Classification of Particles

Hadrons

Hadrons are particles that experience the strong nuclear force.

They are made up of smaller particles called quarks, hence are not a fundamental particle.

Baryons

Baryons are hadrons with 3 quarks. The quarks are all the same type of quark; either 3 normal quarks (baryon) or 3 antiquarks (antibaryon).

Protons and neutrons are both baryons. There are other types of baryons, including sigmas \left(\Sigma\right) and lambdas \left(\Lambda\right), but these have very short half-lives and you don’t need to know them.

All particles have a baryon number \left(B\right). Baryons have a baryon number of B = 1, antibaryons have a baryon number of B = -1, with all other particles having a baryon number of B = 0.

The baryon number is a quantum number and is conserved for all particle interactions. This means that in a particle interaction, both sides of the equation must have the same baryon number. A quantum number can only take integer values.

All quarks have a baryon number of \dfrac{1}{3}, while all antiquarks have a baryon number of -\dfrac{1}{3}. The baryon number for each quark will be given on the datasheet.

Protons are the only stable baryon, all other baryons eventually decay to a proton. This is because protons are the lightest baryon and in general, radioactive decay causes heavier particles to decay to lighter particles. The half life of a proton has not been confirmed experimentally but it is theorised to be 1034 years.

Neutrons decay into protons through beta-minus decay, as shown in the diagram below. This decay is caused by the weak interaction. This occurs in a nucleus that is unstable due to the amount of neutrons or when a neutron is outside the nucleus. An electron and antineutrino are also formed in the decay, however as these are not baryons, they do not affect the total baryon number.

Mesons

Mesons are hadrons that are made up of a quark and an antiquark. They have a baryon number of B = 0 and can interact via the strong nuclear force.

Pions \left(\pi\right) are the lightest and most stable meson. There are different types of pions depending on the charge of the pion: \pi^+ \text{,} \: \pi^0 \text{,} \: \pi^- .

Pions can be produced in high energy particle collisions and were discovered in cosmic rays. They can be observed in cloud chambers.

They are a key component in the action of the strong nuclear force, which is one of the four fundamental interactions and holds nucleons together in the nucleus. Pions are the exchange particles for the strong nuclear force. This means that when the strong force acts between 2 nucleons, a pion is exchanged (passed between) them. In this situation, the pion is a virtual particle and therefore is not observed.

Kaons \left(K\right) are a heavier and less stable meson. They are produced by the strong interaction between pions and protons and eventually decay into pions. There are different types of kaons depending on the charge: K^+ \text{,} \: K^0 \text{,} \: K^-.

Kaons have long lifetimes, which is thought to be due to them containing a strange quark. They decay through the weak interaction and decay into pions.

Leptons

Leptons are a fundamental particle. They are not made up of any other particle.

They do not feel the strong nuclear force and interact with other particles through the electromagnetic, gravitational and weak interactions. Two examples of leptons that you need to know are the electron and muon.

Electrons are stable and have a relative charge of -1. Muons are heavier than electrons, making them more unstable but have the same charge as an electron.

Both leptons have their own neutrinos: electron neutrino and muon neutrino. Neutrinos have negligible mass and zero charge and only interact through the weak interaction. They are the most abundant leptons in the universe.

The lepton number is a quantum number that gives the number of leptons. There are two different types of lepton number, electron lepton number \left(L_e\right) and muon lepton number \left(L_{\mu}\right). Leptons and their corresponding neutrinos have a lepton number of +1, while antileptons and lepton antineutrinos have a lepton number of -1.

Particle	Symbol	Relative Charge	\boldsymbol{L_e}	\boldsymbol{L_{\mu}}
electron	e^-	-1	+1	0
positron	e^+	+1	-1	0
electron-neutrino	v_e	0	+1	0
electron-antineutrino	\bar{v}_e	0	-1	0
muon	\mu^-	-1	0	+1
antimuon	\mu^+	+1	0	-1
muon-neutrino	v_{\mu}	0	0	+1
muon-antineutrino	\bar{v}_{\mu}	0	0	-1