[Rivet] DISLepton Class fails to find outgoing lepton

[Andy Buckley]

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.

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.

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