Originally particles were classified in terms of their masses:

leptons – light particles eg electrons

mesons – particles of intermediate mass eg pions . Mesons are composed of one quark and one antiquark.

baryons – heavy particles eg protons and neutrons. Composed of three quarks according to certain rules. Each quark has a colour, and the quarks combine in a baryon to be colourless.

This classification is superseded by one which classifies particles are classified in terms of their interactions.

Hadron – experience strong interaction eg proton and neutron

Lepton – do not experience strong interactions

Particles may also be classified in terms of their intrinsic spin.

Fermions – have half integer spin egFermions obey the Pauli exclusion principle. Fermions are matter particles, as opposed to bosons which transmit forces.

Bosons – Have no spin or integer spin eg 0, 1, 2,... eg mesons. Bosons do not obey the Pauli exclusion principle. These particles transmit the fundamental forces between other particles.

Note that the spin is always measured in units ofso that spinmeans

Strong Force – the force which holds the nucleus together against the mutual repulsion of the protons. mediated by gluons.

Weak Force – responsible for beta decay and the decay of neutrons into protons. Mediated by theandparticles.

Electromagnetic Force – the most familiar force besides gravity. All charges particles experience the electromagnetic force.

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