ParticleGun Problem

What happens, when, and how to reproduce the problem:

==> Run a Pythia6PtGun to generate Z0->mumu, with pT[0,20]GeV/c, in CMSSW_3_3_4

* The python file is this.

* turned off initial/final state radiation, and modified the pythiaDefault_cff.py also to include mumu decay only, like this.

* run this analyzer with this python file.

* In the analyzer, look at the generated Z0 pT distribution (left figure bellow), and then, loop over the daughter muons, pair them, and retrieve the Z0 parent (right figure bellow).

Basically, what it happens, see also the bellow printout, is that though the parent Z0 has a p!=0, the muons are decayed back-to-back, as if the Z0 is at rest (the boost part of the decay was not propagated properly). It happens only with the Z0 (tried with JPsi and don't get into this issue), and it's not a PYTHIA problem (I ran PYTHIA 6-422, and it never gets into this situation, so the decayer itself is fine).

pt_z0Gen = 12.602                    px = -7.59486   py = 10.0563    pz = 254.636 p = 254.94 E = 268.55 mass = 84.3823
E_mu1 = 184.443 pt1=33.5841 px1 = 6.15032  py1 = -33.0161 pz1 = 181.36   eta1 = 2.38804 phi1 = -1.38662
E_mu2 = 84.1065 pt2=45.1056 px2 = -13.6769 py2 = 42.982    pz2 = 70.9885 eta2 = 1.23503 phi2 = 1.87887
pt_z0Dimu = 12.4888                px = -7.52661   py = 9.96591    pz = 252.348 p = 252.657 E = 268.55 mass = 91.0124

pt_z0Gen = 18.302                    px = -13.6041   py = 12.2437    pz = -2.07556 p = 18.4198 E = 92.4163 mass = 90.5621
E_mu1 = 46.6812 pt1=40.4225 px1 = -11.1639 py1 = 38.8504  pz1 = -23.3484 eta1 = -0.549529 phi1 = 1.85061
E_mu2 = 46.6812 pt2=40.4225 px2 = 11.1639  py2 = -38.8504 pz2 = 23.3484 eta2 = 0.549529 phi2 = -1.29098
pt_z0Dimu = 0                           px = 0               py = 0               pz = 0              p = 0            E = 93.3625 mass = 93.3625

pt_z0Gen = 13.9235                  px = 12.453      py = -6.22797   pz = 42.1019  p = 44.3445 E = 84.7429 mass = 72.2144
E_mu1 = 46.0783 pt1=14.1707 px1 = 4.48367  py1 = -13.4426 pz1 = -43.845 eta1 = -1.8477 phi1 = -1.24886
E_mu2 = 46.0783 pt2=14.1707 px2 = -4.48367 py2 = 13.4426  pz2 = 43.845  eta2 = 1.8477 phi2 = 1.89273
pt_z0Dimu = 0                           px = 0                py = 0              pz = 0              p = 0            E = 92.1566 mass = 92.1566