CP violation

Until the 1950s, parity conservation was believed to be one of the fundamental geometric conservation laws (along with conservation of energy and conservation of momentum). After the discovery of parity violation in 1956, CP-symmetry was proposed to restore order. However, while the strong interaction and electromagnetic interaction seem to be invariant under the combined CP transformation operation, further experiments showed that this symmetry is slightly violated during certain types of weak decay.

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 and occurs at the same rate forwards and backwards.

The combination of CPT is thought to constitute an exact symmetry of all types of fundamental interactions. Because of the long-held CPT symmetry theorem, provided that it is valid, a violation of the CP-symmetry is equivalent to a violation of the T-symmetry. In this theorem, regarded as one of the basic principles of quantum field theory, charge conjugation, parity, and time reversal are applied together. Direct observation of the time reversal symmetry violation without any assumption of CPT theorem was done in 1998 by two groups, CPLEAR and KTeV collaborations, at CERN and Fermilab, respectively.[1] Already in 1970 Klaus Schubert observed T violation independent of assuming CPT symmetry by using the Bell–Steinberger unitarity relation.[2]

"Direct" CP violation is allowed in the Standard Model if a complex phase appears in the CKM matrix describing quark mixing, or the PMNS matrix describing neutrino mixing. A necessary condition for the appearance of the complex phase is the presence of at least three generations of quarks. If fewer generations are present, the complex phase parameter can be absorbed into redefinitions of the quark fields. A popular rephasing invariant whose vanishing signals absence of CP violation and occurs in most CP violating amplitudes is the Jarlskog invariant,

${\displaystyle \,J=c_{12}c_{13}^{2}c_{23}s_{12}s_{13}s_{23}\sin \delta \approx 3\times 10^{-5}\,,}$

for quarks which is ${\displaystyle 3\times 10^{-4}}$ times the maximum value of ${\displaystyle J_{\max }=1/6{\sqrt {3}}\approx 0.1}$. For leptons, only an upper limit exists, ${\displaystyle |J|<0.03}$.

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) ${\displaystyle a}$ and ${\displaystyle b}$, and their antiparticles ${\displaystyle {\bar {a}}}$ and ${\displaystyle {\bar {b}}}$. Now consider the processes ${\displaystyle a\rightarrow b}$ and the corresponding antiparticle process ${\displaystyle {\bar {a}}\rightarrow {\bar {b}}}$, and denote their amplitudes ${\displaystyle M}$ and ${\displaystyle {\bar {M}}}$ respectively. Before CP violation, these terms must be the same complex number. We can separate the magnitude and phase by writing ${\displaystyle M=|M|e^{i\theta }}$. If a phase term is introduced from (e.g.) the CKM matrix, denote it ${\displaystyle e^{i\phi }}$. Note that ${\displaystyle {\bar {M}}}$ contains the conjugate matrix to ${\displaystyle M}$, so it picks up a phase term ${\displaystyle e^{-i\phi }}$.

Now the formula becomes:

${\displaystyle M=|M|e^{i\theta }e^{i\phi }}$

${\displaystyle {\bar {M}}=|M|e^{i\theta }e^{-i\phi }}$

Physically measurable reaction rates are proportional to ${\displaystyle |M|^{2}}$, thus so far nothing is different. However, consider that there are two different routes: ${\displaystyle a{\overset {1}{\longrightarrow }}b}$ and ${\displaystyle a{\overset {2}{\longrightarrow }}b}$ or equivalently, two unrelated intermediate states: ${\displaystyle a\rightarrow 1\rightarrow b}$ and ${\displaystyle a\rightarrow 2\rightarrow b}$. Now we have:

${\displaystyle M=|M_{1}|e^{i\theta _{1}}e^{i\phi _{1}}+|M_{2}|e^{i\theta _{2}}e^{i\phi _{2}}}$

${\displaystyle {\bar {M}}=|M_{1}|e^{i\theta _{1}}e^{-i\phi _{1}}+|M_{2}|e^{i\theta _{2}}e^{-i\phi _{2}}}$

Some further calculation gives:

${\displaystyle |M|^{2}-|{\bar {M}}|^{2}=-4|M_{1}||M_{2}|\sin(\theta _{1}-\theta _{2})\sin(\phi _{1}-\phi _{2})}$

Thus, we see that a complex phase gives rise to processes that proceed at different rates for particles and antiparticles, and CP is violated.

From the theoretical end, the CKM matrix is defined as V_CKM = U_u. U^﹢
_d, where U_u and U_d are unitary transformation matrices which diagonalize the fermion mass matrices M_u and M_d, respectively.

