Testing lepton flavour universality violation at CMS
File(s)
Author(s)
Odedra, Jay
Type
Thesis
Abstract
The Standard Model of particle physics predicts lepton flavour universality, meaning that electroweak bosons couple equally to all lepton flavours, apart from small corrections due to their masses. Rare B± → K±ℓ+ℓ− decays provide a sensitive probe of this symmetry through the observable RK, defined as the ratio of branching fractions for B± → K±μ+μ− and B± → K±e+e−. This ratio is theoretically well understood, making it a particularly clean test of lepton flavour universality. A measured deviation of RK from unity would provide clear evidence for physics beyond the Standard Model.
The CMS experiment at the CERN Large Hadron Collider is currently collecting data during Run 3 with enhanced data-taking capabilities, known as data scouting and data parking. These methods increase the acceptance for new physics searches and improve the precision of Standard Model measurements. In the case of RK, data parking enables CMS to record a large sample of B± → K±e+e− candidate decays. This thesis presents an analysis of the B± → K±e+e− decay channel and reports a blinded measurement of RK in the low-q² region (1.1 < q² < 6.0 GeV²) using data recorded in 2022, corresponding to an integrated luminosity of 33.9 fb⁻¹. Compared with the previous CMS measurement based on Run 2 data, this study achieves nearly an order of magnitude increase in the yield of candidate B± → K±e+e− decays and a substantial improvement in the precision of RK.
The expected Run 3 (2022 data) sensitivity to RK is RK = 1.00 +0.21 −0.17.
This result highlights the capability of the CMS experiment to deliver competitive precision measurements in flavour physics.
The CMS experiment at the CERN Large Hadron Collider is currently collecting data during Run 3 with enhanced data-taking capabilities, known as data scouting and data parking. These methods increase the acceptance for new physics searches and improve the precision of Standard Model measurements. In the case of RK, data parking enables CMS to record a large sample of B± → K±e+e− candidate decays. This thesis presents an analysis of the B± → K±e+e− decay channel and reports a blinded measurement of RK in the low-q² region (1.1 < q² < 6.0 GeV²) using data recorded in 2022, corresponding to an integrated luminosity of 33.9 fb⁻¹. Compared with the previous CMS measurement based on Run 2 data, this study achieves nearly an order of magnitude increase in the yield of candidate B± → K±e+e− decays and a substantial improvement in the precision of RK.
The expected Run 3 (2022 data) sensitivity to RK is RK = 1.00 +0.21 −0.17.
This result highlights the capability of the CMS experiment to deliver competitive precision measurements in flavour physics.
Version
Open Access
Date Issued
2025-10-15
Date Awarded
2026-03-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Bainbridge, Robert
Buchmueller, Oliver
Sponsor
Science and Technology Facilities Council (Great Britain)
Publisher Department
Department of Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)