LHEF for Matching

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The Les Houches Event File accord (LHEF) was introduced in 2006 and was meant as a standard way of representing in an ASCII file the information in the old Les Houches common block accord from 2001. LHEF was constructed using XML tags to be easy to extend (although some additional structure is assumed inside some tags which is not formulated in XML). The standard has been extremely usefull, and has been used a lot to interface matrix element generators and parton shower programs.

The standards are found here:

As the matching and merging of tree-level matrix elements and parton showers are now becoming the state-of-art, it is reasonable to let this be reflected in an updated file format to standardise how relevant information should be given by the matrix element generators (MEGs) in a usable fashion for the parton shower programs (PSGs). Furthermore, looking ahead towards the time when NLO matching and merging with parton showers hopefully will become the standard, it might be worth thinking about which additional information is needed for the parton showers in this case.

Below you will find a rough draft of additional XML tags suggested to be included in the new standard (version 2.0).

Note that the notation is that a tag may contain attributes in the following way:

 <tag attribute1="value" attribute2="value">things marked by the tag</tag>


 <tag attribute1="value" attribute2="value" attribute3="value" />

Run information

This is information about all events in the file to be included inside the "init" tag after the compulsary block of parameters corresponding to the HEPRUP common block.

The xsecinfo tag (required)

The information in the HEPRUP common block is in principle sufficient to figure out the cross sections of the processes involved. However, the way things are specified is a bit confusing and complicated since it was assumed to be used to pass information between the MEG and PSG in both directions. For the event file, the communication is per definition one-way, and the information can be made more easily provided. The suggested attributes for the xsecinfo tag are as follows (R means the attribute is required. D= means a default is assumed if the attribute is absent).

  • neve (R): the number of events in the file
  • totxsec (R): the total cross section (in units of pb) of all processes in the file
  • maxweight (D=1): the maximum weight of any event in the file (in an arbitrary unit)
  • meanweight (D=1): The average weight of the events in the file (same unit as maxweight).
  • negweights (D=no): If yes, the file may contain events with negative weights.
  • varweights (D=no): If yes, the file may contain varying weights (if no, all events are weighted with maxweight=meanweight).

The information given per process in the HEPRUP common block can then safely be ignored and the cross section for each process is assumed to be the sum of all weights of the corresponding events multiplied by totxsec/maxweight/neve.

The cutsinfo tag (optional)

Here the MEG should supply information about the cuts used in the generation. Inside this tag any number of cut tags can be supplied.

The cut tag

The cut tag has information of a particular cut used. The information needed is which particle(s) are affected, what variable is used and the maximum and/or minimum value of that parameter. The attributes are

  • type (R): This defines the variable which is cut. The following values are allowed (the lab frame is assumed in all cases):
    • "invmass" the invariant mass of two particles (or if only one particle type is given, the rest mass of that particle).
    • "kt": the transverse momenta of a particle (in GeV)
    • "eta": the pseudorapidity of a particle
    • "y": the true rapidity of a particle
    • "deltaR": the pseudorapidity--azimuthal-angle difference between two particles.
    • "E": the energy of a particle
    • "ETmiss": the sum of the pt of the neutrinos in the process
    • ...
  • p1 (D=any): The particle for which this cut applies. This can either be a number corresponding to a given particle PDG code or one of the following
    • "any": any particle
    • "jet": any coloured particle
    • "uds": ("udsc") any light quark
    • "guds": ("gudsc") any light quark or gluon
    • "b" ("c"): any b or bbar (c or cbar) quark
    • "l" ("l+" or "l-"): any (positively or negatively) charged lepton
    • ...
  • p2 (D=0): If cut is between pairs of particles, this attribute should be non-zero. Allowed values are the same as for p1
  • min (D=-inf): The minimum value of this variable
  • max (D=+inf): The maximum value of this variable

The procinfo tag (optional)

For each process number used in the NPRUP variable in the HEPEUP common block we can have additional information given in the following attributes:

  • nprup (D=0): The process number for which the information is given. 0 means all processes.
  • loops (D=-1): The number of loops used in calculating this process.
  • qcdorder (D=-1): The number of QCD vertices used in calculating this process.
  • eworder (D=-1): The number of electro-weak vertices used in calculating this process.

In all cases above -1 means that no information is given.

