  Elastic neutrino scattering on electrons is a precisely-known purely leptonic
process that provides a standard candle for measuring neutrino flux in
conventional neutrino beams. Using a total sample of 810 neutino-electron
scatters after background subtraction, the measurement reduces the
normalization uncertainty on the muon neutrino NuMI flux between 2 and 20 GeV
from 7.5% to 3.9%. This is the most precise measurement of neutrino-electron
scattering to date, will reduce uncertainties on MINERvA's absolute cross
section measurements, and demonstrates a technique that can be used in future
neutrino beams such as LBNF.
