CP violation

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In particle physics, CP violation (CP standing for Charge Parity) is a violation of the postulated CP-symmetry (or Charge conjugation Parity symmetry): the combination of C-symmetry (charge conjugation symmetry) and P-symmetry (parity symmetry). CP-symmetry states that the laws of physics should be the same if a particle is interchanged with its antiparticle (C symmetry), and then its spatial coordinates are inverted ("mirror" or P symmetry). The discovery of CP violation in 1964 in the decays of neutral kaons resulted in the Nobel Prize in Physics in 1980 for its discoverers James Cronin and Val Fitch.






Simply speaking, charge conjugation is a simple symmetry between particles and antiparticles, and so CP-symmetry was proposed in 1957 by Lev Landau as the true symmetry between matter and antimatter. In other words a process in which all particles are exchanged with their antiparticles was assumed to be equivalent to the mirror image of the original process.

CP violation in the Standard Model[edit]


The reason why such a complex phase causes CP violation is not immediately obvious, but can be seen as follows. Consider any given particles (or sets of particles) a and b, and their antiparticles \tilde{a} and \tilde{b}. Now consider the processes a \rightarrow b and the corresponding antiparticle process \tilde{a} \rightarrow \tilde{b}, and denote their amplitudes M and \tilde{M} respectively. Before CP violation, these terms must be the same complex number. We can separate the magnitude and phase by writing M=|M|e^{i\theta}. If a phase term is introduced from (e.g.) the CKM matrix, denote it e^{i\phi}. Note that \tilde{M} contains the conjugate matrix to M, so it picks up a phase term e^{-i\phi}. Now we have:


However, physically measurable reaction rates are proportional to |M|^{2}, so far nothing is different. However, consider that there are two different routes (e.g. intermediate states) for a \rightarrow b. Now we have:

M = |M_{1}|e^{i\theta_{1}}e^{i\phi_{1}} + |M_{2}|e^{i\theta_{2}}e^{i\phi_{2}}
\tilde{M} = |M_{1}|e^{i\theta_{1}}e^{-i\phi_{1}} + |M_{2}|e^{i\theta_{2}}e^{-i\phi_{2}}

Some further calculation gives:



Experimental status[edit]

Indirect CP violation[edit]



Direct CP violation[edit]

Kaon oscillation box diagram


In 2001, a new generation of experiments, including the BaBar Experiment at the Stanford Linear Accelerator Center (SLAC)[4] and the Belle Experiment at the High Energy Accelerator Research Organisation (KEK)[5] in Japan, observed direct CP violation in a different system, namely in decays of the B mesons.[6] By now a large number of CP violation processes in B meson decays have been discovered. Before these "B-factory" experiments, there was a logical possibility that all CP violation was confined to kaon physics. However, this raised the question of why it's not extended to the strong force, and furthermore, why this is not predicted in the unextended Standard Model, despite the model being undeniably accurate with "normal" phenomena.

In 2011, a first indication of CP violation in decays of neutral D mesons was reported by the LHCb experiment at CERN.[7]

Strong CP problem[edit]

List of unsolved problems in physics
Why is the strong nuclear interaction force CP-invariant?




{\mathcal L} = -\frac{1}{4} F_{\mu\nu}F^{\mu\nu}-\frac{n_f g^2\theta}{32\pi^2}
F_{\mu\nu}\tilde F^{\mu\nu}+\bar \psi(i\gamma^\mu D_\mu - m

For a nonzero choice of the θ angle and the chiral quark mass phase θ′ one expects the CP-symmetry to be violated. One usually assumes that the chiral quark mass phase can be converted to a contribution to the total effective \scriptstyle{\tilde\theta} angle, but it remains to be explained why this angle is extremely small instead of being of order one; the particular value of the θ angle that must be very close to zero (in this case) is an example of a fine-tuning problem in physics, and is typically solved by physics beyond the Standard Model.


The strong CP problem may also be solved within a theory of quantum gravity.

Little CP problem[edit]


CP violation and the matter–antimatter imbalance[edit]

Main article: Baryogenesis
List of unsolved problems in physics
Why does the universe have so much more matter than antimatter?




If CP violation in the lepton sector is experimentally determined to be too small to account for matter-antimatter asymmetry, some new Physics beyond the Standard Model would be required to explain additional sources of CP violation. Fortunately, it is generally the case that adding new particles and/or interactions to the Standard Model introduces new sources of CP violation since CP is not a symmetry of nature.

See also[edit]


  1. ^ "Evidence for the 2π Decay of the K0
    Meson System". Physical Review Letters 13: 138. 1964. Bibcode:1964PhRvL..13..138C. doi:10.1103/PhysRevLett.13.138.
  2. ^ "Observation of Direct CP Violation in KS,L→ππ Decays". Physical Review Letters 83: 22. 1999. arXiv:hep-ex/9905060. Bibcode:1999PhRvL..83...22A. doi:10.1103/PhysRevLett.83.22. 
  3. ^ NA48 Collaboration, V. Fanti, A. Lai, D. Marras, L. Musa, et al.. (1999). "A new measurement of direct CP violation in two pion decays of the neutral kaon". Physics Letters B 465 (1–4): 335–348. arXiv:hep-ex/9909022. Bibcode:1999PhLB..465..335F. doi:10.1016/S0370-2693(99)01030-8. 
  4. ^ "Measurement of CP-Violating Asymmetries in B0 Decays to CP Eigenstates". Physical Review Letters 86: 2515. 2001. arXiv:hep-ex/0102030. Bibcode:2001PhRvL..86.2515A. doi:10.1103/PhysRevLett.86.2515. 
  5. ^ "Observation of Large CP Violation in the Neutral B Meson System". Physical Review Letters 87. 2001. arXiv:hep-ex/0107061. Bibcode:2001PhRvL..87i1802A. doi:10.1103/PhysRevLett.87.091802. 
  8. ^ I. Bars; C. Deliduman; O. Andreev (1998). "Gauged Duality, Conformal Symmetry, and Spacetime with Two Times". Physical Review D 58 (6): 066004. arXiv:hep-th/9803188. Bibcode:1998PhRvD..58f6004B. doi:10.1103/PhysRevD.58.066004. 
  9. ^ Kadosh, Avihay; Pallante, Elisabetta (2011). "CP violation and FCNC in a warped A4 flavor model". Journal of High Energy Physics 2011 (6). arXiv:1101.5420. Bibcode:2011JHEP...06..121K. doi:10.1007/JHEP06(2011)121. 


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