CP Violation & Phase: Explained

In summary, CP violation in the Standard Model is caused by the presence of phases in the CKM matrix. This is because under CP exchange, some processes keep their phase (EM/strong interactions) while others get a different phase (weak interaction), resulting in an overall asymmetry. The Lagrangian and fields are affected by CP exchange, with particles changing into antiparticles. However, for the local gauge of U(1), the charge is flipped by a transformation of the form \psi \rightarrow e^{-iqa(x)} \psi. This means that the \psi spinor will change to \psi^* in order to flip its charge.
  • #1
Utg
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Hello! I have heard many times that CP violation in the SM is due to the presence of phase in the CKM matrix. Can somebody explain how phase is related to CP violation?

Thanks!
 
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  • #2
If you exchange C and P all the amplitudes of individual processes stay the same. The probability is linked to the squared amplitude. So how do you get an asymmetry at all? The only way to do this is via phases: Some processes keep their phase under CP (EM/strong interaction), some get a different phase (weak interaction). If you have at least two processes that contribute to the overall amplitude, and their phases transform differently under CP, you can get an actual asymmetry.
 
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  • #3
mfb said:
If you exchange C and P all the amplitudes of individual processes stay the same. The probability is linked to the squared amplitude. So how do you get an asymmetry at all? The only way to do this is via phases: Some processes keep their phase under CP (EM/strong interaction), some get a different phase (weak interaction). If you have at least two processes that contribute to the overall amplitude, and their phases transform differently under CP, you can get an actual asymmetry.

Thank you for your response. I think my problem is that I don't understand why EM/strong interactions keep phase under CP and weak interaction not. How can I see this? I understand, for example, that P changes left fields to right fields and vice versa. How does C acts on Lagrangian? fields?
 
  • #4
C changes particles with antiparticles.
 
  • #5
mfb said:
C changes particles with antiparticles.

Does it mean that ##\bar{\psi}## changes into ##\psi## and vise versa?
 
  • #6
I think, the [itex]\psi[/itex] is generally denoting a Dirac spinor, and as such it contains both the particle and antiparticle states. So, you cannot just say that [itex]\bar{\psi}[/itex] becomes [itex]\psi[/itex]. Basically the [itex]\psi[/itex] will some sort change to [itex]\psi^*[/itex] because you want to flip its charge, which in the local gauge of the U(1) is given by a transformation of the form: [itex]\psi \rightarrow e^{-iqa(x)} \psi[/itex] (so you need to change the sign of [itex]q[/itex]).
Of course it will be a little more complicated and for that you need to look at what operation the charge conjugation does on your field.
 
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1. What is CP violation?

CP violation refers to a phenomenon in particle physics where the laws of physics are not symmetric under the combined transformations of charge conjugation (C) and parity (P). This means that the behavior of particles and their antiparticles are not identical, and this violation is observed in certain processes involving subatomic particles.

2. What is phase in the context of CP violation?

In the context of CP violation, phase refers to the difference in the weak interaction between a particle and its antiparticle. This phase can affect the decay rates of particles and is a key factor in understanding CP violation.

3. How is CP violation measured?

CP violation is measured through experiments that analyze the decay rates of particles and their antiparticles. By comparing the decay rates, scientists can determine if there is a difference in behavior between particles and antiparticles, indicating CP violation.

4. What is the significance of CP violation in particle physics?

CP violation is significant because it helps explain why there is more matter than antimatter in the universe. According to the Big Bang theory, equal amounts of matter and antimatter should have been created, but this is not observed. CP violation is one of the mechanisms that could explain this asymmetry.

5. Can CP violation be observed in everyday life?

No, CP violation is a phenomenon that occurs at the subatomic level and cannot be observed in everyday life. It requires specialized equipment and experiments to detect and measure CP violation in particles.

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