Pythia not displacing vertices of long-lived particle decays

Hi,

I do not kown how pythia is handling such information.
Concerning MG5aMC, we can compute such information but this is off by default.
You can turn it on via the run_card

Cheers,

Olivier

> On 6 Feb 2024, at 03:05, Andrew Fee <email address hidden> wrote:
>
> New question #709213 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/709213
>
> Hello,
>
> I am using Madgraph5 Version 2.6.1, with Pythia8 installed internally. I am trying to generate Monte Carlo for a model with a long-lived scalar particle called X, whose particle ID is 5000001. In param_card.dat I have specified a width for X, and have set branching ratio of 1 for it to decay into a b b~ pair, as follows:
>
> ###################################
> ## INFORMATION FOR DECAY
> ###################################
> DECAY 6 1.508336e+00 # WT
> DECAY 23 2.495200e+00 # WZ
> DECAY 24 2.085000e+00 # WW
> DECAY 25 4.070000e-03 # WH
> DECAY 5000001 2.466587e-2 # WX
> 1.000000e+00 2 5 -5 # BR(X > bb~)
>
>
> However, when I run the event and pass it to Pythia for showering and hadronization, the resulting hepmc file displays (more or less) the following at the beginning of each event:
>
> HepMC::Version 2.06.09
> HepMC::IO_GenEvent-START_EVENT_LISTING
> E 0 -1 -1.0000000000000000e+00 -1.0000000000000000e+00 -1.0000000000000000e+00 9999 0 117 1 2 0 1 2.1258189000000002e-07
> N 1 "Weight_MERGING=0.000"
> U GEV MM
> C 3.4013102400000002e-06 2.1258189000000001e-03
> V -1 0 0 0 0 0 1 1 0
> P 1 -13 0 0 6.4999999991412287e+03 6.5000000000000009e+03 1.0566000000000000e-01 4 0 0 -1 0
> P 3 -13 0 0 6.5000000000490663e+03 6.5000000000490663e+03 0 21 0 0 -3 0
> V -2 0 0 0 0 0 1 1 0
> P 2 13 0 0 -6.4999999991412287e+03 6.5000000000000009e+03 1.0566000000000000e-01 4 0 0 -2 0
> P 4 13 0 0 -6.5000000000090668e+03 6.5000000000090668e+03 0 21 0 0 -3 0
> V -3 0 0 0 0 0 0 3 0
> P 5 25 1.0582203214000000e+02 4.3923927812999999e+01 -1.4445180422000001e+03 1.4544364340919378e+03 1.2500141060000000e+02 22 0 0 -4 0
> P 6 14 -5.9919264176000002e+00 -2.1924513121000000e+01 -5.0500949564000002e+03 5.0501461024523351e+03 0 1 0 0 0 0
> P 7 -14 -9.9830105723000003e+01 -2.1999414691999998e+01 6.4946129985999996e+03 6.4954174635537884e+03 0 1 0 0 0 0
> V -4 0 0 0 0 0 0 2 0
> P 8 5000001 2.7602849869000000e+01 4.9963630715000001e+01 -5.8361613541999998e+02 5.8852654683447497e+02 4.9974204258000000e+01 22 0 0 -5 0
> P 9 5000001 7.8219182270999994e+01 -6.0397029013000001e+00 -8.6090190673999996e+02 8.6590988721740928e+02 4.9931165278000002e+01 22 0 0 -18 0
> V -5 0 0 0 0 0 0 2 0
> P 10 5 3.0101059342999999e+01 4.6071039994000003e+01 -3.1339525437999998e+02 3.1822521893716163e+02 4.7000000000000002e+00 23 0 0 -7 1 1 501
> P 11 -5 -2.4982094732000002e+00 3.8925907208999999e+00 -2.7022088103999999e+02 2.7030132789745272e+02 4.7000000000000002e+00 23 0 0 -6 1 2 501
> V -6 0 0 0 0 0 0 2 0
> P 12 -5 7.0754642732199251e+00 1.3277663174263710e+01 -3.2944546206905943e+02 3.2982231733457166e+02 4.7000000000000002e+00 51 0 0 -8 1 2 503
> P 13 21 -3.1393784862583525e+00 4.2923518522685855e-01 -6.4729112194026737e+00 7.2068384038000755e+00 0 51 0 0 -9 2 1 503 2 501
> V -7 0 0 0 0 0 0 1 0
> P 14 5 2.3666764082838732e+01 3.6256732355409987e+01 -2.4769776213154421e+02 2.5149739109624900e+02 4.7000000000000002e+00 52 0 0 -44 1 1 501
> V -8 0 0 0 0 0 0 2 0
> P 15 -5 6.0416150215604700e+00 1.1295401001787816e+01 -3.1475555574969201e+02 3.1505116579875937e+02 4.7000000000000002e+00 51 0 0 -50 1 2 504
> P 16 21 7.3675715324047264e-01 2.0228824361528588e+00 -1.5302464118770313e+01 1.5453163836561096e+01 0 51 0 0 -11 2 1 504 2 503
>
> And so on... Unless I am horribly misunderstanding the contents of this file, if a displaced decay has occurred, the lines beginning with "V" corresponding to that decay should look like
>
> V (integer) # # # #...,
>
> where the #'s are nonzero numbers specifying the position 4-vector of the decay.
>
> What am I doing wrong here? Am I not understanding the contents of the hepmc file properly?
>
> Thanks,
> Andrew
>
>
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Thanks! How do I go about doing this in run_card? And where shall that information be located in the LHE file?

Hi,

For the run_card, you have this line that you can edit:
   -1.0 = time_of_flight ! threshold (in mm) below which the invariant livetime is not written (-1 means not written)

For the lhef file, we do follow the conventions, so the best is to check in the associated paper:
J. Alwall et al., Comput. Phys. Commun. 176 (2007) 300 [hep-ph/0609017]

Cheers,

Olivier

I have changed that line to various values (0.01, 0.1, 1, 10, 100), but still do not see any new entries appearing in the lhe file. At the same time, I have also ran through multiple values of the X particle's width, ranging from roughly 10^-2 to 10^-4. Any suggestions for how to fix this?

Thanks Olivier Mattelaer, that solved my question.