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:

* A standard format for Les Houches event files, hep-ph/0609017
* Generic User Process Interface for Event Generators, hep-ph/0109068

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 suggsted to be included in the new standard (version 2.0).

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 tag 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 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 sumprocw*totxsec/maxweight/neve.

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