AQA A-Level Physics/Quarks and sub-atomic particles

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At GCSE you would have learned about the subatomic particles known as protons and neutrons and electrons. However there are many more subatomic particles than this that can be divided into two groups, Hadrons and Leptons.


Hadrons − Hadrons are particles that interact using the stong nuclear force. Hadrons come in two further groups, Baryons and Mesons.
Baryons are hadrons that can decay into or are protons. These include: protons, neutrons, antiprotons and antineutrons.
Mesons are hadrons that do not decay into protons, such as: pions and kaons. Pions and kaons can be positive, neutral and negative.
Baryons and mesons aren't fundamental particles and so can be split into smaller particles known as quarks.


Leptons − Leptons are particles that interact using the weak nuclear force. Leptons are fundamental particles and so can not be split into any smaller particles. These include: electrons, muons, electron neutrino, muon neutrino, and their respective antiparticles.


Quarks Quarks are the particles that make up hadrons. All quarks have a charge. There are: up, down, strange, anti up, anti down and anti strange quarks.When kaons decayed, scientists observed that the particle took longer to decay than predicted, meaning that the particle decayed under weak interactions. To explain this, they were given a property known as strangeness, which is exhibited by strange quarks (strangeness -1) and anti strange quarks (strangeness +1).

Up Down Strange Anti Up Anti Down Anti Strange
Charge +2/3 -1/3 -1/3 -2/3 +1/3 +1/3
Stangeness 0 0 -1 0 0 +1


Baryons are made up of three quarks. The proton is made from uud quarks (charge +2/3 +2/3 -1/3 = 1), and the neutron is made from udd quarks (charge +2/3 -1/3 -1/3 = 0). Anti protons are made from anti up anti up anti down quarks, Anti neutons are made from anti up anti down anti down.

Mesons are made from 2 quarks, and always a quark and an anti quark.