Point of terminology: quarks aren't hadrons. Hadrons are particles composed of quarks.
For example, the pi+ decays first into a muon+ and a neutrino, then the muon decays into a positron (anti-electron) and an anti-neutrino?
A mu+ decays into a positron, an electron neutrino, and a muon anti-neutrino.
Hadrons decaying into leptons is thoroughly confusing to me, and implies that the former are composed of the latter somehow.
Well, no. Notice that in the pion decay you mentioned, an anti-muon and a muon neutrino were created. What happened was that when the pion decayed, the quark and antiquark--which were of different flavors, an up quark and an anti-down quark--annihilated into a virtual W+ boson. The W+ boson then "decayed" (manifested itself, really) in the form of a mu+ (i.e., an anti-muon) and a muon neutrino. One of these particles (the neutrino) carried one unit of "muon-ness", while the other (the mu+) carried negative one units of "muon-ness". The total muon-ness of the system was zero both before and after the pion decay.
It's the same deal with the decay of the mu+. The anti-muon-ness is conserved when it decays into the form of a muon anti-neutrino. It also emits a (virtual) W+ which instantly decays into a positron and an electron neutrino.
The conversion of a W+ into a positron and an electron neutrino (or a mu+ and a muon neutrino) is directly analogous to a high energy photon (i.e., a gamma ray) converting into an electron positron pair. The photon doesn't "contain" an electron and a positron prior to the conversion. Rather, at some point in time, the photon just connects to a single conserved electron current, doing a U-turn in time.
Hadrons are particles composed of quarks.That should never be typed by people who tranpsose letters in wodrs. >:)
-Eric