Heavy fourth-generation neutrino and flavour-changing neutral currents

We study the flavour-changing neutral currents in the case that the fourth-generation neutrino exists and the known three left-handed neutrino masses are at the experimental limits of the direct measurements. The fourth-generation neutrino has the mass of order a few ten GeV and the flavour-changing processes of the heavy neutrinos are expected to be observed onZ ⁰ ine ⁺ e ^? collisions. The heavy fourth-generation neutrino is significant to reveal the nature of the neutrino; Dirac or Majorana, the see-saw mechanism and the right-handed scale.