In particle physics, neutral particle oscillation is the transmutation of a particle with zero electric charge into another neutral particle due to a change of a non-zero internal quantum number, via an interaction that does not conserve that quantum number. Neutral particle oscillations were first investigated in 1954 by Murray Gell-mann and Abraham Pais. For example, a neutron cannot transmute into an antineutron as that would violate the conservation of baryon number. But in those hypothetical extensions of the Standard Model which include interactions that do not strictly conserve baryon number, neutron–antineutron oscillations are predicted to occur. Such oscillations can be classified into two types: ⇄ oscillation]], [[B–Bbar oscillation| ⇄ oscillation]], ⇄ oscillation ). ⇄ ⇄ oscillation]]). In those cases where the particles decay to some final product, then the system is not purely oscillatory, and an interference between oscillation and decay is observed.

History and motivation

CP violation

After the striking evidence for parity violation provided by Wu et al. in 1957, it was assumed that CP (charge conjugation-parity) is the quantity which is conserved. However, in 1964 Cronin and Fitch reported CP violation in the neutral Kaon system. They observed the long-lived KL (with ) undergoing decays into two pions (with ) thereby violating CP conservation. In 2001, CP violation in the [[B–Bbar oscillation| ⇄ system]] was confirmed by the BaBar and the Belle experiments. Direct CP violation in the ⇄ system was reported by both the labs by 2005. The [[Kaon#Oscillation| ⇄ ]] and the ⇄ systems can be studied as two state systems, considering the particle and its antiparticle as two states of a single particle.

The solar neutrino problem

The pp chain in the sun produces an abundance of . In 1968, R. Davis et al. first reported the results of the Homestake experiment. Also known as the Davis experiment, it used a huge tank of perchloroethylene in Homestake mine (it was deep underground to eliminate background from cosmic rays), South Dakota. Chlorine nuclei in the perchloroethylene absorb to produce argon via the reaction which is essentially The experiment collected argon for several months. Because the neutrino interacts very weakly, only about one argon atom was collected every two days. The total accumulation was about one third of Bahcall's theoretical prediction. In 1968, Bruno Pontecorvo showed that if neutrinos are not considered massless, then (produced in the sun) can transform into some other neutrino species ( or ), to which Homestake detector was insensitive. This explained the deficit in the results of the Homestake experiment. The final confirmation of this solution to the solar neutrino problem was provided in April 2002 by the SNO (Sudbury Neutrino Observatory) collaboration, which measured both flux and the total neutrino flux. This 'oscillation' between the neutrino species can first be studied considering any two, and then generalized to the three known flavors.

Description as a two-state system

Special case that only considers mixing

Let \ H_0\ be the Hamiltonian of the two-state system, and and be its orthonormal eigenvectors with eigenvalues \ E_1\ and \ E_2\ respectively. Let be the state of the system at time \ t ~. If the system starts as an energy eigenstate of \ H_0, for example, say then the time evolved state, which is the solution of the Schrödinger equation will be But this is physically same as since the exponential term is just a phase factor: It does not produce an observable new state. In other words, energy eigenstates are stationary eigenstates, that is, they do not yield observably distinct new states under time evolution. Define to be a basis in which the unperturbed Hamiltonian operator, \ H_0, is diagonal: It can be shown, that oscillation between states will occur if and only if off-diagonal terms of the Hamiltonian are not zero. Hence let us introduce a general perturbation \ W\ imposed on \ H\ 0\ such that the resultant Hamiltonian \ H\ is still Hermitian. Then where and and The eigenvalues of the perturbed Hamiltonian, \ H, then change to \ E+\ and \ E_-\ , where Since \ H\ is a general Hamiltonian matrix, it can be written as The following two results are clear: ! Proof ! Proof With the following parametrization (this parametrization helps as it normalizes the eigenvectors and also introduces an arbitrary phase \phi making the eigenvectors most general) and using the above pair of results the orthonormal eigenvectors of \ H'\ and consequently those of \ H\ are obtained as Writing the eigenvectors of \ H_0\ in terms of those of \ H\ we get Now if the particle starts out as an eigenstate of \ H_0\ (say, ), that is then under time evolution we get which unlike the previous case, is distinctly different from We can then obtain the probability of finding the system in state at time \ t\ as which is called Rabi's formula. Hence, starting from one eigenstate of the unperturbed Hamiltonian \ H_0, the state of the system oscillates between the eigenstates of \ H_0\ with a frequency (known as Rabi frequency), From equation (6), for we can conclude that oscillation will exist only if So \ W_{12}\ is known as the coupling term as it connects the two eigenstates of the unperturbed Hamiltonian \ H_0\ and thereby facilitates oscillation between the two. Oscillation will also cease if the eigenvalues of the perturbed Hamiltonian \ H\ are degenerate, i.e. But this is a trivial case as in such a situation, the perturbation itself vanishes and \ H\ takes the form (diagonal) of \ H_0\ and we're back to square one. Hence, the necessary conditions for oscillation are:

The general case: considering mixing and decay

If the particle(s) under consideration undergoes decay, then the Hamiltonian describing the system is no longer Hermitian. Since any matrix can be written as a sum of its Hermitian and anti-Hermitian parts, \ H\ can be written as, The eigenvalues of \ H\ are The suffixes stand for Heavy and Light respectively (by convention) and this implies that \Delta m is positive. The normalized eigenstates corresponding to and respectively, in the natural basis are \ p\ and \ q\ are the mixing terms. Note that these eigenstates are no longer orthogonal. Let the system start in the state That is Under time evolution we then get Similarly, if the system starts in the state, under time evolution we obtain

CP violation as a consequence

If in a system and represent CP conjugate states (i.e. particle-antiparticle) of one another (i.e. and ), and certain other conditions are met, then CP violation can be observed as a result of this phenomenon. Depending on the condition, CP violation can be classified into three types:

CP violation through decay only

Consider the processes where decay to final states, where the barred and the unbarred kets of each set are CP conjugates of one another. The probability of decaying to is given by, and that of its CP conjugate process by, If there is no CP violation due to mixing, then. Now, the above two probabilities are unequal if, . Hence, the decay becomes a CP violating process as the probability of a decay and that of its CP conjugate process are not equal.

