LHEF for Matching

From Wiki Les Houches 09

(Difference between revisions)
Jump to: navigation, search
(The cutsinfo tag (optional))
(The cut tag)
Line 77: Line 77:
the maximum and/or minimum value of that parameter. The contents should be the minimum and maximum allowed values of this variable (in that order). If only one number is given, there is no maximum. If the minumum is larger or equal to the maximum, there is no minimum. The attributes are
the maximum and/or minimum value of that parameter. The contents should be the minimum and maximum allowed values of this variable (in that order). If only one number is given, there is no maximum. If the minumum is larger or equal to the maximum, there is no minimum. The attributes are
 +
* <tt>p1</tt> (D=0): The particles for which this cut applies. This can either be a number corresponding to a given particle PDG code or 0 for any particle or the name of a previously defined <tt>ptype</tt> tag
 +
* <tt>p2</tt> (D=0): If cut is between pairs of particles, this attribute should be non-zero. Allowed values are the same as for <tt>p1</tt>
* <tt>type</tt> (R): This defines the variable which is cut. The following values are allowed (the lab frame is assumed in all cases):
* <tt>type</tt> (R): This defines the variable which is cut. The following values are allowed (the lab frame is assumed in all cases):
** "m" the invariant mass of two particles (or, if only one particle type is given, the invariant mass of that particle).
** "m" the invariant mass of two particles (or, if only one particle type is given, the invariant mass of that particle).
-
** "kt": the transverse momenta of a particle (in GeV)
+
** "kt": the transverse momenta of a particle matching <tt>p1</tt> (in GeV)
-
** "eta": the pseudorapidity of a particle
+
** "eta": the pseudorapidity of a particle matching <tt>p1</tt>
-
** "y": the true rapidity of a particle
+
** "y": the true rapidity of a particle matching <tt>p1</tt>
-
** "deltaR": the pseudorapidity--azimuthal-angle difference between two particles.
+
** "deltaR": the pseudorapidity--azimuthal-angle difference between two particles matching <tt>p1</tt> and  <tt>p2</tt> respectively.
-
** "E": the energy of a particle
+
** "E": the energy of a particle matching <tt>p1</tt>
** "ETmiss": the sum of the pt of the non-neutrinos in the process
** "ETmiss": the sum of the pt of the non-neutrinos in the process
-
** "HT": the scalar sum of the transverse momentum of the particles in <tt>p1</tt>
+
** "HT": the scalar sum of the transverse momentum of the particles matching <tt>p1</tt>
** ...
** ...
-
* <tt>p1</tt> (D=0): The particles for which this cut applies. This can either be a number corresponding to a given particle PDG code or 0 for any particle or the name of a previously defined <tt>ptype</tt> tag
 
-
* <tt>p2</tt> (D=0): If cut is between pairs of particles, this attribute should be non-zero. Allowed values are the same as for <tt>p1</tt>
 
== The <tt>procinfo</tt> tag (optional) ==
== The <tt>procinfo</tt> tag (optional) ==

Revision as of 14:25, 13 June 2009

Contents

Introduction

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>

or

 <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 and ptype tags can be supplied.

The ptype

This tag can be used to define a group of particles to which a cut can be applied. Its contents should be a white-space separated list of PDG particle codes. The only attribute is

  • name (R): The string used to represent this ptype in a cut.

The cut tag

This 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 contents should be the minimum and maximum allowed values of this variable (in that order). If only one number is given, there is no maximum. If the minumum is larger or equal to the maximum, there is no minimum. The attributes are

  • p1 (D=0): The particles for which this cut applies. This can either be a number corresponding to a given particle PDG code or 0 for any particle or the name of a previously defined ptype tag
  • p2 (D=0): If cut is between pairs of particles, this attribute should be non-zero. Allowed values are the same as for p1
  • type (R): This defines the variable which is cut. The following values are allowed (the lab frame is assumed in all cases):
    • "m" the invariant mass of two particles (or, if only one particle type is given, the invariant mass of that particle).
    • "kt": the transverse momenta of a particle matching p1 (in GeV)
    • "eta": the pseudorapidity of a particle matching p1
    • "y": the true rapidity of a particle matching p1
    • "deltaR": the pseudorapidity--azimuthal-angle difference between two particles matching p1 and p2 respectively.
    • "E": the energy of a particle matching p1
    • "ETmiss": the sum of the pt of the non-neutrinos in the process
    • "HT": the scalar sum of the transverse momentum of the particles matching p1
    • ...

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:

  • iproc (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.

  • iproc (D=0): The process number for which the information is given. "0" means all processes.
  • scheme (R): The reweight scheme used. 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.
  • clustering (R): The clustering used to find the scales in the Sudakov form factors (and αS). The following values are valid
    • "None": No clustering has been done
    • "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.


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 weight tag (optional)

An event can be associated with a number of different weights given in weight tags. The content of these tags are simply a sequence of weights corresponding to different PDFs, αS values, etc. The first weight in the first of these tags should correspond to the main weight as described in the xsecinfo tag. All weights should be given in the same unit as the maxweight attribute in the xsecinfo tag. For each weight tag, further information can be given in the following attributes

  • name (D=""): an arbitrary sting describing this set of weights
  • 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)

Personal tools