Particle Physics Theory

The Glasgow Particle Physics Theory group researches fundamental particles and their interactions.  We are principally interested in phenomena that can be probed at current and next generation particle colliders, such as the Large Hadron Collider, SuperKEKB, and the International Linear Collider.  We use our current model of particle physics, the Standard Model, to make predictions that can be tested by our experimental colleagues.  We also examine models of exotic new physics beyond the Standard Model.

In particular, we focus on the behaviour of the strong force as described by Quantum Chromodynamics, both at high energies (via perturbation theory) and at low energies (via lattice QCD); the physics of the Higgs boson; and models beyond the Standard Model such as supersymmetry, extra dimensions, and little Higgs.

The main areas of research pursued by our group are:

 

The Standard Model

The Standard Model of particle physics is at present our best theory for explaining how the universe works on a fundamental level. It describes the interactions of the fundamental particles via three of the four fundamental forces.

The particles themselves are divided into two groups, bosons and fermions. The fermions (spin-half particles) are "matter" particles that makes up the elements around us, and are further divided into quarks and leptons. The quarks are the constituents of the proton and neutron, while the electron is an example of a lepton. The bosons are sometimes referred to as the "force carriers" and are responsible for the forces between particles. A force carrier is exchanged between two particles, transferring momentum and providing a force. The final particle is the Higgs boson, which is intimately linked to the origin of particle masses.

Standard Model Particles

The three forces described by the Standard Model are:

  • Electromagnetism: This force effects particle which have electric charge, such as the electron, and is responsible for both electricity and magnetism. In the Standard Model it is described by the theory of Quantum ElectroDynamics (QED), where the force is passed from one particle to another by exchanging a photon (a particle of light).

  • The strong nuclear force: This force is felt by particles which have "color" charge. It is responsible for holding together three quarks to form a proton or neutron. The proton consists of two up quarks and a down quark, while the neutron is two down quarks with an up quark. This force is mediated by the exchange of gluons and is described by the theory of Quantum ChromoDynamics (QCD).

  • The weak nuclear force: This force is not so apparent in everyday life as the other three. It is manifest in beta decays, for example, where a neatron decays to a proton, electron and neutrino. It is mediated by the exchange of W and Z bosons. This force is unusual because the W and Z bosons are rather heavy, making them difficult to produce and the force very short range. It is believed that the Higgs boson is linked to their mass, as explained by the Higgs mechanism of Electroweak Symmetry breaking, but this has not yet been experimentally confirmed.

The remaining force, the force of Gravity is absent from the Standard Model. It is much much weaker than the other three, and is not relevant to the interactions of particles at low energies.

The Standard Model

High Energy Colliders

In order to experimentally investigate these forces, machines have been built that accelerate charged particles to ever-higher energies and collide them together.  These high-energy collisions produce the fundamental particles and allow us to study their interactions, testing the Standard Model.  Until its shutdown in 2011, the highest energies achieved were at the Tevatron collider at Fermilab (just outside of Chicago, USA), which accelerated protons and anti-protons to energies of almost 1000 times their rest mass, 1 TeV.  These energies were surpassed when the Large Hadron Collider (at CERN in Geneva, Switzerland) began running in 2009.  Data was initially taken at a total energy of 7 TeV.  In 2015, after an upgrade, the LHC began operating at 13 TeV.

On 4 July 2012, CERN announced the discovery of the last remaining unseen particle in the Standard Model -- the Higgs boson.  This discovery confirmed the mechanism responsible for providing mass to the fundamental particles.  Two theorists, Francois Englert and Peter Higgs, were awarded the 2013 Nobel Prize in Physics for its prediction.  Though the Higgs boson decays almost instantaneously, it can be detected by observing the decay products.  Its discovery and the search for physics beyond the Standard Model rely heavily on theoretical predictions in order to disentangle new physics effects from those of the Standard Model.

Aerial view of CERN and surroundings

 

Recent student theses