set decay time in the right place for the antiparticle in Pythia8PtGun

[slava77]

oddly enough, before this fix the not only the anti-particle doesn't decay,
but also the decaying particle comes out with ctau=0 (prompt decays).

K-gun with decays allowed:
Before:

eo->Scan("recoGenParticles_genParticles__GEN.obj.pdgId():recoGenParticles_genParticles__GEN.obj.vx():recoGenParticles_genParticles__GEN.obj.vy():recoGenParticles_genParticles__GEN.obj.vz():recoGenParticles_genParticles__GEN.obj.pt()")
***********************************************************************************
*    Row   * Instance * recoGenPa * recoGenPa * recoGenPa * recoGenPa * recoGenPa *
***********************************************************************************
*        0 *        0 *      -321 * 0.1056949 * 0.1692507 * 1.3009399 * 60.489666 *
*        0 *        1 *       321 * 0.1056949 * 0.1692507 * 1.3009399 * 60.489666 *
*        0 *        2 *       -13 * 0.1056949 * 0.1692507 * 1.3009399 * 26.971918 *
*        0 *        3 *        14 * 0.1056949 * 0.1692507 * 1.3009399 * 33.517959 *

After:

e->Scan("recoGenParticles_genParticles__GEN.obj.pdgId():recoGenParticles_genParticles__GEN.obj.vx():recoGenParticles_genParticles__GEN.obj.vy():recoGenParticles_genParticles__GEN***********************************************************************************
*    Row   * Instance * recoGenPa * recoGenPa * recoGenPa * recoGenPa * recoGenPa *
***********************************************************************************
*        0 *        0 *      -321 * 0.1052539 * 0.1674712 * -4.651619 * 52.327102 *
*        0 *        1 *       321 * 0.1052539 * 0.1674712 * -4.651619 * 52.327102 *
*        0 *        2 *       -13 * 14.896501 * -98.68802 * 8.3533515 * 36.057064 *
*        0 *        3 *        14 * 14.896501 * -98.68802 * 8.3533515 * 16.271556 *

note vx,vy,vz for muons
(both +13 and -13 show up).

[cmsbuild]

A new Pull Request was created by @slava77 (Slava Krutelyov) for CMSSW_9_0_X.

It involves the following packages:

GeneratorInterface/Pythia8Interface

@cmsbuild, @govoni, @perrozzi, @thuer, @davidlange6 can you please review it and eventually sign? Thanks.
@agrohsje, @mkirsano this is something you requested to watch as well.
@slava77, @smuzaffar you are the release manager for this.

cms-bot commands are listed here #13028

[slava77]

@cmsbuild please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/16571/console

[cmsbuild]

+1
Tested at: 4763674
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-16743/16571/summary.html

[cmsbuild]

Comparison job queued.

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-16743/16571/summary.html

[perrozzi]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_9_0_X IBs (tests are also fine). This pull request requires discussion in the ORP meeting before it's merged. @slava77, @davidlange6, @smuzaffar