Activity: Particle physics


  • Introduce the classification of particles into hadrons (mesons and baryons), leptons and gauge bosons.
  • Introduce the conservation of lepton number, baryon number. spin and strangeness.
  • Understand how the spin of baryons and mesons leads to the quark model of hadrons.
  • Apply simple conservation principles to particle interactions.
  • Deduce particle properties from their constituent quarks.
  • Introduce colour charge and gluons
  • Draw Feynman diagrams for strong interactions.

What you know so far

So far the picture isn't that complicated

Neutrons and protons
affected by the strong force
mass about 900 MeVc-2
charge +e or 0
spin 1/2

affected by electric force
mass about 0.5 MeVc-2
charge -e
spin 1/2

affected by weak force
mass about 0
charge 0
spin 1/2

affected by strong force
mass about 200 MeVc-2
Charge -e
spin 0

takes part in electromagnetic interactions
mass 0
charge 0
spin 1

We can classify these particles into two groups. Hadrons that do interact with the strong force and leptons that don't

  • Classify these particles as Hadrons and Leptons.

A further classification of hadrons is into heavy baryons and medium sized mesons.

  • Classify the particles into baryons and mesons.

Conservation principles

When looking at nuclear reactions we can use the conservation of charge and mass number to determine whether reactions are possible.

  • Why is this equation impossible? U presubscript 92 presuperscript 236 rightwards arrow B presubscript 56 presuperscript 142 a plus K presubscript 36 presuperscript 92 r
  • What must be added to balance the equation?
  • Why isn't this possible? H presubscript 1 presuperscript 2 space plus H presubscript 1 presuperscript 2 space rightwards arrow space H presubscript 2 presuperscript 3 e
  • What should the right hand side be?

Lepton number

In particle physics we find that mass and charge is not enough for example no → p+ + e- is OK as far as conservation of charge and nucleon number goes but it's not possible. It seems we can't make leptons out of baryons. To balance this equation we can add an anti neutrino.

n to the power of o rightwards arrow p to the power of plus space plus space e to the power of minus space plus space nu with bar on top

This is possible since the antineutrino (anti lepton) cancels out the electron (lepton). To take this into account we can introduce a new number that must be conserved, the lepton number.

In any interaction lepton number must be conserved

leptons have lepton number = 1
Anti leptons have lepton number -1

Baryon number

It is also true that Baryons can't be made out of leptons or mesons so we introduce the baryon number

In any interaction baryon number must be conserved

baryons have baryon number = 1
mesons and leptons have baryon number = 0

Fill in the quantum numbers

symbol Baryon no. Lepton no. Spin
nu with bar on top      


Hidden explanation (optional)


Total Score:

Do exercises 38 to 44 on page 318

The particle explosion

During the 1960's improving technology meant it was possible to produce particles with very high energy. If high energy particles are collided into each other some of the energy of the collision turns to mass and new particles are formed. As a result a lot of new particles were discovered. See these tables of baryons and mesons. This is a good example of how advances in technology often precede advanced in science (NOS).

As always scientists are on the look out for a simple model and it would be much simpler if all the hadrons were made of different combinations of some fundamental particles - quarks.


If quarks are the building blocks of the hadrons then they must have mass, charge and spin that can add up to the mass charge and spin of the hadrons. Let's start with spin.


When two particles add, their spins can either align in the same way so they add, or opposite, so they cancel.

The diagram shows combinations of particles with spin 1/2 so if quarks have spin 1/2 then they could be used to make particles with different spin

  • What is the total spin of each combination?

Have a look at the meson table and note what the spin on the different mesons is.

  • How many quarks must there be in each meson?

Have a look at the baryon table and note their spin.

  • How many quarks make up the baryons?


  • How many quarks are there in a proton?
  • Guess the charge of each quark that makes up a proton.

A proton is a baryon so has 3 quarks (since its spin is 1/2)
a reasonable guess would be that each quark has charge +1/3 however this is wrong. The proton is made of two different quarks with charge +2/3, +2/3 and -1/3.


There are 6 flavours of quark


Protons and neutrons are made of two different combinations of u and d quarks.Determine the quark combination of a

  • proton
  • neutron

If the flavour of a quark changes then this is a weak interaction.


Some particle interactions that look possible in terms of baryon number, charge and spin are found not to be possible. This is strange and the strangeness quantum number was introduced to solve the problem.

The strangeness number of hadrons is determined by the number of strange quarks

+1 for antistrange -1 for strange

Strangeness is not conserved in weak interactions

  • a Λo is made of uds, what is its charge and strangeness?


Baryons are made of 3 quarks
Mesons are made quark + antiquark pairs
proton is uud
neutron is ddu
π+ is u anti d

Try making baryons and mesons with this applet, remember the charge must be an integer.

Quark confinement

The quark model is very neat but do quarks exist? Well, they've never been found on their own but there is a very good reason for that.

  • The force holding the quarks together is very strong, what does that imply about the energy required to pull them apart?
  • What happens to the energy transferred to the quarks.

So every time two quarks are pulled apart two more quarks are created.

  • Why are so many mesons created when two protons are collided at high energy?

Colour force

The force between quarks is called the colour force and the exchange particle is the gluon. Different types of quark interact with different types of quark but its all too complicated to model with numbers so colours are used. This is how it works:

Quarks are red green and blue
Antiquarks antired antigreen and antiblue

All particles are made of combinations of quarks that are colourless

gluons are colour anticolour combinations e.g. blue-antiblue

  • Why is antiblue yellow?

When a gluon exchanges between two quarks the colour of the quarks change.

  • Why does the green quark lose its green colour?
  • Why does the green quark turn blue?
  • Why does the blue quark lose its blue colour?
  • Why does the blue quark turn green?


Quark Jigsaw puzzle

Here is a game you can play quarknet . You will need Algodoo to run it but you should have that installed by now. Make baryons and mesons out of the quarks. The rule is that there mustn't be any small gaps inside the figures. Full details here.

Feynman diagrams for quark interactions

This is the interaction between two quarks.

  • What is the name and colour of the exchange particle.

  • What does this diagram represent?
  • What is the exchange particle?
  • Which force is involved in this interaction?

Gauge Bosons

There is one group of particles that we haven't classified yet, the exchange particles. There is a different one for each type of interaction and they all have spin 1. This group is known as Gauge Bosons.


The Standard Model

This is a way of organising the fundamental particle into groups so that we can easily see which interact with each other. The model is represented by a table where the particles are ordered into generations according to their mass.

Top row quarks have charge +2/3 and second row -1/3 if quark flavour changes then the change must be ±1 so quarks can only change between the rows. We can see this in the example given in the previous Feynman diagram for beta decay.

  • How much charge was transfered to the electron?
  • What is the change in charge when a down quark changes to an up?

Each top row lepton has its own neutrino. Leptons can only interact with neutrinos of the same generation (same column).

And then the Higgs was found, part 1

and part 2

That's almost the theory of everything, this is the theory of everything:

Try to drag and drop the labels to the right place.

Particle physics multiple choice

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