Width calculation error

Hi,

Did you look at the README of the model?
I do not know that model, but it is likely that you set the coupling of h to two photon to zero.
You can look at the log of the computation to see the value of that coupling.

Federico the author of that model might be more specific

Cheers,

Olivier

Hi Dylan,

I've just generated and launched x0 > a a (you don't need to specify QCD=0 for this) with the latest version of MG5 and the model, and I haven't encountered any problem.

However I'm confused from your message.

1) Are lambda and cosa really the only non-zero parameters in the frblock of the param_card?
If this is true, I'm not surprised, since then it means that you have turned *off* all the x0-photons interactions by setting kHaa and kAaa =0.
So you get a zero width, as the error message says.

2) kHda does not enter the x0-a-a interaction, it enters the x0-a-z interaction, so I don't understand how it can affect your result (I've checked it is not among the couplings in the x0>aa code).
I agree this is not intuitive, but I'm not the one who chose the nomenclature of the parameters and, in any case, before playing with the parameters you should learn what they mean.
Look for example at eq. (2.4) in the first HC paper http://arxiv.org/abs/1306.6464 (which is cited in the README along with the other papers).
The only parameters that affect the x0-a-a interaction are cosa, kHaa and kAaa.

Let me know if this solves your problem,
cheers,
Federico Demartin

Hello Federico,

I would like to respond to each point. To point 1: The only other non-zero param is kHda. I am trying to calculate widths with just kHda. To point 2: In the reference you provided http://arxiv.org/abs/1306.6464 in the Lagrangian (eq 2.4), there is a term kHdyZ... and this article http://arxiv.org/abs/1311.1829 in the Lagrangian (eq 1) they have the term kHdyA... So perhaps there is a mistake in one of the articles which may fix what corresponds to what. Also on that note I followed your recommendation and checked the width for x0 > z a, with the previous values for lambda and cosa along with kHda = 1. I got a number with an order of magnitude of 10^-36, which seems very low. Can this be correct? All of calculations I got had the smallest order of 10^-7. Thank you for your help.

Best,

Dylan

Hi Dylan,

in the UFO at NLO model "HC_NLO_X0", that I've implemented, I'm sure that kHda belongs to the x0-a-z interaction and not to the x0-a-a one.
You can easily check this by looking into the UFO model:
couplings.py -> you can see that kHda enters only the coupling GC_3006h1,
vertices.py -> you can see that GC_3006h1 enters only the interaction vertex V_3006, which connects particles a, z and x0.

So, as I said before, the only parameters that affect the x-a-a interaction are cosa, kHaa and kAaa.
Since you said that both kHaa and kAaa were set to zero, this means that all the x0-a-a interactions have been turned off and the partial width x0>aa is zero (10^-36 is just a funny way the machine returns a zero, to me), so both the UFO model and madgraph are working correctly.

A different question is whether this damn parameter kHda has actually been put in the right place, i.e. if it is supposed to belong to the x0-a-a interaction or to the x0-a-z interaction.
As you said, there is an inconsistency between the two Lagrangian's in http://arxiv.org/abs/1306.6464 and http://arxiv.org/abs/1311.1829 .
So, I've downloaded the latest version of the original LO UFO model "HC" (which has *not* been implement by me, but by other authors of the first HC paper) and, looking into it, I find that kHda belongs to the x0-a-z interaction, same as in my "HC_NLO_X0" model.
Therefore, it is likely that there is a typo in the second HC paper http://arxiv.org/abs/1311.1829 and not in the first one, since no fix has been implemented in the "HC" UFO model.

To be sure, I suggest to contact one of the authors of http://arxiv.org/abs/1311.1829 (Kentarou Mawatari) and ask him about this inconsistency between the Lagrangian's in the two papers and what kHda is supposed to do.
Please put me in cc, so that I can be informed about this point as well.

Cheers,
Federico

Btw, please ignore my comment when I said "(10^-36 is just a funny way the machine returns a zero, to me)", since I was talking about the x0>aa width and not the x0>az width you've checked after.

