The aMC@NLO package [Fri02] attempts to automate the MC@NLO matching
procedure [Fri02]. MC@NLO interprets the parton shower as NLO
subtraction method, and removes unwanted parton-shower contributions by
extending the subtraction scheme used to generate NLO fixed-order results.
Upon showering, an NLO accurate prediction for inclusive observables is
achieved. This makes MC@NLO a convenient NLO+PS matching scheme. A
consistent extended subtraction in the NLO fixed-order result makes
analytic knowledge of the shower emission probability necessary. Once
this is known, interfacing the (parton-shower specific) NLO calculation
with the shower is straightforward.
To allow for a fast, automatic generation of shower subtractions that are
used in the fixed-order calculation, Pythia allows to generate emissions
with a "global" recoil scheme, in which the recoil of an emission is shared
among all final state particles. When using aMC@NLO, this global recoil must
be switched on. Please see
Timelike Showers for details and options.
A minimal set of settings necessary for a consistent treatment of aMC@NLO
SpaceShower:pTmaxMatch = 1
SpaceShower:pTmaxFudge = 1.
TimeShower:pTmaxMatch = 1
TimeShower:pTmaxFudge = 1.
SpaceShower:MEcorrections = off
TimeShower:MEcorrections = off
TimeShower:globalRecoil = on
TimeShower:weightGluonToQuark = 1
and further (process-specific) settings related global recoils.
Some comments are in order. The settings
SpaceShower:pTmaxMatch = 1,
SpaceShower:pTmaxFudge = 1.,
TimeShower:pTmaxMatch = 1,
TimeShower:pTmaxFudge = 1. are included to ensure that the
correct parton shower starting scale (i.e. the scale set when generating the
subtractions in MC@NLO) is used within Pythia. Note that the last three
options are default in Pythia8, and that the first option differs from the
default only if the input state does not contain final state partons.
Matrix element corrections to the parton shower splitting kernels have to be
switched off by
SpaceShower:MEcorrections = off and
TimeShower:MEcorrections = off . This is necessary because
the matrix element corrections are not suitable for showers in the
global recoil scheme, and because it is not viable to include process-specific
shower probabilities in an automatic framewrok like aMC@NLO.
TimeShower:globalRecoil = on is necessary. Formally, it
is allowed to switch back to a local recoil treatment beyond the first
proposed emission of any of the hard scattering partons in Born-type events.
Pythia offers three choices at which stage the global recoil is dropped in
favour of a local strategy. It is necessary to supplement the setting
TimeShower:globalRecoil = on by additional settings specifying
which global recoil strategy should be used. As these choices are up to the
user, please consult Timelike Showers
for details on these options.
TimeShower:weightGluonToQuark = 1 is not default any
longer, but was it at the time the subtractions were first implemented,
and so is required for consistency until further notice.