AS AQA Phys Unit 1- Classification of Particles

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Hadrons protons, neutrons, pi-mesons, K mesons

Hadrons are particles which are acted on by the strong nuclear force e.g. protons and neutrons. Since protons are positively charged they need the strong nuclear force/strong interactions to hold them together. Hadrons are non-fundamental particles- they are made up of smaller particles called quarks. There are two types of hadrons; baryons and mesons. These are classified according to the number of quarks they're made up of.

Hadrons interact through the strong interaction and through electromagnetic interaction if charged. Apart from the proton, which is stable, hadrons decay through the weak interaction.


Nucleons (protons and neutrons) are baryons. -There are other baryons, such as short-lived sigmas, which aren't found in normal matter (these other baryons are not needed for AS).

The proton is the only stable baryon. All other baryons are unstable so can decay to become other particles.  The particles a baryon becomes depends upon the baryon but always includes a proton. Protons are the only stable baryons and do not decay. Therefore all baryons except protons decay to a proton.

Antibaryons are the antiparticles of protons and neutrons. However antibaryons are not found in ordinary matter due to annihilation.

Baryon number

Protons and neutrons are baryons so have a baryon number of +1. Antibaryons are -1 and anything which isn't a baryon is 0. During an interaction baryon number on either side of the equation remains the same. This can be used to predict whether an interaction will happen. If the baryon numbers are not equal on either side the interaction cannot happen. The total baryon number in any interaction never changes.

Neutron decay

Neutrons decay into protons. Beta decay involves a neutron changing into a proton which occurs when there are more neutrons than protons in a nucleus. Beta decay is caused by the weak interaction. When a neutron decays it forms a proton, electron and antineutrino;

n ----> p + e^- + (anti) Ve

Electrons and antineutrinos are not baryons they are leptons. Therefore their baryon number = 0. Neutrons and protons have a baryon number of +1 therefore this interaction can happen as on both sides of…


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