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【正文】 001||an d||)|(|21|)|(|21|kkCPkkCPkkkkkkIf CP is conserved in the decay of k1 and k2 then we expect the following decay modes: k1? two pions (p+p or p0p0 ) (CP = +1 states) k2? three pions (p+pp0 or p0p0p0 ) (CP = 1 states) In 1964 it was found that every once in a while (?1/500) k2?two pions ! This is just a QM system in two different basis. Mamp。S 250252 Spring 2022 L6 Richard Kass Neutral Kaons, Mixing, and CP violation The k0 and k0 are eigenstates of the strong interaction. These states have definite strangeness, are NOT CP eigenstates, and are particle/antiparticle. They are produced in strong interactions (collisions) . 000 or ???? kpkp ppThe k1 and k2 are eigenstates of the weak interaction, assuming CP is conserved. These states have definite CP but are not strangeness eigenstates. Each is its own antiparticle. These states decay via the weak interaction and have different masses and lifetimes. 0002022 or ppppp ?? kkIn 1955 (!) GellMann and Pais pointed out that the k1 and k2 lifetimes should be very different since there was more decay energy (phase space) available for k1 than k2. mk2mp ? 219 MeV/c2 mk3mp ? 80 MeV/c2 Thus, expect k1 to have the shorter lifetime. The lifetimes were measured to be: t1 ? 9x1011 sec (194753) t2 ? 5x108 sec (Lande et. al. Phys. Rev. V103, 1901 (1956)) We can use the lifetime difference to produce a beam of k2’s from a beam of k0’s. 1) produce k0’s using pp?k0?. 2) let the beam of k0’s propagate in vacuum until the k1 ponent dies out. 1 GeV/c k1 travels on average ? cm 1 GeV/c k2 travels on average ? 3100 cm Need to put detector 100200 m away from target. m2 m1 = eV Mamp。S pages 252255 Spring 2022 L6 Richard Kass Neutral Kaons, Mixing, and CP violation How do we look for CP violation with a k2 beam? Look for decays that have CP = +1 ! k2? (p+p or p0p0 ) Experimentally we find: k2? p+p about % of the time (Christenson et. al. PRL V13, 138 (1964)) k2? p0p0 about % of the time Can also look for differences in decays that involve matter and antimatter: )()( )()()(2222 ????? ??? ?? ??eeee ekBRekBR ekBRekBRe ?p?p ?p?p?0 2 0 )()( )()()(2222 ????? ??? ?? ??mmmm ?mp?mp ?mp?mpm? kBRkBR kBRkBRNature differentiates between matter and antimatter ! CP violation has recently (2022) been unambiguously measured in the decay of Bmesons. This is one of the most interesting areas in HEP and we will discuss this topic later in the course. Observing CP violation with Bmesons is much more difficult than with kaons! Unfortunately (for us) the BS and BL have essentially the same lifetime and as a result there is no way to get a beam of BL and look for “forbidden” decay modes. In addition, it is much harder to produce large quantities of Bmesons than kaons.
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