One of the most compelling mysteries surrounding the Standard Model is that our existence itself is a clear demonstration of the failings of the theory. We live in an asymmetric universe where matter has a profound dominance over anti-matter (everything is made of matter), although this is not observed with anti-matter or matter created in the laboratory where they have almost uniform properties. The Standard Model has been confirmed to incredible precision in a number of different tests. However these clear discrepancies, between the universe it would allow, and the one that does in fact exist, leads to the conviction that this theory is only a low energy approximation to a higher energy theory. Nearly all of these theories predict the existence of further families of fundamental particles that are heavy with respect to the known particles.
Central to nearly all particle physics research is the desire to find or characterize this new physics. The approach I will take within this project is to observe this new physics through the study of CP violation, that is, the difference in behaviour of matter and anti-matter. The lack of any direct observation of new physics particles so far at the higher collision energy of the LHC Run-2 suggests that perhaps new physics is at a mass scale beyond the reach of the LHC. In that case, the need for indirect searches for new physics through CP violation only intensifies. The aim of this project is a measurement of the Cabibbo-Maskawa-Kobayashi (CKM) angle γ with a precision of 1°. Increasing the precision on the least well known angle, γ, by almost an order of magnitude could for the first time, provide laboratory evidence of physics beyond the Standard Model.
Direct measurements of γ are made using decays such as B^∓→DK^∓ (other similar decays can be used too), where the charmed meson (D) is a superposition of the D^0 and (D^0 ) ̅. In the next few years a substantial number of these decays will be collected by the LHCb experiment. It is the interference of the decay paths B^∓→D^0 K^∓ and B^∓→(D^0 ) ̅K^∓ that gives sensitivity to the phase γ. Interference between two decay paths only occurs when the initial and final states are identical and hence it is required that both the D^0 and (D^0 ) ̅ decay to the final state. Hence to determine γ, information is required not only about the B decay, but the D decay too. The large samples of B decays will provide good information on the B decay but can only contribute weakly to the information on the D decay. The relevant D decay information is related to the difference in amplitudes and phases between the D^0 and (D^0 ) ̅ decay and is referred to as the charm strong-phase parameters.
It will not be possible to make full use of future datasets at either LHCb or Belle II if there is not a significant expansion and reoptimisation of the measurements of the charm strong-phase related parameters. The only experiment at which these charm strong-phases can be improved is BESIII. With a dedicated and coordinated effort, that brings the power and information contained within the BESIII and LHCb data sets together, I will make the most precise measurement of γ.
The project ushered in a new era of precision when in comes to charm strong-phase related parameters and the measurement of γ, with many decay channels fully exploited. At the end, 21 publications arise from this project with a further 3 in peer review. New, robust, techniques that will continue to be used as the research continues beyond this project have been pioneered through this project. Delays to operating schedules meant that there was insufficient data to reach the 1° target, but the project closed with a precision of 2.5°, with the expectation that the target precision will soon be reached.
The central value of γ turned out to be consistent with the Standard Model – therefore the mysteries of the universe only continue to deepen.