Thus, there are two necessary conditions for getting a complex CKM matrix:

1. At least one of U_u and U_d is complex, or the CKM matrix will be purely real.
2. If both of them are complex, U_u and U_d mustn’t be the same, i.e., U_u ≠ U_d, or CKM matrix will be an identity matrix which is also purely real.

There is no experimentally known violation of the CP-symmetry in quantum chromodynamics. As there is no known reason for it to be conserved in QCD specifically, this is a "fine tuning" problem known as the strong CP problem.

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 one trillionth that size.

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

${\displaystyle {\mathcal {L}}=-{\frac {1}{4}}F_{\mu \nu }F^{\mu \nu }-{\frac {n_{f}g^{2}\theta }{32\pi ^{2}}}F_{\mu \nu }{\tilde {F}}^{\mu \nu }+{\bar {\psi }}(i\gamma ^{\mu }D_{\mu }-me^{i\theta '\gamma _{5}})\psi }$

For a nonzero choice of the θ angle and the chiral phase of the quark mass 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 ${\displaystyle \scriptstyle {\tilde {\theta }}}$ 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.

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.[27]

The non-dark matter universe is made chiefly of matter, rather than consisting of equal parts of matter and antimatter as might be expected. 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 antimatter if CP-symmetry was preserved; as such, there should have been total cancellation of both—protons should have cancelled with antiprotons, electrons with positrons, neutrons with antineutrons, and so on. This would have resulted in a sea of radiation in the universe with no matter. Since this is 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 at least three sources of CP violation. The first of these, involving the Cabibbo–Kobayashi–Maskawa matrix in the quark sector, has been observed experimentally and can only account for a small portion of the CP violation required to explain the matter-antimatter asymmetry. The strong interaction should also violate CP, in principle, but the failure to observe the electric dipole moment of the neutron in experiments suggests that any CP violation in the strong sector is also too small to account for the necessary CP violation in the early universe. The third source of CP violation is the Pontecorvo–Maki–Nakagawa–Sakata matrix in the lepton sector. The current long-baseline neutrino oscillation experiments, T2K and NOνA, may be able to find evidence of CP violation over a small fraction of possible values of the CP violating Dirac phase while the proposed next-generation experiments, Hyper-Kamiokande and DUNE, will be sensitive enough to definitively observe CP violation over a relatively large fraction of possible values of the Dirac phase. Further into the future, a neutrino factory could be sensitive to nearly all possible values of the CP violating Dirac phase. If neutrinos are Majorana fermions, the PMNS matrix could have two additional CP violating Majorana phases, leading to a fourth source of CP violation within the Standard Model. The experimental evidence for Majorana neutrinos would be the observation of neutrinoless double-beta decay. The best limits come from the GERDA experiment. CP violation in the lepton sector generates a matter-antimatter asymmetry through a process called leptogenesis. This could become the preferred explanation in the Standard Model for the matter-antimatter asymmetry of the universe if CP violation is experimentally confirmed in the lepton sector.

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. Adding new particles and/or interactions to the Standard Model generally introduces new sources of CP violation since CP is not a symmetry of nature.

Sakharov proposed a way to restore CP-symmetry using T-symmetry, extending spacetime before the Big Bang. He described complete CPT reflections of events on each side of what he called the "initial singularity". Because of this, phenomena with an opposite arrow of time at t < 0 would undergo an opposite CP violation, so the CP-symmetry would be preserved as a whole. The anomalous excess of matter over antimatter after the Big Bang in the orthochronous (or positive) sector, becomes an excess of antimatter before the Big Bang (antichronous or negative sector) as both charge conjugation, parity and arrow of time are reversed due to CPT reflections of all phenomena occurring over the initial singularity:

We can visualize that neutral spinless maximons (or photons) are produced at t < 0 from contracting matter having an excess of antiquarks, that they pass "one through the other" at the instant t = 0 when the density is infinite, and decay with an excess of quarks when t > 0, realizing total CPT symmetry of the universe. All the phenomena at t < 0 are assumed in this hypothesis to be CPT reflections of the phenomena at t > 0.

— Andrei Sakharov, in Collected Scientific Works (1982).[28]

• B-factory
• Parity (physics) § Parity violation
• Charge conjugation
• T-symmetry
• CPT symmetry
• BTeV experiment
• Cabibbo–Kobayashi–Maskawa matrix
• LHCb
• Penguin diagram
• Neutral particle oscillation
• Electron electric dipole moment

• Cern Courier article