Use of rare decay modes to probe for physics beyond the Standard Model

Studying the most basic constituents of matter, and how they interact with each other, is the primary challenge of particle physics. Our current knowledge is embodied in a mathematical description called 'the Standard Model' (SM). This has been spectacularly successful, with many measurements confirming its predictions to extraordinary precision. However, we know the SM cannot be a full description of the physical world because it provides no explanation for the origin of particles masses, or for the relationship between the different families of particles that are seen. It also doesn't explain the nature of the 'dark matter' that is inferred to exist from astronomical observations, or the observed dominance of matter over anti-matter in the universe. A number of new models have been proposed to overcome these problems. This year, CERN's latest particle accelerator, the Large Hadron Collider (LHC), will smash together two beams of protons at the highest energies and rates ever observed. There are a number of hints from existing experiments that something new will be seen in the interactions that will occur and we will be able to explore the more fundamental theory beyond the SM. We hope to find new particles, new interactions and get a deeper insight into the nature of matter and fundamental interactions. Particle physicists can search for new particles in collisions either by looking for specific decays of the new particles, which tend to be too short-lived to observe directly, or by carefully studying the properties of decays which should be accurately described by the SM. In the second case, processes which occur only very rarely are particularly interesting. Such rare decays typically involve a relatively light particle decaying into another particle through a 'quantum loop'. Within such a loop, intermediate particles, that are heavier than the amount of mass-energy available from the original particle, can be briefly created, as long as they are destroyed again very quickly. This makes the processes involved very rare but allows new heavy particles in the loop to influence the decay. These heavy particles can cause very large deviations from the SM predictions of various properties of the decays. For example, the decay rate of Bs particles into two muons can be enhanced by a factor of up to 10 by a neutral Higgs boson in the relevant loop. The Higgs is postulated to give rise to particle masses but has not yet been observed. Similarly, the angular distribution of the decay products in certain Bd decays can be used to probe for new heavy particles or even extra spatial dimensions that are curled up too tightly for us to see. By measuring these properties I will have a powerful probe of new particles and their interactions. However, there are enormous practical problems with studying such processes. Around 10^12 Bd decays will occur in the LHCb experiment every year. Isolating a few tens or hundreds of interesting signal events in this data, from the millions that just happen to look like signal events, will be extremely challenging. The goal of my research is to make just such a separation and to use the signal events to investigate the physics beyond the SM. Even if no deviation from the SM predictions is seen, I will able to exclude the existence of certain types of new particles or new physics models. Studying these decays will therefore be an essential part of exploiting the LHC.