Simple Showers

## The Simple Shower

### Introduction

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 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

- Matrix elements corrections for the first ("hardest") gluon emission in most two-body resonance decays, effectively making the FSR in these decays NLO accurate.
- There is no corresponding NLO accuracy for ISR in any processes, but several examples where reasonably accurate kinematics spectra are available over the full phase space, by input of partial higher-order information.
- The default dipole-recoil scheme for FSR can be switched to a global-recoil option for the first few emissions, in order to simplify matching and merging to higher-order calculations (e.g. as done in MadGraph_aMC@NLO).
- The default global-recoil scheme for ISR can be replaced by a dipole-recoil scheme, where the other colour dipole end may be in the final state (as needed e.g. for DIS studies).
- Showers off massive objects, within and beyond the Standard Model, including e.g. octet onium states.
- Showers interleaved with multiparton interactions, and set up to handle two predefined hard interactions.
- QED showers, where photons can be emitted and then branch into fermion pairs that shower further.
- Weak radiation of W^+- and Z^0 off fermions.
- Radiation also in some hadronic decays.
- Possibility to handle both abelian and nonabelian showers in a hidden valley sector, where relevant fully interleaved with normal QCD and QED radiation.
- A wide selection of further switches and parameters to vary shower assumptions: running of alpha_s, p_Tmin value, scale choices, gluon polarization effects, mass effects in g → q qbar, etc.
- Most existing tunes to LEP and LHC data are based on this choice of showers.

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:

- The final-state Timelike Showers cover all aspects of QCD and QED FSR.
- The initial-state Spacelike Showers cover all aspects of QCD and QED ISR.
- While the main switches for weak radiation of W^+- and Z^0 are found in the two previous FSR and ISR pages, there also a few common technical Weak Showers settings.
- There is a special framework to produce uncertainty bands from Automated Variations of basic parameters, such as factorization and renormalization scales, or choice of parton distributions.
- The settings for final-state showers in a Hidden Valleys are stored along with the switches for such hard processes.
- There is a wide selection of Matching and Merging approaches that have been implemented so as to work well with these showers.
- Tunes that include ISR and FSR parameters are described on the Tunes page.
- The shower evolution can be interrupted or modified with the help of User Hooks.