  We discuss how to reduce theoretical uncertainties in the neutrino-deuteron
breakup cross-sections crucial to the Sudbury Neutrino Observatory's efforts to
measure the solar neutrino flux. In effective field theory, the dominant
uncertainties in all neutrino-deuteron reactions can be expressed through a
single, common, isovector axial two-body current parameterized by $L_{1,A}$.
After briefly reviewing the status of fixing $L_{1,A}$ experimentally, we
present a constraint on $L_{1,A}$ imposed by existing reactor
antineutrino-deuteron breakup data. This constraint alone leads to an
uncertainty of 6-7% at 7 MeV neutrino energy in the cross-sections relevant to
the Sudbury Neutrino Observatory. However, more significantly for the Sudbury
experiment, the constraint implies an uncertainty of only 0.7% in the ratio of
charged to neutral current cross-sections used to verify the existence of
neutrino oscillations, at the same energy. This is the only direct experimental
constraint from the two-body system, to date, of the uncertainty in these
cross-sections.
