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 KrkNLOEventReweight: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 KrkNLOEventReweight: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, KrkNLOEventReweight:AS_R, and the virtual+soft correction, KrkNLOEventReweight: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

The options available for the type of PDF are MCDY, and MC which refer to the type of MC-scheme PDF. These are selected via the interface KrkNLOEventReweight:PDF, as shown here

set KrkNLOEventReweight:PDF MC 

Note

The PDF type here must match the chosen PDF.

Using Monte Carlo scheme PDFs

Obtaining the 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

Setup

To set the MC-scheme PDF in Herwig one can use the following snippet, replacing MC_MMHT2014lo68cl.LHgrid with the required MC-scheme PDF:

  cd /Herwig/Partons

  set HardLOPDF:PDFName MC_MMHT2014lo68cl.LHgrid
  set HardNLOPDF:PDFName MC_MMHT2014lo68cl.LHgrid

  set ShowerLOPDF:PDFName MC_MMHT2014lo68cl.LHgrid
  set ShowerNLOPDF:PDFName MC_MMHT2014lo68cl.LHgrid

  set MPIPDF:PDFName MC_MMHT2014lo68cl.LHgrid
  set RemnantPDF:PDFName MC_MMHT2014lo68cl.LHgrid
  

The file $HERWIG_ENV/share/Matchbox/MMHT2014.in can be used a a reference on how to set PDFs within Herwig.