Neutrino oscillation

Theory

Neutrino oscillation arises from mixing between the flavor and mass eigenstates of neutrinos. That is, the three neutrino states that interact with the charged leptons in weak interactions are each a different superposition of the three (propagating) neutrino states of definite mass. Neutrinos are produced and detected in weak interactions as flavour eigenstates but propagate as coherent superpositions of mass eigenstates.

As a neutrino superposition propagates through space, the quantum mechanical phases of the three neutrino mass states advance at slightly different rates, due to the slight differences in their respective masses. This results in a changing superposition mixture of mass eigenstates as the neutrino travels; but a different mixture of mass eigenstates corresponds to a different mixture of flavor states. For example, a neutrino born as an electron neutrino will be some mixture of electron, mu, and tau neutrino after traveling some distance. Since the quantum mechanical phase advances in a periodic fashion, after some distance the state will nearly return to the original mixture, and the neutrino will be again mostly electron neutrino. The electron flavor content of the neutrino will then continue to oscillate – as long as the quantum mechanical state maintains coherence. Since mass differences between neutrino flavors are small in comparison with long coherence lengths for neutrino oscillations, this microscopic quantum effect becomes observable over macroscopic distances.

In contrast, due to their larger masses, the charged leptons (electrons, muons, and tau leptons) have never been observed to oscillate. In nuclear beta decay, muon decay, pion decay, and kaon decay, when a neutrino and a charged lepton are emitted, the charged lepton is emitted in incoherent mass eigenstates such as | e⁻
⟩, because of its large mass. Weak-force couplings compel the simultaneously emitted neutrino to be in a "charged-lepton-centric" superposition such as | ν
_e ⟩, which is an eigenstate for a "flavor" that is fixed by the electron's mass eigenstate, and not in one of the neutrino's own mass eigenstates. Because the neutrino is in a coherent superposition that is not a mass eigenstate, the mixture that makes up that superposition oscillates significantly as it travels. No analogous mechanism exists in the Standard Model that would make charged leptons detectably oscillate. In the four decays mentioned above, where the charged lepton is emitted in a unique mass eigenstate, the charged lepton will not oscillate, as single mass eigenstates propagate without oscillation.

The case of (real) W boson decay is more complicated: W boson decay is sufficiently energetic to generate a charged lepton that is not in a mass eigenstate; however, the charged lepton would lose coherence, if it had any, over interatomic distances (0.1 nm) and would thus quickly cease any meaningful oscillation. More importantly, no mechanism in the Standard Model is capable of pinning down a charged lepton into a coherent state that is not a mass eigenstate, in the first place; instead, while the charged lepton from the W boson decay is not initially in a mass eigenstate, neither is it in any "neutrino-centric" eigenstate, nor in any other coherent state. It cannot meaningfully be said that such a featureless charged lepton oscillates or that it does not oscillate, as any "oscillation" transformation would just leave it the same generic state that it was before the oscillation. Therefore, detection of a charged lepton oscillation from W boson decay is infeasible on multiple levels.

Pontecorvo–Maki–Nakagawa–Sakata matrix

The idea of neutrino oscillation was first put forward in 1957 by Bruno Pontecorvo, who proposed that neutrino–antineutrino transitions may occur in analogy with neutral kaon mixing. Although such matter–antimatter oscillation had not been observed, this idea formed the conceptual foundation for the quantitative theory of neutrino flavor oscillation, which was first developed by Maki, Nakagawa, and Sakata in 1962 and further elaborated by Pontecorvo in 1967. One year later the solar neutrino deficit was first observed, and that was followed by the famous article by Gribov and Pontecorvo published in 1969 titled "Neutrino astronomy and lepton charge".

