Tests of lepton universality in $B^{0} \to K_{S}^{0} l^{+} l^{-}$ and $B^{+} \to K^{*+} l^{+} l^{-}$ decays at the LHCb experiment

The decays B⁰ \to K_S⁰ l⁺ l^- and B⁺ \to K^*+ l⁺ l^- - where l⁺ l^- represents an electron-positron or muon-antimuon pair - are examples of b \to s l⁺ l^- processes. Numerous analyses of such decays have found tensions with predictions under the Standard Model (SM) of particle physics. These tensions form a coherent pattern, which could be caused by a novel physical effect. Ratios of branching fractions for muon-mode and electron-mode b \to s l⁺ l^- processes can be predicted with O(1%) precision under the SM, and can provide stringent experimental tests for novel physical processes which violate lepton universality. This dissertation presents new measurements of such ratios using B⁰ \to K_S⁰ l⁺ l^- and B⁺ \to K^*+ l⁺ l^- decays, denoted R^-1_KS⁰ and R^-1_K^*+. These measurements used data from pp collisions collected using the LHCb detector between 2011 and 2018, with a total sample size of 9 fb^-1 . The results are: R^-1_KS⁰ = 1.51 ^+0.40_-0.35 (\text{stat.}) ^+0.09_-0.04 (\text{syst.}) R^-1_K^*+ = 1.44 ^+0.32_-0.29 (\text{stat.}) ^+0.09_-0.06 (\text{syst.}) Here, the first uncertainties are statistical, and the second systematic. Both R^-1_KS⁰ and R^-1_K^*+ consistent with SM expectations, at significances of 1.5σ and 1.4σ , respectively. In addition, differential branching fractions for the decays B⁰ \to K_S⁰ e⁺ e^- and B⁺ \to K^*+ e⁺ e^- are measured in bins of the dilepton invariant mass squared. The first ever observations of these decays are reported.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2022

Author dc:creator

• Smeaton, John

Advisor dc:contributor.advisor

• Gibson, Valerie

• B Physics
• CERN
• Flavour Physics
• High-Energy Physics
• Large Hadron Collider
• LHCb Experiment
• Particle Physics

Licence

• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.88673

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/341247