* Pythia Tips

• MSEL (process selection): * MSEL = 0: compose subprocesses "a la carte", using the subprocess numbers (ISUB), setting MSUB(ISUB)=1. Example to activate subprocesses 14 and 18:

   MSEL=0
   MSUB(14)=1
   MSUB(18)=1

* MSEL = n: choose a fixed menu of subprocesses. For hadron-hadron collisions: * MSEL = 1: QCD high-pT processes (ISUB = 11, 12, 13, 28, 53, 68); additionally low-pT production if CKIN(3) < PARP(81) or PARP(82), depending on MSTP(82) (ISUB = 95). If low-pT is switched on, the other CKIN cuts are not used * MSEL = 2: all QCD processes, including low-pT, single and double diffractive and elastic scattering, are included (ISUB = 11, 12, 13, 28, 53, 68, 91, 92, 93, 94, 95). The CKIN cuts are not used. * MSEL = 4-8: qqbar production with massive matrix elements (ISUB = 81, 82, 84, 85). MSEL=4: ccbar; 5: bbbar; 6: ttbar; 7:b'b'bar; 8: t't'bar * MSEL = 16: h0 production (ISUB = 3, 102, 103, 123, 124)

• ISUB (subprocess number): * tree-level 2-2 QCD processes, O(alpha_s^2). All the matrix elements in this group are for massless quarks. As a consequence, cross sections are divergent for pT-->0 * ISUB = 11 : qi qj --> qi qj * ISUB = 12 : qi qibar --> qk qkbar * ISUB = 13 : qi qi --> g g * ISUB = 28 : qi g --> qi g * ISUB = 53 : g g --> qk qkbar * ISUB = 68 : g g --> g g * 96 semihard QCD 2 --> 2 * heavy-flavor production. The matrix elements in this group differ from the corresponding ones in the group above in that they correctly take into account the quark masses. * ISUB = 81 : qi qibar --> Qk Qkbar * ISUB = 82 : g g --> Qk Qkbar * ISUB = 1 : fi fibar --> gamma*/Z0 --> Fk Fkbar * ISUB = 2 : fi fjbar --> W+ --> Fk Flbar * prompt photon production * ISUB = 14 : qi qi bar --> g gamma * ISUB = 18 : fi fibar --> gamma gamma * ISUB = 29 : qi g --> qi gamma * ISUB = 114 : g g --> gamma gamma * ISUB = 115 : g g --> g gamma

• CKIN: kinematic cuts at generation level. Example to generate hard scatterings with 50 GeV < pThat < 100 GeV:

       CKIN(3)=50D0 
       CKIN(4)=100D0 
  

  * CKIN(1), CKIN(2) : range of allowed mhat = sqrt(shat) * CKIN(3), CKIN(4) : range of allowed pThat values for hard 2 --> 2 processes, with transverse momentum phat defined in the rest frame of the hard interaction.

Links

• Pythia web page

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