LHE file results for BSM processes

Asked by VC on 2020-11-07

Hello Whizard experts,

I used Whizard to simulate interactions for a BSM model, and then exported the events to an LHE file with the following code:
simulate (myprocess) {n_events = 10000 sample_format = lhef $sample = $filename}

Some final state particles were unstable, and I hoped to decay the unstable particles separately in Pythia. However, maybe because the resulting LHE file didn't contain general information (particle mass, decay width... that would appear in a parameter card) about the BSM model, the unstable particles couldn't be decayed in Pythia. Is there a way to include information about a BSM model in the resulting LHE file in addition to simulation results, so that unstable final state particles can then be decayed separately in Pythia?

Thanks a lot!

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Juergen Reuter (j.r.reuter) said : #1

Hi VC,
from your question I deduce that you tried to do a BSM heavy particle with Pythia, but Pythia did not do anything? Or did it stop with an error code? Did you use the internal Pythia (v6.4) or an external Pythia, e.g. 8.3? Normally, you need to make sure that Pythia knows what to do with the resonances?

VC (vv-cc) said : #2

Thanks for your reply! I tried to use external Pythia 8.3 to decay a BSM particle produced by Whizard simulation. There was no error message but Pythia did nothing, just gave back my original initial and final state particles in the Whizard process.

Simon Braß (sbrass) said : #3

Dear VC,

IMO you have two possible options: you either use the internal decay feature of WHIZARD, or you have to add manually the particles and their decays to Pythia8, however, I never tried the latter one and I'm not sure whether this will work in my intended way.

For the `unstable` feature please take a look at p. 65 for the keyword, and Sec. 5.8.2 for a detailed explanation of the decay handler in WHIZARD.
In summary, you integrate your process as usual, but for event generation the `unstable` marked particles are decayed inside WHIZARD.
WHIZARD uses all available information from the underlying model to project the correct decays, however, you have to fix the maximal multiplicity by yourself, which defaults to 2, i.e. 1 -> 2 decays are only produced.

I hope I could point you in the right direction.

VC (vv-cc) said : #4

I successfully decayed the final state particles inside Whizard. Thank you for answering my questions!