= Solution
Strike a target with high-energy <protons>, collect negative <pions> and <kaons> with magnetic horns, and let them decay in a long evacuated <decay tunnel>: $\pi^-\to\mu^-\bar\nu_\mu$ and $K^-\to\mu^-\bar\nu_\mu$. Absorb residual <hadrons> and range out <muons> in shielding. Focusing selects the negative parent sign and therefore enriches <muon antineutrinos>; other parent decays leave a finite contamination. Monitor the parent flux and use a near detector to constrain the unoscillated spectrum.
A massive segmented <calorimeter> supplies interaction probability and measures the <hadronic shower>. Alternate dense absorber plates with <scintillation counters> or tracking planes, and magnetize the iron or add a downstream magnetic <muon detector>. The signal is a positive <muon> plus a <hadronic shower>; the sign rejects much of the <muon neutrino> contamination. For a contained <muon>, its range gives <energy>; for a penetrating track, curvature gives $p_\perp[\mathrm{GeV}]=0.2998 B[\mathrm T]R[\mathrm m]$. Multiple measurements and a field map determine the fitted <momentum> and sign. Summed calibrated shower response estimates the struck <quark>'s visible <hadronization> energy, not the energy of a free observable <quark>. Correct for invisible nuclear losses, noncompensating response, leakage and acceptance. Neglecting target recoil corrections, $E_\nu\simeq E_\mu+E_{\rm had}$ and $y\simeq E_{\rm had}/E_\nu$.
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