David Bacher

Abstract:

This paper presents the first measurement of the forward-backward asymmetry, AFB, and the flat term, FH, that parameterise the angular distribution of the B+ → π+μ+μ− decay. The proton-proton collision dataset used in the analysis corresponds to an integrated luminosity of 9 fb−1, collected with the LHCb experiment between 2011 and 2018. The analysis is performed in two intervals of dimuon mass squared, one above and one below the region containing the J/ψ and ψ(2S) narrow charmonium resonances. The Standard Model predictions lie within the obtained 68% confidence level interval in the high-mass and within the 99% interval in the low-mass region.

Abstract:

A search for the rare decay $B^{+}\to \overline{\mathrm{\Lambda }}p{\mu }^{+}{\mu }^{-}$ is performed using proton-proton collision data recorded by the LHCb experiment at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$ , corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$ . An excess of events is found with respect to the background-only expectation, with a signal significance of 3.5 standard deviations, in the low invariant-mass region of $m(\overline{\mathrm{\Lambda }}p)<2.8\mathrm{GeV}/c^2$ . The branching fraction is measured to be ${\mathcal{B}}_{\mathrm{low}}(B^{+}\to \overline{\mathrm{\Lambda }}p{\mu }^{+}{\mu }^{-})=(1.70_{-0.56}^{+0.65}(\mathrm{stat})\pm 0.17(\mathrm{syst})\pm 0.14(\mathrm{ext}))\times {10}^{-8}$ , where the last uncertainty is due to external inputs on $\mathcal{B}(B^{+}\to J/\psi \overline{\mathrm{\Lambda }}p)\times \mathcal{B}(J/\psi \to {\mu }^{+}{\mu }^{-})$ . With no significant signal observed in the high $m(\overline{\mathrm{\Lambda }}p)$ region above $2.8\mathrm{GeV}/c^2$ , an upper limit is set to be ${\mathcal{B}}_{\mathrm{high}}(B^{+}\to \overline{\mathrm{\Lambda }}p{\mu }^{+}{\mu }^{-})<2.8(3.7)\times {10}^{-9}$ at the 90% (95%) confidence level.

Abstract:

Muon-electron universality is tested in $B^0\to K^{*0}{\ell }^{+}{\ell }^{-}$ decays, in the dilepton-invariant-mass region above the $\psi (2S)$ resonance. The analysis uses beauty mesons produced in proton-proton collisions recorded by the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$ . The ratio of branching fractions between the muon and electron channels $R_{K^{*0}}$ is measured to be ${1.08}_{-0.12}^{+0.14}(\mathrm{stat})\pm 0.07(\mathrm{syst})$ for a dilepton-invariant-mass squared above $14.0{\mathrm{GeV}}^2/c^4$ , consistent with the standard model prediction. This result represents the most precise measurement of $R_{K^{*0}}$ in this region and the first such measurement performed at a hadron collider.

Abstract:

Two machine learning techniques for jet measurements at the LHCb experiment are presented: a regression-based method for jet-energy calibration and a deep neural network algorithm for jet flavour tagging, distinguishing between b-quark, c-quark, and light parton jets. These techniques are applied to a search for inclusive H→bb¯$$ H\to b\overline{b} $$ and H→cc¯$$ H\to c\overline{c} $$ decays using a LHCb dataset corresponding to an integrated luminosity of 1.6 fb−1. The observed (expected) 95% confidence level upper limits correspond to 6.6 (11.1) times the SM cross-section for the H→bb¯$$ H\to b\overline{b} $$ process, and 1003 (1834) times the SM cross-section for the H→cc¯$$ H\to c\overline{c} $$ process.

Abstract:

An analysis of the $B^0\to K^{*0}(\to K^{+}{\pi }^{-}){\mu }^{+}{\mu }^{-}$ decay is presented using proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of $8.4{\mathrm{fb}}^{-1}$ . The full set of $CP$ -averaged and $CP$ -asymmetric angular observables is determined in bins of the invariant mass squared of the dimuon system, as well as the branching fraction relative to the $B^0\to J/\psi (\to {\mu }^{+}{\mu }^{-})K^{+}{\pi }^{-}$ decay. For the first time, the full set of observables pertaining to the $K^{+}{\pi }^{-}$ S-wave contribution to the final state are presented and consideration is given to effects arising from the mass of the muons. The extracted $CP$ -asymmetry observables show no significant deviations from zero. The measurements of the $CP$ -averaged observables and the branching fractions continue to exhibit the pattern of tensions with the standard model predictions that have been seen in previous analyses that use part of the dataset considered in this Letter.