Measurement of CP asymmetries in Λb⁰ → pK− and Λb⁰ → pπ− decays analysing LHCb Run 3 data

Understanding the origin of the matter-antimatter asymmetry observed in the Universe is one of the major open questions in modern physics. A necessary ingredient to explain this asymmetry is CP violation. While the Standard Model (SM) predicts CP violation through the complex phase of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, the amount it provides is far too small to account for the observed imbalance. The search for additional sources of CP violation is therefore a key objective of particle physics. CP violation has been observed in several meson decays (K⁰, D⁰, B⁰, B⁺, Bs⁰), whereas in the baryonic sector it has so far been observed only in Λb⁰ decays. The study of baryonic decays therefore provides a complementary probe of CP violation. In particular, Λb⁰ → pK⁻ and Λb⁰ → pπ⁻ are of interest because they proceed through mechanisms analogous to those of B⁰ → K⁺π⁻ and Bs⁰ → π⁺K⁻, for which CP violation has been observed. However, no evidence of CP violation has so far been found in the corresponding Λb⁰ decays. This thesis presents a measurement of the difference between the CP asymmetries in Λb⁰ → pK⁻ and Λb⁰ → pπ⁻, ΔA_CP = A_CP(pK⁻) − A_CP(pπ⁻). The difference allows common instrumental and production asymmetries to cancel. The analysis uses approximately 1.5 fb⁻¹ of proton-proton collision data recorded by LHCb during Run 3 at a centre-of-mass energy of 13.6 TeV. The data are divided according to the two opposite magnet polarities, MagUp and MagDown, which are analysed separately and subsequently combined. The results are ΔA_CP^MagUp = (7.9 ± 3.1 ± 0.9)%, ΔA_CP^MagDown = (−0.7 ± 2.4 ± 0.4)%, and ΔA_CP^Avg = (2.4 ± 1.9 ± 0.4)%. The MagUp and MagDown measurements are compatible within 2.1σ, while the weighted average is compatible with zero within 1.2σ. No evidence of CP violation is observed. This work constitutes the first measurement of ΔA_CP in these decay channels using Run 3 data collected by LHCb.