Physic Labs

Particle physics

The weak interaction and CP violation

Illustrate the discrete symmetry transformations C, P, T and CP violation in the weak interaction. Compare the system before and after each transformation, and tune the CP asymmetry to see P(Γ)≠P(Γˉ)P(\Gamma) \neq P(\bar\Gamma).

Research

Equipment

  • Transformation buttons: Original, C, P, T, CP
  • Sliders for CP asymmetry and animation speed
  • Display area showing particle and antiparticle processes

Procedure

  1. Apply each transformation

    Click "Original", "C", "P", "T" in turn and watch the simulation: C swaps particle ↔ antiparticle, P is mirror reflection (spatial inversion), T reverses time. Note which cases look identical to the original (symmetric) and which differ.

  2. Test the combined CP symmetry

    Press "CP" to apply both operations. Set "Illustrative CP asymmetry" to 0: the CP-transformed system matches the original — CP conserved. Raise the slider gradually: the two conjugate processes' rates split, ACP=Γ−ΓˉΓ+Γˉ≠0A_{CP} = \frac{\Gamma - \bar\Gamma}{\Gamma + \bar\Gamma} \neq 0.

  3. Relate to the CPT theorem

    Compare the CP states at different asymmetry values; adjust "Animation speed" to track details. Conclusion: a small CP violation (a few parts per thousand in K mesons) suffices to distinguish matter from antimatter, while combined CPT is still believed exactly conserved in any local Lorentz-invariant field theory.

Simulation

Experiment history

In 1956 Tsung-Dao Lee and Chen Ning Yang pointed out that mirror symmetry P had never been tested in the weak interaction; Chien-Shiung Wu confirmed P violation in 1957 via beta decay of polarized cobalt-60. The community initially trusted that CP survived — until 1964, when James Cronin and Val Fitch found the long-lived K meson decaying into two pions, violating CP (Nobel 1980). In 1967 Andrei Sakharov showed CP violation is one of three conditions needed to explain a matter-rich universe. In 1973 Kobayashi and Maskawa proved that three quark generations allow an irremovable CP phase in the CKM matrix — predicting the third generation before its discovery (Nobel 2008). The B factories (BaBar, Belle) later measured direct CP violation in B mesons.

Related physicists

Richard FeynmanPaul DiracTsung-Dao LeeChen Ning YangMakoto KobayashiToshihide Maskawa

Related library topics

The weak interaction and CP violation