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##

Parton Distributions

The parton distribution function parameterizes the
probability to find a parton with a fraction of the beam energy
when the beam particle is probed by a hard scattering at virtuality
scale . Usually the momentum-weighted combination
is used, for which the normalization condition
normally applies.
The dependence of parton distributions is perturbatively
calculable, see section .

The parton distributions in PYTHIA come in many shapes, as shown in the
following.

**Subsections**

Stephen Mrenna
2007-10-30