Progress on the Pythia 8 event generator

Multiparton interactions (MPI’s)
Many parton-parton interactions
per pp event: MPI.
Problem:
∫∫∫
σ int =
Most have small p ⊥ , ∼ 2 GeV
⇒ not visible as separate jets,
but contribute to event activity.
Solid evidence that MPIs play
central role for event structure.
dx 1 dx 2 dp 2 ⊥ f 1(x 1 , p 2 ⊥ ) f 2(x 2 , p 2 ⊥ ) dˆσ
dp 2 ⊥
= ∞
since ∫ dx f (x, p 2 ⊥ ) = ∞ and dˆσ/dp2 ⊥ ≈ 1/p4 ⊥ → ∞ for p ⊥ → 0.
Requires empirical dampening at small p ⊥ ,
owing to colour screening (proton finite size).
Many aspects beyond pure theory ⇒ model building.
Torbjörn Sjöstrand <strong>Progress</strong> on the Pythia 8 event generator slide 15/46