Compute width problem

Hi,

This is completely normal. You just need to be informed that some of the partial width are lower than QCD scale (and therefore can not be trusted)
Since those width can not be trusted (since QCD is not perturbative anymore)

Cheers,

Olivier

On 11 May 2015, at 18:41, Tiago Vale <email address hidden> wrote:

> New question #266787 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/266787
>
> Greetings,
>
> I'm trying to use the compute_widhts command to calculate the widths and branching ratios of a particle, using a parameter card from the UFO model VLQ, given in this link: http://feynrules.irmp.ucl.ac.be/wiki/VLQ. The command I write is "compute_widths bp --path=~/param_card.dat --output=~/output.dat". It takes some time to calculate and the writes the following message:
> "partial width of particle bp lower than QCD scale:0.0692829923736. Set it to zero. ((5, 25))
> partial width of particle bp lower than QCD scale:0.0650640086544. Set it to zero. ((5, 23))
> partial width of particle bp lower than QCD scale:0.039916998972. Set it to zero. ((-24, 6))
> Results written to ~/output.dat"
> The problem should not be the parameter card I use, because other people, using the same parameter card and the same version of MG5, have no problem at all. Sometimes it calculates the widths and BR, and then I tweak the xi parameters, it then gives me that message, and if I use the same parameters that I used in the start(that MG5 calculated) it gives me the same message anyway.
>
> I have tried a lot of things, like reinstalling MG5, removing and adding the UFO model again, and nothing worked...
>
> I would be very thankful for any help.
> Thank you for the attention.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

No, but it is not possible to be lower than qcd scale. Sometimes it gives that message for the same parameter card it previously calculated. And I checked with other people using the same parameter card, model and mg5 version and it calculates with no problem.

This question was expired because it remained in the 'Open' state without activity for the last 15 days.

looks like I miss your answer.

Do you mean that MG returns different number the first and second time that the code runs? If yes, could you indicate to me how to reproduce your problem?

Thanks,

Olivier

Well, I just run it twice and it gives me different outputs each time. I wouldn't know how to indicate you that, because it is something that just happens when I do things normally... And once it gives me that message it doesn't calculate anymore, even if I put the same parameters I started with. It's a weird thing.

Thank you for your effort :)

Hi Tiago,

The problem is that I do not reproduce your behavior.

I have imported the model and computed twice in a row the width for bp for the default benchmark point.
and in both case, it returns me 6.8219e+1.

Could you send me by email the benchmark point that you use?

Cheers,

Olivier