[EvtGen] Problem with EvtGen R01-07-00

[Martin Ritter]

Hi John,

of course, here you go: https://evtgen.hepforge.org/trac/ticket/1

I could not really fill out component/milestones as they are not 
configured yet but everything else worked fine.

Cheers,

Martin

On 09/03/18 15:35, John Back wrote:
> Hi Martin,
> 
> Thanks for your report and I agree with your assessments:
> 
> a) EvtPDL::getMass() needs to be replaced with EvtPDL::getMeanMass().
> 
> The getMass() function returns a mass sampled from the particle's 
> resonance shape, but we need the pole mass. I'm not sure how this 
> mistake was done, but I guess it was only tested on particles with very 
> small or zero widths, so getMass() would be the same as getMeanMass(), 
> and we should have done more testing.
> 
> b) For particles with Pythia id = 0, we should add a check in the
> entry_generic and entry_alias conditional statements to also require the 
> id to be non-zero. This will then avoid the issue of redefining the void 
> particle many times.
> 
> Could you please create a bug report for these issues on our HepForge 
> webpage:
> 
> http://evtgen.hepforge.org/trac/report/3
> 
> using the "guest" account (password = evtgen)? This is so that we can i) 
> test that our bug reporting feature works for users such as yourself and 
> we can ii) keep a record of the bug and its required fix. Let us know if 
> you are having problems creating the bug report.
> 
> Thanks,
>      John.
> 
> On 09/03/18 13:01, Martin Ritter wrote:
>> Dear EvtGen Developers,
>>
>> I was trying to use EvtGen R01-07-00 for Belle II and I found some
>> problems with the synchronization of particle properties between EvtGen
>> and Pythia which was introduced with the following commit:
>>
>>> commit 48a86cf7d8c45cf026beac84df6e204d6eb42b1d
>>> Author: John Back <J.J.Back at warwick.ac.uk>
>>> Date:   Fri Apr 7 16:03:30 2017 +0100
>>>
>>>     Make sure Pythia8 uses the same particle properties as EvtGen.
>>>     Modified EvtGenExternal/EvtPythiaEngine to ensure that the
>>> EvtGen-based
>>>     instances of Pythia8 (for generic and alias decays) use the same
>>>     particle properties as defined by EvtGen, courtesy Patrick Robbe
>>> (LHCb).
>>
>> I modified the evtgen code to provide some additional output (see
>> attached output.patch). As you can see from the attached log output
>> (evtgen_output.txt) this produces very weird results (with Pythia
>> 8.215):
>>
>> 1. The Pythia particle with id 0 (void) gets redefined ~160 times,
>>     probably because the particles are not known to Pythia.
>> 2. The assigned masses are not correct. For example in line 142 the rho0
>>     is set to 1.023 GeV. This is due to the fact that the code uses
>>     EvtPDL::getMass() instead of EvtPDL::getMeanMass().
>>
>> I was wondering if (1) is an effect of the Pythia version we use but I
>> haven't seen anything in the Pythia Changelog or in the EvtGen README so
>> maybe you know more.
>>
>> For (2) I really believe this is a bug in the code.
>>
>> Best Regards,
>>
>> Martin
>>

-- 
Dr. Martin Ritter

LMU München, Excellence Cluster Universe
Boltzmannstrasse 2, 85748 Garching

Tel: (+49) 89 35831-7152
Fax: (+49) 89 3299-4002