  Understanding the charged current quasielestic (CCQE) neutrino-nucleus
interaction is important for precision studies of neutrino oscillations. The
theoretical description of the interaction depends on the combination of a
nuclear model with the knowledge of form factors. While the former has received
considerable attention, the latter, in particular the axial form factor, is
implemented using the historical dipole model. Instead, we use a
model-independent approach, presented in a previous study, to analyze the muon
antineutrino CCQE mineral oil data published by the MiniBooNE collaboration. We
combine the cross section for scattering of antineutrinos off protons in carbon
and hydrogen, using the same axial form factor for both. The extracted value of
the axial mass parameter $m_A = 0.84^{+0.12}_{-0.04} \pm {0.11} \, {\rm GeV}$
is in very good agreement with the model-independent value extracted from
MiniBooNE's neutrino data. Going beyond a one-parameter description of the
axial form factor, we extract values of the axial form factor in the range of
$Q^2=0.1...1.0$ GeV$^2$, finding a very good agreement with the analogous
extraction from the neutrino data. We discuss the implications of these
results.
