= 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 [wiki:MCDY MCDY], or [wiki:MC 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 [wiki:MC 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 [https://krknlo.hepforge.org/hg/mc-pdf this repository], and are stored as [https://lhapdf.hepforge.org 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.