= Solution
The Fermilab primary <proton> beam strikes a target, producing secondary <pions> and <kaons>. Horn focusing selects positive parents; their decays $\pi^+\to\mu^+\nu_\mu$ and $K^+\to\mu^+\nu_\mu$ produce a predominantly <muon neutrino> beam. A <decay tunnel> permits decay before the remaining charged secondaries encounter an absorber. A near detector constrains the flux and interaction spectrum, while the distant <MINOS> detector measures its change.
The alternating <scintillation counters> and magnetized iron give a sampling <calorimeter> and a magnetic tracking volume. A <charged current> $\nu_\mu N\to\mu^-X$ event has a long negative <muon> track and a localized <hadronic shower>. Estimate $E_\mu$ by range if contained or magnetic curvature if exiting, and $E_X$ by the calibrated scintillator pulse-height sum. Reconstruct $E_\nu\simeq E_\mu+E_X$, with corrections for nuclear binding, invisible <energy> and detector response. The <muon> charge separates most wrong-sign <muon antineutrino> events; short showers without the track are primarily <neutral current> candidates. The source's $730\,\mathrm{km}$ baseline is used in the calculations below.
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