Home >> Nuclear, fundamental particles

Particles & Anti-particles

Simply put, an anti-particle has the opposite charge to the original particle, but the same mass.

proton p⁺    anti-proton p^-

electron e^-    anti-electron(positron) e⁺

More generally, an antiparticle is signified by a short line drawn above it.

Anti-matter has the same structure as ordinary matter, but all the signs of the particles are reversed. So anti-matter hydrogen would be an anti-proton nucleus (p^-) with a positron (e⁺) in orbit around it.

However, it must be borne in mind that 'sign change' is often the result of changes at a more basic level. This will become more apparent when 'quarks' are discussed.

Particles and anti-particles are created and destroyed in pairs. The two processes are pair production and pair annihilation.

Pair Production is the creation of a particle and its anti-particle when a gamma ray photon passes close to a nucleus.

Pair Annihilation is when a particle and its anti-particle collide. The result is energy in the form of gamma ray photons and/or other particles being produced.

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Quarks

By definition, a fundamental particle cannot be split up into anything simpler. So an electron is a fundamental particle, but a proton is not.

When high energy electrons are fired at protons and neutrons, the electrons are deflected(scattered) through a wide range of angles.

The explanation is that an electron penetrates the nucleon (proton/neutron) and makes an inelastic collision with other particles within it. The electron can do this because it is unaffected by the Strong Force. The phenomenon is called deep inelastic scattering.

These 'other particles' are what we now call quarks. They are fundamental particles that have fractional charges on them, positive or negative, with masses much greater than electrons.

There appear to be 6 basic types of quark(sometimes called their flavour).

up (u) ...........................down (d)

charm (c) ....................strange (s)

top (t) .........................bottom (b)

So with their anti-particles, there are 12 quarks in total.

Quarks & anti-quarks are described as having the following properties:

charge e
Each has either + or - values of 2e/3 or e/3 (e: electronic charge). Charge is always conserved in interactions.

baryon number B

Each has a baryon number of 1/3. Like charge, this is also conserved in interactions.

strangeness S

This property is conserved in 'strong' and 'electromagnetic' interactions. The values can however be altered in 'weak' interactions.

* signifies anti-particle

The structure of protons, neutrons, anti-protons and anti-neutrons can now be described:

u ... up quark         d ... down quark

proton	u u d	( +2/3 +2/3 -1/3 = +1 )
neutron	u d d	( +2/3 -1/3 -1/3 = 0 )
anti-proton		( -2/3 -2/3 +1/3 = -1 )
anti-neutron	.	( -2/3 +1/3 +1/3 = 0 )

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Leptons

Leptons, like quarks, are also fundamental particles. They cannot be split up into anything simpler. Leptons are only affected by the 'weak' nuclear force.

There are 3 families of lepton:

The Electron Family

The Muon Family

The Tauon Family

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Hadrons

Hadrons are not fundamental particles. Hadrons are exclusively composed of quarks. Further, hadrons are affected by both the 'strong' and the 'weak' nuclear forces.

Hadrons fall into 2 groups called baryons and mesons.

Baryons

Baryons are composed of 3 quarks. There are only two baryons with quarks in stable configurations. These are the proton(uud) and the neutron(udd).

Mesons

Mesons are composed of 2 quarks (a quark and an anti-quark). Note, the quarks and anti-quarks can be of different flavours.

Mesons fall into two families called pions* and kaons. each family(including anti-particles) has 6 members.

*sometimes called pi-mesons

Pions

Kaons

kaon	structure	charge e	baryon no. B	strangeness S
ko		0	0	+1
k+		+1	0	+1
k-		-1	0	-1

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