In the shower formulation, the kinematics of each branching is given in terms of two variables, and . Somewhat different interpretations may be given to these variables, and indeed this is one main area where the various programs on the market differ. has dimensions of squared mass, and is related to the mass or transverse momentum scale of the branching. gives the sharing of the energy and momentum between the two daughters, with parton taking a fraction and parton a fraction . To specify the kinematics, an azimuthal angle of the around the direction is needed in addition; in the simple discussions is chosen to be isotropically distributed, although options for non-isotropic distributions currently are the defaults.
The probability for a parton to branch is given by the evolution
equations (also called DGLAP or Altarelli-Parisi [Gri72,Alt77]).
It is convenient to introduce
The splitting kernels
Persons familiar with analytical calculations may wonder why the `+ prescriptions' and terms of the splitting kernels in eq. () are missing. These complications fulfil the task of ensuring flavour and energy conservation in the analytical equations. The corresponding problem is solved trivially in Monte Carlo programs, where the shower evolution is traced in detail, and flavour and four-momentum are conserved at each branching. The legacy left is the need to introduce a cut-off on the allowed range of in splittings, so as to avoid the singular regions corresponding to excessive production of very soft gluons.
Also note that is given here with a factor in front, while it is sometimes shown with . The confusion arises because the final state contains two identical partons. With the normalization above, is interpreted as the branching probability for the original parton . On the other hand, one could also write down the probability that a parton is produced with a fractional energy . Almost all the above kernels can be used unchanged also for this purpose, with the obvious symmetry . For , however, the total probability to find a gluon with energy fraction is the sum of the probability to find either the first or the second daughter there, and that gives the factor of 2 enhancement.