In particle physics, CP violation is a violation of the postulated CP symmetry of the laws of physics. It plays an important role in theories of cosmology that attempt to explain the dominance of matter over antimatter in the present Universe. 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. The study of CP violation remains a vibrant area of theoretical and experimental work today.

What is CP?

CP is the product of two symmetries: C for charge conjugation, which transforms a particle into its antiparticle, and P for parity, which creates the mirror image of a physical system. The strong interaction and electromagnetic interaction seem to be invariant under the combined CP transformation operation, but this symmetry is slightly violated during certain types of weak decay. Historically, CP-symmetry was proposed to restore order after the discovery of parity violation in the 1950s.

The idea behind parity symmetry is that the equations of particle physics are invariant under mirror inversion. This leads to the prediction that the mirror image of a reaction (such as a chemical reaction or radioactive decay) occurs at the same rate as the original reaction. Parity symmetry appears to be valid for all reactions involving electromagnetism and strong interactions. Until 1956, parity conservation was believed to be one of the fundamental geometric conservation laws (along with conservation of energy and conservation of momentum). However, in 1956 a careful critical review of the existing experimental data by theoretical physicists Tsung-Dao Lee and Chen Ning Yang revealed that while parity conservation had been verified in decays by the strong or electromagnetic interactions, it was untested in the weak interaction. They proposed several possible direct experimental tests. The first test based on beta decay of Cobalt-60 nuclei was carried out in 1956 by a group led by Chien-Shiung Wu, and demonstrated conclusively that weak interactions violate the P symmetry or, as the analogy goes, some reactions did not occur as often as their mirror image.

Overall, the symmetry of a quantum mechanical system can be restored if another symmetry S can be found such that the combined symmetry PS remains unbroken. This rather subtle point about the structure of Hilbert space was realized shortly after the discovery of P violation, and it was proposed that charge conjugation was the desired symmetry to restore order.

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.

Experimental status

Indirect CPV

In 1964, James Cronin, Val Fitch with co-workers provided clear evidence (which was first announced at the 12th ICHEP conference in Dubna) that CP symmetry could be broken, too, winning them the 1980 Nobel Prize. This discovery showed that weak interactions violate not only the charge-conjugation symmetry C between particles and antiparticles and the P or parity, but also their combination. The discovery shocked particle physics and opened the door to questions still at the core of particle physics and of cosmology today. The lack of an exact CP symmetry, but also the fact that it is so nearly a symmetry created a great puzzle.

Only a weaker version of the symmetry could be preserved by physical phenomena, which was CPT-symmetry. Besides C and P, there is a third operation, time reversal (T), which corresponds to reversal of motion. Invariance under time reversal implies that whenever a motion is allowed by the laws of physics, the reversed motion is also an allowed one. The combination of CPT is thought to constitute an exact symmetry of all types of fundamental interactions. Because of the CPT-symmetry, a violation of the CP-symmetry is equivalent to a violation of the T-symmetry. CP violation implied nonconservation of T, provided that the long-held CPT theorem was valid. In this theorem, regarded as one of the basic principles of quantum field theory, charge conjugation, parity, and time reversal are applied together.

Direct CPV

The kind of CP violation discovered in 1964 was linked to the fact that neutral kaons can transform into their antiparticles (in which each quark is replaced with the other's antiquark) and vice versa, but such transformation does not occur with exactly the same probability in both directions; this is called indirect CP violation. Despite many searches, no other manifestation of CP violation was discovered until the '90s, when the NA31 experiment at CERN suggested evidence for CP violation in the decay process of the very same neutral kaons, so-called direct CP violation. The observation was somehow controversial, and final proof for it came in 1999 from the KTeV experiment at Fermilab and the NA48 experiment at CERN.

In 2001, a new generation of experiments, including the BaBar Experiment at the Stanford Linear Accelerator Center (SLAC) and the Belle Experiment at the High Energy Accelerator Research Organisation (KEK) in Japan, observed direct CP violation in a different sector of particle physics, namely in decays of the B mesons. By now a large number of CP violation processes in B-meson decays have been discovered. Before these "B-factory" experiments, it 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.

The CP violation is incorporated in the Standard model by including a complex phase in the CKM matrix describing quark mixing. In such scheme a necessary condition for the appearance of the complex phase, and thus for CP-violation, is the presence of at least three generations of quarks.

CPV in strong sector

There is no experimentally known violation of the CP-symmetry in quantum chromodynamics which presents the strong CP problem (see below).

Strong CP problem

In particle physics, the strong CP problem is the puzzling question why quantum chromodynamics (QCD) does not seem to break the CP-symmetry.

