[EvtGen] Problem with EvtGen R01-07-00

[John Back]

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
>