Anthony Downes – Phd’s defense – Sep. 28, 2023

The primary focus of this thesis is the study of the decays of B0(s) → J/ψη(′) with the LHCb experiment at CERN’s Large Hadron Collider (LHC). These decays can occur either via b →ccs transitions for the B0s decays or the Cabibbo suppressed b →ccd transitions for the B0 decays. These neutral B mesons can oscillate between particle B and antiparticle B states. In the B0s → J/ψη(′) decays it would be possible to observe violation of charge-parity (CP) symmetries. This violation arises from the interference in the decays with and without B0s -B0s oscillations. This can be measured in the time distributions of B0s → J/ψη(′) and B0s → J/ψη(′) which is related to the decay width difference ∆Γs and the phase φs. A future measurement of φs in these channels can then be used to test the Standard Model. Yet there is a contribution to this measurement that arises from decay topologies named penguin diagrams. 

It is therefore necessary to have constraints on the penguin parameters, which is explored in this thesis through the measurements of the branching ratios of the B0(s) → J/ψη(′) decays and the mixing between the η and η′ mesons. In the measurement of this mixing there also enters a small mixing contribution from a possible meson formed from valence gluons, named gluonium or glueballs. This amounts to a measurement of two mixing angles that can be determined in different fractions of branching ratios. One angle is associated with the mixing between the quark states of the η and η′ mesons and another for their mixing with the gluonium state. The strategy of this thesis was to measure these branching ratios and the mixing angles using J/ψ → μ+μ− final states and complementary η and η′ final states: η → π+π−γ, η → π+π−(π0 → γγ), η′→(ρ0 →π+π−)γ and η′→π+π−(η →γγ). This physics analysis was performed using a dataset corresponding to 8.76 fb−1 at centre of mass energies of √s = 7, 8 and 13 TeV which was collected in the first and second runs of the LHCb detector. The measurements of the η and η′ final states were challenging due to the inclusion of photons. It was therefore necessary to also perform an analysis of the simulation performance for the determination of the reconstruction efficiencies of photons and π0.