[Rivet] electroweak results used to tune/validate theory

[Heidi Schellman]

One issue here - we do use photos in our MC, but that is partially 
because, we also have real photon radiation from the detector - which is 
almost certainly a larger effect.  At least for the total Z cross 
section, we found that if we use a .2 cone for electrons in D0, photos 
or not photos made very little effect in our results.

And we are going to quote for our fiductial PT, eta range, as well as 
for the full extrapolated range.



Emily Nurse wrote:
> Dear CDF and D0 electroweak convenors,
>
> You may already know about this, but incase you don't .... Rivet is a 
> toolkit for the validation and tuning of MC event generators. The idea 
> is to produce a routine for every relevant analysis from any 
> experiment that can be interfaced with any MC output and plot 
>  distributions that can be directly compared with the experimental 
> result. More information is here:
> http://projects.hepforge.org/rivet/
>
> We are currently including many Tevatron Run I and Run II 
> measurements. One important example is the Z pT measurement, which CDF 
> has a Run I result for and D0 a Run I and Run II result. All the 
> results have the feature that they use theoretical models to correct 
> the data. While this is unavoidable at some level it is really 
> preferable to keep this "contamination" of the data to an absolute 
> minimum.
>
> For example, all of these results have used Photos to correct the 
> di-lepton pT so that the measurement gives the true Z pT. This is of 
> course only true if Photos gets QED 100% correct, and it means that 
> for event generators that include QED we have to somehow un-include it 
> when we write the routine (by e.g. clustering back photons and 
> leptons, for both electrons and muons). While it may be desirable for 
> other reasons to correct for some theoretical effects, it would be 
> really helpful if all such publications also included a measurement of 
> what was actually observed. This is not only essential for us but also 
> any other theorists who want to make comparisons to the data.
>
> Since, for electrons, collinear photons are usually indistinguishable 
> in the calorimeter, this would probably mean publishing the dilepton 
> pT where an electron is a cluster of a certain size.
>
> The same point is also true when correcting for acceptance cuts, it 
> would actually be preferable to publish the dilepton pT for electrons 
> with |eta| < X etc, rather than relying on models of the lepton decay 
> distributions to correct for these cuts.
>
> We are keen to make sure that any future Tevatron results take this 
> approach to make using them for MC validation more robust. In general 
> data that has been interpreted using the best theoretical model at the 
> time age as the theory develops, but experimental results, corrected 
> only for detector effects, last forever.
>
> Please let us know your thoughts on this issue and whether you are 
> willing to make this change to future publications. 
>
> Best regards,
>
> Emily (for the Rivet team).
>