Update of LHE file format

We intend to have a discussion on extensions of the Les Houches event file format, gearing towards an update which merges proposed and de facto in use extensions of the standard in a coherent way.

(L. Lönnblad, S. Plätzer + A. Buckley + …)

More material on extension for reweighting is here

Some extra concerns:

• Support for weight matrices to correlate multiple simultaneous scale/PDF variations (Maria Vittoria Garzelli)
• Identification of meaning of weights, esp for experiment use where the users know little about how the generator was run (Andy Buckley)

Need to forward the matrix element PDF info to the shower generator, if not already done. HepMC needs to be able to record a separate shower PDF identity (at present MC samples which use an LO shower PDF on top of an NLO LHE event file will report that they only used the LO PDF when the shower generator is queried).

alpha_s(MZ) and the running order need to be specified so that the shower generator has the option to match the ME coupling definition automatically.

Extension for the mundane but important issue of being able to track the generator(s) name and version(s) responsible for the LHE file, and to add the same information to HepMC is also needed, particularly for event provenance in the experiments. This can probably be achieved by adding a list of pairs of (genname, genversion) strings to each format.