Progress on the Pythia 8 event generator

Multiparton interactions modelling
Regularise cross section with p ⊥0 as free parameter
dˆσ
dp 2 ⊥
∝ α2 s (p 2 ⊥ )
p 4 ⊥
→ α2 s (p 2 ⊥0 + p2 ⊥ )
(p 2 ⊥0 + p2 ⊥ )2
with energy dependence
( ) ɛ
p ⊥0 (E CM ) = p⊥0 ref × ECM
ECM
ref
Matter profile in impact-parameter space
gives time-integrated overlap which determines level of activity:
simple Gaussian or more peaked variants
ISR and MPI compete for beam momentum → PDF rescaling
+ flavour effects (valence, qq pair companions, . . . )
+ correlated primordial k ⊥ and colour in beam remnant
Many partons produced close in space–time
⇒ colour rearrangement; reduction of total string length
⇒ steeper 〈p ⊥ 〉(n ch )
Torbjörn Sjöstrand <strong>Progress</strong> on the Pythia 8 event generator slide 16/46