The Simple Shower


Up until the end of the PYTHIA 8.2 version these simple shower codes were the only ones distributed as part of the PYTHIA package. They were originally only called TimeShower and SpaceShower, with no further qualification necessary. In version 8.240 (December 2018) a transformation was begun where this physics code was transferred to SimpleTimeShower and SimpleSpaceShower, while TimeShower and SpaceShower remained as bare-bones base classes. This paved the way for the inclusion of the VINCIA and Dire showers as core parts of the PYTHIA 8.3 distribution.

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 better coherence and higher-logarithmic 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

When the Time/SpaceShower became SimpleTime/SpaceShower the related settings names were retained, for reasons of backwards compatibility of user code, e.g. in command files. Thus shower setting names beginning with TimeShower:, SpaceShower:, WeakShower:, UncertaintyBands: or HiddenValley: refer to the current baseline "simple" showers.

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: