Tau neutrino mass

The Standard Model has no predictive power on the neutrino masses. However, a non-zero neutrino mass is predicted in many of the fashionable extensions of Standard Model such as compositeness, Grand Unified Theory, and the left-right symmetry model[58]. A sufficiently massive neutrino is also a popular solution to the solar neutrino and dark matter problems. Since there are only three generations of leptons, is the only potential candidate to solve these problems. The upper limit on the mass of is 31 MeV/c at 95%confidence level[59]; for comparison, the mass limits on and are 17 eV/c and 270 KeV/c, respectively[42]. In some models, an assumption is made for the mass hierarchy Given , is expected to be 206 MeV/c. Therefore, the limit on already exceeds the sensitivity of and limits in these models. A sensitivity to of 3 MeV/c corresponds to a sensitivity of 0.25 eV/c on . Measuring the neutrino mass, therefore, can be a sensitive probe of the phenomenon of neutrino mass.

The best probe of neutrino mass is the shape of the end-point mass spectrum of the hadronic decay products. One of the decay modes with sensitivity to is . In general, the mass spectrum of the hadronic system depends on the unknown intermediate resonance structure. Fortunately, the end-point mass spectrum[61][60] in this decay is dominated by the weak decay matrix element and the phase space factor , where is the mass of the hadronic system, and

The candidates for can be selected with an or tag with no photon above a certain threshold energy. Based on the experience with CLEO II, we expect a detection efficiency of . For a data sample of , this corresponds to at least 2000 events in the final sample which allows the detailed examination and fitting of the full mass spectrum. This is much more satisfactory than previous methods which are dominated by the highest mass event (which may actually be background). With the expected mass resolution of MeV/c, the sensitivity on is a few MeV/c.

While has historically been the favored decay mode to probe , there are several other candidate reactions which may play an important role in this area that are accessible with the detector upgrades proposed here. In Table we list several of these modes. In particular, the mode is attractive due to the large minimum hadronic mass in the decay. At the present time the CLEO experiment is unable to use this mode as it lacks the necessary particle identification. A phase space Monte Carlo of shows that only 1%of the decays have both kaons in the momentum interval GeV/c where kaons can be unambiguously identified in CLEO II. Raising the momentum cutoff to 3 GeV/c in CLEO III improves the acceptance to 90%. Thus with the proposed CLEO III improvements in high momentum kaon identification, the mode will compete with as the standard channel to use for measuring .