CP violation through mixing only

The probability (as a function of time) of observing starting from is given by, and that of its CP conjugate process by, The above two probabilities are unequal if, Hence, the particle-antiparticle oscillation becomes a CP violating process as the particle and its antiparticle (say, and respectively) are no longer equivalent eigenstates of CP.

CP violation through mixing-decay interference

Let be a final state (a CP eigenstate) that both and can decay to. Then, the decay probabilities are given by, and, From the above two quantities, it can be seen that even when there is no CP violation through mixing alone (i.e. ) and neither is there any CP violation through decay alone (i.e. ) and thus the probabilities will still be unequal, provided that The last terms in the above expressions for probability are thus associated with interference between mixing and decay.

An alternative classification

Usually, an alternative classification of CP violation is made:

Specific cases

Neutrino oscillation

Considering a strong coupling between two flavor eigenstates of neutrinos (for example, –, –, etc.) and a very weak coupling between the third (that is, the third does not affect the interaction between the other two), equation gives the probability of a neutrino of type \alpha transmuting into type \beta as, where, E_+ and E_- are energy eigenstates. The above can be written as, Thus, a coupling between the energy (mass) eigenstates produces the phenomenon of oscillation between the flavor eigenstates. One important inference is that neutrinos have a finite mass, although very small. Hence, their speed is not exactly the same as that of light but slightly lower.

Neutrino mass splitting

With three flavors of neutrinos, there are three mass splittings: But only two of them are independent, because. This implies that two of the three neutrinos have very closely placed masses. Since only two of the three \Delta m^2 are independent, and the expression for probability in equation is not sensitive to the sign of \Delta m^2 (as sine squared is independent of the sign of its argument), it is not possible to determine the neutrino mass spectrum uniquely from the phenomenon of flavor oscillation. That is, any two out of the three can have closely spaced masses. Moreover, since the oscillation is sensitive only to the differences (of the squares) of the masses, direct determination of neutrino mass is not possible from oscillation experiments.

Length scale of the system

Equation indicates that an appropriate length scale of the system is the oscillation wavelength. We can draw the following inferences:

Neutral kaon oscillation and decay

CP violation through mixing only

The 1964 paper by Christenson et al. provided experimental evidence of CP violation in the neutral Kaon system. The so-called long-lived Kaon (CP = −1) decayed into two pions (CP = (−1)(−1) = 1), thereby violating CP conservation. and being the strangeness eigenstates (with eigenvalues +1 and −1 respectively), the energy eigenstates are, These two are also CP eigenstates with eigenvalues +1 and −1 respectively. From the earlier notion of CP conservation (symmetry), the following were expected: Since the two pion decay is much faster than the three pion decay, was referred to as the short-lived Kaon, and as the long-lived Kaon. The 1964 experiment showed that contrary to what was expected, could decay to two pions. This implied that the long lived Kaon cannot be purely the CP eigenstate, but must contain a small admixture of , thereby no longer being a CP eigenstate. Similarly, the short-lived Kaon was predicted to have a small admixture of. That is, where, \varepsilon is a complex quantity and is a measure of departure from CP invariance. Experimentally,. Writing and in terms of and, we obtain (keeping in mind that ) the form of equation : where,. Since, condition is satisfied and there is a mixing between the strangeness eigenstates and giving rise to a long-lived and a short-lived state.

CP violation through decay only

The and have two modes of two pion decay: or. Both of these final states are CP eigenstates of themselves. We can define the branching ratios as, Experimentally, and. That is, implying and , and thereby satisfying condition. In other words, direct CP violation is observed in the asymmetry between the two modes of decay.

CP violation through mixing-decay interference

If the final state (say f_{CP}) is a CP eigenstate (for example ), then there are two different decay amplitudes corresponding to two different decay paths: CP violation can then result from the interference of these two contributions to the decay as one mode involves only decay and the other oscillation and decay.

Which then is the "real" particle

The above description refers to flavor (or strangeness) eigenstates and energy (or CP) eigenstates. But which of them represents the "real" particle? What do we really detect in a laboratory? Quoting David J. Griffiths:

The mixing matrix - a brief introduction

If the system is a three state system (for example, three species of neutrinos ⇄ ⇄ , three species of quarks ⇄ ⇄ ), then, just like in the two state system, the flavor eigenstates (say, , ) are written as a linear combination of the energy (mass) eigenstates (say , , ). That is, In case of leptons (neutrinos for example) the transformation matrix is the PMNS matrix, and for quarks it is the CKM matrix. The off diagonal terms of the transformation matrix represent coupling, and unequal diagonal terms imply mixing between the three states. The transformation matrix is unitary and appropriate parameterization (depending on whether it is the CKM or PMNS matrix) is done and the values of the parameters determined experimentally.

Footnotes