Combinatorics error with identical decays
Affects | Status | Importance | Assigned to | Milestone | |
---|---|---|---|---|---|
MadGraph5_aMC@NLO |
Invalid
|
Undecided
|
FabioMaltoni |
Bug Description
I have a MG model where the higgs can decay to a "dark Z", called Zp, which has similar decays to the regular Z.
I can set parameters so that there is no h_Z_Zp vertex, just h_Zp_Zp. Furthermore, I set masses such that mZp < mh/2 (though that last part isn't important for the bug).
If I then calculate the partial width
h > zp ll, zp > ll
I get TWICE the partial width that I get from
h > zp zp, zp > ll
The latter is correct (verified by analytical calculation). The former is wrong by an extra factor of two.
This factor of two likely arises because MG does not realize that the 'first' MG decay (which happens in the 1->3 matrix element) is the same as the 'second' decay (of the 'on-shell Zp', though in this case both Zp are on-shell, and any way this shouldn't matter), so when it does the 'first' decay MG puts in an extra factor of two to account for the possibility that either of the two Zp could undergo that first decay.
Changed in mg5amcnlo: | |
status: | New → Triaged |
Changed in mg5amcnlo: | |
assignee: | nobody → FabioMaltoni (fabio-maltoni) |
Changed in mg5amcnlo: | |
status: | Triaged → Invalid |
Note: if my interpretation is correct, this bug should also arise when calculating
h > z ll, z > ll
vs
h > 4l
(which is dominated by h>z*z > 4l)