Lesson 2.4 - Neutrinos

VIDEO LESSON (34:31 min)
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THE STANDARD MODEL

So far we have three subatomic particles:
- Proton
- Neutron
- Electron

The current "Standard Model" of subatomic particles contains 17 subatomic particles.

What are they and how did they get discovered?

Image result for standard model

BETA DECAY

A neutron is very slightly unstable and decays to a proton and an electron by the process called beta decay. This can be seen in a cloud chamber as below
Image result for cloud chamber neutrino discovery

WHAT HAPPENS?

Expected result:

Experimental result:


THE NEUTRINO

To conserve momentum Wolfgang Pauli theorized that there had to be a weak interacting, light particle present in the collision. He called it a neutrino (small neutral particle in Italian).


It took until 1956 for this particle to be discovered even though Pauli theorized it in 1930. Also, beta decay produces an electron-antineutrino, the antiparticle of an electron neutrino since lepton number has to be conserved.

WEAK INTERACTIONS

Beta decay is a very rare process that involves the change of nuclei. Why is it so rare?

Beta decay is mediated by a new fundamental force

Recall: Four Fundamental Forces
- Electromagnetism (photons)
- Strong Nuclear Force (gluon)
- Weak Nuclear Force (W\( \pm \), \( Z^0 \))
- Gravity (Gravitron?)

MODERN PICTURE OF BETA DECAY


NEUTRINO DETECTION

Capturing neutrinos uses a similar process.

The electron can be detected and the energy of the neutrino can be inferred. This is mediated by a \( W^- \) boson.


Another possible neutrino reaction is mediated by the \( Z^0 \) boson and is thus doesn't change the quark flavour.



SUDBURY NEUTRINO OBSERVATORY



CHERENKOV RADIATION



Last modified: Sunday, 18 May 2025, 8:05 PM