Particle physics

The weak interaction and CP violation

The weak interaction changes quark and lepton flavor; CP violation in some processes shows that matter and antimatter are not perfectly symmetric under charge conjugation combined with parity.

The weak interaction is the Standard Model interaction that changes quark flavor; it also drives beta decay and neutrino interactions. CP transforms particles into antiparticles and reverses space. Weak processes can violate CP: a process and its CP-conjugate need not have equal probabilities.

P(A\to B)\ne P(CP(A)\to CP(B))\quad\text{(CP violation)}

Definition: The CKM matrix and CP phase

Weak and mass quark eigenstates differ; the CKM matrix describes flavor mixing in charged-current interactions. With three generations it contains an irreducible complex phase, which permits CP violation in quark weak processes. Neutrinos have an analogous PMNS mixing matrix.

Toggle a schematic process through its original, C (particle to antiparticle), P (mirror reversal), and T (time reversal) views. This is a symbolic illustration, not measured decay data.

CP and the cosmic imbalance

CP violation has been observed in K- and B-meson systems, consistent with the CKM phase. It is a necessary ingredient in scenarios generating a baryon asymmetry in the early universe, but Standard Model CP violation appears too small to account for the observed excess without additional physics and suitable conditions.

Example: A CP test

A particle and its antiparticle decay to CP-related final states. If their measured decay rates differ after detector-efficiency corrections, what is the appropriate conclusion?

Solution

It indicates CP violation in that channel, provided statistical uncertainty and detector asymmetries are controlled; it does not mean every interaction violates CP.

To quantify an asymmetry, compare decay rates of a particle and its antiparticle into conjugate final states. If an amplitude contains two contributions with different magnitudes and a relative phase other than $0$ or $\pi$ , interference can make the probabilities differ. Measurements must correct for sample-dependent efficiencies and backgrounds; only an effect that survives these checks is interpreted as CP violation.

Which transformations are combined in CP?

Does CP violation in one decay channel by itself prove CPT violation?

References

I.I. Bigi, A.I. Sanda (2009). CP Violation