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Deep inelastic scattering

Strange new particles in cosmic rays

These lessons are about radioactivity rather than particle physics so this is just a tiny taster.

Try Why Do Astronauts Float by Julian Hamm

In the 1950s strange new particles began to be discovered in cosmic rays that shared some of the properties of protons and neutrons.  The theoretical physicists Murray Gell-Mann and George Zweig independently suggested that there was a more fundamental particle that could explain them.

Every time you think there's nothing smaller...

But what to call it?

‘A-tom’ was ancient Greek for ‘un-cuttable’ but everyone now accepted that atoms were made up of protons, neutrons and electrons.  So Gell-Mann wanted to avoid any name that suggested that no more fundamental particle would ever be found.

He chose the nonsense word ‘quark’ from James Joyce’s final novel, Finnegan’s Wake.  A chorus of seagulls sing ‘Three quarks for Muster Mark’.  Gell-Mann wanted it pronounced ‘kwork’ not ‘kwark’ but it was a losing battle.

The names are just for fun

The idea was that quarks came in three flavours called ‘up’, ‘down’ and ‘strange’.  At the time it was considered just to be an elegant theory with no obvious way of testing it.

The higher the energy the better the 'microscope'

In the late 60s scientists suggested scattering electrons off a target of protons and seeing if the pattern revealed the presence of quarks.

Remember how Rutherford used the scattering of alpha particles off gold atoms to find out about the nucleus.  The problem was that the speed of the alphas just depended on your source and you couldn’t do much to change it.

Quantum theory suggests that the more energetic your particles the finer the detail you ought to be able to see.  By 1968 the huge Stanford Linear Accelerator could accelerate electrons to ten thousand times the energy of Rutherfords’s alphas.

The electrons were scattered off protons and the scattering angles were measured.

If energy seems to be lost there must be something smaller

Imagine two balls colliding.  Some energy is converted into the movement of the particles that make up the balls.  If you measure only the speeds of the balls then there seems to be some loss of energy.

This is called an inelastic collision.

However if two particles have no internal structure then when they collide no energy can be lost in this internal movement.  In this case the collisions are called ‘elastic’ because the speeds don’t seem to show any energy being lost.

If electrons and protons have no internal structure then they should always make elastic collisions with each other.

Deep inelastic scattering

At low energies electrons and protons make elastic collisions and the scattering angles are quite small.  At very high energies however there was an apparent loss of energy and the scattering angle was big.

And if the energy was high enough the proton broke apart and a shower of particles appeared.

This is called deep inelastic scattering and it suggested that the protons had some internal structure.

Quantum theory allows particles to be created from nowhere if there is enough energy available to pay for them.  So the particles were created during the collision and were paid for by the electron’s energy.

A careful study of the results confirmed that the shower of particles were made of pairs of quarks, consisting of an up or a down bound to their antiparticle.  These are now called mesons because of their intermediate mass.

back to Lesson 10: Changes to the Nucleus