Neutrinos oscillate between flavors (electron, muon, tau) as they propagate. The probability depends on mixing angles, mass squared differences, and distance traveled.
Formula (2-flavor approx): P(νₑ→νμ) = sin²(2θ)·sin²(1.27·Δm²·L/E)
where L is distance in km, E is energy in GeV, Δm² in eV².
Physical context: Solar neutrinos (θ₁₂ ≈ 33.4°), reactor neutrinos (θ₁₃ ≈ 8.5°). The oscillation length L_osc = 4π·E/(Δm²·1.27). Nobel Prize 2015: Takaaki Kajita & Arthur McDonald confirmed neutrino mass via oscillation.