The Underlying-Event Model in PYTHIA (6&8) - Peter Skands - Cern

P. Skands
pQCD
2→2
= Sum of
≈ Rutherford
(t-channel gluon)
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MPI a la PYTHIA
Multiple Perturbative Parton-Parton Interactions
A) Start from perturbative model (dijets) and extend to IR
Multiparton interactions
Regularise cross section with p ⊥0 as free parameter
IR Regularization
dˆσ
dp 2 ⊥
with energy dependence
Energy Scaling
∝ α2 s (p 2 ⊥ )
p 4 ⊥
p ⊥0(E CM) =p ref
⊥0 ×
→ α2 s (p 2 ⊥0 + p2 ⊥ )
(p 2 ⊥0 + p2 ⊥ )2
ECM
Eref ɛ CM
Matter profile in impact-parameter space
gives time-integrated overlap which determines level of activity:
simple Gaussian or more peaked variants
Normalize to
ISR total and cross MPI section: compete for beam momentum → PDF rescaling
+ flavour effects (valence, qq pair companions, . . . )
+ Resum/Unitarize → Probability
+ correlated primordial k⊥ and colour in beam remnant
for a 2→2 interaction at xT1 =
Many partons produced close in space–time ⇒ colour rearrangement;
reduction → This is of now total our string basic length (UV & ⇒IR) steeper 2→2 cross 〈p section
⊥〉(nch) See, e.g., new MCnet Review: “General-purpose event generators for LHC physics”, arXiv:1101.2599
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