There are various approaches available for current simulations:

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Output from the Hidden Valley Generator can currently be either as

I) 
Les Houches Accord Event Records at v-hadron level, PRIOR to v-hadron
decays, which can be read in to Pythia with 
  (A) a corresponding Pythia card or 
  (B) a PYUPDA call [which cannot be done using a card]
Pythia performs the v-hadron decays and ensuing 
QCD showering/hadronization etc.


II) Under Development (two-four weeks?)
Les Houches Accord Event Records at parton level, AFTER v-hadron
decays, which can be read in to Pythia with methods (A) or (B) above.
Pythia now performs only QCD showering/hadronization etc.

III) 
HEPEVT output at ordinary hadron level, convertible to HEPMC.

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In each of these cases, v-hadrons must appear in the event record.  In
the long run, v-hadrons will have to be defined with their PDG codes,
masses, decays etc.  In the short run, only v-pions appear, and these
can be encoded as known particles H0 and A0.  Therefore there are
currently two options for how the v-pions appear

1) store v-hadrons as H0 and A0 (codes 35 and 36)
2) store v-hadrons as vpi+ and vpi0 (codes 6000211 and 6000111); note
     that no distinction is made between vpi+ and vpi- (both are 
     electrically neutral despite the name, and both are stable here)

You will have to tell me what your software prefers, but this easily
changed.

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For the various cases currently available, here is what you need to do:

IA.1) input to Pythia the file CMS36.lha.dat with the use of
the Pythia card CMS36.lha.pycard

[IA.2 is not available since the Pythia card option does not work with new particle codes 6000211 etc.]

IB.1) input to Pythia the LHA event record file CMS36.lha.dat and use 

      OPEN(UNIT=60,file='pyupda.36',status='old'
      CALL PYUPDA(3,60)
      CLOSE(60)

or its equivalent to upload the information about the decaying vpions.

IB.2) input to Pythia the LHA event recrord file CMS60.lha.dat and use

      OPEN(UNIT=60,file='pyupda.60',status='old'
      CALL PYUPDA(3,60)
      CLOSE(60)

to upload the information about the decaying vpions.

III.1) hepevt output is in CMS36.hepevt.dat ; this was written
in text format using the write command stored in HEPEVT.IO.txt 
All particles are familiar to GEANT

III.2) hepevt output is in CMS60.hepevt.dat ; this was written
in text format using the write command stored in HEPEVT.IO.txt
New particles must be defined:

6000111   40.000   # vpi0 -- unstable to decay to b bbar, c cbar, taus...
6000211   41.363   # vpi+