Question about warning related to partial width lower than QCD scale

Hi,

I'm not really expert here, my best two point of comparison is the b quark and the tau lepton.
For the b quark, we typically keep his width to zero and let the hadronization model handling it.
For the tau, we do decay it, but never use the BR that is calculated from the standard model, but effective BR where the tau decay directly to pion/hadron (and typically to three of them which I would not be able to explain by looking at the BR computed within the perturbative theory).

>By giving partial width manually from the theoretical calculation, it was able to generate samples which gave us consistent cross-sections. However, we are not sure if forcing to put values in the param_card.dat is meaningful because we know mg5 rejected those cases because it considers them as the cases which don't follow the perturbative QCD theory.

I guess it depends of what you mean by meaningfull. If you are sensitive to the fact that MG5 set those BR to zero then they are probably a lot of part of the simulation where you need to check the hyppothesis made and check if you do not have any issue.
In particular in the fit of the PDF and the fit of the hadronization model.

Now they are no problem per say to simply fix the width within the param_card, this is the point to have that as a free parameter such that one can use either the experimental value, the LO value, the most theoretical prediction or ... zero.

> Can we use that modified param_card.dat using the calculated width or it makes more sense that we say we can exclude our model in the region of the mg5 limitation?

This is likely a question that I do not have the answer, I can only recommend to be careful in the hypothesis made before doing any statement for such type of benchmark.

Cheers,

Olivier

It seems that the warning about small partial width is prompting you to set it to zero automatically. While you managed to achieve consistent cross-sections by manually providing the partial width, the concern is whether it's meaningful to do so, given that MG5 rejected those cases as non-perturbative.

Considering the warning and MG5's rejection criteria, it's crucial to assess the reliability of your model within the limitations of the MG5 framework. While forcing values in the param_card.dat may provide consistency in cross-sections, it's essential to acknowledge and potentially exclude the model in regions where it doesn't align with perturbative QCD theory. Seeking expert opinion and understanding the implications of the warning in the context of your specific model can guide the decision on whether to use the modified param_card.dat or adhere to MG5's limitations.