Matching and Merging

Starting from a Born-level leading-order (LO) process, higher orders can be included in various ways. The three basic approaches would be Given the pros and cons, much of the effort in recent years has involved the development of different prescriptions to combine the methods above in various ways.

The common traits of all combination methods are that matrix elements are used to describe the production of hard and well separated particles, and parton showers for the production of soft or collinear particles. What differs between the various approaches that have been proposed are which matrix elements are being used, how doublecounting is avoided, and how the transition from the hard to the soft regime is handled. These combination methods are typically referred to as "matching" or "merging" algorithms. There is some confusion about the distinction between the two terms, and so we leave it to the inventor/implementor of a particular scheme to choose and motivate the name given to that scheme.

PYTHIA comes with methods, to be described next, that implement or support several different kind of algorithms. The field is open-ended, however: any external program can feed in Les Houches events that PYTHIA subsequently showers, adds multiparton interactions to, and hadronizes. These events afterwards can be reweighted and combined in any desired way. The maximum pT of the shower evolution is set by the Les Houches scale, on the one hand, and by the values of the SpaceShower:pTmaxMatch, TimeShower:pTmaxMatch and other parton-shower settings, on the other. Typically it is not possible to achieve perfect matching this way, given that the PYTHIA pT evolution variables are not likely to agree with the variables used for cuts in the external program. Often one can get close enough with simple means but, for an improved matching, User Hooks can be inserted to control the steps taken on the way, e.g. to veto those parton shower branchings that would doublecount emissions included in the matrix elements.

Zooming in from the "anything goes" perspective, the list of relevent approaches actively supported is as follows.