**Proposed updates for HepMC event record**

__GenRun__

There is a need to store the names and version numbers of all generators used in producing an event. Propose to define a GenRun object to contain cross section (GenCrossSection) and other run-level quantities discussed below. Each GenEvent should link to the GenRun.

__Generator names and versions__

Add string array to Genrun with pairs of strings:

"generator name"  "generator version" 

All generators used in the production should be listed, with full version numbers.


__Vertex ID__

The GenVertex.id() is not currently used/specified. We propose to define six codes:


  * **0 -- undefined** (no information)
  * **1 -- Matrix Element vertex** (i.e. the hard process which defines the PDF arguments). (Same as signal vertex)
  * **2 -- secondary partonic scatters** (useful for studies of MPI jet production, e.g. for suppressing jets from MPI while retaining the low pT tail of the underlying event, as in [[http://inspirehep.net/record/1216670|this paper]].
  * **3 -- Hard decay** (prehadronisation) e.g. of W, H, Z, t. Useful e.g. for optimisation studies of mass resolution
  * **4 -- parton shower** (including QED radiation) Useful for e.g. identifying g->bb splitting probability; ME/PS matching studies; identifying photons for dressing leptons. NB any parton shower after a hadronisation is ignored, and the hadrons from it 
are flagged as being produced in a hadron decay.
  * **5 -- primary hadron formation** Useful for e.g redoing hadroisation, applying BEC corrections, studying/tracking decay chains. 
  * **6 -- hadron, tau and muon decays**

We propose that 10-99 be reserved for generator-specific types.

__Weight names__

Some standardisation of weight names is desirable:

"MUF*** MUR*** PDF10801 PDF20123"     (last numbers are the LHAPDF/PDFLIB unique PDF member IDs)

names for each weight, to go in the GenRun, linked to GenWeightContainer?