Origin of the Forward Pi0s in Pythia 6.22 and 6.4

During the analysis meeting at Austin, we presented a result from Pythia+GSTAR that shows vastly different energy slope for the forward Pi0s depending on the version of Pythia. I also commented that most of the forward Pi0s seem to be coming from the breaking of strings formed by two beam remnants. I was asked to verify my statement, especially regarding version 6.22, and this page is meant to address it.

Both versions of Pythia were run with the following setting.

  int msel=0;
  int isub=29;
  float ckin3=0.0;
  float ckin4=-1.0;
  float ckin56=1.0;

  processes included:11, 12, 13, 28, 53, 68, 94, 95, 14, 18, 29, 114, 115 (This is basically all 2->2 hard QCD + all prompt photons + double diffractive + low pT)

Example Pythia particle record of a qg->qg process. (process id=28)

LINE 0 & 1: Beam 1 and 2

j=2 pdg=-2  mo1=0  mo2=27  e=3.817349 px=1.135377 py=1.058231 pz=3.487581                --> LINE 2: Beam 1 Initial State before ISR
j=3 pdg=2   mo1=1  mo2=27  e=59.963612 px=0.409830 py=0.474441 pz=-59.960335          --> LINE 3: Beam 2 Initial State before ISR
j=4 pdg=21 mo1=2  mo2=27  e=1.747272 px=1.641245 py=-0.597236 pz=-0.050801             --> LINE 4: Beam 1 Initial State after ISR
j=5 pdg=2   mo1=3   mo2=27  e=54.623997 px=0.373336 py=0.432193 pz=-54.621014         --> LINE 5: Beam 2 Initial State after ISR
j=6 pdg=21 mo1=-1  mo2=27 e=3.355057 px=-2.782330 py=-0.048495 pz=-1.874219           --> LINE 6: Final State 1 before FSR
j=7 pdg=2   mo1=-1  mo2=27  e=53.016212 px=4.796911 py=-0.116549 pz=-52.797596       --> LINE 7: Final State 2 before FSR 

Initial State before ISR --> Initial State after ISR --> Final State before FSR --> Many Final State daughters from FSR --> String --> Pi0

Pythia Version 6.22

Fig. 1. (Estring-parent / Epi0) vs. (String Parent-pi0 angle) for all parents of the string that produced our Pi0

Fig. 2. Initial State x-momentum for the incoming beam in GeV. FPD ES has positve x coordinate.   

Fig. 3. Correlation between initial state px and Pi0 px

Fig. 4. Initial state px for HS and BR (break down of Fig. 2)

Fig. 5. Correlation between ~xB(initial state px/100GeV) vs. energy of Pi0

Fig. 6. Correlation between ~Z(Epi0/Eparent) vs. energy of the Pi0

Fig. 7. Y-axis projection of Fig. 6

Fig. 8.

Fig. 9.  Process Fraction. BLUE: Hard Scattering Total, GREEN: qg->qg, MAGENTA: qq->qq, YELLOW: gg->gg, RED: Beam Remnant

Pythia Version 6.4

Fig. 10.

Fig. 11.

Fig. 12. Process Fraction. BLUE: Hard Scattering Total, GREEN: qg->qg, MAGENTA: qq->qq, YELLOW: gg->gg, RED: Beam Remnant