Madgraph auto decay width calculations not working, returns 0 even when decay should be possible.
Hi,
Madgraph is refusing to calculate decay widths of some of the particles I've added in a BSM model when I write "set decay [ID] auto" in a script Some particles' widths are calculated but others' aren't, even though I believe should be able to be calculated.
Specifically, my model adds new fermions N, N2 and N3, as well as a fermion X and a real scalar Ph. N, N2, and N3 act like three generations of a new field so that their interactions with other particles are identical, up to coupling constants.
N, N2, and N3 all have a vertex with W and lepton, and a vertex with Z and neutrino, so they should all be able to decay to e.g. 3 leptons. There are no other couplings between my new particles and the SM.
Additionally, there is a Yukawa type coupling between Ph, X and each of the N, N2, and N3 particles. Summarizing, I have included these vertices in my model:
N-W-lepton and N-Z-neutrino (and same with N -> N2, N -> N3)
N-X-Ph (and same with N -> N2, N -> N3)
The masses are such that N2 > Ph > N > X, with X a stable particle, such that I expect N2 to be able to decay to Ph + X. I have double checked that this is the N2 mass is greater than the final states'. For concrete numbers, the masses I'm using are:
N: 5 GeV
N2: 13.6 GeV
Ph: 9.5 GeV
X: 2.7 GeV
Now, when I ask Madgraph to automatically calculate the widths of all these particles, it calculates a width for the N and the N3 just fine, but ignores N2 and Ph and sets their widths to 0 instead. This is confusing because I'm certain that Ph is able to decay to N + X and there exists a direct coupling to do so. It also ignores that N2 can decay to Ph + X.
Moreover, N2 has the same couplings to e.g. W+lepton as the N and the N3. And even more puzzlingly, if I actually remove the N2-Ph-X coupling in my model, Madgraph will automatically calculate the three body decay width of the N2 decaying to SM leptons, the same final states as the N has. In other words, I expect N2 to have both 2-body decays to Ph + X and the same 3-body decays as N and N3. I suspect there is something amiss somehow with Madgraph's attempt to include N2 -> Ph + X in the width, and that Madgraph gives up on 3-body decays afterward for some reason.
I can ask Madgraph to just generate the decay widths I'm interested in directly, e.g. "generate N2 > Ph X," and Madgraph runs and outputs events with no problem. But the automatic calculations seem to not work correctly when I want to generate other events.
My questions are:
1. Why is it that Madgraph will sometimes ignore certain kinematically allowed decay channels for particles (but not ignore them for heavier and lighter generations of the same kind of particle), and is this an intentional behavior?
2. Why can disabling a coupling actually cause Madgraph to calculate a particle's decay width through other, previously ignored, decay channels?
3. If all of this is the correct behavior, is there a way to have Madgraph calculate the widths of particles automatically without needing to do separate Madgraph runs just decaying the particles?
I am happy to provide more details or model files if needed. I apologize if this is asked in the wrong place.
Thank you in advance,
-Niral
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