# A Basic Introduction to Particle Physics

Part 1 of the ‘How to Spot a Neutrino from a Hole in the Ground’ series, based on the talk by Art Mcdonald as well as my own work based on Particle Physics.

Introduction to this series

At the start of the talk, Art gave the audience a brief overview of Particle Physics, in order to allow him to explain his work at the Sudbury Neutrino Observatory, which proved that neutrinos have mass. Notes from this talk will be built upon by my own studies in order to give ‘A Basic Introduction to Particle Physics.’ Knowledge on GCSE-Level Physics will be needed.

The Atom

Atoms are composed of a nucleus orbited by electrons, organised in shells. The nucleus contains protons and neutrons, the protons being positively charged & neutrons having no charge; thus, they are neutral. Electrons are negatively charged. Their properties can be shown in the table below:

Every atom has the same number of electrons and protons, so it has no overall charge – if you look at the table, the charge of the proton is +1.6*10^-19, whilst the charge of the electron is -1.6*10^-19. The charge of the neutron is 0, so when you add up all three values, you get a value of 0.
You can have different variations on the atom – isotopes are atoms with a different number of neutrons but the same number of electrons and protons. Ions have a different number of electrons.

Models

There have been various models of the atom throughout time. In 1810, the Billiard Ball model was created, which suggested that the atom was the smallest piece of an element possible – based on the Greek idea of ‘atomos’ – and it was a solid sphere. Each of these atoms of the element had the same mass, so different elements had different masses. They could not be created or destroyed, much like what is seen in the Law of Conservation of Energy.

The Plum Pudding model suggested that the atom was a sphere of positive charge with electrons embedded inside – the so called “plums – and their properties were the same as this Billiard model. The Rutherford Scattering experiment aimed to test this by firing alpha particles at a gold leaf. However, only one in 8000 deflected over 90 degrees, proving that the mass was in fact concentrated in a positively charged region at the centre of the atom, surrounded by electrons. This left us with the Rutherford-Bohr model, that is taught as the generally accepted structure of the atom during high school – one that I too have learnt about, but isn’t all that correct, as we will find out.

The Smallest?

Now is where we enter the realms of fundamental particles – particles that cannot be broken down further, and also appear to have no structure. The Standard Model shows these 12 fundamental particles, alongside exchange particles – also known as force carrier particles – and the Higgs Boson & gravitons. To be able to explain each and every one of these fundamental particles would take a long time and require me to get into degree level physics, a stage that I am not at yet, so I will instead explain one of the groups that can be identified in the table.

Exchange Particles – there are four fundamental forces that allow particles to interact with each other, which is what makes up the known universe. These forces are carried by exchange particles, with a different particle for a different force: photons carry the electromagnetic force,  gluons carry the strong nuclear force, W+, W- and Z bosons carry the weak force, and gravitons carry gravity, so to speak.

Quarks – fundamental particles that have a fractional electrical charge. Much of the observed universe is made of the combination of 3 subatomic particles. For example, protons are made up of 2 up quarks and 1 down quark. The up quark has a rough charge of 2/3, whilst the down quark has a rough charge of -1/3. Thus, (2*2/3) – 1/3 = 1, hence the integer charge value given to protons of 1.

There are 6 known type of quarks that can be seen in the Standard Model, and they are held together by gluons under its strong nuclear force – think glue – with their existence coming and going as they exchange between quarks. The energy needed to do this is part of the mass of the proton and neutron, and each type of quark can be observed under different conditions e.g. the top quarks only being observable in particle accelerators. Quarks can be distinguished by their masses, that are all different.

Feynman showed the gluon-quark interactions in his diagrams, seen below. As the two quarks interact with the gluon, they are converted into a different type, as seen by their colour changes.

Leptons – fundamental particles that do not undergo strong nuclear forces, but instead the weak forces. They are not made up of quarks – instead, they are categorised into the electron, muon and tau particles, each with their own neutrino, antineutrino and antiparticle (more on antiparticles later on). An example of what leptons make up include the fact that muon particles make up half the cosmic radiation at sea level.

Mr Mcdonald’s talk was about detecting these leptons – specifically, the neutrinos – that are created from the sun – also known as solar neutrinos, and the impact of the results of this experiment. I will discuss leptons and neutrinos in greater detail in my next post, which will be about the SNOLAB work.

Antiparticles

In 1928, Paul Dirac presented the case for a positron – a subatomic particle that had the exact same mass of an electron (9.11 × 10-31kg), but its properties were the opposite. Therefore, it was essentially a positively charged electron. Further experiments proved that the positron not only existed, but all particles have their own positron, so to speak. Their own ‘antiparticle’. By having the same mass, this also means they have the same rest mass energy – the amount of energy that would be released if you converted the mass of the particle at rest into energy, using Einstein’s famous E=mc2 equation, where m is the rest mass and c is the speed of light.

When a particle meets its antiparticle, it will annihilate, resulting in the mass of the particle pair being converted into energy and producing photons. This is done to conserve its momentum – leptons have their own Law of the Conservation of Leptons, meaning the energy before the event has to be the same after the event, as well as the momentum. I will create a post later on describing this in detail, but I can recommend a brilliant blog that helped me understand the process:

Particle/Anti-Particle Annihilation | Of Particular Significance

For now, this is all I have to share about the basics of Particle Physics. There is far more that needs to be explained – for example, how hadrons enter the picture and the real movement of electrons and how it relates to quantum physics. As I continue to learn more, I will update this post, until I have a detailed introduction to Particle Physics.