The Standard Model of Particle Physics: An Introduction

Developed in the early 1970s, the standard model is the reigning theory of particle physics, classifying all known elementary particles and describing 3 of the 4 fundamental forces of nature – namely the electromagnetic, weak and strong interactions – and how they interact.

Fundamental Forces

All forces observed in everyday life arise from one of these fundamental forces; they govern every interaction that happens in the universe. Most of us are familiar with the gravitational and electromagnetic (EM) forces, since the effects of these are visible on large scales. Although the EM force is much stronger, the gravitational force dictates the motion of bodies in astronomical systems because there tends to be an equal amount of negative and positive charge in gigantic systems, so that they cancel themselves out.

The effects of the weak and strong forces, on the other hand, are observed on a subatomic scale. They act over far smaller ranges of 10-15m and 10-18m respectively; to put this into perspective, the average size of an atom is around 10-10m. The weak force is responsible for the decay of particles, including radioactive decay that is used to produce energy via nuclear fusion in nuclear reactors and stars. The strong force binds fundamental particles called quarks together to create larger particles called hadrons, such as protons and neutrons; this force also binds the protons and neutrons together, holding together atomic nuclei.

The strong force overcomes the electrostatic repulsion of protons to hold the nuclei of atoms together.

Gauge Bosons

The fundamental forces’ interactions are mediated by the exchange of force carriers known as gauge bosons. There are 4 kinds of gauge bosons, corresponding to the 3 fundamental forces described by the Standard Model, as shown in the table below. 

ForceGauge BosonRangeActs on
ElectromagneticPhoton (massless)InfiniteCharged particles
StrongGluon (massless)3×10-15Hadrons 
WeakW and Z bosons10-18 and 10-15All particles
Table showing the corresponding gauge bosons and range for each of the fundamental forces described by the Standard Model, as well as the particles that each force affects.

Mass and Range

The range over which a fundamental force has influence is inversely proportional to the mass of its corresponding gauge boson. Photons are massless, so the range of the electromagnetic force is infinite. The W and Z bosons, however, have relatively large masses of approximately 80 times that of the proton, which severely limits the influence of the weak force and explains why its effects are only seen over very small scales.

Quantum Field Theories

A quantum field theory is a theory that employs quantum physics to describe the fundamental forces of nature; they describe particles, or quanta, as excitations of fields that fill the expanse of the universe. 

Quantum electrodynamics (QED), i.e. Feynmann’s theory of electromagnetism, and quantum chromodynamics (QCD) are the quantum field theories that lie at the heart of the standard model.  QED is also thought of as the electroweak theory, which unifies the theory of both the electromagnetic and the weak nuclear forces into one, containing one electron field and one photon field. QCD offers a theory for the workings of the strong nuclear force and has a field associated with each type of quark.

It is predicted that, at high enough energies, further unification of the forces will occur; the strong and electroweak forces are expected to merge into a single force. Models of particle physics that amalgamate force theory in this manner are known as Grand Unified Theories (GUT). Unfortunately, the energies required to observe a GUT in action are unlikely to be accessible any time soon.

Higgs Boson 

In 1964, physicist Peter Higgs first theorised that gauge bosons (and all elementary particles) acquire their mass from interaction with the Higgs field, that exists throughout space. The Higgs boson is a vibration of the Higgs field, and possesses neither mass nor spin. The discovery of this Higgs Boson in 2012, at CERN’s Large Hadron Collider (LHC), was a ground-breaking success for the Standard model. Peter Higgs and Francois Englert were jointly awarded the Nobel prize in physics in 2013 following the triumph.

To date, the Standard Model has managed to successfully explain almost all experimental results. In addition to the discovery of the Higgs boson, the detection of the top quark in 1995 and tau neutrino in 2000 provided further credibility for the theory, since they were predicted by the Standard Model prior to their discovery.

Reservations

We still lack a quantum field theory of gravity, hence gravity is the only fundamental force that does not have any associated particles. Currently, it is best explained by Einstein’s General Theory of Relativity but these theories are mathematically incompatible. The Standard Model is also unable to explain dark matter (matter that cannot be seen directly but accounts for 27% of the universe) and what happened to antimatter after the Big Bang. CERN currently has plans to build a new particle accelerator called the Future Circular Collider (FCC), in the hope of finding more answers to the puzzling questions in the world of particle physics.

A digital rendering of the Future Circular Collider. This new hadron collider will span 100km and be up to 6 times more powerful than the current LHC. Credit: CERN

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