Particle classification for OCR A-level Physics
This page covers the following topics:
2. Pions and Kaons
4. Strange particles
5. Quarks and antiquarks
6. The conservation laws
Hadrons are the heaviest particle known and is made up of smaller particles called quarks. Hadrons come in two categories: baryons and mesons. Baryons are hadrons made up of three quarks, including neutrons and protons. The only stable hadron is the proton, and all other hadrons will eventually decay into one. All baryons have a baryon number of 1, whereas all other particles have a baryon number of 0. Mesons are hadrons made up of 2 quarks, a quark and an antiquark. Mesons will never decay into protons. Antihadrons are made up of the antiquarks of the quarks that the corresponding hadron is made of.
Pions and kaons are types of mesons. Pions can be positive (π⁺), negative (π⁻) or neutral (π⁰). The antiparticle of π⁺ is π⁻, whereas the antiparticle of π⁰ is itself. Pions are stable mesons since they are the lightest. The strong nuclear force is caused by a particle exchange, where the pion is the exchange particle. The strong nuclear force is transmitted between a proton and neutron by exchanging a pion. Kaons can be positive (K⁺), negative (K⁻) or neutral (K⁰). The antiparticle of K⁺ is K⁻, whereas the antiparticle of K⁰ is itself. Since kaons are heavy, they are unstable and thus eventually decay into pions. Kaons tend to have very long lifetimes, due to the fact that they contain a strange quark.
Leptons are a family of fundamental particles, meaning that they are not made up of any other particles. Leptons are the lightest particles that exist. All leptons have a corresponding neutrino lepton, and the neutrino of the corresponding particle will be involved in reactions of the particle. The most common leptons are electrons, muons, tauons, electron neutrinos, muon neutrinos and tauon neutrinos, and their antiparticles. Neutrinos are chargeless particles with negligible mass, which makes them almost impossible to detect. Leptons do not interact with the strong nuclear force, but instead with the weak gravitational or electromagnetic interactions. The lepton number is the number of leptons in an interaction, where leptons have a lepton number of 1, antileptons have a lepton number of −1 and all other particles have a lepton number of 0. The lepton number is conserved in particle equations.
Strange particles are ones made up of strange or antistrange quarks. They are produced through the strong interaction and decay through the weak interaction. Strangeness is a number conserved in all interactions, except weak interactions, meaning that strange particles are always produced in pairs. Particles with a strange quark have strangeness −1, ones with an antistrange quark have strangeness +1 and ones with no strange quark have strangeness 0.
Quarks and antiquarks are the fundamental particles which make up hadrons. There are 6 types of quarks and their corresponding antiquarks: up, down, charm, strange, top, bottom. All quarks have a baryon number of 1/3. All quarks also have strangeness of 0, except for the strange quark, which has strangeness −1. Protons and neutrons are both made up of quarks: a proton is made of two up quarks and one down quark (uud) and a neutron is made of one up quark and two down quarks (udd).
Baryon number must be conserved in particle reactions. The law of baryon number conservation states that the baryon number before and after the reaction must be equal. Lepton number must also be conserved. The law of lepton number conservation states that the lepton number before and after the reaction must be equal. Strangeness is also conserved in particle reactions. The law of conservation of strangeness states that the strangeness before and after the reaction must be equal. However, this law does not hold for weak decays. Another quantity which is conserved in particle reactions is charge. If any reaction does not conserve the above quantities, it cannot occur.
State what quarks are.
Quarks are the fundamental particles which make up hadrons.
State the quark combination of an antiproton.
A proton's quark combination is uud.
An antiproton will be made up of the antiquarks of a proton.
So, the quark combination of an antiproton is ūūđ.
What is the antiparticle of π⁰?
The antiparticle of π⁰ is itself.
An omega particle is made up of three strange quarks. Calculate the charge, the baryon number and the strangeness of the particle.
Charge = − 1/3 − 1/3 − 1/3 = −1.
Baryon number = 1/3 + 1/3 + 1/3 = 1.
Strangeness = − 1 − 1 − 1 = −3.
Define what leptons are.
Leptons are a family of fundamental particles, meaning that they are not made up of any other particles.
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