First direct observation of the tau neutrino

Abstract

History I n 1930 W. Pauli, in a famous letter to his colleagues, postulated the existence of a new particle the neutrino. Pauli presented the neutrino as an explanation ofthe observation that the decay of the neutron produced a continuous B-spectrum. Since then, great progress has been made in particle physics to describe the most fundamental constituents and interaction of matter. Nevertheless, many fundamental questions about neutrinos need to be answered. For example: Are neutrinos massive? Do they have a magnetic moment? Do they oscillate? Does a sterile neutrino exist? Is there direct evidence for the existence ofthe tau neutrino? The last question was finally answered in July 2000 by a group of 52 physicists from the DONUT Collaboration (Ferrnilab experiment E872) at Ferrnilab near Chicago. The tau neutrino was postulated in 1975 after the discovery of the tau lepton by M. 1. Perl et al. as the third generation neutrino in the Standard Model ofelectroweak interactions [1]. Glashow, Salam and Weinberg proposed the Standard Model in the late 1960s as a unified theory of the electromagnetic and weak interactions based on the gauge group SU(2) xU(l). This model has been successful in describing all recent experimental observations. Neutrinos enter this model as massless, neutral fermions. From measurements ofthe ZO decay width, it was determined that there are three light neutrinos in nature. The charged and neutral leptons in the Standard Model are represented by three left-handed doublets ofweak isospin.