In light of current knowledge (quarks were not yet theorized), the Cabibbo angle is related to the relative probability that down and strange quarks decay into up quarks (|Vud|2 and |Vus|2 respectively). In particle physics parlance, the object that couples to the up quark via charged-current weak interaction is a superposition of down-type quarks, here denoted by d′. Mathematically this is:
or using the Cabibbo angle:
Using the currently accepted values for |Vud| and |Vus| (see below), the Cabibbo angle can be calculated using
When the charm quark was discovered in 1974, it was noticed that the down and strange quark could decay into either the up or charm quark, leading to two sets of equations:
where the various |Vij|2 represent the probability that the quark of j flavor decays into a quark of i flavor. This 2 × 2 rotation matrix is called the Cabibbo matrix.
A pictorial representation of the six quarks' decay modes, with mass increasing from left to right.
In 1973, observing that CP-violation could not be explained in a four-quark model, Kobayashi and Maskawa generalized the Cabibbo matrix into the Cabibbo–Kobayashi–Maskawa matrix (or CKM matrix) to keep track of the weak decays of three generations of quarks:
On the left is the weak interaction doublet partners of up-type quarks, and on the right is the CKM matrix along with a vector of mass eigenstates of down-type quarks. The CKM matrix describes the probability of a transition from one quark i to another quark j. These transitions are proportional to |Vij|2.
As of 2010, the best determination of the magnitudes of the CKM matrix elements was:
The choice of usage of down-type quarks in the definition is a convention, and does not represent a physically preferred asymmetry between up-type and down-type quarks. Other conventions are equally valid, such as defining the matrix in terms of weak interaction partners of mass eigenstates of up-type quarks, u′, c′ and t′, in terms of u, c, and t. Since the CKM matrix is unitary, its inverse is the same as its conjugate transpose.
General case construction
To generalize the matrix, count the number of physically important parameters in this matrix, V which appear in experiments. If there are N generations of quarks (2Nflavours) then
An N × N unitary matrix (that is, a matrix V such that VV† = I, where V† is the conjugate transpose of V and I is the identity matrix) requires N2 real parameters to be specified.
2N − 1 of these parameters are not physically significant, because one phase can be absorbed into each quark field (both of the mass eigenstates, and of the weak eigenstates), but an overall common phase is unobservable. Hence, the total number of free variables independent of the choice of the phases of basis vectors is N2 − (2N − 1) = (N − 1)2.
Of these, N(N − 1)/2 are rotation angles called quark mixing angles.
The remaining (N − 1)(N − 2)/2 are complex phases, which cause CP violation.
For the case N = 2, there is only one parameter which is a mixing angle between two generations of quarks. Historically, this was the first version of CKM matrix when only two generations were known. It is called the Cabibbo angle after its inventor Nicola Cabibbo.
For the Standard Model case (N = 3), there are three mixing angles and one CP-violating complex phase.
Observations and predictions
Cabibbo's idea originated from a need to explain two observed phenomena:
the transitions u ↔ d, e ↔ νe, and μ ↔ νμ had similar amplitudes.
the transitions with change in strangeness ΔS = 1 had amplitudes equal to 1/4 of those with ΔS = 0.
Cabibbo's solution consisted of postulating weak universality to resolve the first issue, along with a mixing angle θc, now called the Cabibbo angle, between the d and s quarks to resolve the second.
For two generations of quarks, there are no CP violating phases, as shown by the counting of the previous section. Since CP violations were seen in neutral kaon decays already in 1964, the emergence of the Standard Model soon after was a clear signal of the existence of a third generation of quarks, as pointed out in 1973 by Kobayashi and Maskawa. The discovery of the bottom quark at Fermilab (by Leon Lederman's group) in 1976 therefore immediately started off the search for the missing third-generation quark, the top quark.
Note, however, that the specific values of the angles are not a prediction of the standard model: they are open, unfixed parameters. At this time, there is no generally accepted theory that explains why the measured values are what they are.
The constraints of unitarity of the CKM-matrix on the diagonal terms can be written as
for all generations i. This implies that the sum of all couplings of any of the up-type quarks to all the down-type quarks is the same for all generations. This relation is called weak universality and was first pointed out by Nicola Cabibbo in 1967. Theoretically it is a consequence of the fact that all SU(2) doublets couple with the same strength to the vector bosons of weak interactions. It has been subjected to continuing experimental tests.
The unitarity triangles
The remaining constraints of unitarity of the CKM-matrix can be written in the form
For any fixed and different i and j, this is a constraint on three complex numbers, one for each k, which says that these numbers form the sides of a triangle in the complex plane. There are six choices of i and j (three independent), and hence six such triangles, each of which is called a unitary triangle. Their shapes can be very different, but they all have the same area, which can be related to the CP violating phase. The area vanishes for the specific parameters in the Standard Model for which there would be no CP violation. The orientation of the triangles depend on the phases of the quark fields.
A popular quantity amounting to twice the area of the unitarity triangle is the Jarlskog invariant, . For Greek indices denoting up quarks and Latin ones down quarks, the 4-tensor is doubly antisymmetric,
Up to antisymmetry, it only has 9 = 3 × 3 non-vanishing components, which, remarkably, from the unitarity of V, can be shown to be all identical in magnitude, that is,
Since the three sides of the triangles are open to direct experiment, as are the three angles, a class of tests of the Standard Model is to check that the triangle closes. This is the purpose of a modern series of experiments under way at the Japanese BELLE and the American BaBar experiments, as well as at LHCb in CERN, Switzerland.
Four independent parameters are required to fully define the CKM matrix. Many parameterizations have been proposed, and three of the most common ones are shown below.
The original parameterization of Kobayashi and Maskawa used three angles (θ1, θ2, θ3) and a CP-violating phase (δ). Cosines and sines of the angles are denoted ci and si, respectively. θ1 is the Cabibbo angle.
A "standard" parameterization of the CKM matrix uses three Euler angles (θ12, θ23, θ13) and one CP-violating phase (δ13). Couplings between quark generation i and j vanish if θij = 0. Cosines and sines of the angles are denoted cij and sij, respectively. θ12 is the Cabibbo angle.
The currently best known values for the standard parameters are:
A third parameterization of the CKM matrix was introduced by Lincoln Wolfenstein with the four parameters λ, A, ρ, and η. The four Wolfenstein parameters have the property that all are of order 1 and are related to the "standard" parameterization:
λ = s12
Aλ2 = s23
Aλ3(ρ − iη) = s13e−iδ
The Wolfenstein parameterization of the CKM matrix, is an approximation of the standard parameterization. To order λ3, it is:
The CP violation can be determined by measuring ρ − iη.
Using the values of the previous section for the CKM matrix, the best determination of the Wolfenstein parameters is:
λ = 0.2257+0.0009 −0.0010, A = 0.814+0.021 −0.022, ρ = 0.135+0.031 −0.016, and η = 0.349+0.015 −0.017.
In 2008, Kobayashi and Maskawa shared one half of the Nobel Prize in Physics "for the discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature". Some physicists were reported to harbor bitter feelings about the fact that the Nobel Prize committee failed to reward the work of Cabibbo, whose prior work was closely related to that of Kobayashi and Maskawa. Asked for a reaction on the prize, Cabibbo preferred to give no comment.