# Is the macroscopic decay length of particles raken into account when generating events?

Dear experts,

My question is "Is the macroscopic decay length of particles taken into account when MadGraph is computing the cross section?". To be more specific, suppose madgraph is generating events for the process e- e+ > A B, A > e X where X is a BSM particle and A,B are SM particles. Suppose that the coupling of the decay A > e X is very small (based on experimental limits). The decay length of A corresponding to this BSM decay is thus very large. As a result, most of A particles may decay OUTSIDE the detector. Now the question arises: is the cross section that madgraph computes the cross section of events with A decaying INSIDE the detector? or it is just the cross section of all events (some of which with decays inside and some with decays outside the detector.) If the latter is the case, one needs to multiply the madgraph cross section by a factor (regarding the probability that A decays inside the detector) to find the cross section of the desired events.

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Solved by:
Olivier Mattelaer
Solved:
2020-11-15
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2020-11-15
2020-11-14

## This question was reopened

 Olivier Mattelaer (olivier-mattelaer) said on 2020-11-14: #1

Hi,

The computation returned within MG5aMC does not have any detector effect. We compute the hard-cross section.
(we do not have jet at this level). We do have the possibility to have cuts but those are NOT detector cuts and those should be loose cuts since observable can be impacted by parton-shower, by jet reconstruction and by detector effect.

Concerning the decay length of the particles, by default we do not compute the distance travelled by each particles.
But you have a parameter in the run_card to tell madgraph to add such (trivial) computation.
Note that the parton-shower do not do any bending for such particles (and they might have some additional hyppothesis/restriction)

Cheers,

Olivier

 HH (hh-madgrapher) said on 2020-11-15: #2

Thanks Olivier Mattelaer, that solved my question.