The concept of neutrino mixing is a natural outcome of gauge theories with massive neutrinos, and its structure can be characterized in general. In its simplest form it is expressed as a unitary transformation relating the flavor and mass eigenbasis and can be written as

${\displaystyle |{\nu }_i⟩=\sum _{\alpha }{U}_{\alpha i}^{\ast }|{\nu }_{\alpha }⟩}$

${\displaystyle |{\nu }_{\alpha }⟩=\sum _i{U}_{\alpha i}|{\nu }_i⟩}$

where

${\displaystyle |{\nu }_{\alpha }⟩}$ is a neutrino with definite flavor ${\displaystyle \alpha }$ = e (electron), μ (muon) or τ (tauon)

${\displaystyle |{\nu }_i⟩}$ is a neutrino with definite mass ${\displaystyle m_i}$ with ${\displaystyle i}$ = 1, 2 or 3

the superscript asterisk (${\displaystyle {}^{\ast }}$) represents a complex conjugate; for antineutrinos, the complex conjugate should be removed from the first equation and inserted into the second.

The symbol ${\displaystyle U_{\alpha i}}$ represents the Pontecorvo–Maki–Nakagawa–Sakata matrix (also called the PMNS matrix, lepton mixing matrix, or sometimes simply the MNS matrix). It is the analogue of the CKM matrix describing the analogous mixing of quarks. If this matrix were the identity matrix, then the flavor eigenstates would be the same as the mass eigenstates. However, experiment shows that it is not.

When the standard three-neutrino theory is considered, the matrix is 3 × 3. If only two neutrinos are considered, a 2 × 2 matrix is used. If one or more sterile neutrinos are added (see later), it is 4 × 4 or larger. In the 3 × 3 form, it is given by

${\displaystyle \begin{array}{cc}U& =\left[\begin{array}{ccc}{U}_{e1}& U_{e2}& U_{e3}\\ U_{\mu 1}& U_{\mu 2}& U_{\mu 3}\\ U_{\tau 1}& U_{\tau 2}& U_{\tau 3}\end{array}\right]\\ & =\left[\begin{array}{ccc}1& 0& 0\\ 0& c_{23}& s_{23}\\ 0& -s_{23}& c_{23}\end{array}\right]\left[\begin{array}{ccc}{c}_{13}& 0& s_{13}{e}^{-i\delta }\\ 0& 1& 0\\ -s_{13}{e}^{i\delta }& 0& c_{13}\end{array}\right]\left[\begin{array}{ccc}{c}_{12}& s_{12}& 0\\ -s_{12}& c_{12}& 0\\ 0& 0& 1\end{array}\right]\left[\begin{array}{ccc}{e}^{i{\alpha }_1/2}& 0& 0\\ 0& e^{i{\alpha }_2/2}& 0\\ 0& 0& 1\end{array}\right]\\ & =\left[\begin{array}{ccc}{c}_{12}{c}_{13}& s_{12}{c}_{13}& s_{13}{e}^{-i\delta }\\ -s_{12}{c}_{23}-c_{12}{s}_{23}{s}_{13}{e}^{i\delta }& c_{12}{c}_{23}-s_{12}{s}_{23}{s}_{13}{e}^{i\delta }& s_{23}{c}_{13}\\ s_{12}{s}_{23}-c_{12}{c}_{23}{s}_{13}{e}^{i\delta }& -c_{12}{s}_{23}-s_{12}{c}_{23}{s}_{13}{e}^{i\delta }& c_{23}{c}_{13}\end{array}\right]\left[\begin{array}{ccc}{e}^{i{\alpha }_1/2}& 0& 0\\ 0& e^{i{\alpha }_2/2}& 0\\ 0& 0& 1\end{array}\right],\end{array}}$

where c_ij ≡ cos θ_ij, and s_ij ≡ sin θ_ij. The phase factors α₁ and α₂ are physically meaningful only if neutrinos are Majorana particles—i.e. if the neutrino is identical to its antineutrino (whether or not they are is unknown)—and do not enter into oscillation phenomena regardless. If neutrinoless double beta decay occurs, these factors influence its rate. The phase factor δ is non-zero only if neutrino oscillation violates CP symmetry; this has not yet been observed experimentally. If experiment shows this 3 × 3 matrix to be not unitary, a sterile neutrino or some other new physics is required.

Propagation and interference

Since ${\displaystyle |{\nu }_j⟩}$ are mass eigenstates, their propagation can be described by plane wave solutions of the form

${\displaystyle |{\nu }_j(t)⟩=e^{-i\left(E_{j}t-{\overrightarrow{p}}_j\cdot \overrightarrow{x}\right)}|{\nu }_j(0)⟩,}$

where

• quantities are expressed in natural units (${\displaystyle c=1,\hslash =1}$), and ${\displaystyle i^2\equiv -1}$,
• ${\displaystyle E_j}$ is the energy of the mass-eigenstate ${\displaystyle j}$,
• ${\displaystyle t}$ is the time from the start of the propagation,
• ${\displaystyle {\overrightarrow{p}}_j}$ is the three-dimensional momentum,
• ${\displaystyle \overrightarrow{x}}$ is the current position of the particle relative to its starting position

In the ultrarelativistic limit, ${\displaystyle \left|{\overrightarrow{p}}_j\right|=p_j\gg m_j,}$ we can approximate the energy as

${\displaystyle E_j=\sqrt{p_j^2+m_j^2}\simeq p_j+\frac{m_j^2}{2p_j}\approx E+\frac{m_j^2}{2E},}$

where E is the energy of the wavepacket (particle) to be detected.

This limit applies to all practical (currently observed) neutrinos, since their masses are less than 1 eV and their energies are at least 1 MeV, so the Lorentz factor, γ, is greater than 10⁶ in all cases. Using also t ≈ L, where L is the distance traveled and also dropping the phase factors, the wavefunction becomes

${\displaystyle |{\nu }_j(L)⟩=e^{-i{m}_j^{2}L/(2E)}|{\nu }_j(0)⟩.}$

Eigenstates with different masses propagate with different frequencies. The heavier ones oscillate faster compared to the lighter ones. Since the mass eigenstates are combinations of flavor eigenstates, this difference in frequencies causes interference between the corresponding flavor components of each mass eigenstate. Constructive interference causes it to be possible to observe a neutrino created with a given flavor to change its flavor during its propagation. The probability that a neutrino originally of flavor α will later be observed as having flavor β is

${\displaystyle P_{\alpha \to \beta }=|⟨{\nu }_{\beta }|{\nu }_{\alpha }(L)⟩{|}^2={\left|\sum _j{U}_{\alpha j}^{\ast }{U}_{\beta j}{e}^{-i{m}_j^{2}L/(2E)}\right|}^2.}$

This is more conveniently written as

${\displaystyle \begin{array}{cc}{P}_{\alpha \to \beta }={\delta }_{\alpha \beta }& -4\sum _{j>k}{\mathcal{R}}_{\mathcal{e}}\left\{U_{\alpha j}^{\ast }{U}_{\beta j}{U}_{\alpha k}{U}_{\beta k}^{\ast }\right\}{\sin }^2\left(\frac{\mathrm{\Delta }{m}_{jk}^{2}L}{4E}\right)\\ & +2\sum _{j>k}{\mathcal{I}}_{\mathcal{m}}\left\{U_{\alpha j}^{\ast }{U}_{\beta j}{U}_{\alpha k}{U}_{\beta k}^{\ast }\right\}\sin \left(\frac{\mathrm{\Delta }{m}_{jk}^{2}L}{2E}\right),\end{array}}$

where ${\displaystyle \mathrm{\Delta }{m}_{jk}^2\equiv m_j^2-m_k^2.}$

The phase that is responsible for oscillation is often written as (with c and ${\displaystyle \hslash }$ restored)

${\displaystyle \frac{\mathrm{\Delta }{m}_{jk}^2{c}^{4}L}{4\hslash cE}=\frac{\mathrm{G}\mathrm{e}\mathrm{V}\mathrm{f}\mathrm{m}}{4\hslash c}\times \frac{\mathrm{\Delta }{m}_{jk}^2}{{\mathrm{e}\mathrm{V}}^2}\frac{L}{\mathrm{k}\mathrm{m}}\frac{\mathrm{G}\mathrm{e}\mathrm{V}}{E}\approx 1.27\times \frac{\mathrm{\Delta }{m}_{jk}^2}{{\mathrm{e}\mathrm{V}}^2}\frac{L}{\mathrm{k}\mathrm{m}}\frac{\mathrm{G}\mathrm{e}\mathrm{V}}{E},}$

where 1.27 is unitless. In this form, it is convenient to plug in the oscillation parameters since:

• The mass differences, Δm², are known to be on the order of 10⁻⁴ (eV/c²)² = (10⁻² eV/c²)²
• Oscillation distances, L, in modern experiments are on the order of kilometres
• Neutrino energies, E, in modern experiments are typically on order of MeV or GeV.

If there is no CP-violation (δ is zero), then the second sum is zero. Otherwise, the CP asymmetry can be given as

${\displaystyle A_{\mathsf{C}\mathsf{P}}^{(\alpha \beta )}=P({\nu }_{\alpha }\to {\nu }_{\beta })-P({\overline{\nu }}_{\alpha }\to {\overline{\nu }}_{\beta })=4\sum _{j>k}{\mathcal{I}}_{\mathcal{m}}\left\{U_{\alpha j}^{\ast }{U}_{\beta j}{U}_{\alpha k}{U}_{\beta k}^{\ast }\right\}\sin \left(\frac{\mathrm{\Delta }{m}_{jk}^{2}L}{2E}\right)}$

In terms of Jarlskog invariant

${\displaystyle {\mathcal{I}}_{\mathcal{m}}\left\{U_{\alpha j}^{\ast }{U}_{\beta j}{U}_{\alpha k}{U}_{\beta k}^{\ast }\right\}=J\sum _{\gamma ,\ell }{\epsilon }_{\alpha \beta \gamma }{\epsilon }_{jk\ell },}$

the CP asymmetry is expressed as

${\displaystyle A_{\mathsf{C}\mathsf{P}}^{(\alpha \beta )}=16\sin \left(\frac{\mathrm{\Delta }{m}_{21}^{2}L}{4E}\right)\sin \left(\frac{\mathrm{\Delta }{m}_{32}^{2}L}{4E}\right)\sin \left(\frac{\mathrm{\Delta }{m}_{31}^{2}L}{4E}\right)J\sum _{\gamma }{\epsilon }_{\alpha \beta \gamma }}$

Two-neutrino case

The above formula is correct for any number of neutrino generations. Writing it explicitly in terms of mixing angles is extremely cumbersome if there are more than two neutrinos that participate in mixing. Fortunately, there are several meaningful cases in which only two neutrinos participate significantly. In this case, it is sufficient to consider the mixing matrix

${\displaystyle U=\left(\begin{array}{cc}\cos \theta & \sin \theta \\ -\sin \theta & \cos \theta \end{array}\right).}$

Then the probability of a neutrino changing its flavor is

${\displaystyle P_{\alpha \to \beta ,\alpha \ne \beta }={\sin }^2(2\theta ){\sin }^2\left(\frac{\mathrm{\Delta }{m}^{2}L}{4E}\right)\text{ [natural units] .}}$

Or, using SI units and the convention introduced above

${\displaystyle P_{\alpha \to \beta ,\alpha \ne \beta }={\sin }^2(2\theta ){\sin }^2\left(1.27\frac{\mathrm{\Delta }{m}^{2}L}{E}\frac{[\mathrm{e}{\mathrm{V}}^2][\mathrm{k}\mathrm{m}]}{[\mathrm{G}\mathrm{e}\mathrm{V}]}\right).}$

This formula is often appropriate for discussing the transition ν_μ ↔ ν_τ in atmospheric mixing, since the electron neutrino plays almost no role in this case. It is also appropriate for the solar case of ν_e ↔ ν_x, where ν_x is a mix (superposition) of ν_μ and ν_τ. These approximations are possible because the mixing angle θ₁₃ is very small and because two of the mass states are very close in mass compared to the third.

Classical analogue of neutrino oscillation

The basic physics behind neutrino oscillation can be found in any system of coupled harmonic oscillators. A simple example is a system of two pendulums connected by a weak spring (a spring with a small spring constant). The first pendulum is set in motion by the experimenter while the second begins at rest. Over time, the second pendulum begins to swing under the influence of the spring, while the first pendulum's amplitude decreases as it loses energy to the second. Eventually all of the system's energy is transferred to the second pendulum and the first is at rest. The process then reverses. The energy oscillates between the two pendulums repeatedly until it is lost to friction.

The behavior of this system can be understood by looking at its normal modes of oscillation. If the two pendulums are identical then one normal mode consists of both pendulums swinging in the same direction with a constant distance between them, while the other consists of the pendulums swinging in opposite (mirror image) directions. These normal modes have (slightly) different frequencies because the second involves the (weak) spring while the first does not. The initial state of the two-pendulum system is a combination of both normal modes. Over time, these normal modes drift out of phase, and this is seen as a transfer of motion from the first pendulum to the second.

The description of the system in terms of the two pendulums is analogous to the flavor basis of neutrinos. These are the parameters that are most easily produced and detected (in the case of neutrinos, by weak interactions involving the W boson). The description in terms of normal modes is analogous to the mass basis of neutrinos. These modes do not interact with each other when the system is free of outside influence.

When the pendulums are not identical the analysis is slightly more complicated. In the small-angle approximation, the potential energy of a single pendulum system is ${\displaystyle \frac{1}{2}\frac{mg}{L}{x}^2}$, where g is standard gravity, L is the length of the pendulum, m is the mass of the pendulum, and x is the horizontal displacement of the pendulum. As an isolated system the pendulum is a harmonic oscillator with a frequency of ${\displaystyle \sqrt{g/L}}$. The potential energy of a spring is ⁠1/2⁠kx², where k is the spring constant and x is the displacement. With a mass attached it oscillates with a period of ${\displaystyle \sqrt{k/m}}$. With two pendulums (labeled a and b) of equal mass but possibly unequal lengths and connected by a spring, the total potential energy is

${\displaystyle V=\frac{m}{2}\left(\frac{g}{L_{\text{a}}}{x}_{\text{a}}^2+\frac{g}{L_{\text{b}}}{x}_{\text{b}}^2+\frac{k}{m}(x_{\text{b}}-x_{\text{a}}{)}^2\right).}$

This is a quadratic form in x_a and x_b, which can also be written as a matrix product:

${\displaystyle V=\frac{m}{2}\left(\begin{array}{cc}{x}_{\text{a}}& x_{\text{b}}\end{array}\right)\left(\begin{array}{cc}\frac{g}{L_{\text{a}}}+\frac{k}{m}& -\frac{k}{m}\\ -\frac{k}{m}& \frac{g}{L_{\text{b}}}+\frac{k}{m}\end{array}\right)\left(\begin{array}{c}{x}_{\text{a}}\\ x_{\text{b}}\end{array}\right).}$

The 2 × 2 matrix is real symmetric and so (by the spectral theorem) it is orthogonally diagonalizable. That is, there is an angle θ such that if we define

${\displaystyle \left(\begin{array}{c}{x}_{\text{a}}\\ x_{\text{b}}\end{array}\right)=\left(\begin{array}{cc}\cos \theta & \sin \theta \\ -\sin \theta & \cos \theta \end{array}\right)\left(\begin{array}{c}{x}_1\\ x_2\end{array}\right)}$

then

${\displaystyle V=\frac{m}{2}\left(\begin{array}{c}{x}_1{x}_2\end{array}\right)\left(\begin{array}{cc}{\lambda }_1& 0\\ 0& {\lambda }_2\end{array}\right)\left(\begin{array}{c}{x}_1\\ x_2\end{array}\right)}$

where λ₁ and λ₂ are the eigenvalues of the matrix. The variables x₁ and x₂ describe normal modes which oscillate with frequencies of ${\displaystyle \sqrt{{\lambda }_1}}$ and ${\displaystyle \sqrt{{\lambda }_2}}$. When the two pendulums are identical (L_a = L_b), θ is 45°.

The angle θ is analogous to the Cabibbo angle (though that angle applies to quarks rather than neutrinos).

When the number of oscillators (particles) is increased to three, the orthogonal matrix can no longer be described by a single angle; instead, three are required (Euler angles). Furthermore, in the quantum case, the matrices may be complex. This requires the introduction of complex phases in addition to the rotation angles, which are associated with CP violation but do not influence the observable effects of neutrino oscillation.

Theory, graphically

Two neutrino probabilities in vacuum

In the approximation where only two neutrinos participate in the oscillation, the probability of oscillation follows a simple pattern:

The blue curve shows the probability of the original neutrino retaining its identity. The red curve shows the probability of conversion to the other neutrino. The maximum probability of conversion is equal to sin² 2θ. The frequency of the oscillation is controlled by Δm².

Three neutrino probabilities

If three neutrinos are considered, the probability for each neutrino to appear is somewhat complex. The graphs below show the probabilities for each flavor, with the plots in the left column showing a long range to display the slow "solar" oscillation, and the plots in the right column zoomed in, to display the fast "atmospheric" oscillation. The parameters used to create these graphs (see below) are consistent with current measurements, but since some parameters are still quite uncertain, some aspects of these plots are only qualitatively correct.

The illustrations were created using the following parameter values:

• sin²(2θ₁₃) = 0.10 (Determines the size of the small wiggles.)
• sin²(2θ₂₃) = 0.97
• sin²(2θ₁₂) = 0.861
• δ = 0 (If the actual value of this phase is large, the probabilities will be somewhat distorted, and will be different for neutrinos and antineutrinos.)
• Normal mass hierarchy: m₁ ≤ m₂ ≤ m₃
• Δm²
  ₁₂ = 7.59×10⁻⁵ (eV/c²)²
• Δm²
  ₃₂ ≈ Δm²
  ₁₃ = 2.32×10⁻³ (eV/c²)²

Observed values of oscillation parameters

• sin²(2θ₁₃) = 0.086±0.002. PDG combination of Daya Bay, RENO, and Double Chooz results.
• sin²(2θ₁₂) = 0.845±0.018. This corresponds to θ_sol (solar), obtained from KamLAND, solar, reactor and accelerator data.
• sin²(θ₂₃) = 0.56+0.03
  −0.05. corresponding to the joint analysis of the T2K and NOvA Collaborations.
• Δm²
  ₂₁ ≡ Δm²
  _sol = 7.41+0.021
  −0.020×10⁻⁵ (eV/c²)²
• |Δm²
  ₃₁| ≈ |Δm²
  ₃₂| ≡ Δm²
  _atm = 2.437+0.028
  −0.027×10⁻³ (eV/c²)² (in the normal mass hierarchy)
• δ, α₁, α₂, and the sign of Δm²
  ₃₂ are currently unknown.

Solar neutrino experiments combined with KamLAND have measured the so-called solar parameters Δm²
_sol and sin² θ_sol. Atmospheric neutrino experiments such as Super-Kamiokande together with the K2K and MINOS long baseline accelerator neutrino experiment have determined the so-called atmospheric parameters Δm²
_atm and sin² θ_atm. The last mixing angle, θ₁₃, has been measured by the experiments Daya Bay, Double Chooz and RENO as sin²(2θ₁₃).

For atmospheric neutrinos the relevant difference of masses is about Δm² = 24×10⁻⁴ (eV/c²)² and the typical energies are ~ 1 GeV; for these values the oscillations become visible for neutrinos traveling several hundred kilometres, which would be those neutrinos that reach the detector traveling through the earth, from below the horizon.

The mixing parameter θ₁₃ is measured using electron anti-neutrinos from nuclear reactors. The rate of anti-neutrino interactions is measured in detectors sited near the reactors to determine the flux prior to any significant oscillations and then it is measured in far detectors (placed kilometres from the reactors). The oscillation is observed as an apparent disappearance of electron anti-neutrinos in the far detectors (i.e. the interaction rate at the far site is lower than predicted from the observed rate at the near site).

From atmospheric and solar neutrino oscillation experiments, it is known that two mixing angles of the MNS matrix are large and the third is smaller. This is in sharp contrast to the CKM matrix in which all three angles are small and hierarchically decreasing. The CP-violating phase of the MNS matrix is as of April 2020 to lie somewhere between −2° and −178°, from the T2K experiment.

If the neutrino mass proves to be of Majorana type (making the neutrino its own antiparticle), it is then possible that the MNS matrix has more than one phase.

Since experiments observing neutrino oscillation measure the squared mass difference and not absolute mass, one might claim that the lightest neutrino mass is exactly zero, without contradicting observations. This is however regarded as unlikely by theorists.

Origins of neutrino mass

The question of how neutrino masses arise has not been answered conclusively. In the Standard Model of particle physics, fermions only have intrinsic mass because of interactions with the Higgs field (see Higgs boson). These interactions require both left- and right-handed versions of the fermion (see chirality). However, only left-handed neutrinos have been observed so far.

Neutrinos may have another source of mass through the Majorana mass term. This type of mass applies for electrically neutral particles since otherwise it would allow particles to turn into anti-particles, which would violate conservation of electric charge.

The smallest modification to the Standard Model, which only has left-handed neutrinos, is to allow these left-handed neutrinos to have Majorana masses. The problem with this is that the neutrino masses are surprisingly smaller than the rest of the known particles (at least 600000 times smaller than the mass of an electron), which, while it does not invalidate the theory, is widely regarded as unsatisfactory as this construction offers no insight into the origin of the neutrino mass scale.

The next simplest addition would be to add into the Standard Model right-handed neutrinos that interact with the left-handed neutrinos and the Higgs field in an analogous way to the rest of the fermions. These new neutrinos would interact with the other fermions solely in this way and hence would not be directly observable, so are not phenomenologically excluded. The problem of the disparity of the mass scales remains.

Seesaw mechanism

The most popular conjectured solution currently is the seesaw mechanism, where right-handed neutrinos with very large Majorana masses are added. If the right-handed neutrinos are very heavy, they induce a very small mass for the left-handed neutrinos, which is proportional to the reciprocal of the heavy mass.

If it is assumed that the neutrinos interact with the Higgs field with approximately the same strengths as the charged fermions do, the heavy mass should be close to the GUT scale. Because the Standard Model has only one fundamental mass scale, all particle masses must arise in relation to this scale.

There are other varieties of seesaw and there is currently great interest in the so-called low-scale seesaw schemes, such as the inverse seesaw mechanism.

The addition of right-handed neutrinos has the effect of adding new mass scales, unrelated to the mass scale of the Standard Model, hence the observation of heavy right-handed neutrinos would reveal physics beyond the Standard Model. Right-handed neutrinos would help to explain the origin of matter through a mechanism known as leptogenesis.

Other sources

There are alternative ways to modify the standard model that are similar to the addition of heavy right-handed neutrinos (e.g., the addition of new scalars or fermions in triplet states) and other modifications that are less similar (e.g., neutrino masses from loop effects and/or from suppressed couplings). One example of the last type of models is provided by certain versions supersymmetric extensions of the standard model of fundamental interactions, where R parity is not a symmetry. There, the exchange of supersymmetric particles such as squarks and sleptons can break the lepton number and lead to neutrino masses. These interactions are normally excluded from theories as they come from a class of interactions that lead to unacceptably rapid proton decay if they are all included. These models have little predictive power and are not able to provide a cold dark matter candidate.

Oscillations in the early universe

During the early universe when particle concentrations and temperatures were high, neutrino oscillations could have behaved differently. Depending on neutrino mixing-angle parameters and masses, a broad spectrum of behavior may arise including vacuum-like neutrino oscillations, smooth evolution, or self-maintained coherence. The physics for this system is non-trivial and involves neutrino oscillations in a dense neutrino gas.