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• PYTHIA (Sjostrand, Lund) (JETSET)

• Events are considered in the CM frame of initial

• partons, z is a energy fraction in this frame

• t = ln (ma2 /L2) Q2evol = ma2

• mass evolution

• Angular ordering imposed at each branching

• 1st shower branching º ME of 3- jets

• Obs.: Initial State Radiation X Final State Radiation

• the shower structures are ¹

• INITIAL STATE: The sequence of partons from the shower

• initiator to the one that enters the hard interaction is

• characterized by Q2 = - m2 > 0

• Use ``backward evolution '' [HERWIG, PYTHIA, ISAJET]

• FINAL STATE: All partons have m2 ³ 0; parton coming from

• hard interaction has greatest mass, branching into others with

• smaller mass and energy until every one is on the mass shell