[Rivet] DISLepton Class fails to find outgoing lepton

[Andy Buckley]

Unfortunately that DIS definition doesn't correspond to something
well-defined in HepMC. There is no standard for hard-process
interaction representation.

It would be good to know if this works with the Sherpa tree-like mode,
but generally we want to avoid such sensitivities. I am more inclined
to sacrifice some accuracy for robustness... and an observable
definition that cannot be reproduced is a fundamentally problematic
thing.

Andy


On 14 November 2017 at 17:05, Andrii Verbytskyi
<andrii.verbytskyi at desy.de> wrote:
> 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
>> >>
>> >>
>> >>
>> >
>> >
>>
>>
>>
>
>



-- 
Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow