## 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.

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.

# 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.

# Recent student theses

- Euan McLean, 2019, supervisor Prof. C. Davies, Semileptonic b -> c Form Factors from Lattice Quantum Chromodynamics
- John McDowall, 2019, supervisor Dr. D. Miller, High scale boundary conditions in extension of the Standard Model
- Andres Luna Godoy, 2018, supervisors Dr. C. White and Dr. D. Miller, The double copy and classical solutions
- Ben Andrew Galloway, 2017, supervisor Prof. C. Davies, Properties of charmonium and bottomonium from lattice QCD with very fine lattices
- Karl Anders Mattias Nordström, 2017, supervisor Dr. C. Englert, Phenomenology for the Large Hadron Collider
- Michael Russell, 2017, supervisor Dr. C. Englert, Top quark physics in the Large Hadron Collider era
- Bipasha Chakraborty, 2016, supervisor Prof. C. Davies, Precision tests of the standard model using lattice QCD
- Stacey Elizabeth Melville, 2016, supervisor Dr. C. White, Next-to-soft radiative corrections in QCD and quantum gravity
- Liam Ronald Moore, 2016, supervisor Dr. D. Miller, Top quark physics in the standard model eﬀective field theory
- Brian Colquhoun, 2015, supervisor Prof. C. Davies, Bottomonium and B physics with lattice NRQCD b quarks
- António Pestana Morais, 2013, supervisor Dr. D. Miller, Grand unification phenomenology at the LHC and beyond
- Daniel Coumbe, 2013, supervisor Dr. J. Laiho, Exploring a formulation of lattice quantum gravity
- Gordon Donald, 2013, supervosor Prof. C. Davies, Semileptonic and radiative meson decays from lattice QCD with improved staggered fermions
- Iain Kendall, 2010, supervisor Prof. C. Davies, Lattice QCD studies of Upsilon physics
- Luo Rui, 2010, supervisor Dr. D. Miller, Neutrino masses and Baryogenesis via Leptogenesis in the Exceptional Supersymmetric Standard Model
- Peter Athron, PhD, 2008, supervisor Dr. D. Miller, Aspects of electroweak symmetry breaking in physics beyond the standard model
- David Thomson, PhD, 2008, supervisor Prof. C. Froggatt, Low energy consequences of some non-standard Higgs models
- Ian Allison, PhD, 2006, supervisor Prof. C. Davies, Dynamical lattice QCD determinations for heavy quark physics
- Greig Cowan, PhD, 2005, supervisor Dr. M. Alford, Single-Colour and Single-Flavour Colour Superconductivity
- Jack Cheyne, PhD, 2005, supervisor Dr. M. Alford, Colour Superconductivity and steps beyond the mean field approximation
- Alan Gray, PhD, 2003, supervisor Prof. C. Davies, Upsilon Spectroscopy and Leptonic Decays using Fully Unquenched Lattice QCD [Ogden prize 2004 for best UK PhD in particle physics phenomenology]
- Josef Dubicki, PhD, 2002, supervisor Prof. C. Froggatt, Renormalization Group Study of Four Generation Models
- Laurence Marcantonio, PhD, 2001, supervisor Prof. C. Davies, Unquenched Lattice Upsilon Spectroscopy
- Alessandro Tiesi, PhD, 2003, supervisors Prof. C. Froggatt and Dr. A. Davies, Higgs boson masses in a Non-Minimal Supersymmetric Model
- Gordon Aird, PhD, 2000, supervisor Dr. A. Watt, Modelling the Induced Magnetic Signature of Naval Vessels
- Alessandro Usai, PhD, 2000, supervisor Dr. A. Davies, Spontaneous CP violation in the Next-to-Minimal Supersymmetric Standard Model
- Mark Gibson, PhD, 1999, supervisor Prof. C. Froggatt, The Scalar and Neutrino Sectors of the Anti-Grand Unification Theory and Related Abelian Models
- Nektarios Psycharis, PhD, 1999, supervisor Dr. I. Barbour, Analysis of the Lee-Yang Zeros in Lattice Compact QED with Scalars and Fermions in 3D and in Lattice Non-Compact QED in 4D
- Gordon Jenkins, PhD, 1997, supervisor Dr. A. Davies, Electroweak Baryogenesis in Two Higgs Models
- Mark Campbell, MSc (by research), 1997, supervisor Dr. S. Collins, A Study of D-State Upsilon Spectroscopy Using Lattice QCD
- Paul McCallum, PhD, 1997, supervisor Dr. C. Davies, Upsilon Spectroscopy using Lattice QCD
- Susan Morrison, PhD, 1997, supervisor Dr. I. Barbour, Lattice QCD at Finite Baryon Density with an Implementation of Dynamical Fermions