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Simulation programs – also dubbed event generators – like Sherpa are indispensable work horses for current particle physics phenomenology and are (at) the interface between theory and experiment. In the following their necessity and some of their construction principles are briefly highlighted.

Precision tests of the Standard Model and the quest for new physics in many cases relies on the confrontation of theoretical calculations with experimental findings. But such a procedure is far from being trivial because

• many signals under investigation rely on non-trivial final states involving many particles;
• experimentally, these final states have to fulfill some (detector) cuts, leading to considerable difficulties when integrating over the appropriate phase space;
• the objects in the calculations are often quarks and gluons, whereas experimentally only hadrons, i.e. their bound states, are detected, which necessitates to model the quantitatively poorly understood (phase-)transition from the perturbative to the non-perturbative regime of QCD, the theory of strong interactions;
• apart from the already mentioned hadronization also detector effects wash out signals.

One of the most popular options to treat the theoretical difficulties sketched above lies in the usage of event generators. They are based on a kind of “divide et impera” (divide and rule) strategy. The key idea is to separate events - as they could be seen through the detector - into different stages according to some characteristic energy scale. Starting from the highest scales, i.e., the shortest distances, subprocesses are added which will populate different phases of particle emission and creation at lower scales or larger distances. Schematically this translates into

1. considering the production of heavy and/or highly energetic particles through appropriate matrix elements that are exact at some perturbative order and respect quantum interferences etc.,
2. radiating lighter particles - like for instance gluons and photons - which are softer and tend to be collinear with their emitters through the parton shower, which correctly resums the leading terms to all orders in perturbation theory,
3. the eventual decay of heavy unstable particles, again through matrix elements, again dressed by parton showers to fill the phase space of multiple soft emission in the perturbative regime,
4. the hadronization of the quarks and gluons into hadrons, which in many cases may not be stable,
5. their decays into long-lived, i.e. practically stable, hadrons that finally hit the detector.

For a pictorial representation, check this out.

However appealing, this nice picture is somewhat spoiled by various additional problems, for instance:

• more than often divergencies appear in (loop-level) calculations that cancel analytically when considering full perturbation theory but are extremely difficult to handle numerically;
• especially for strong interacting initial states effects like multiple interactions of beam remnants or further beam particles, rescattering and generally the pile-up of these effects contribute significantly to the final state multiplicity and are thus important, their qualitative and quantitative understanding, however, is quite limited.

These problems are - to some extent - currently investigated.