Add lifetime in pythia8 using MG5 generated LHE with prompt particles

Asked by Wei Shi on 2019-06-12

Dear MG5 experts,

I was using two different LHEs (from two models) each containing a prompt parent particle decaying into oppositely charged muons. The two LHEs follow,

*LHE #1* is from an axion-like-particle model:
In this case, the parent particle is 9000005, it decays to a pair of muons. There are two such parent particles in each event, so at the end we have four muons in total.

*LHE #2* is from a dark SUSY model:
In this case, the parent particle is 3000022, it also decays to a pair of muons and there are two such parent particle in each event, similar to the above case.

For both LHE files, the parent particles inside are prompt. Following suggestions from Pythia8 experts, I added lifetime for the parent particle using Pythia8 settings in [1] . I found that with LHE #1 as input, the muons are simulated displaced as expected. For LHE #2, the lifetime is also set successfully for the parent particle at GEN level, but at the meantime, the muons are not simulated as displaced mostly. It also get simulated twice in some cases. I also described the issue earlier before I made the comparison between these two LHEs:

After this comparison, it's natural to think the issue arises from LHE, since LHE #1 worked, but not #2 with the same procedure. Skimming over the two LHE files, I only can tell that LHE #1 header contains a decay table for the parent particle while LHE #2 doesn't. I tried added a decay table in #2 but didn't work.

The other difference is that LHE #1 does the whole decay process in MG5 proc card, but for LHE #2, the main decay process is done via madspin. I am also not sure if this will cause any confusion. These are the major differences I could spot in the header and SLHA block.

Another relevant information is ,when generating LHE #2, there was this warning,

WARNING: Order of particle in the event did not agree with parent/child order. This might be problematic for some code

I am not sure if experts here has any insights on this?


            'ParticleDecays:limitTau0 = on',
            'ParticleDecays:tau0Max = 1000.1',
            'LesHouches:setLifetime = 2',
            'PDGID:all = void void 1 0 0',
            'PDGID:tau0 = 50' #unit: [mm]

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This sounds more a question for the pythia author since you are using their feature and not the feature from MG5aMC to generate displaced vertex. Now, the width is not the same in both model (in particular particle 3000001 has a width of 0 which sounds not physical) .

> WARNING: Order of particle in the event did not agree with parent/child order. This might be problematic for some code

I do not think that this is an issue for pythia. This is just that some line in the lhe file write the decaying particle before the particle it decay from. In general the good parser does not have any issue with that.



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