In fact, I confirm that for x0>za the kHda interaction with those parameters returns a width of 10^-36 which, if correct, just means that this kHda interaction is rather irrelevant...
I'll do some extra checks just to make sure this result is correct.

F.

I confirm I find 10^-36 also using the HC model, so this means to me that kHda is practically irrelevant to the x0-z-a interaction... F.

Hello Federico,

Thank you for your response again. But the problem still persists. I have checked for X0 > z a, kHda = 1 where I have got 10^-36. You checked it for X0 > a a. Here is a part of my previous reply: "Also on that note I followed your recommendation and checked the width for x0 > z a, with the previous values for lambda and cosa along with kHda = 1. I got a number with an order of magnitude of 10^-36, which seems very low. Can this be correct? All of calculations I got had the smallest order of 10^-7."

Best regards,

Dylan

Hello Dylan,

apparently my chain of replies was confusing, so let me recap in 4 key points:

(1)
"I am trying to calculate the decay width of the process H to gamma gamma."
kHda enters the x0-a-Z interaction, not the x0-a-a one, so it does not affect your process.

(2)
If you generate x0 > a a and set
kHaa=kAaa=0,
then the width is zero (you turned off all the relevant interactions).
The only parameters that affect the x0-a-a interaction are cosa, kHaa and kAaa.

(3)
If you generate x0 > a Z and set the only non-zero parameters to
Lambda=10^3, cosa=1 and kHda=1,
then you get a width of O(10^-36), which is ridiculously low. In fact, it is just a weird way the code returns a zero.

(4)
If you are not sure where kHda should belong (i.e. which Lagrangian is correct between http://arxiv.org/abs/1306.6464 and http://arxiv.org/abs/1311.1829 ), ask Kentarou Mawatari and let me know his reply.

EXTRA COMMENT ON (3):
If you are able to compute tree-level Feynman diagrams, you can easily check the fact in (3) by computing analytically the amp^2 for the part of interaction vertex X0-a-Z proportional to kHda.
The expression is extremely simple and you can find it putting together the relevant pieces in vertices.py, couplings.py and lorentz.py (to understand the syntax look also to http://arxiv.org/abs/1108.2040 ).
You will see that the amp^2 is proportional to the photon virtuality p_A^2, and thus it is ZERO if the photon is on-shell.
This happens for example in your case: you compute the decay x0 > a Z, where all the 3 particles in the vertex are external and so on-shell.

I think I've given you a very extensive and complete reply to all your points (already in my previous comments), so to me the case is closed.
Best,
Federico

Dear Federico,

Thank you very much for your response. You have answered my questions. Also, here is the response I got from Kentarou Mawatari:

My initial email:

Hello Dr. Mawatari,

My name is Dylan Gray and I am doing some research. I came across a problem to which I posted on the MadGraph5 Launchpad forum, here is the link: https://answers.launchpad.net/mg5amcnlo/+question/274924. My question is, does the kHda coupling belong to H > gamma gamma or H > z gamma? The confusion comes from the differences in Lagrangians in these two papers: http://arxiv.org/abs/1306.6464 eq. 2.4 and http://arxiv.org/abs/1311.1829 eq. 1. The kHdy terms are different. Thank you in advance for your response.

Best regards,

Dylan Gray

His response:

Dear Dylan,

eq.2.4 in 1306.6464 is correct (i.e., eq.1 in 1311.1829 has a typo...)
since there is no derivative term for H-photon-photon. Very sorry
for causing this confusion. We will correct the arxiv (1311.1829) version.

Best wishes,
Kentarou

Hi Dylan,

thanks for the follow-up.
I'm glad that the expression coded in HC_NLO_X0 is indeed the correct one and I don't have to update the model! :-)

In the end, before you contacted me, I've never investigated the physics behind that 6-dimensional x0-a-z interaction...
so, I'm also happy because we have learned that the kHda term in the interaction cannot be constrained by measuring the Higgs decay to Z gamma. I think it probably needs processes like VBF and VH, where the photon is not a real particle but an internal off-shell propagator, for that interaction not to be zero.

Best wishes,
Federico