simplified framework, our arguments and conclusions are applicable, with some reservations, to processes both at lepton and hadron colliders. We also show how an improved understanding of the origin of linear power corrections allows us to simplify their calculation. As an application, we compute the leading non-perturbative corrections to the C-parameter and the thrust in ee annihilation in a generic three-jet configuration
A POWHEG generator for deep inelastic scattering
We present a new event generator for the simulation of both neutral- and
charged-current deep inelastic scattering (DIS) at next-to-leading order in QCD
matched to parton showers using the POWHEG method. Our implementation builds on
the existing POWHEG BOX framework originally designed for hadron-hadron
collisions, supplemented by considerable extensions to account for the
genuinely different kinematics inherent to lepton-hadron collisions. In
particular, we present new momentum mappings that conserve the special
kinematics found in DIS, which we use to modify the POWHEG BOX implementation
of the Frixione-Kunszt-Signer subtraction mechanism. We compare our predictions
to fixed-order and resummed predictions, as well as to data from the HERA ep
collider. Finally we study a few representative distributions for the upcoming
Electron Ion Collider.Comment: 54 pages, 25 figures, code obtainable from
svn://powhegbox.mib.infn.it/trunk/User-Processes-RES/DI
Linear power corrections to ee shape variables in the three-jet region
We use an abelian model to study linear power corrections which arise from infrared renormalons and affect event shapes in ee annihilation into hadrons. While previous studies explored power corrections in the two-jet region, in this paper we focus on the three-jet region, which is the most relevant one for the determination of the strong coupling constant. We show that for a broad class of shape variables, linear power corrections can be written in a factorised form, that involves an analytically-calculable function, that characterises changes in the shape variable when a soft parton is emitted, and a constant universal factor. This universal factor is proportional to the so-called Milan factor, introduced in earlier literature to describe linear power corrections in the two-jet region. We find that the power corrections in the two-jet and in the three-jet regions are different, a result which is bound to have important consequences for the determination of the strong coupling constant from event shapes. As a further illustration of the power of the approach developed in this paper, we provide explicit analytic expressions for the leading power corrections to the C-parameter and the thrust distributions in the N -jet region for arbitrary N, albeit in the abelian model