Neutrino-pair bremsstrahlung and the number of lepton generations

Neutrino pair creation in bremsstrahlung processes of the type l → lv anti v contains vital information on the number of lepton generations, and is catalyzed by the coherent nuclear Coulomb effect, or other forms of intense fields. Of particular interest is the ratio Rsub(v anti v) = sigma [lsub(A) → l(v anti v)]/sigma [lsub(A) → l'(v anti v)] (where l, l' are distinct charged leptons). It is sensitive to the number of neutrino types and their couplings in the same way that the ratio Rsub(q anti q) = sigma [e+ e- → hadrons]/sigma [e+e- → μ+μ-] is to those of quarks. In the Weinberg-Salam model with N lepton generations, the ratio Rsub(v anti v) is approximately given by [(N + 3/2) + 5(1 - 4sin2 thetasub(w))]/8cos2 thetasub(w). (orig.)