QCD does not violate the CP-symmetry as easily as the electroweak theory; unlike the electroweak theory in which the gauge fields couple to chiral currents constructed from the fermionic fields, the gluons couple to vector currents. Experiments do not indicate any CP violation in the QCD sector. For example, a generic CP-violation in the strongly interacting sector would create the electric dipole moment of the neutron which would be comparable to 10-18 e·m while the experimental upper bound is roughly a trillion times smaller.

This is a problem because at the end, there are natural terms in the QCD Lagrangian that are able to break the CP-symmetry.

{mathcal L} = -frac{1}{4} F_{munu}F^{munu}-frac{n_f g^2theta}{32pi^2}
F_{munu}tilde F^{munu}+bar psi(igamma^mu D_mu - m e^{itheta'gamma_5})psi

For a nonzero choice of the QCD theta-angle and the chiral quark mass phase theta' 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 tildetheta-angle, but it remains to be explained why Nature chooses an unbelievably small value of this angle instead of an angle of order one; the special choice of the theta-angle that must be very close to zero (in this case) is an example of fine-tuning in physics.

There are several proposed solutions to solve the strong CP problem. The most well-known is Peccei-Quinn theory, involving new scalar particles called axions. A newer, more radical approach not requiring the axion is a theory involving two-time dimensions first proposed in 1998 by Bars, Deliduman, and Andreev.

CP violation and the matter-antimatter imbalance

One of the unsolved theoretical questions in physics is why the universe is made chiefly of matter, rather than consisting of equal parts of matter and antimatter. It can be demonstrated that to create an imbalance in matter and antimatter from an initial condition of balance, the Sakharov conditions must be satisfied, one of which is the existence of CP violation during the extreme conditions of the first seconds after the Big Bang. Explanations which do not involve CP violation are less plausible, since they rely on the assumption that the matter-antimatter imbalance was present at the beginning, or on other admittedly exotic assumptions.

The Big Bang should have produced equal amounts of matter and anti-matter if CP-symmetry was preserved; as such, there should have been total cancellation of both. In other words, protons should have cancelled with anti-protons, electrons with positrons, neutrons with anti-neutrons, and so on for all elementary particles. This would have resulted in a sea of photons in the universe with no matter. Since this is quite evidently not the case, after the Big Bang, physical laws must have acted differently for matter and antimatter, i.e. violating CP symmetry.

The Standard Model contains only two ways to break CP symmetry. The first of these, discussed above, is in the QCD Lagrangian, and has not been found experimentally; but one would expect this to lead to either no CP violation or a CP violation that is many, many orders of magnitude too large. The second of these, involving the weak force, has been experimentally verified, but can account for only a small portion of CP-violation. It is predicted to be sufficient for a net mass of normal matter equivalent to only a single galaxy in the known universe.

Since the Standard Model does not accurately predict this discrepancy, it would seem that the current Standard Model has gaps (other than the obvious one of gravity and related matters) or physics is otherwise in error. Moreover, experiments to probe these CP-related gaps may not require the practically impossible-to-obtain energies that may be necessary to probe the gravity-related gaps (see Planck mass).

See also



  • Sozzi, M.S. (2008). Discrete symmetries and CP violation. Oxford University Press. ISBN 978-0-19-929666-8.
  • G. C. Branco, L. Lavoura and J. P. Silva (1999). CP violation. Clarendon Press, Oxford. ISBN 0-19-850399-7.
  • I. Bigi and A. Sanda (1999). CP violation. Cambridge University Press. ISBN 0-521-44349-0.
  • Michael Beyer (Editor) (2002). CP Violation in Particle, Nuclear and Astrophysics. Springer. ISBN 3-540-43705-3. (A collection of essays introducing the subject, with an emphasis on experimental results.)
  • L. Wolfenstein (1989). CP violation. North-Holland, Amsterdam. 0444-88081X. (A compilation of reprints of numerous important papers on the topic, including papers by T.D. Lee, Cronin, Fitch, Kobayashi and Maskawa, and many others.)
  • David J. Griffiths (1987). Introduction to Elementary Particles. Wiley, John & Sons, Inc. ISBN 0-471-60386-4.
  • I. Bigi, CP violation, an essential mystery in Nature's grand design. Invited lecture given at the XXV ITEP Winter school of Physics, February 18-27, 1997, Moscow, Russia, at 'Frontiers in Contemporary Physics', May 11-16, 1997, Vanderbilt University, Nashville, USA, and at the International School of Physics 'Enrico Fermi', CXXXVII Course 'Heavy Flavour Physics: A Probe of Nature's Grand Design', Varenna, Italy, July 8-18, 1997. hep-ph/9803479.
  • Davide Castelvecchi, What is direct CP-violation?, Stanford Linear Accelerator (SLAC)

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