Hi Costas,

Since we didn't have a chance to discuss this further this morning:

> a point was made before that no one actually runs with free nucleon
> targets. A change to reproduce ANL/BNL in the default tune is by
> itself unwarranted if not supported by comparisons to nuclear data.
> So we need to study the suggested model configurations against all
> data, before deciding whether we want to introduce a similar change
> in one or more configurations (default or not).
>
> The issue was not unknown to GENIE before the Rodrigues paper, and
> their tuning can not be copied into GENIE as a host of things have
> changed wrt to the version of GENIE used by Rodrigues et al. So we
> need to investigate our own independent solution. For one, Rodrigues
> et al have not investigated -I think- the impact of their tune on
> other data/MC comparisons where GENIE did well.

I'll note that anecdotally that the correction Rodrigues et al. suggest 
via the RvnCC1pi knob does improve agreement with both MINERvA and NOvA 
ND data.

But I suppose whether it is the right thing to do or not also depends on 
how the current tuning of the underlying parameters originated.  If the 
underlying parameters were only tuned with bubble chamber data in the 
first place, or if they correspond simply to free nucleon parameters, 
then there's basically no reason not to update them, even if they will 
be corrected again with a more sophisticated tuning effort later.

Which leads me back to the questions from before: what parameters do the 
R{v,vbar}{n,p}{CC,NC}{1,2}pi knobs actually correspond to?  Are there 
even corresponding parameters that one could directly change in the 
configuration to get the same effect as these reweight knobs?  Or are 
they somehow related to the Bodek-Yang model parameters that set the DIS 
production rates?  Does that mean there is overlap with the BY reweight 
knobs?  I haven't been able to figure any of this out from reading the 
code or the documentation, so I was hoping for somebody to offer the 
history.

I'll echo the comment that Gabe made further down in this thread.  The 
reweighting infrastructure is critical to our Fermilab experiment users, 
at least, because it's traditionally the only mechanism they use to 
obtain uncertainties on the neutrino interaction parameters.  (You know 
at least as well as I do, I think, that they can't do a Professor-style 
generation with different configurations because they can't afford to 
re-do the expensive detector response simulation and reconstruction 
stages they need to get fully simulated events.)  So if the knobs that 
we provide are unreasonable, don't correspond to changes that we can 
identify with parameters we want to change, or overlap with other knobs, 
I think we're offering the community misinformation and have something 
of a duty to clean them up.


-Jeremy