Narrow width approximation for small width particles

Hi,

My first guess is that your UFO model has the width of x hardcoded to a given value (likely 0 or 1).
In principle if it is the case, the default param_card should have a comment for that parameter like
: 0.0
instead of a variable name.

You should also see some warning in the log of the computation such that you try to modify an moot parameter.

If it is not that, I would need to know which BSM model you are using in order to reproduce the issue

Cheers,

Olivier

> On 3 Jan 2021, at 12:50, Shuer Yang <email address hidden> wrote:
>
> New question #694769 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/694769
>
> Dear MG Authors
> We are trying to study exotic decay at future Z factory in the framework of some BSM by using MG5_2.7.3 (Actually we firstly use the version 2.6 which suffer the small width calculation problem). I have read the post https://answers.launchpad.net/mg5amcnlo/+faq/3053 . But I am still confused about the handle on small width particles.
> 1) I generate e+ e- > Z > x vm vm~, x > mu+ mu- at the Z pole Ecm =91 GeV . Here, the x is a new particle with small width and mx =5 gev. I set the width of mx as 9.00e-5 and get the xsection 0.000345. Furthermore, I found that when I set the width of x to auto or some value such as 0 or 0.5e-6*mx, there is no obvious change. It is wired.
> 2) I generate e+ e- > Z > x vm vm~ and get the xsection 0.223. I expect the xsection of it times the branching ratio of x > mu mu Br =2.44e-5 should equals to the xsection 0.000345 of the above process under narrow width approximation. But they are very different. I wonder whether my realization is correct.
> 3) I generate e+ e- > z > x vm vm~ and set madspin on. And then set decay x > mu+ mu. I get an error PRWIDTH(-1)=ZERO Error: Symbol ‘zero’ at (1) has no IMPLICIT type. I have no idea that how to fix this problem.
>
> I would be grateful if you could provide some help.
> I am looking forward to your reply.
> Best regards,
> Shuer
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Dear Oliveier
Thanks for your quick reply.
I used the axion like particle effective field theory model (linear ). Please kindly find the attached model file and some tag files generated by MG5.

I generate e+ e- > z > x vm vm~ again and set madspin on. And then set decay x > mu+ mu. I find that even if I set the width of ax to a value, it is still set to 0 in the calculation as recored in the tag file.

In addition, I generate e+ e- > z , z > x vm vm~ and use madspin to decay x to mu mu. And I get a warning "WARNING: For consistency, the width of particle 9000005 (ax) is changed to 0.0." and a report as " 1) A massive s-channel particle has a width set to zero."
As you pointed out, these problems might be because that the width of x is hardcoded to 0. But I have no idea that how to turn off the hardcode and modify the width of x as a assumped value.

Best regards,
Shur

At 2021-01-04 03:45:52, "Olivier Mattelaer" <email address hidden> wrote:
>Your question #694769 on MadGraph5_aMC@NLO changed:
>https://answers.launchpad.net/mg5amcnlo/+question/694769
>
> Status: Open => Answered
>
>Olivier Mattelaer proposed the following answer:
>Hi,
>
>My first guess is that your UFO model has the width of x hardcoded to a given value (likely 0 or 1).
>In principle if it is the case, the default param_card should have a comment for that parameter like
>: 0.0
>instead of a variable name.
>
>You should also see some warning in the log of the computation such that
>you try to modify an moot parameter.
>
>If it is not that, I would need to know which BSM model you are using in
>order to reproduce the issue
>
>Cheers,
>
>Olivier
>
>> On 3 Jan 2021, at 12:50, Shuer Yang <email address hidden> wrote:
>>
>> New question #694769 on MadGraph5_aMC@NLO:
>> https://answers.launchpad.net/mg5amcnlo/+question/694769
>>
>> Dear MG Authors
>> We are trying to study exotic decay at future Z factory in the framework of some BSM by using MG5_2.7.3 (Actually we firstly use the version 2.6 which suffer the small width calculation problem). I have read the post https://answers.launchpad.net/mg5amcnlo/+faq/3053 . But I am still confused about the handle on small width particles.
>> 1) I generate e+ e- > Z > x vm vm~, x > mu+ mu- at the Z pole Ecm =91 GeV . Here, the x is a new particle with small width and mx =5 gev. I set the width of mx as 9.00e-5 and get the xsection 0.000345. Furthermore, I found that when I set the width of x to auto or some value such as 0 or 0.5e-6*mx, there is no obvious change. It is wired.
>> 2) I generate e+ e- > Z > x vm vm~ and get the xsection 0.223. I expect the xsection of it times the branching ratio of x > mu mu Br =2.44e-5 should equals to the xsection 0.000345 of the above process under narrow width approximation. But they are very different. I wonder whether my realization is correct.
>> 3) I generate e+ e- > z > x vm vm~ and set madspin on. And then set decay x > mu+ mu. I get an error PRWIDTH(-1)=ZERO Error: Symbol ‘zero’ at (1) has no IMPLICIT type. I have no idea that how to fix this problem.
>>
>> I would be grateful if you could provide some help.
>> I am looking forward to your reply.
>> Best regards,
>> Shuer
>>
>> --
>> You received this question notification because you are an answer
>> contact for MadGraph5_aMC@NLO.
>
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>know that it is solved:
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>
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>following page to enter your feedback:
>https://answers.launchpad.net/mg5amcnlo/+question/694769
>
>You received this question notification because you asked the question.

Thanks Olivier Mattelaer, that solved my question.

Hi Shuer,

I am wondering how did you solve the problem with the error PRWIDTH(-1)=ZERO when you were trying to run Madspin. I have the same error and I have no idea how to fix it. I read the conversation above but I am not sure if I understand what I have to do.

Thanks by advance.

The first point to check is if your width is set to zero due to a restriction of your model:
For that you have to check if you have a file restrict_default.dat in your UFO model.
(or restrict_xxxx.dat if you import the model via import model MOdELNAME-xxxx)

If that width is set to zero in that file then changing that value might fix the issue.

Otherwise the bet is to recreate the UFO model within FeynRules. If this is not an option, you have to read the UFO convention paper to add new parameter to the model by hannd.

Cheers,

Olivier

Thanks Olivier. I understood now.