AQA A-Level Physics/Particles and Anti-particles/Constituents of the particle

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You may have learnt that matter is made up out of three types of particles; protons and neutrons in the nucleus and electrons orbiting around outside. You might think that these particles are the basic building blocks for all matter. While it is true that the electrons cannot be broken down into any smaller particles, it is possible to smash up protons and neutrons into smaller bits. For this reason we say that the electron is a fundamental particle and that protons and neutrons are not fundamental.

The electron belongs to the family of fundamental particles called Leptons. These small particles typically have very small masses. The proton and the neutron belong to a family of heavier particles called Baryons.


What makes up the proton and the neutron?[edit]

Protons and neutrons are made up of smaller, fundamental particles known as quarks. Just like all of the other baryons, protons and neutrons consist of three quarks. The three quarks add together to make the baryon, so in the case of a neutron we require the charges on the quarks to cancel out. In the case of the proton the charges must add up to give e.

To keep things simple in Particle Physics, we can call the proton charge +1 and give each quark a fractional charge as shown in the table below.

Name Charge Baryon Number Strangeness
Up (u) +2/3 1/3 0
Down (d) -1/3 1/3 0
Charm (c) +2/3 1/3 0
Strange (s) -1/3 1/3 -1
Top (t) +2/3 1/3 0
Bottom (b) -1/3 1/3 0

Explaining the properties of quarks[edit]

The first thing you'll be unfamiliar with is the term Baryon Number.This is a term that's basically either as "yes" or "no", and it's called boolean, so yes is +1 and no is 0. If there's an anti-particle, the number will be -1.

Anti-particles exist, and so do anti-quarks! They're represented with what's known as a bar, which is drawn above the shorthand name, so for instance, we have \bar{u} for a u-bar, an anti-up-quark! So, let's look at how a proton is made of quarks! Look:

An example of calculating quarks[edit]

A proton, is made up of 3 quarks, and so is a neutron, because it's a pretty big particle in comparison to others. So, we remember from before that a proton is a baryon, and it must therefore have a baryon number of +1 and a charge of +1. So, let's start off.

U = Charge of +2/3 and a baryon number of 1/3. So, we add that to:
D = Charge of -1/3 and a baryon number of 1/3, so we have...  +2/3 - -1/3 = +1/3 charge and a baryon number of 2/3.
U = Charge of +2/3, so we add that to the +1/3 of U + D, and we get +1 (the charge of a proton) and a baryon number of +1!