Return to 2 KS5 Particles and Radiation

2 Quarks and leptons

 This topic is about learning what is inside a nucleus and essentially how does that make an atom behave. You really need to learn a lot of basic facts.  You can also find loads of youtube movies, just search for the key terms and you will learn lots of things.

Main Resources

02 Quarks and Leptons Student Booklet

2.1 The particle zoo

Particle Physics Reference Booklet (Particle reference sheet)

2.2 Particle Sorting

2.2 ParticleAccelerators  (Information sheet from Cern on Accelerators)

2.2 LHC (Information sheet from Cern on Accelerators)

2.5 Conservation rules

Strangeness (Strangeness Summary)

Cloud Chamber Cosmic Rays

Pb210 Source showing background alpha and neutrino, muons and other cosmic rays in a cloud chamber.

AS Chapter 2 Quarks and Leptons

This is a test designed to check if you understand Chapter 2 of the work you have completed. Only attempt this quiz when you have revised or you will not get a top score. When you get a score you are happy with let you teacher know so they can put it in their markbook.
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Use these quick notes to help you revise each topic from the Chapter.

2.1 The Particle Zoo

Yukawa & the False Meson!

In 1934 Hideki Yukawa predicted the existence and the approximate mass of a particle call the “meson” as the carrier of the strong force that holds the atom together.  Yukawa called his carrier particle the meson, from mesos, the Greek word for intermediate, because its predicted mass was between that of the electron and that of the proton, which has about 1,836 times the mass of the electron.

 Muon Discovery

Carl Anderson found the “mu meson” (or muon) in 1936 with other decay products of cosmic ray interactions. The mu meson had about the right mass to be Yukawa’s carrier of the strong nuclear force, but over the course of the next decade, it became evident that it was not the right particle.

It was eventually found that the mu meson did not participate in the strong nuclear interaction at all, but rather behaved like a heavy version of the electron, and is in fact a lepton rather than a meson. They had found the muon!

Muon Facts

The muon (mu μ is an elementary particle similar to the electron, with a -1.6 x 10-19C negative charge and mass 200x that of the electron.

Together with the electron, the tau, and the three neutrinos, it is classified as a lepton.

As is the case with other leptons, the muon is a fundamental particle.

The muon is unstable with a mean lifetime of 2.2 µs.

This comparatively long decay life time (the second longest known) is due to being mediated by the weak interaction.

Muon Decay

All muons decay to three particles (an electron plus two neutrinos of different types)

But the daughter particles are believed to originate newly in the decay. (Learn this diagram & decay)

Muon -> electron + electron-antineutrino + muon-neutrino


Anti-muon -> positron + electron-neutrino + muon-antineutrino

Feynman Diagram Skills….

  • Charge is conserved bottom to top.
  • Muon converts to muon neutrino of same type (or antiparticle version).
  • Electron & anti neutrino pair produced. (or reversed with e+ and ve)

Pions or Pi-mesons

In particle physics, pion (short for pi meson) is the collective name for three subatomic particles: π0, π+ and π.

Pions are the lightest mesons and play an important role in explaining low-energy properties of the strong nuclear force.

Pions are made of a quark and antiquark a down or up combination.

Decay via weak interaction

Kaons or K-mesons

In particle physics, a kaon also called K-meson is any one of a group of four mesons distinguished by the fact that they carry a quantum number called strangeness.

In the quark model they are understood to contain a single strange quark (or anti strange quark) and either an up or down or anti up or anti down.

In our modern understanding, strangeness is conserved during the strong (collisions) and the electromagnetic interactions, but not during the weak interactions.

Kaons decay “strangely” or last longer than they should for their mass.

Particle Accelerator

Particle accelerators are areas of space where there are strong magnetic and electric fields. Particles can be made to move very fast in circular or straight paths. Then they are collided into each other smashing into pieces. These pieces or “showers” contain other particles made from the original particles and their kinetic energy.

CERN and the Large Hadron Collider can accelerate particles to over 7000GeV or 7000 x 109eV.

2.2 Particle Sorting

Matter & Antimatter

The universe is consists of matter which splits into hadrons (have quarks inside) and leptons (which are empty).

Hadrons then split into baryons (3 quarks or 3 antiquarks) and mesons (1 quark and 1 antiquark. All have fractional charges which add to up to whole numbers.


Hadrons are unstable with the exception being the proton-the only stable Hadron.

Hadrons are composed of smaller fundamental particles called Quarks.

Meson have 2 Quarks and Baryons 3. Hence mesons don’t decay to protons or neutrons.

They all have masses much larger than that of leptons.

Some carry charge i.e. (p, Kˉ, K+)

Some have no charge i.e. (n, Ko)

If charged are effected by electromagnetic interaction, if not they won’t.

