A-level Physics (Advancing Physics)/Leptons

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Leptons are particles which interact with all the fundamental forces except for the strong nuclear force. There are two types of leptons: electrons and neutrinos.


Electrons are particles with a charge of -1.6 x 10−19C. There are three types of leptons: the electron (e-), the muon (μ-), and the tauon(τ-), one for each generation. These leptons have antiparticles, each with a charge of +1.6 x 10−19C: the positron (e+), the antimuon (μ+), and the antitauon (τ+), respectively.


Neutrinos are chargeless, and almost massless. Loads of them travel around the universe and through you at speeds close to the speed of light. The symbol for a neutrino is the Greek letter nu (ν), with its generation (e, μ or τ) in subscript. If it is an antineutrino, the symbol has a bar above it. So, the symbol for a muon-antineutrino is .

Lepton Number[edit]

All leptons have a lepton number of 1. All antileptons have a lepton number of -1. In a nuclear reaction, the lepton number before the reaction must equal the lepton number after the reaction. This necessitates the existence of neutrinos. When a nucleus gives off a beta particle (electron), the lepton number before the emission is 0. Without neutrinos, the lepton number after the emission would be 1, not 0. In reality, an electron-antineutrino is also emitted, with a lepton number of -1, and so the total lepton number both sides of the reaction is 0.

The situation is actually slightly more complicated, as the lepton number from each generation of particles must also be conserved. The lepton number from the beta particle cannot be balanced out by a tauon-antineutrino, since this is from a different generation.


1. An electron is produced by a nuclear reaction, but an electron-antineutrino is not produced. What other particle is produced?

2. Why do electrons not make up part of the nucleus?

3. Why did it take until the 1950s to detect the first antineutrino?

4. Complete the following equation for the emission of a beta particle from a nucleus:

5. Complete the following equation for the emission of an antielectron from a nucleus:

6. Complete the following equation for the capture of an electron by a nucleus:

Worked Solutions