Neutrinos – Properties, Discovery, and Origins

What are Neutrinos?

• Neutrinos (symbol: ν) are electrically neutral, elementary particles that interact only via the weak force and gravity.

• They do not interact via the strong or electromagnetic forces, allowing them to pass through matter virtually undetected.

Key Properties:

• Neutrinos have very small mass — once thought to be zero.

• They are fermions with spin ½ ħ.

• Neutrinos come in three types (flavors):

  • Electron neutrino (νₑ)

  • Muon neutrino (ν_μ)

  • Tau neutrino (ν_τ)

• Each flavor is linked with a corresponding charged lepton.

Flavor Oscillation:

• Neutrinos can oscillate between different flavors during flight.

• This means a neutrino created as νₑ may later behave as ν_μ or ν_τ — a quantum superposition of mass states.

• This proves they have mass, but the exact mass values remain unknown (only mass differences and upper limits are known).

Antineutrinos:

• Every neutrino has a corresponding antiparticle (antineutrino), also electrically neutral but with:

  • Opposite lepton number and weak isospin

  • Right-handed chirality (vs. left-handed for neutrinos)

Production Sources:

• Neutrinos are produced in:

  • Beta decay of nuclei or hadrons

  • Nuclear reactions in stars (e.g., Sun), reactors, and accelerators

  • Supernovae, cosmic ray interactions, and neutron star activity

• Earth receives a solar neutrino flux of ~65 billion neutrinos per second per cm².

Historical Context:

• Proposed in 1930 by Wolfgang Pauli to explain conservation in beta decay.

• Pauli initially called it a "neutron", before the actual neutron was discovered by James Chadwick in 1932.

• The term “neutrino” (Italian for “little neutral one”) was coined by Edoardo Amaldi and popularized by Enrico Fermi to distinguish it from Chadwick’s neutron.

Applications:

• Neutrinos are used in Earth tomography and offer insights into cosmic events.

• Their elusive nature and unique properties make them key to understanding physics beyond the Standard Model.