Theoretical particle physics at the University of Edinburgh

## What is particle physics?

Particle physics studies the most elementary building blocks of matter. By studying the basic forces of nature, we aim to understand how these fundamental particles interact and bind together to form more familiar objects (like the proton or the neutron), how they decay from one form to another and how they influenced the evolution of the very early Universe. The goal of the subject is to discover the law of nature in their most fundamental form, and to understand how the Universe we see around us emerges from these fundamental laws.

The subject relies on experiments ranging from huge particle accelerators (such as the LHC), orbiting satellites (e.g. WMAP and Planck), terrestrial detectors, underground laboratories and simulations using powerful supercomputers, as well as on the creative ideas of individial scientists.

## What is the role of theory in particle physics?

Particle physics is broadly split into two groups: experimental physicists, who gather data about the universe, and theoretical physicists. Theoretical particle physics uses mathematical techniques to understand the results of particle physics experiments. It then uses this understanding to predict how particles would behave in other situations.

Over the last century, this has led to the realisation that there are four fundamental interactions in nature, called the strong, weak, electromagnetic and gravitational forces. The Standard Model of particle physics governs three of these (all except gravity) and the elementary particles that take part in these interactions. The Standard Model is a quantum field theory - a quantum mechanical generalisation of the ideas pioneered by Michael Faraday, James Clerk Maxwell and Albert Einstein.

Whilst very successful in explaining most, if not all, of current experimental measurements, we do not believe that the Standard Model is complete. As well as not explaining gravity, there are various theoretical reasons for believing that the Standard Model is incomplete.

Theoretical particle physics today moves in two basic directions. One is to use the Standard Model to make precise predictions that can be tested against experimental data, with a view to confirming or disproving the correctness of the current theory. The other direction is to use mathematical reasoning to deduce what would be the logical extension of the Standard Model.

One key prediction of the Standard Model that has only recently been verified is the existence of the Higgs boson. This particle is needed to allow other elementary particles to have a non-zero mass without breaking the important symmetries of the Standard Model. One of the theoretical physicists to suggest such a particle was Peter Higgs, after whom it was eventually named. Peter Higgs has been a faculty member in Physics at the University of Edinburgh since 1960 and at present is an Emeritus Professor in our group. Professor Higgs's important work was rewarded with the Nobel Prize in Physics in 2013.

## Our group's research interests

Particle physics phenomena start at the length scale of the proton and continue to ever tinier scales. Our group interests span all these length scales. The proton is not an elementary particle, but is itself built out of elementary particles called quarks. The theory of strong interactions is called "Quantum chromodynamics" (or QCD for short). QCD is believed to explain how quarks bind to form the proton and other similar particles (neutrons and more exotic particles), while forbidding the appearance of free quarks, a property called confinement. Solving QCD is currently beyond our reach, and better understanding of the confinement mechanism is one of the prime goals of theoretical particle physics. Our group is involved in answering this question using both analytic and computational methods.

The typical experiment for testing particle physics involves smashing very high energy particles together. The higher the energy of the two colliding particles, the closer they come to each other, thus the tinier the length scale that is probed. This is why the study of particle physics at very short lengths scales is so closely tied to high energy (and large scale) experiments such as the 27km-long LHC.

Another way to examine particles at high energy is in the early universe, when the universe was a lot hotter and thus particles were much more energetic. This is why cosmology is also an important testing ground for particle physics. Our group also has research interest in this direction.

Our group activities can be broken into five categories, which you can explore by following the links.

- Collider Physics
- Flavour Physics
- Lattice gauge theory
- Fundamentals of quantum field theory
- Particle cosmology

Some of our well-known papers can be found here.