Progress on the Pythia 8 event generator

An x-dependent proton size – 2
Convolution of two incoming protons gives impact parameter shape
Õ(b; x 1 , x 2 ) = 1 (
1
π a 2 (x 1 ) + a 2 (x 2 ) exp b 2 )
−
a 2 (x 1 ) + a 2 (x 2 )
b 2 eik [fm]
1.2
1.1
1
0.9
0.8
a1 = 0.00
a1 = 0.15
a1 = 1.00
(b)
0.7
10 2 10 3 10 4 10 5
E CM [GeV]
norm
(1 / N) dN / db MPI
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
(a)
SG
DY
Z 0
Z’
0
0 0.5 1 1.5 2 2.5
norm
b MPI
Consequence: collisions at large x will have to happen at small b,
and hence further large-to-medium-x MPIs are enhanced,
while low-x partons are so spread out that it plays less role.
Torbjörn Sjöstrand <strong>Progress</strong> on the Pythia 8 event generator slide 20/46