The MSW Effect in Quantum Field Theory

Recent results from the Super-Kamiokande experiment [1] have confirmed, with high statistics, the reality of the solar [2] and atmospheric [3] neutrino anomalies. In the case of the atmospheric neutrino anomaly, the SuperKamiokande data can be interpreted as providing evidence that the anomaly is due to neutrino flavor mixing [4], and “long baseline” accelerator and reactor neutrino experiments may be able to confirm this conclusion [5]. Data from Super-Kamiokande and the Sudbury Neutrino Observatory (SNO) eventually should allow tests to determine if the solar neutrino anomaly is also due to neutrino oscillations [6]. These developments are bringing to a critical test the possibility suggested by earlier observations of solar and atmospheric neutrinos—and by a persistent signal in an accelerator neutrino experiment, LSND [7]—that neutrino flavor mixing may provide one of the first experimental windows on physics beyond the Standard Model. In a generic neutrino oscillation experiment the event rate for the detection of νβ from a flux of να from a point source at distance L takes a form like the following: