The Particle Zoo

Too Many Particles

• Far from just finding• Protons• Neutrons• Electrons• Neutrinos

and their antiparticles,

during the 20th century many new particles were discovered having different properties

New discoveries

New particles were discovered in cosmic rays that collide with our upper atmosphere.

Most of these particles are very short lived breaking up into other particles very quickly.

A single proton colliding with the atmosphere produces showers of hadrons of new types. These sorts of collisions are abundant in our uper atmosphere

The Aurora Borealis (The Northern Lights) is the product of billions of particle collisions in the upper atmosphere. It can be seen in the far north.

Particle accelerators

The Van Der Graaf Generator used for its original purpose

Particle accelerators were produced to give collisions which were reproducible instead of just looking at accidental collisions in the atmosphere.

The Van Der Graaf Generator was one of the first particle accelerators.

Particle accelerators

The Cyclotron

Early Linear Accelerator

Too many hadrons• Most of the particles

produced in these collisions were hadrons. That is they respond to the strong force.

A LOOK AT THE HADRONS FOUND (Don’t Panic!)

Note that there seem to be two main types of particle.

Those with “spin” which is a whole number or 0 these are mesons

These are the and those where the number is a half or a multiple of ½ we call these baryons

Baryons

From the Greek for “heavy”

Mesons

From the Greek for “middle”

Remember lepton means light

The Particle Zoo

• There were now so many particles that they were referred to as the particle zoo

• The problem is that not all of these particles can be classed as fundamental.

• When things get this complicated it means that there must be some simpler building blocks which make up these particles!

Note that all of the Baryons are unstable

Except (as far as we know) the PROTON

In fact all baryons decay producing a proton

You can think of most baryons as almost explosive!

A characteristic time for the decay is 10-23 seconds. This is the time it takes for a light wave to cross the nucleus!!!

An exception is the (free) neutron which takes on average around 15 minutes to decay

If all baryons decay producing a proton does that mean there is a rule?

Conservation of Baryon Number

• It turned out that if the baryons were allocated a number

• 1 for baryons

• -1 for antibaryons

• In interactions baryon number was conserved.

The Rules So Far

1. energy is conserved.

2. (Electric) charge is conserved

3. Baryon number is conserved

4. Lepton number is conserved

n p + e- + νe

∆+ π+ + π0

Ξ0 p + π0

Possible and impossible decays

Λ p + p (bar)Λ p + π -Σ0 Λ + γ

Using conservation of charge and conservation of baryon number can you say which of these decays is possible and which is impossible?

Hadrons you need to know

• Your specification says:

• baryons (proton, neutron) and antibaryons (antiproton and antineutron)

• mesons (pion, kaon).

This means all other details must be given in the question or on the data sheet