Field-theoretical approach to coherence in neutrino oscillations - art. no. 013011

We study the conditions for the existence of neutrino oscillations in the f ield-theoretical approach which combines neutrino production and detection processes in a single Feynman graph. The ''oscillating neutrino" is represe nted by an inner line of this graph where, due to the macroscopic distance L between source and detector, the neutrino propagators for neutrinos with definite mass are replaced by the projection operators into the neutrino st ates on mass shell. We use as a concrete model reaction the neutrino source and detector as given in the LSND experiment and we carefully take into ac count the finite lifetime of the stopped muons which provide the <(upsilon) over bar>(mu), beam. We show that the field-theoretical approach provides a solid method to locate all possible conditions and allows one to separate unambiguously their different origins. Some of these conditions are indepen dent of L whereas others state that coherence is lost when L exceeds a cert ain "coherence length." Also it turns out that, at least in the concrete si tuation considered here, the concept of neutrino wave packets is not suppor ted by the field-theoretical approach for realistic experimental conditions , i.e., the neutrino energy spread is incoherent in origin.