We develop a general formalism for describing the eν and μν semileptonic decays of arbitrary unpolarized particles, and apply it in a discussion of the dilepton events produced in neutrino experiments. The formalism holds for mesons, baryons, or heavy leptons, with arbitrary spin and quantum numbers, decaying into exclusive or inclusive channels. Results are obtained for single-particle decay distributions. Specific examples of experimental interest are evaluated explicitly; in particular, for the decay Y-->Keν, we calculate the Ke invariant-mass distribution and all single-particle momentum distributions transverse to either a plane or a line. To obtain the distributions of secondary leptons from the decaying Y particles in neutrino processes, we adopt the current-fragmentation quark-parton model for Y-meson production. Distribution broadening from Y transverse momentum is calculated. Estimates of the Y-particle mass are obtained from experimental transverse-momentum distributions of the slow muon in neutrino dimuon events. The true dimuon rate is estimated to be 2-3 times larger than the observed rate, because of the experimental acceptance cutoff at low muon energy.