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

P. Skands
CR in PYTHIA
pT-Ordered Model (in PYTHIA 6.4): Colour Annealing
Consider each color-anticolor pair
If (reconnect), sever the color connection
Different variants use different reconnect probabilities
Fundamental string-string reconnect probability PARP(78)
Enhanced by either nMPI (Seattle type) or local string density (Paquis type)
For all severed connections, construct new color topology:
Fig. 2: Type I colour annealing in a schematic scattering. Black dots: beam remnants. Smaller dots:
Consider the parton which is currently “furthest away” (in λ) from all others
gluons emitted in the perturbative cascade. All objects here are colour octets, hence each dot must be connected to
two string pieces. Upper: the first connection made. Lower: the final string topology.
M. Sandhoff & PS, in hep-ph/0604120
“Sees” the sum of the others → connect it to the closest severed parton to it.
where runs over the number of colour-anticolour pairs (dipoles) in the event, , is the
Strike it off the list and consider the next-furthest parton, etc.
invariant mass of the ’th dipole, and is a constant normalisation factor of order the hadronisation
scale. The average multiplicity produced by string fragmentation is proportional to the
logarithm of . Technically, the model implementation starts by erasing the colour connections
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