I am working with histfactory, and I would like to add a systematic uncertainty on the signal yield.
To give more explanation: for the time being, I just have a normalization factor corresponding to my signal strength which is allowed to float in my fit and is my parameter of interest. So my total signal yield is given by:
N_{tot} = \mu * N_{0}
In order to do that, I’m using the AddNormFactor function of histfactory.
What I would like to do is to add a second term in the formula above, such as to have :
N_{tot} = \mu * N_{0} + N_{1} * \theta
Where N_{1} is equal to a constant (fixed by me before the fit) and \theta is a nuisance parameter with a Gaussian constraint.
I was thinking to use the AddOverallSys function of histfactory, but it does not correspond to what I would like to do.
Would you please give me a hint on how I could do that?
I don’t think this is directly possible in the HistFactory, but at the end the HistFactory builds a workspace and in principle you can modify the function defining the overall normalisation by a function as above. You might able to do this using the EDIT function of the workspace, or otherwise using a script to modify your model.
Otherwise another possibility if you just need to add an extra term, you could consider it as an additional background component to your signal.
Thank you so much for answering!
Your first option sounds very good to me.
However, I’m not sure to understand your second option, because here, I would like to add a nuisance parameter to the signal yield. So adding an other background would be equivalent to do this?
I’m very sorry to come back to this topic again, but I would have one follow-up and one more question if I may:
I can’t modify my model because it’s not possible to overwrite objects in my workspace once it’s created: with EDIT I can create a clone with a different name, but not overwrite what is already there.
When you say a “flat background component”, is it really a flat background in the observable I’m considering, or do you mean that I should add a background with the same shape as my signal, but with +N_{1} events in every bin?
Thank you very much again for the very helpful comments.
Best regards,
Nathan