Madgraph auto decay width calculations not working, returns 0 even when decay should be possible.

Asked by Niral Desai on 2020-12-11

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

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Niral Desai
Solved:
2020-12-18
Last query:
2020-12-18
Last reply:
2020-12-16

Did you check the associated paper?

> 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.

what is the value returned by MadEvent in that case? Is this above or below QCD scale?
One likely reason for you trouble is that your partial width are below QCD scale and therefore the auto-width set them to zero since your particle will hadronise before going trough such decay.

Cheers,

Olivier

> On 11 Dec 2020, at 08:55, Niral Desai <email address hidden> wrote:
>
> New question #694464 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/694464
>
> 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
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Niral Desai (npd393) said : #2

Hi Olivier,

> what is the value returned by MadEvent in that case? Is this above or below QCD scale?
One likely reason for you trouble is that your partial width are below QCD scale and therefore the auto-width set them to zero since your particle will hadronise before going trough such decay.

Madgraph gives a 2-body width for the N2 of 0.0002757 ± 1.2e-08 GeV for the parameters I described when I manually ask it to generate N2 > ph X decays. However, I'm able to calculate the width of N automatically with no problem, which decays to just SM final states and has a width of 3.070405e-14 GeV, much smaller than the N2's width (the small width should be correct). None of my particles directly couple to QCD, I'm not sure I understand what you mean here, since none of them should hadronise.

I checked the paper associated with Madgraph, arxiv:1405.0301, if that's what you mean, but I wasn't able to find an answer to my question. To reiterate, what I am confused about is why Madgraph is not looking at decays of N2 to N2 > Ph + X, and why it's also ignoring the 3-body decay of N2 > 3 SM states when it doesn't ignore this for N. As I understand it, Madgraph should check 2-body decays first and if it finds none of relevance, it moves onto 3-body decays. So the two things I am confused about is why Madgraph isn't seeing the 2-body decays to my new physics particles that should be allowed, and even if it ignores those, why it doesn't see the 3-body decays to SM particles next.

I apologize if I missed something potentially obvious or something that was explained elsewhere.

Thank you,
-Niral

Hi,

The paper where the auto-width computation is described is here:
https://arxiv.org/pdf/1402.1178.pdf <https://arxiv.org/pdf/1402.1178.pdf>

> None of my particles directly couple to QCD, I'm not
> sure I understand what you mean here, since none of them should
> hadronise.

Ok so this is not your issue if your particles does have a QCD coupling.

Could you send me the model (<email address hidden> <mailto:<email address hidden>>) and the benchmark point such that I can take a look?

Cheers,

Olivier

> On 11 Dec 2020, at 18:41, Niral Desai <email address hidden> wrote:
>
> Question #694464 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/694464
>
> Status: Answered => Open
>
> Niral Desai is still having a problem:
> Hi Olivier,
>
>> what is the value returned by MadEvent in that case? Is this above or below QCD scale?
> One likely reason for you trouble is that your partial width are below QCD scale and therefore the auto-width set them to zero since your particle will hadronise before going trough such decay.
>
> Madgraph gives a 2-body width for the N2 of 0.0002757 ± 1.2e-08 GeV for
> the parameters I described when I manually ask it to generate N2 > ph X
> decays. However, I'm able to calculate the width of N automatically with
> no problem, which decays to just SM final states and has a width of
> 3.070405e-14 GeV, much smaller than the N2's width (the small width
> should be correct). None of my particles directly couple to QCD, I'm not
> sure I understand what you mean here, since none of them should
> hadronise.
>
> I checked the paper associated with Madgraph, arxiv:1405.0301, if that's
> what you mean, but I wasn't able to find an answer to my question. To
> reiterate, what I am confused about is why Madgraph is not looking at
> decays of N2 to N2 > Ph + X, and why it's also ignoring the 3-body decay
> of N2 > 3 SM states when it doesn't ignore this for N. As I understand
> it, Madgraph should check 2-body decays first and if it finds none of
> relevance, it moves onto 3-body decays. So the two things I am confused
> about is why Madgraph isn't seeing the 2-body decays to my new physics
> particles that should be allowed, and even if it ignores those, why it
> doesn't see the 3-body decays to SM particles next.
>
> I apologize if I missed something potentially obvious or something that
> was explained elsewhere.
>
> Thank you,
> -Niral
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

fixed offline (model issue)

Niral Desai (npd393) said : #5

Thank you very much Olivier! The widths are being calculated automatically now and with no problems.

In the unlikely event anyone else has a similar problem, here was the (very simple) fix: I was editing my mg5_aMC model files by hand and left in the default SM decays.py file. Just deleting that file fixed the issue.

Thank you again,
-Niral