Neutrino-pair bremsstrahlung and the number of lepton generations

Abstract

Neutrino pair creation in bremsstrahlung processes of the type$l \to l{\text{ }}v{\text{ }}\bar v$ 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$R_{v\bar v} = \sigma [1\mathop \to \limits_A l(v\bar v)]/\sigma [1\mathop \to \limits_A l'(v\bar v)]$ (wherel, l′ are distinct charged leptons). It is sensitive to the number of neurino types and their couplings in the same way that the ratio$R_{q\bar q} = \sigma [e^ + e^ - \to {\text{hadrons}}]/\sigma [e^ + e^ - \to \mu ^ + \mu ^ - ]$ is to those of quarks. In the Weinberg-Salam model withN lepton generations, the ratio$R_{v\bar v}$ is approximately given by$[(N + 4) + 4(1 - 4\sin ^2 \theta _W )]/8$.