In 1964, Christenson, Cronin, Fitch, and Turlay observed that the long-lived neutral kaon (K_L, which should be CP-odd) decays to two pions (a CP-even state) at a rate of about 2 x 10^{-3}. What does this observation imply?
AThat pions are not CP eigenstates
BThat the weak interaction violates CP symmetry -- the physical K_L state is not a pure CP eigenstate but contains a small admixture of the CP-even component, parameterized by epsilon ~ 2.2 x 10^{-3}, and the decay K_L -> pi pi proceeds through this admixture (indirect CP violation) and/or through a direct CP-violating decay amplitude (epsilon-prime)
CThat the strong interaction violates CP
DThat the kaon mass measurement was incorrect
In the absence of CP violation, the neutral kaon mass eigenstates would be exact CP eigenstates: K_1 (CP-even, decaying to 2 pions) and K_2 (CP-odd, decaying to 3 pions). The observation of K_L -> 2 pions means K_L is not purely CP-odd. The parameter epsilon measures the CP impurity in the K_L state (indirect CP violation from K-Kbar mixing), while epsilon-prime measures CP violation directly in the decay amplitude. Both have been measured: |epsilon| ~ 2.2 x 10^{-3} and Re(epsilon'/epsilon) ~ 1.7 x 10^{-3}.
Question 2 Short Answer
There are three types of CP violation: (1) in mixing, (2) in decay (direct), and (3) in the interference between mixing and decay. The golden measurement of CP violation in B physics is the asymmetry in B_d -> J/psi K_S, which measures sin(2*beta). Which type is this?
Think about your answer, then reveal below.
Model answer: This is type (3): CP violation in the interference between B_d -> J/psi K_S (direct decay) and B_d -> B_d-bar -> J/psi K_S (mixing followed by decay). The time-dependent CP asymmetry is A_CP(t) = sin(2*beta) * sin(Delta m * t), where beta is an angle of the unitarity triangle and Delta m is the B_d mixing frequency. This measurement is theoretically clean because the J/psi K_S final state is accessible to both B and B-bar, and the hadronic uncertainties cancel in the asymmetry. The BaBar and Belle experiments measured sin(2*beta) = 0.699 +/- 0.017, establishing CP violation in the B system and confirming the CKM mechanism.
The measurement of sin(2*beta) was the primary physics goal of the B factory program. Its agreement with the CKM prediction (from the sides of the unitarity triangle measured independently) was a major validation of the three-generation Standard Model. This earned Kobayashi and Maskawa a share of the 2008 Nobel Prize.
Question 3 True / False
The Standard Model predicts CP violation from the CKM phase, but the amount is far too small to explain the observed matter-antimatter asymmetry of the universe.
TTrue
FFalse
Answer: True
The Sakharov conditions for baryogenesis require: (1) baryon number violation, (2) C and CP violation, and (3) departure from thermal equilibrium. The Standard Model satisfies all three in principle (baryon number violation from electroweak sphalerons, CP violation from the CKM phase, non-equilibrium from the electroweak phase transition). However, quantitative calculations show that the CP violation from the CKM matrix produces a baryon asymmetry roughly 10 orders of magnitude too small. Additionally, for m_H = 125 GeV, the electroweak phase transition is a smooth crossover rather than the first-order transition needed for departure from equilibrium. New sources of CP violation beyond the Standard Model are therefore required.