[Rivet] Born and dressed level in Rivet

[Tim Martin]

Hi all,

I have tried to summarise this discussion for next weeks ATLAS
Higgs+Top+SM unfolding meeting.

Try and make everyone aware of the little problem and how we're
proposing to fix it.

For ATLAS people, the slides are on
https://indico.cern.ch/conferenceDisplay.py?confId=273212

Cheers, Tim.

On 9 October 2013 07:34, Ulla Blumenschein <ublumenschein at googlemail.com> wrote:
> 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
>
>
>
> --
> /_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
>     Ulla Blumenschein
>     II Physik, Uni Goettingen
>     Friedrich-Hund-Platz 1, D01.110
>     phone: 0049-551-397645
> /_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/



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