Hadron classification

Hadrons are particles that experience the strong interaction. They are classified into two groups: baryons and mesons.

• Baryons include the proton and neutron. Their corresponding antiparticles are the antiproton and antineutron, which are antibaryons.
• Mesons include pions and kaons.

Stability of baryons

The proton is the only stable baryon: it does not spontaneously decay. Other free baryons eventually decay, producing particles that include a proton. The particles produced in any decay must satisfy the relevant conservation laws.

For example, a free neutron eventually decays and produces a proton among its decay products. The statement about decay applies to free baryons; it does not mean that every neutron inside a nucleus must decay.

Baryon number conservation

Baryon number is a quantum number assigned to each particle. A baryon has baryon number $$+1$$, an antibaryon has baryon number $$-1$$, and a particle that is neither has baryon number $$0$$.

In a particle interaction or decay, the total baryon number is conserved: the sum of the baryon numbers before the process equals the sum afterwards. To check a proposed process, add the baryon numbers of all particles on each side and compare the totals.

For example, a proton and an antiproton can annihilate to produce two photons. The proton has baryon number $$+1$$ and the antiproton has $$-1$$, so the total before is $$+1 + (-1) = 0$$. Each photon has baryon number $$0$$, so the total afterwards is also $$0$$. Baryon number is conserved even though the particles have changed.

Pion exchange in the strong nuclear force

Protons and neutrons, collectively called nucleons, are held together in nuclei by the strong nuclear force. One way to model this force is to treat it as arising from the exchange of particles called pions.

In this model, one nucleon emits a pion and another nucleon absorbs it. The exchange transfers momentum between the nucleons, mediating the force between them. This force binds protons and neutrons together in a nucleus.

The exchanged pion is a virtual particle: it is part of the interaction model, not a free pion that can be directly detected.

Lepton classification

Leptons are a group of particles that includes the electron, the muon, the electron neutrino and the muon neutrino. The electron and muon each have a negative electric charge. Their neutrinos are electrically neutral.

Each of these leptons has a corresponding antiparticle. The electron’s antiparticle is the positron, and the muon’s is the antimuon; both have positive charge. The electron neutrino and muon neutrino have corresponding antineutrinos, which are also electrically neutral.

Muon decay

A muon is unstable: it can decay into an electron, an electron antineutrino and a muon neutrino. The decay can be written as:

Here, $$\mu^-$$ is the muon, $$e^-$$ is the electron, $$\bar{\nu}e$$ is the electron antineutrino and $$\nu\mu$$ is the muon neutrino.

Lepton number conservation

Each particle is assigned a number for each lepton family. These lepton numbers allow us to check whether a particle interaction or decay preserves the total for each family.

• Electron lepton number, $$L_e$$: an electron or electron neutrino has $$+1$$; its antiparticle has $$-1$$. Particles outside the electron family have $$L_e = 0$$.
• Muon lepton number, $$L_\mu$$: a muon or muon neutrino has $$+1$$; its antiparticle has $$-1$$. Particles outside the muon family have $$L_\mu = 0$$.

Electron and muon lepton numbers are conserved separately. To check a process, add the relevant lepton numbers of all particles before it and compare the total with the sum afterwards. Both family totals must stay the same.

Lepton numbers in negative muon decay

A negative muon decays into an electron, an electron antineutrino and a muon neutrino:

The initial muon has $$L_e = 0$$ and $$L_\mu = +1$$. After the decay:

• Electron lepton number: the electron has $$+1$$ and the electron antineutrino has $$-1$$. The muon neutrino has $$0$$, so the total is $$+1 - 1 + 0 = 0$$.
• Muon lepton number: the electron and electron antineutrino each have $$0$$, while the muon neutrino has $$+1$$. The total is $$0 + 0 + 1 = +1$$.

Each total is therefore the same before and after the decay.

Strange particles and strangeness

Strange particles, including kaons, can be produced through the strong interaction but decay through the weak interaction. A kaon, for example, can decay into pions.

The quantum number strangeness, conventionally written $$S$$, helps account for which production and decay processes are allowed. Each particle has a strangeness value, and the total strangeness of a state is the sum of the values for all its particles.

Strangeness in strong interactions

Strangeness is conserved in a strong interaction: the total strangeness after the interaction must equal the total before it. If the initial total is zero, the final total must also be zero.

Thus, when a strong interaction produces a strange particle from an initial state with zero strangeness, the final state must include balancing strangeness. For example, a particle with $$S=+1$$ could be produced alongside one with $$S=-1$$, giving a final total of zero.

Strangeness in weak interactions

In a weak interaction, total strangeness can change by $$0$$, $$+1$$ or $$-1$$. This allows a strange particle to decay into particles whose total strangeness differs from its own.

For example, a positive kaon can decay into a positive pion and a neutral pion:

The $$K^+$$ has strangeness $$+1$$, while each pion has strangeness $$0$$. The total strangeness therefore changes from $$+1$$ to $$0$$: a change of $$-1$$, which is allowed in a weak interaction but not in a strong interaction.

Collaboration in particle physics

Particle physics experiments depend on large teams of scientists and engineers. Their work combines different kinds of expertise: engineers help develop the equipment, while scientists contribute to designing experiments, collecting data and analysing the results.

Once data have been collected, team members examine what the measurements show and assess whether the results support a proposed explanation. Different people can check the analysis and question the conclusions, helping the team identify errors or weaknesses.

This collaboration produces evidence that can be examined and checked. When results withstand such assessment, they provide a basis for validating new knowledge about particles.