The mergeinfo tag (optional)

For some merging schemes (eg. for CKKW) it is possible to reweight the the events with Sudakov form factors already in the MEG. This can be described in the following attributes.

  • nprup (D=0): The process number for which the information is given. "0" means all processes.
  • reweightscheme (R): valid values are
    • "None": no reweighting done
    • "CKKW-NLL": events are reweighted by the standard CKKW NLL Sudakov form factor
    • "CKKW-NLL1": same as above but weight is required to be less than 1
    • "VetoSudakov": events are reweighted with 0 or 1 with a probability given by a (non-analytic) form factor.
  • reconstruction (R): The clustering used to find the scales in the Sudakov form factors (and αS). The following values are valid
    • "ktalg": the kt-algorithm used in the original CKKW scheme
    • ...
  • maxmult (D=no): If yes the corresponding process is reweighted as if it is the maximum multiplicity process (ie. the Sudakov for the last step down to the merging scale is not included.

The nloinfo tag (optional)

If the events were produced by a NLO generator the following information can be supplied as attributes:

  • scheme (R): The NLO (subtraction) scheme used. The following values are valid:
    • "CSdipole": Catani-Seymour subtraction.
    • "POWHEG": Nasons scheme for giving one extra jet with Sudakov form factors correct to NLO.
    • "MC@NLO": "parton-shower subtracted"
    • "Exclusive": The 1-loop events are generated according to the exclusive NLO cross section (according to the cuts), while 0-loop events are pure tree-level.
  • rfscheme (D=MSbar): The factorization and renormalization scheme used (MSbar, DIS)

Note that for NLO generators (except for the Exclusive scheme), events with real emission corrections will be produced which will not respect the cuts. For the POWHEG scheme there is an additional cut imposed on the emission, which can be specified by "cut" tags (see the cutsinfo tag above) inside the the nloinfo tag.

The pdferrors tag (optional)

It is sometimes efficient to supply different weights for the same event using a sets of error PDFs. If this is done, each event must have this information and the only attribute in this tag specifies the number of sets used.

  • nsets (R): the number of error sets used.

Event information

This is information about a particular event to be included inside the event tag after the compulsory block of parameters corresponding to the HEPEUP common block.

The weightinfo tag (optional)

Here some information can be supplied to help the PSG to reweight events according to its needs. The following attributes may be given

  • weight (D=XWGTUP): the weight of the event as described in the xsecinfo tag. The default is the XWGTUP variable of the HEPEUP common block. Must be given in the same unit as the maxweight attribute in the xsecinfo tag.
  • born (D=0): the relative size of the tree-level cross section for this event. Summing born*weight and multiplying with totxsec/neve/maxweight will give the born cross section for this process. If 0 this information is not given.
  • sudakov (D=0): The sudakov form factor used to weight this event (see the reweightscheme attribute of the mergeinfo tag).
  • subtraction (D=0): The relative size of the subtraction term used for a 0-loop event (is typically negative)

The relationships between these terms are typically born*sudakov=1 and born+subtraction=1.

The alphas tag (optional)

Information about the αS used in generating this event. Attributes are

  • scale (D=SCALUP): The scale in GeV used in the αS (default is the SCALUP variable in the HEPEUP common block).
  • value (D=AQCDUP): The value of αS used (default is the AQCDUP variable in the HEPEUP common block).

If different values of αS has been used in different vertices in this even a white-space separated list of values can be given in this tag. The order of the values is dictated by the "reconstruction" attribute in the mergeinfo tag above (falling scales) if present.

The pdfinfo tag (optional)

Here the MEG can supply information about the PDFs used for this event. The following attributes are available

  • p1 (R): the type of the incoming particle 1.
  • p2(R): the type of the incoming particle 2.
  • x1 (R): the x-value used for the incoming particle 1.
  • x2(R): the x-value used for the incoming particle 2.
  • xf1 (R): the value of the pdf for the incoming particle 1.
  • xf2 (R): the value of the pdf for the incoming particle 2.
  • scale (D=SCALUP): The scale in GeV used in the PDFs (default is the SCALUP variable in the HEPEUP common block).

The pdfinfo tag may contain a white-space separated list of additional relative weights corresponding to reweighting this event according to the error sets of the LHAPDF set used (see the pdferrors tag above).

--Lonnblad 13:46, 10 June 2009 (UTC)

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