ZHee vs. ZHvv: difference when computing on-shell cross sections?

Asked by Philipp Windischhofer on 2020-04-24

Dear experts,

I am trying to understand the results I am getting for the cross sections of the ZH(e+ e-) and ZH(ve ve~) processes, computed in different ways (see below). I am using a fresh installation of MG 2.6.7 out of the box as well as the default "sm" model.

Without further ado, here are the cross sections I obtain with this setup:

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> generate p p > H e+ e-
0.0126 +- 4.945e-05 pb

> generate p p > Z H, Z > e+ e-
0.01906 +- 2.466e-05 pb

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> generate p p > H ve ve~
0.03808 +- 6.823e-05 pb

> generate p p > Z H, Z > ve ve~
0.03767 +- 7.368e-05 pb

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There are some features that confuse me, and I would like to understand if there is a problem in this way of using MG, or whether this is an actual physics effect.

* Restricting the Z to be onshell only has a small effect on the ZH(ve ve~) cross section, but modifies the ZH(e+ e-) cross section much more severely.

* In case the Z is forced to be onshell, the ratio ZH(ve ve~) / ZH(e+ e-) is about 2, which agrees well with the ratio of the branching fractions Br(Z -> ve ve~) / Br(Z -> e+ e-), as I would expect. In case there is no restriction on the Z, this ratio is about 3. The diagrams that MG prints for both cases are identical.

At this point, I would greatly appreciate any input as to where this might be coming from, or what the origin of the effect could be.

Many thanks for your help!

Best,
Philipp

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
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Solved by:
Philipp Windischhofer
Solved:
2020-04-24
Last query:
2020-04-24
Last reply:
2020-04-24

Olivier Mattelaer suggests this article as an answer to your question:
FAQ #2442: “why production and decay cross-section didn't agree.”.

Ha! Thanks a lot for your answer. For posterity, the culprit was the "etal" cut that I had missed and that only applies to *charged* leptons. Disabling this cut leads to the expected outcome.

Closing.