Simple Showers

Overview of recent changes

PYTHIA comes with a complete parton-shower machinery, but also allows external shower programs to be linked in to it, see the Implement New Showers page. Notably the VINCIA and DIRE codes have been structured to make use of this functionality. Currently these codes are distributed separately, but the intention is to integrate them into the PYTHIA distribution in the future.

Originally the TimeShower and SpaceShower classes implemented the default PYTHIA showers, but also acted as base classes from which the external showers derived. This has some disadvantages, so the two aspects are now split. The TimeShower and SpaceShower classes remain as simple base classes from which the actual showers are derived. The physics code has been moved to the new derived SimpleTimeShower and SimpleSpaceShower. An external shower that does not use any of the existing shower algorithms will therefore work as before, which would be the normal case, but alternatively a shower could of course derive from the new classes and then reuse relevant code in them.

Settings names have been retained, again for reasons of backwards compatibility of user code, e.g. in command files. Thus setting names beginning with TimeShower:, SpaceShower:, WeakShower: or UncertaintyBands: refer uniquely to the current baseline "simple" ones. In the future some of them may become common with VINCIA and DIRE, notably the uncertainty bands ones, whereas ones specific to those two programs will have names that spell it out.

The prepending of "Simple" was a minimalistic choice under the circumstances; more fancy names could have been chosen. What it refers to is that showers like VINCIA and DIRE aim higher, in striving to achieve full NLL accuracy, whereas the Simple ones operate in an improved LL approximation. In other respects the Simple showers can do more different physics than the other two, at least currently. Some examples of the broad approach are

Shower components

The Master Switches for ISR and FSR in general, and a switch for QED radiation in Particle Decays to leptons, are intended to be common for all shower programs, where applicable.

The full description of settings in the Simple Shower framework is spread across several pages: