Chirality and Flavour of Leptons, Neutrino Masses and Oscillations

Leptons are fundamental fermions that do not participate in strong interactions. They exhibit important internal properties such as chirality and flavour, which play a crucial role in weak interactions. The discovery of neutrino masses and flavour oscillations has provided clear evidence of physics beyond the Standard Model.

1. Flavour of Leptons

Leptons occur in three generations (or flavours), each consisting of a charged lepton and its corresponding neutrino.

Each flavour is associated with a conserved lepton number (L_e, L_μ, L_τ) in the Standard Model.

2. Chirality of Leptons

Chirality is a quantum property related to how a fermion transforms under the weak interaction. It is defined using the γ⁵ operator.

• Left-handed leptons participate in weak interactions.
• Right-handed leptons do not couple to W bosons.

* Right-handed neutrinos are hypothetical and may explain neutrino masses.

3. Neutrino Masses

In the original Standard Model, neutrinos were assumed to be massless. However, experimental evidence now confirms that neutrinos possess very small but non-zero masses.

• Masses are much smaller than charged leptons
• Require physics beyond the Standard Model
• Possible mechanisms: Dirac mass, Majorana mass, Seesaw mechanism

4. Neutrino Flavour Oscillations

Neutrino oscillation is a quantum phenomenon in which a neutrino created with a definite flavour changes into another flavour as it propagates through space.

This occurs because flavour eigenstates are superpositions of mass eigenstates.

5. Significance

• Confirms neutrinos have mass
• Violates lepton flavour conservation
• Strong evidence for physics beyond the Standard Model
• Important for cosmology and matter–antimatter asymmetry

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