The History of “Anomalous” Atmospheric Neutrino Events: A First Person Account

The modern picture of the neutrino as a multiple-mass, highly mixed neutral particle has emerged over forty years of study. Best known of the issues leading to this picture was the apparent loss of neutrinos coming from the sun. This article describes another piece of evidence that supports the picture; the substantial reduction of high-energy muon-type neutrinos observed in nature. For much of the forty-year period before the modern picture emerged, this observation was known as the “atmospheric neutrino anomaly,” since these neutrinos originate in the Earth’s atmosphere. This paper describes the discovery of the atmospheric neutrino anomaly. I explore the scientific context and motivations in the late 1970s, from which this work emerged. The gradual awareness that the observations of atmospheric neutrinos were not as expected took place in the 1983–1986 period.