[Rivet] DISLepton Class fails to find outgoing lepton

[Andrii Verbytskyi]

Hi Marian, Andy,

0) The event contains loops. It is better to generate events with Sherpa
with HEPMC_TREE_LIKE=1 or so. See docs.  Have you used it?


1)
On Tue, 2017-11-14 at 16:43 +0000, Andy Buckley wrote:
> Hi Andrii,
> 
> If the particle status codes are not 1 or 2, there is no guarantee
> that HepMC vertices correspond to physical processes. They may just be
> bookkeeping devices, e.g. to absorb parton shower recoils (or in this
> case perhaps the QED radiation treatment) -- this wouldn't be a Sherpa
> bug, but a valid use of the freedoms in the HepMC standard. So we
> can't have projection code that assumes particular vertex structures,
> because there will always be such edge-cases.

In general you are right, but not for electron in DIS that is coming
from hard process. The only thing one expects from it is e->e+gamma  or
e-> W nu ( not in Rivet anyway). But no hadronisation vertices.


Sherpa mixes everything together, in one vertex.
I haven't said that is a bug, but a *problem*. A problem for kinematics
reconstruction.




> Can you explain what the logic of your DIS lepton finder is? (The code
> is not super-easy to follow.) Hopefully an understanding of the
> intention will help us to find a safer definition.

The logic comes from  DIS definition: scattered lepton is the beam
electron that went through e->e gamma/Z0 or e-> W nu vertices.


Best regards,
Andrii

> Thanks,
> Andy
> 
> 
> On 14 November 2017 at 16:32, Andrii Verbytskyi
> <andrii.verbytskyi at desy.de> wrote:
> > Hi Marian, Andy,
> >
> > 0) I see no attachment. It is hard to guess which kind of process will
> > produce electron+ some leptons from a single electron.
> > If that is not SM process, the code was not designed to handle BSM.
> > 1) Yes, it is complicated with Sherpa. It merges too many things
> > together in one vertex. Just skip events with no electron/bad electron.
> > 2) Everything "works" in "some way" with some conditions.
> > 3) I cannot say without looking at the event that this is Sherpa
> > problem, but I suspect it is, so it is not clear if any fix is needed.
> >
> >
> > Best regards,
> > Andrii
> >
> >
> > On Tue, 2017-11-14 at 15:06 +0000, Andy Buckley wrote:
> >> Hi Marian,
> >>
> >> Thanks for the report. I've CC'd Andrii, who provided that updated
> >> DISLepton logic. Andrii, can you comment on how we can fix this
> >> behaviour in a generator-independent way?
> >>
> >> Andy
> >>
> >>
> >> On 10 November 2017 at 16:32, Marian Heil <marian.heil at durham.ac.uk> wrote:
> >> > Dear rivet authors,
> >> >
> >> > I think I found a bug in the DISLepton class in rivet version 2.5.4: It does
> >> > not find the outgoing lepton for a DIS scattering generated with Sherpa (the
> >> > corresponding HepMC file is attached).
> >> >
> >> > As far as I understand it in DISLepton::project the iteration over all
> >> > vertices and fails on the vertex "-6", because there are two outgoing
> >> > leptons ("10009" and "10015"). The first electron loops over vertex "-5"
> >> > back into vertex "-6" (for what ever reason), so it is not an actual final
> >> > state particle. The old code from version 2.5.3 does actually work for the
> >> > event.
> >> >
> >> > Cheers,
> >> > Marian
> >> >
> >> >
> >> > _______________________________________________
> >> > Rivet mailing list
> >> > Rivet at projects.hepforge.org
> >> > https://www.hepforge.org/lists/listinfo/rivet
> >>
> >>
> >>
> >
> >
> 
> 
>