= Christenson-Cronin-Fitch-Turlay kaon CP violation experiment
{c}
<James H. Christenson>, <James Watson Cronin>, <Val Logsdon Fitch> and <René Turlay> reported the result in 1964. At Brookhaven's AGS, 30 GeV <protons> struck beryllium; <collimators> selected a neutral <particle beam> at 30 degrees. A downstream helium decay volume reduced interactions.<Evidence for the 2π Decay of the K₂⁰ Meson by James H. Christenson and others>[p.138, preparation and helium volume]{ref}
The flight distance suppressed the short-lived <kaon> component. Each arm of a two-arm <magnetic spectrometer> had <spark chambers> before and after its magnet. <Cherenkov detectors> and <scintillation counters> triggered photographic recording in coincidence.https://www.nevis.columbia.edu/~haas/documents/physics_nobel_lectures/physics71-80-3.pdf[Fitch, The Discovery of Charge-Conjugation Parity Asymmetry, pp.598–600, apparatus and reconstruction]{ref}
\Image[/neutral-kaon-beam-and-two-track-reconstruction.png]
{title=From a neutral <kaon> beam to a two-track <invariant mass>}
{description=Original functional schematic, not a scale drawing. The apparatus functions follow the 1964 paper p.138 and Fitch's lecture pp.598–600. The lower example is dimensionless synthetic <four-momentum conservation>, not a historical event.}
The helium sample yielded $45\pm9$ signal events; $(2.0\pm0.4)\times10^{-3}$ was the ratio to all charged <decay channels>.<Evidence for the 2π Decay of the K₂⁰ Meson by James H. Christenson and others>[pp.139–140, forward peak and charged-mode ratio]{ref} That denominator must be retained when comparing this result with a total <branching fraction>.
The evidence was kinematic. Later regenerated-beam <quantum interference> established that the short- and long-lived states reached the same two-<pion> final state, excluding alternatives such as distinct pion-like particles.https://www.nevis.columbia.edu/~haas/documents/physics_nobel_lectures/physics71-80-3.pdf[Cronin, CP Symmetry Violation — The Search for Its Origin, pp.570–573, original inference and subsequent interference]{ref} The 1964 observation established evidence for <CP violation>; it did not determine every parameter of the <CKM matrix>. Inferring <time-reversal symmetry> violation from <CP violation> invokes <CPT symmetry>, rather than a direct comparison of time-reversed transitions in that experiment.
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