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Particle Physics Experiments

Annihilation

When particles and antiparticles meet, they annihilate each other, releasing their combined mass as energy in the form of photons:

Because momentum and energy have to be conserved, two or three photons are created.  If there is sufficient energy, other particles may be created as well.  For example, the collision between an electron and a positron may give rise to two muons:

                        e-   +   e+  ---->  m+      +     m-

The reverse process can apply as well.  Electrons and positrons can be formed when a gamma ray passes through matter.  This pair production is a good illustration of the interchangeability of mass and energy.  We can see this by applying a magnetic field.  The opposite charges make circular paths in opposite directions.

Beta decay

Beta emission is completely different to alpha.  Beta particles can be negative (b-), which is more common, or b+ in which a positively charged particle is given off.  For beta b- emission we know the following:

• b- particles are given off by neutron-rich nuclei

•  b- particles are electrons

• b- emission is accompanied by the simultaneous emission of an antineutrino, <n_e>.

It is worth noting that the electron and antineutrino are NOT present in the nucleus before the beta decay.  We can write the same for a b+ decay.  A neutron decays to give a b+ with a neutrino.  The b+ particle is called a positron, which is the same size as an electron but has a charge of +1.  We can write general equations to describe beta decay.

b-                    ^AX        ---->         ^AY      +      ⁰e-     +    ⁰n-bar

                        ^{Z                              Z + 1            -1                  0}

                                                               electron   electron antineutrino

b+                          ^AX        ---->        ^AY       +       ⁰e+    +   ⁰n

                        ^{Z                              Z - 1               +1                 0}        

The key thing to be aware of at the particle level is that beta decay is due to the weak interaction, whereby a neutron is turned into a proton:

Feynman Diagrams

We can show this using a Feynman diagram, which we will use as a pictorial representation of what is going on.  They were first devised by Richard Philips Feynman (1918 – 1988) who was an American particle physicist.  At the nucleon level we see:

At the quark level we see:

Beta plus decay is mediated by the W+ particle, to release a positron and an electron neutrino.

There are many other interactions that can be summed up with Feynman diagrams, for example:

• electron capture

• neutrino-neutron collisions

• antineutrino-proton collisions

• electron proton collisions.

When we attempt these, we need to know what the interaction is in terms of the four fundamental forces.

Photons

The forces between electrically charged particles are thought to be transmitted by photons, which are emitted and absorbed by the particles.  We normally associate photons with the particle properties of electromagnetic waves. 

Gluons

As separate particles, gluons have never been directly identified.  They are however the mediators of the strong nuclear force and there is compelling indirect evidence for them.  They are thought to be fundamental particles.  There are eight gluons that have been identified theoretically from quantum chromodynamics, each having a different “colour”, although all have zero rest mass and zero charge.  The nucleons in the nucleus are thought to be held together by mesons such as the pi-meson.

W+, W-, and Z

These are thought to mediate the weak force.  W+ mediates beta plus decay, W- beta minus.   The weak force is little understood, but is thought to be responsible for fusion in stars.  The role of the Z boson is unclear.