Classification Of Particles

When the universe was created, matter and antimatter were made in almost equal amounts. The Universe is made of matter: protons, neutrons, electrons and other such particles. Antimatter has the exact same mass as matter but the opposite charge. For example, positrons are the antiparticle of the electron. You have learned about positrons before in beta-plus decay, in which a proton in the nucleus decays into a neutron.

When matter meets its corresponding antimatter, they annihilate each other, producing high-energy EM waves. In the beginning, it was thought that there was a tiny excess of matter over antimatter. As a result, when most matter and antimatter annihilated each other, the leftover matter became the building blocks of the Universe. Antimatter cannot therefore be stored in a normal container, because as soon as it interacts with normal matter, they annihilate each other. Antimatter must be suspended in a vacuum in a magnetic field so it does not come into contact with its surroundings.

In annihilation, two photons are produced to conserve momentum. We can calculate the energy of the photons produced by using Einstein's famous equation, . Einstein showed us that matter and energy are interchangeable. We can then calculate the energy of the photons produced if an electron and a positron met, assuming they both had zero initial kinetic energy.

First, we calculate the rest mass energy of the electron:

As the positron has the same mass but opposite charge, its energy is the same as that of the electron.

Total energy is conserved, so this is the total energy of the two photons. The energy of one photon is therefore half of this = .

We can find the wavelength of the photon produced using ,

We know that antimatter exists, as it can be made in particle accelerators like those at CERN. The process is called pair production, where a high-energy photon produces a particle-antiparticle pair.

We can calculate the energy of the beam of light needed to create the two particles using Einstein's equation, as particles and antiparticles have the same mass.

We will assume the proton has no kinetic energy, so its energy is due solely to its mass.

As particle and anti-particle are produced, the total energy of the photon is double this number,

Protons and neutrons are not the smallest particles that can exist. Protons and neutrons are all made of quarks. Quarks are known as fundamental particles as they cannot be broken down into anything simpler.

There are three main types of quarks that you need to be aware of: the up quark, down quark and strange quark. They make up particles known as Hadrons.

All matter and antimatter can be classified into two groups, hadrons and leptons.

• Hadrons are made of quarks and can be subdivided into baryons and mesons.

• Baryons are made of 3 quarks (either all real quarks, or all antiquarks, ). An example of a baryon is a proton.

• Mesons are made of 2 quarks (1 real quark and 1 antiquark, ). Kaons and pions are types of mesons.

We can figure out which quarks are present in a hadron by knowing its charge and type of hadron.

For example, a proton is a baryon. It is therefore made of three real quarks. It has a charge of , so it requires three quarks that have a total charge that sums to .

The only logical solution is uud.

If a meson has a strange or anti-strange quark it is called a kaon.

is a positively charged kaon that has a strangeness of . It is a type of meson, so it is made of two quarks, . As it has a strangeness of , it must have an antistrange quark, , which has a charge of , so to get a total charge of , the other quark present must be an up quark.

The quark composition of is .

Leptons are fundamental particles, as they cannot be broken down into anything simpler. The main lepton that you already know about is the electron. It has a lepton number of . Its antiparticle, the positron, has a lepton number of as it is an antiparticle. Other leptons you were introduced to when learning about beta decay are neutrinos, . Neutrinos have negligible mass and zero charge.

There are 4 fundamental forces that exist in nature. You are already very familiar with the force of gravity, and you have previously been introduced to the strong nuclear force. The electromagnetic force is the unified magnetic force and electrostatic force, and the weak nuclear force is responsible for most decays.

Particle interactions are only possible if all conservation laws are obeyed.

• In all interactions, mass-energy, momentum, charge, baryon number, and lepton number must be conserved.

• Strangeness is only conserved in strong nuclear interactions, but in weak nuclear interactions, it can change by or .

• Strange particles (e.g. kaons) are produced via the strong nuclear force in pairs and decay via the weak nuclear force.

To determine whether an interaction is possible, we first write the equation and then check whether charge, lepton, and baryon numbers are conserved, as shown below for the interaction between a neutrino and a neutron that forms an antiproton and a positron.

This interaction is not possible, as lepton and baryon numbers are not conserved.

Strangeness conservation only needs to be checked if the interaction is due to the strong nuclear force, but here we see that, because leptons are involved, the interaction would be due to the weak interaction.