In the leading-logarithmic picture, a shower may be viewed as a sequence of branchings . Here is called the mother and and the two daughters. Each daughter is free to branch in its turn, so that a tree-like structure can evolve. We will use the word `parton' for all the objects , and involved in the branching process, i.e. not only for quarks and gluons but also for leptons and photons. The branchings included in the program are , , , and . Photon branchings, i.e. and , have not been included so far, since they are reasonably rare and since no urgent need for them has been perceived. Furthermore, the branching is intimately related to the issue of the hadronic nature of the photon, which requires a much more sophisticated machinery to handle, see section .
A word on terminology may be in order. The algorithms described here are customarily referred to as leading-log showers. This is correct insofar as no explicit corrections from higher orders are included, i.e. there are no terms in the splitting kernels, neither by new processes nor by corrections to the ones. However, it would be grossly misleading to equate leading-log showers with leading-log analytical calculations. In particular, the latter contain no constraints from energy-momentum conservation: the radiation off a quark is described in the approximation that the quark does not lose any energy when a gluon is radiated, so that the effects of multiple emissions factorize. Therefore energy-momentum conservation is classified as a next-to-leading-log correction. In a Monte Carlo shower, on the other hand, energy-momentum conservation is explicit branching by branching. By including coherence phenomena and optimized choices of scales, further information on higher orders is inserted. While the final product is still not certified fully to comply with a NLO/NLL standard, it is well above the level of an unsophisticated LO/LL analytic calculation.