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This is in reference to a MC generator called Pythia, but has wider applicability because the phrases are used widely in particle phyiscs, even though I have not managed to find a clear, understandable definition.

I have come across the phrase "hard process" multiple times while trying to study proton-proton collisions. My understanding of it is that any non-diffractive, non-elastic collision is a hard process. Or in loose terms, probably any event where something new is produced. Is that correct?

Secondly, Pythia coins another term - "second hard process", which too I don't understand.

Also, I am assuming that all these terms pertain only to QCD processes. Is that true?

Thanks :)

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Boy, that term gets thrown around in a number of ways.

In a MC generator context it sometimes means "everything but radiative corrections", but I don't know if that is the way the authors of Pythia mean it.

On that assumption a "second hard process" would be a final state interaction that is modeled separately of the corrections; re-scattering in a nuclear environment or similar.

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  • $\begingroup$ What do you mean by a final state interaction? $\endgroup$
    – Man
    Commented Feb 13, 2015 at 17:17
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    $\begingroup$ D'oh! That's another term that varies a bit, but if you are writing a naive generator you often start by writing a model for some set of processes and then applying corrections. Corrections associated with times after the "main" process are final state interactions (i.e. interactions with the final state of the process). But the big problem here is that I am not deeply familiar with Pythia and don't actually know what nomenclature that authors have in mind. I wouldn't have answered except we have few particle experimenters and I didn't want you to go unanswered. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Feb 13, 2015 at 17:24

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