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I have Pythia Monte Carlo (MC) samples where I can't understand the parton showering model. If I print out full decay chains from the events, each event contains multiple string objects with pdgId 92. There can be up to 19 strings which are exactly the same and have exactly the same further decay chains, but have different mothers. And this causes a problem that it's impossible to go up in the decay chain from each final state particle and find for example from which quark or gluon the particle is coming. It's possible to identify mothers of each particle up to the string, and then this string has multiple different mothers.

So how does this string-related part work in Pythia?

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Not sure if this would be better in Scientific Computation or not. It's not really about how to write some code but how to interpret the output of an existing one. Does it violate the FAQ's prohibition on "computational questions"? –  dmckee Oct 31 '12 at 14:21
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My guess is this is done on purpose simulating the assumed for the moment model of nature. After all quantum mechanically there is no real identity other than the quantum numbers. energy and momentum conservation laws allow for some discrimination, for example "forward going" particles are mainly coming from the corresponding forward quark, but in a probabilistic fashion, not a one to one fashion. –  anna v Oct 31 '12 at 15:57
    
@dmckee I think this is perfectly fine here because, as you said, it is really about interpretation, not implementation - and IMO it is quite a good question. (I wish I had some idea how to answer it, but I have only minimal experience with Pythia) –  David Z Nov 1 '12 at 17:42
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From what I have found so far, Pythia just combines all particles that go in the same direction (cone of particular radius) creating a string or cluster and then producing the shower from it. It is seen from the momenta of all mother particles of any string, that they have very close Eta and Phi even though they are coming from different particles.

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