The Large Hadron Collider (LHC) has allowed us to make measurements of high energy particle interactions with unprecedented accuracy. Recently, previously unexplored particle masses and decays are being measured with such high precision that they are starting to show signs of disagreement with predictions from the Standard Model (SM) of particle physics. As the precision of experimental results from the LHC continues to improve, and new measurements are made, the precision of the corresponding SM predictions will need to be improved as well in order to provide a stringent test of the theory. However, many previous and current SM predictions rely on approximations which are not expected to be valid at the precision the LHC is starting to reach. Moreover, many of these decays involve composite particles; hadrons made up of two or three elementary particles called quarks. Studying these from first principles, without approximations, requires a special computational technique known as lattice quantum chromodynamics (lattice QCD).
In lattice QCD, the continuous space-time of the SM is discretised onto a grid or "lattice", with quarks living on the points and gluons, the particles which carry force between the quarks, living on the lines connecting the points. The physics of the quarks can then be studied using lattices with different edge lengths (or "lattice spacings") in order to obtain results in the continuum limit at which the lattice spacing goes to zero. This technique requires the use of large supercomputing facilities, and has worked very well for studying the physics of lighter quarks, such as up, down, strange or charm quarks.
Many of the signs of disagreement with theory, and hints of new physics beyond the SM, seen at the LHC are in the physics of bottom quarks. These are much heavier than up, down, strange or charm quarks and require the use of lattices with a very small lattice spacing. This is in turn much more computationally expensive and, until only very recently, doing these calculations without additional approximations remained intractable. However, advances that I have made allow for the high precision study of bottom quark decays using lattice QCD, reaching the level of accuracy required for comparison to projected LHC measurements.
Utilising the new upgrade to the UK's STFC high performance computing resources, DiRAC-3, the first objective of my project is to apply these new, state of the art techniques to six different, but complementary bottom quark decays. These decays are under intense ongoing scrutiny by the experimental and theoretical physics community with upcoming measurements at the LHC. I will analyse my results for these decays, incorporating the results from the LHC, searching for hints of new physics and providing a guide for possible future measurements.
The second focus of my project is the study of bottom quarks appearing in exotic particles known as "tetraquarks". These are composite particles with four quarks rather than the normal two or three that we see predominantly in nature. The LHC experiment has very recently observed both a four-charm tetraquark and a one-charm tetraquark, while a two-charm tetraquark was observed back in 2003 by the Belle experiment. It has become clear that observations of other types of tetraquark are likely to be made in the future, with compelling theoretical arguments for the existence of tetraquarks with two bottom quarks. However these have not yet been observed, no precise theoretical predictions for their masses have been made, and, furthermore, their internal structure is currently completely unknown. My aim is to make high precision predictions for the masses of these states, as well as to study their internal structure using a novel method of adding electric charge to lattice simulations.
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