IB DP Physics: HL复习笔记7.3.2 Quarks & Leptons

Quarks & Leptons


  • Quarks are fundamental particles that make up other subatomic particles such as protons and neutrons
  • Protons and neutrons are in a category of particles called hadrons

Hadrons are defined as any particle made up of quarks

  • Fundamental means that quarks are not made up of any other particles. Another example is electrons
  • Quarks have never been observed on their own, they’re either in pairs or groups of three
  • There are six flavours (types) of quarks that exist:


The six flavours of quarks

  • The charge of a hadron is determined by the sum of the charges of its quarks
  • Each flavour of quark has a certain relative charge:


Each flavour of quark has a charge of either +⅔e or -⅓e

  • For example, a proton is made up of two up quarks and a down quark. Adding up their charges gives the charge of a proton:

+⅔e + ⅔e - ⅓e = +1e

  • The equivalent antiparticle of the quark is the anti-quark
  • These are identical to quarks except with opposite relative charges


Each flavour of anti-quark has a charge of either -⅔e or +⅓e. The quark composition of anti-protons and anti-neutrons changes to anti-quarks

  • Quarks have a baryon number of +1/3
  • Anti-quarks having a baryon number of –1/3
  • Strange quarks have a strangeness of –1
  • Anti-strange quarks have a strangeness of +1
    • This is unique to the strange quark

Worked Example

Particles are made up of a combination of three quarks or two quarks. Which quark combination would not give a particle a charge of -1 or 0?

A. up, strange, strange

B. charm, charm, down

C. top, anti-up

D. anti-up, anti-up, anti-strange


Worked Example

A K- particle has a strangeness of –1. Determine the quark structure of this particle.



  • Leptons are a group of fundamental (elementary) particles
  • This means they are not made up of any other particles (no quarks)
  • There are six leptons altogether:


The six leptons are all fundamental particles

  • The muon and tau particle are very similar to the electron but with slightly larger mass
  • Electrons, muon, and tau particles all have a charge of -1e and a mass of 0.0005u
  • There are three flavours (types) of neutrinos (electron, muon, tau)
  • Neutrinos are the most abundant leptons in the universe
    • They have no charge and negligible mass (almost 0)
  • Leptons interact with the weak interaction, electromagnetic and gravitational forces
  • However, they do not interact with the strong force
  • Although quarks are fundamental particles too, they are not classed as leptons
  • Leptons do not interact with the strong force, whilst quarks do

Worked Example

Circle all the anti-leptons in the following decay equation.WE-Leptons-question-image


Lepton Number

  • Similar to baryon number, the lepton number, L is the number of leptons in an interaction
  • L depends on whether the particle is a lepton, anti-lepton or neither
    • Leptons have a lepton number L = +1
    • Anti-leptons have a lepton number L = –1
    • Particles that are not leptons have a lepton number L = 0
  • Lepton number is a quantum number and is conserved in all interactions
  • This is helpful for knowing whether an interaction is able to happen


The lepton number depends if the particle is a lepton, anti-lepton or neither

Worked Example

If the lepton number is conserved in the following decay, identify whether particle X should be a neutrino or anti-neutrinoScreenshot-2021-03-04-at-12.48.35-pm

Step 1: Determine the lepton number of all the particles on both sides of the equation

    • 0 + (–1) = 0 + X

Step 2: Identify the lepton number of X

    • If the lepton number must be conserved, X must also have a lepton number of –1

Step 3: State the particle X

    • Particle X is an anti-neutrino

Protons & Neutrons

Protons and Neutrons

  • Protons and neutrons are not fundamental particles. They are each made up of three quarks
  • Protons are made up of two up quarks and a down quark
  • Neutrons are made up of two down quarks and an up quark


Protons and neutrons are made up of three quarks

  • You will be expected to remember these quark combinations for exam questions

Protons as Baryons

  • The proton is the most stable baryon
  • This means it has the longest half-life of any baryon and is the particle which other baryons eventually decay to
  • It is the most stable baryon because it is also the lightest baryon
    • Radioactive decay occurs when heavier particles decay into lighter particles
    • A decay of the proton would therefore violate the conservation of baryon number
  • It is theorized that the proton has a half-life of around 1032 years and research experiments are still underway that are designed to detect proton decay

Worked Example


Step 1: Calculate number of protons:

    • The number of protons is from the proton number = 26 protons

Step 2: Calculate number of neutrons:

    • The number of neutrons = nucleon number - proton number = 56 - 26 = 30 neutrons

Step 3: Up quarks in a proton:

    • Protons are made up of uud quarks = 2 up quarks

Step 4: Up quarks in a neutron:

    • Neutrons are made up of udd quarks = 1 up quark

Step 5: Total number of up quarks:

    • 26 protons x 2 up quarks = 52 up quarks
    • 30 neutrons x 1 up quark = 30 up quarks
    • 52 + 30 = 82 up quarks

β– and β+ decay

  • Beta decay happens via the weak interaction
    • This is one of the four fundamental forces and it’s responsible for radioactive decays

Quark Composition: β- decay

  • Recall that β- decay is when a neutron turns into a proton emitting an electron and anti-electron neutrino
  • More specifically, a neutron turns into a proton because a down quark turns into an up quark


Beta minus decay is when a down quark turns into an up quark

Quark Composition: β+ decay

  • Recall that β+ decay is when a proton turns into a neutron emitting a positron and an electron neutrino
  • More specifically, a proton turns into a neutron because an up quark turns into a down quark


Beta plus decay is when an up quark turns into a down quark

Worked Example

The equation for β decay is2.3.3-Beta-Minus-EquationUsing the quark model of beta decay, prove that the charge is conserved in this equation.