Determination of the \(r_{J/ψ}^{K}\) and \(r_{J/ψ}^{K^*}\) Ratios for Lepton Universality Measurements at LHCb

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Calladine, Ryan Bernard (2021). Determination of the \(r_{J/ψ}^{K}\) and \(r_{J/ψ}^{K^*}\) Ratios for Lepton Universality Measurements at LHCb. University of Birmingham. Ph.D.

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Abstract

Lepton Universality is an integral part of the Standard Model of Particle Physics. This thesis describes new measurements carried out as part of a programme to test this by comparing ratios of branching fractions of rare B meson decays that include either electrons or muons in their otherwise equivalent final states. The ratios, denoted \(R_{K^*}\) and \(R_K\) for \(B^0 \rightarrow K^{∗0} l^+ l^−\) and \(B^+ \rightarrow K^+ l^+ l^−\) decays respectively, have been measured previously by LHCb, showing tension with the Standard Model expectation at a level of 2–3σ.

This thesis investigates the simultaneous extraction of both of these ratios and focuses on measurements of \(r_{J/ψ}^{K}\) and \(r_{J/ψ}^{K^*}\). These have the same final state particles as the rare decay but are not expected to be sensitive to any physics beyond the Standard Model as they are made in intervals of the squares of the dilepton invariant masses corresponding to the decay of J/ψ mesons. Hence, they are the ideal and mandatory environment in which to study all aspects of the analysis to avoid bias in the primary measurements or \(R_{K^*}\) and \(R_K\).

All pp collision data recorded by the LHCb detector, corresponding to an integrated luminosity of approx. 9 \(fb^{−1}\) at energies 7–13 TeV, are used. Measurements of the integrated \(r_{J/ψ}\) value generally agree within 3σ, while \(r_{J/ψ}\) itself is flat as a function of most kinematic and topological variables considered. Results obtained using corrections derived from \(B^+\) or \(B^0\) modes are consistent. These detailed measurements of \(r_{J/ψ}\) demonstrate that it is now acceptable to proceed with the high-profile determination of \(R_K\) and \(R_{K^∗}\) .

In addition, as validation of the LHCb detector simulation is vitally important for reliable physics measurements, a test of the muon multiple scattering in the LHCb detector is presented and its implementation into LHCbPR, a browser-based monitoring system which collates regular test results, is presented.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):