projectiles.](NuNu.eps){width="15cm"}

![\[fig:NuNuBar\]The permissible tree-level Feynman graphs for the elastic scattering of an electron neutrino (projectile) off the antineutrini of the electron, muon, and $\tau$-lepton generations of matter (targets). Graphs (a) and (b) relate to the $t$-channel scattering off the electron antineutrino, graph (a) via the exchange of the weak neutral current, graph (b) via the exchange of the weak charged current. Graphs (c)-(e) relate to the $s$-channel scattering off the electron antineutrino (annihilation graphs); in all such cases, only the weak neutral current is involved. Graph (f) relates to the $t$-channel scattering processes $\nu_e \bar{\nu}_{\mu,\tau} \to \nu_e \bar{\nu}_{\mu,\tau}$, where only the weak neutral current may be exchanged. Similar graphs are applicable in case of the muon and $\tau$-lepton neutrini as projectiles, save for graph (b) which cannot contribute in the energy range explored in this paper.](NuNuBar.eps){width="15cm"}

![\[fig:NuElPos\]The permissible tree-level Feynman graphs for the elastic scattering of an electron neutrino (projectile) off an electron and a positron (targets). Graphs (a) and (b) relate to the scattering off an electron, graph (a) via the $t$-channel exchange of the weak neutral current, graph (b) via the $u$-channel exchange of the weak charged current. Graphs (c) and (d) relate to the scattering off a 