[Rivet] Born and dressed level in Rivet

[Ulla Blumenschein]

Hi Andy,

On Tuesday, October 8, 2013, Andy Buckley wrote:

> On 08/10/13 15:52, Ulla Blumenschein wrote:
> > Hi David,
> >
> >
> > On Tuesday, October 8, 2013, David Grellscheid wrote:
> >
> >     Hi Ulla,
> >
> >         We didn't identify the FSR photons experimentally but we
> >         unfolded to a
> >         reference where a  lepton was dressed with the respective
> technical
> >         FSR implementation.
> >
> >
> >     I'm sorry, I don't understand fully what you did there. The
> >     "reference" you refer to is a certain MC run with a certain setup?
> >     In that case isn't your unfolding dependent on that MC author's
> >     choice of where to put photon emissions in that specific version of
> >     the code?
> >
> >
> > we depend on at least three authors of three generators' choices.
> >
> >
> >         We checked that we get a similar correction in the
> >         standard generators, (pythia, herwig, sherpa..).
> >
> >
> >     That doesn't mean it makes physical sense to split FSR from ISR
> photons.
> >
> >
> > No of course not. We are talking here merely in terms of dominating and
> > negligible contributions to the collinear photon radiation.
>
> Recent QED modelling studies by Steve Muanza showed that there is a
> substantial "ISR" QED structure in Z->l+l- events only in Pythia8 (with
> or without Photos++ to produce the QED FSR). We didn't see the same
> structure in invariant masses for Sherpa or Herwig++. Was Py8 included
> in the three models used to derive the dressing correction?
>
>
No, only Pythia6 so far.


> In Steve's study there was still relatively little difference between
> that and other models once observables were constructed from IR-safe
> clustering (and there we/he explicitly excluded photons from hadron
> decays as discussed here), so I suspect that as suggested the FSR will
> turn out to be dominant.
>
> If there is no significant distinction between the two approaches, then
> this sort of separation of photon origins is, I think, preferable to the
> dependence so far on particular generators' schemes for representing QED
> FSR. It's a lot more portable (unless a generator standard
> representation is agreed upon -- good luck!) and I think physically less
> contentious.
>
>
I personally agree.

Cheers, Ulla


> Cheers,
> Andy
>
> --
> Dr Andy Buckley, Royal Society University Research Fellow
> Particle Physics Expt Group, University of Glasgow / PH Dept, CERN
>


-- 
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    Ulla Blumenschein
    II Physik, Uni Goettingen
    Friedrich-Hund-Platz 1, D01.110
    phone: 0049-551-397645
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