The 2015 Nobel Prize in Physics was co-awarded to Art McDonald at the Sudbury Observatory for experimental confirmation of this phenomenon. For 10 points each:
[10m] Name this phenomenon described by the PMNS matrix. It was proposed to resolve a problem in which the Homestake experiment observed a much-lower-than-expected flux of a certain particle from the Sun.
ANSWER: neutrino oscillations [or neutrino flavor oscillation; prompt on neutrino mixing or lepton mixing]
[10e] Due to neutrino oscillation, Homestake observed only a third of expected neutrinos of this flavor. During beta decay, one of these negatively-charged particles is emitted along with a neutrino.
ANSWER: electron [or electron neutrinos]
[10h] The bicycle model and puma model are theories of neutrino oscillation that violate this property, as part of the Standard Model Extension. Physicists observe ultra-high energy cosmic rays to detect violations of this continuous property.
ANSWER: Lorentz invariance [or Lorentz invariant, or Lorentz symmetry; prompt on Lorentz covariance or Lorentz covariant; prompt on invariant or invariance]
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