Effective electromagnetic form factor of the neutrino

The conceptual difficulties affecting the definition of the neutrino electromagnetic form factor are briefly reviewed in the context of the standard model. It is then shown that the radiative corrections ${\Delta }^{(\nu ;l)}$($q^2$) and ${\Delta }^{(\nu ;h)}$($q^2$) relevant to ν-lepton and ν-hadron scattering in the renormalization scheme in which ${\sin }^2$ ${\mathrm{\theta }}_{\mathrm{W}}$≡1-${\mathrm{m}}_{\mathrm{W}}^2$/${\mathrm{m}}_{\mathrm{Z}}^2$, can be separated into two finite and gauge-invariant parts. One part ${\Delta }^{(\nu )}$($q^2$) is target independent and the associated function f($q^2$)≡-$q^2$ ${\Delta }^{(\nu )}$ ($q^2$)/(2$m_W^2$) can be interpreted as an effective electromagnetic form factor of the neutrino in the framework of the low-energy theory derived from the standard model at invariant mass scales ≪$m_W^2$. The observability of this quantity and its dependence on the top-quark mass and the neutrino flavor are discussed. Updated estimates of the hadronic contributions to ${\Delta }^{(\nu )}$($q^2$) are given. A strategy to search for ν structure beyond the standard model is outlined.