KrkNLO

The KrkNLO method provides NLO QCD corrections to hard processes using LO parton-shower Monte Carlo events. It does so by making use of dedicated Monte Carlo (MC) scheme Parton Distribution Functions (PDFs) combined with a simple, postitive-definite, per-event kinematic weight.

Supported Processes

Drell-Yan

The process p p -> Z is supported, and can be used together with the MCDY?, or MC? PDFs.

This mode is selected by choosing p p -> e+ e- as your process in Herwig, and setting

set KrkNLO:Mode Z

Higgs production via Gluon Fusion

The process g g -> H is supported and can used together with the MC? PDFs. This is performed in the large top-mass limit.

This mode is selected by choosing p p -> H as your process in Herwig, and setting

set KrkNLO:Mode H

as well as read Matchbox/HiggsEffective.in

Note

the qq-initiated tree-level diagram must be added manually

Options

Scale Settings

There are options that set the argument of AlphaS for the real emission correction, KrkNLO:AS_R, and the virtual+soft correction, KrkNLO:AS_V. The possible options are:

• AS_R: Q2, M2, Freeze
• AS_V: Q2, M2

The options correspond to:

• Q2 is the scale used in the shower (i.e. pT)
• M2 is the nominal mass of the boson (H/Z)
• Freeze is Q2 below the mass M2, and fixed at M2 above

PDF Type

TO BE DOCUMENTED

Using Monte Carlo scheme PDFs

The MC-scheme PDFs are available in ​this repository, and are stored as ​LHAPDF .LHgrid files.

The repository contains MC-scheme PDF, which are converted from their correspondingly named MSbar PDF, and are prefixed with either MCDY_, or MC_, with the former denoting the Drell-Yan only variant and the latter denoting the full conversion.

To download the MC-scheme PDFs run

hg clone https://krknlo.hepforge.org/hg/mc-pdf

and add the PDFs location to your LHAPDF environment path LHAPATH or LHAPDF_DATA_PATH. For example:

export LHAPATH=/path/to/mc-pdf:$LHAPATH