[Halld-offline] [EXTERNAL] new HOWTO on changing the standard particle decay branching ratios in hdgeant4

[Richard Jones]

Hello Mariana,

Yes, you are right.  Changing the decay tables in hdgeant4 should not be
considered if you can already achieve what you need by simply restricting
the allowed decay modes when you generate the final-state particles at the
primary vertex.  However that approach is not adequate in circumstances
like the one I showed in the HOWTO example. In that case, one of the
products from the primary vertex is a neutral kaon, and we were looking to
reconstruct it in the final state where the k0 decays to two pi0. In that
case you cannot attach the two pi0 from the k0 decay to the primary vertex
at the event generator stage because the k0 will travel a
measurable distance in the detector away from the Sigma+ decay vertex
before it decays. Along the way there are material effects that might
scatter the k0 or interfere with its decay distribution through
regeneration. I am not sure how well the currently selected G4 physics
lists simulate the effects of regeneration on a k0 in flight, but here I
just argue from a general principle. If the decay happens in 10 microns or
less from the primary vertex then you might as well do the decay in the
event generator and spare yourself the trouble.  But if the decay happens a
detectable distance from the primary vertex or if there are material
effects between the primary and secondary decay vertices that might change
the momentum or the internal state of the particle, you will want to let G4
do the decay.

-richard jones

On Wed, Dec 1, 2021 at 10:37 AM Mariana Khachatryan <marianak at jlab.org>
wrote:

> *Message sent from a system outside of UConn.*
>
> Hi Richard,
>
> as I understand this is an alternative to using decay_evtgen.
> What are the advantages of using one or the other?
>
> Thank you,
> Mariana.
>
> On Nov 30, 2021, at 3:18 PM, Richard Jones <richard.t.jones at uconn.edu>
> wrote:
>
> Hello all,
>
> Alexander pointed out that my wiki page "HOWTO  force hdgeant4 to decay
> particles in certain channels" was somewhat cryptic and hard to follow.
>
> I have done a rewrite of the page, with an example session you can try to
> test it out.
>
>
> https://halldweb.jlab.org/wiki/index.php/HOWTO_force_hdgeant4_to_decay_particles_in_certain_channels
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__nam10.safelinks.protection.outlook.com_-3Furl-3Dhttps-253A-252F-252Fhalldweb.jlab.org-252Fwiki-252Findex.php-252FHOWTO-5Fforce-5Fhdgeant4-5Fto-5Fdecay-5Fparticles-5Fin-5Fcertain-5Fchannels-26data-3D04-257C01-257C-257C8f9080bab2004a88e38208d9b4e06af4-257C17f1a87e2a254eaab9df9d439034b080-257C0-257C0-257C637739698223750721-257CUnknown-257CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0-253D-257C2000-26sdata-3DbDubbnuBrdkLuZ-252BU8EpTpdKzt6MqQYT3sqq2APUhb-252Fg-253D-26reserved-3D0&d=DwIFaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=Te_hCR4EUlJ6iCDYLJ8Viv2aDOR7D9ZZMoBAvf2H0M4&m=ME6ku2YGRP00W-B5RqpDjp0tk9vf-KxkXZmHAjOf1iIibDw9zWRB5v4y8dq-X566&s=sxbRvSvgCuAIwWuzX5tgKgvQ49zDNfnY93RoStp_H54&e= >
>
> -Richard Jones
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