Fundamental Forces Recap

The gauge bosons for electromagnetic forces and gravity are without mass. These forces have infinite range.

The gauge bosons for the strong and weak interactions have mass. These forces are short range.

The strong interaction only acts between hadrons (particles made of quarks)

The weak interaction acts on all particles.


Leptons are just points of charge so don’t interact through strong interaction. They are also lightweight compared to hadrons as they have quarks inside which are quite massive!

2.3 Leptons at Work

Lepton & Collisions

The Lepton family of particles consist of the electron, muon and the neutrinos. They have a lepton number of +1 (antileptons are -1).

They are fundamental particles, with no internal structure (i.e. Quarks).

They do not interact via strong force.

Each of the charged leptons has an antiparticle with identical mass but whose other properties are opposite.

Each lepton has an associated neutrino, and antineutrino

Leptons can be accelerated and then collide with each other or other particles to produce hadrons.

Neutrino Facts

Neutrinos are elementary particles that travel close to the speed of light, lack an electric charge, are able to pass through ordinary matter almost undisturbed and are thus extremely difficult to detect. Neutrinos have a minuscule, but nonzero mass. They are usually denoted by the Greek letter  (nu) ν.

There are three types, or “flavors”, of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos

Neutrino Creation

Created as a result of certain types of radioactive decay or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms.

Are generated whenever neutrons change into protons or vice versa, the two forms of beta decay. Interactions involving neutrinos are generally mediated by the weak force (rad decay)

Lepton Decay

When leptons decay we can check to see if it will happen by looking at the conservation of charge, lepton electron number, lepton muon numbers. The system is simple. If you are it then you get a “1” if you are not you get a “0” and if you are an anti you get a “-1”

For example….

Muon -> muon neutrino -> electron + electron-antineutrino

Le…… 0 = 1 + 1 + (-1)

Lµ….. 1 = 1 + 0 + 0

So this decay occurs as both are “conserved”. You must show it like this in your exam.


2.4 Quarks and Antiquarks

Basic Quark to Learn

There are 6 quarks and many particles in the word. However, you are only an AS student so you only need to know of….

Up (u) / down (d) / strange (s)

Learn combinations of quarks and antiquarks required to make;

  •  for baryons (proton and neutron only),
  • antibaryons (antiproton and antineutron only)
  • mesons (pion and kaon)

Quark Facts

While an atom is tiny, the nucleus is ten thousand times smaller than the atom and the quarks and electrons are at least ten thousand times smaller than that.

We don’t know exactly how small quarks and electrons are; they are definitely smaller than 10-18 meters, and they might literally be points, but we do not know.

Quarks have a fractional charge which you are given on the data sheet.

u=2/3,  d=-1/3, s=-1/3

If you logically reason out the charge and if they are “strange” then you can work out all particle constituents from this.

Quarks also carry another type of charge called color charge. (Not required for AS)

Meson Wheel

The Pi-meson and K-mesons can be placed in what is called the 8-fold way. This is confusing at AS as there are six corners and 1 particle to learn in the middle (Pi-Zero). However, if can help to think about the particle anti-particle symmetry. You need a diagram for this one!

Evidence of Quarks

Scientists created very high energy electrons and fired them at protons or neutrons. They found that they were scattered around three main points. This led to the concept of three quarks in hadrons.

A more advanced experiment was to cause the annihilation of electrons and positrons. The annihilation can produce muon-antimuon pairs or quark-antiquark pairs which in turn produce hadrons. The hadron events are evidence of quark production. The ratio of the number of hadron events to the number of muon events gives a measure of the number of “colours” of the quarks, and the evidence points to five quarks with three colours. With the more recent evidence for the top quark, these experiments provide support for the standard model of six quarks with three colours. (Remember colour is not required for AS)

2.5 Conservation Rules

Conservation of Charge

Any particle interaction must conserve charge. This works from start to end and cannot occur if they don’t add up.

Conservation of Baryon Number

This must be conserved with interactions between Baryons (as anything that is not a Baryon has a Baryon Number =0. All Baryons have a B Number=1 and all Anti-baryons have a B Number=-1).

Conservation of Strangeness

Any Hadron that is made up a Strange quark has a Strangeness = -1. Any made up an Anti-strange quark has S=1. Therefore  anything that is not a Hadron has a S=0. This is the opposite of all the other numbers as the “strange” is negative which seems “strange”.

The Lepton Number

There are however two types of Lepton Number each associated with the Electron and Muon. Any Electron or Electron-neutrino has an Le Number =1 with their anti-particles having -1. This pattern also continues with the Lepton-muon number (Lµ) and the Lepton-tau number (Lt).  (tau not required at AS)

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