David Bacher

Abstract:

A test of lepton universality between muons and electrons is performed using B+ → K+ℓ+ℓ− decays (where ℓ = e, μ), in the dilepton invariant-mass-squared region above 14.3 GeV2/c4. The data used for the measurement consists of beauty meson decays produced in proton-proton collisions, corresponding to an integrated luminosity of 9 fb−1, collected by the LHCb experiment between 2011 and 2018. The ratio of branching fractions for B+→ K+μ+μ− and B+→ K+e+e− decays is measured to be RK=1.08−0.09+0.11stat−0.04+0.04syst, which is consistent with the Standard Model prediction of unity. This constitutes the most precise test of lepton flavour universality using B+ → K+ℓ+ℓ− decays with dilepton invariant-mass-squared above the ψ(2S) mass, whilst being the first of its kind at a hadron collider.

Abstract:

A study of the charmless baryonic decays Bs0→K0pp¯ is presented, where Bs0 denotes either a B0 or a Bs0 meson. The analysis is based on proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb−1. The decay Bs0→K0pp¯ is observed for the first time, with a measured branching fraction of (9.14 ± 1.69 ± 0.90 ± 0.33 ± 0.20) × 10−7 and a significance of 5.6σ. The uncertainties respectively account for statistical and systematic contributions, the precision of the branching fraction of the normalisation channel B0→ K0π+π− and the fragmentation fraction ratio fs/fd. The branching fraction determined for B0→K0pp¯ is (2.82 ± 0.08 ± 0.12 ± 0.10) × 10−6, which is the most precise measurement to date.

Abstract:

An angular analysis of the decay Bs0→ ϕe+e− is presented, using proton-proton collision data collected with the LHCb detector between 2011 and 2018 at centre-of-mass energies of 7, 8 and 13 TeV. The combined dataset corresponds to an integrated luminosity of 9 fb−1. Observables are determined by fitting time-integrated projections of the angular distribution in three bins of dielectron mass squared, q2, corresponding to [0.1, 1.1], [1.1, 6.0] and [15.0, 19.0] GeV2/c4. The results are compatible with predictions based on the Standard Model of particle physics.

Abstract:

An angular analysis of B0→ K*0e+e− decays is presented using proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7, 8 and 13 TeV, corresponding to an integrated luminosity of 9 fb−1. The analysis is performed in the region of the dilepton invariant mass squared of 1.1–6.0 GeV2/c4. In addition, a test of lepton flavour universality is performed by comparing the obtained angular observables with those measured in B0→ K*0μ+μ− decays. In general, the angular observables are found to be consistent with the Standard Model expectations as well as with global analyses of other b → sℓ+ℓ− processes, where ℓ is either a muon or an electron. No sign of lepton-flavour-violating effects is observed.

Abstract:

A search for the decay Bc+→ χc1(3872)π+ is reported using proton-proton collision data collected with the LHCb detector between 2011 and 2018 at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb−1. No significant signal is observed. Using the decay Bc+ → ψ(2S)π+ as a normalisation channel, an upper limit for the ratio of branching fractionsRψ2Sχc13872=BBc+→χc13872π+BBc+→ψ2Sπ+×Bχc13872→J/ψπ+π−Bψ2S→J/ψπ+π−<0.050.06, is set at the 90 (95)% confidence level.