From inauguration to upgrade: studying charmless beauty at LHCb

Abstract

The LHCb experiment at CERN is designed for the study of b-hadrons as a laboratory to test the Standard Model of particle physics. Studies of charmless B-decays are an avenue for such studies. The B0 → ϕϕ decay is a highly suppressed process that is allowed by the Standard Model that is not yet observed at LHCb using data collected in the period 2011 - 2016 inclusive. The rare nature of this mode makes it sensitive to beyond Standard Model physics which can manifest itself in the form of an enhanced branching fraction. This thesis presents a dedicated search for this decay mode in the full LHCb dataset, collected in the period 2011 - 2018 inclusive. No significant signal is observed and a new limit is set on the branching fraction at 1.3 × 10−8 at 90% confidence level.

The topologically identical and much more abundant decays of B0 s → ϕϕ are also studied as a probe of beyond Standard Model physics in CP violation. The B0 s → ϕϕ mode occurs via loop-level penguin processes and give access to the CP-violating phase, ϕ sss¯ s . The B0 s → ϕϕ decays in particular are sensitive to any beyond Standard Model physics that may manifest in the loop contributions. This thesis presents the expected sensitivity to ϕ sss¯ s with B0 s → ϕϕ decays that may be achieved in the Upgrade I phase of LHCb, assuming 25 fb−1 of data are collected in Run 3.

Looking beyond Upgrade I and into the future, the LHCb experiment will begin Upgrade II; research and development is ongoing with a target to begin installation in 2033. Studies of the acceptance of the LHCb detector are presented; this informs the development of the Upgrade II subdetectors by setting a common working point. The effect of reducing acceptance is presented for three modes of complimentary topology. In addition, momentum parameterisation in fast simulation is used to give preliminary sensitivities to flagship measurements like the search for B0 → ϕϕ in the Upgrade II era of the experiment.