David Bacher

Abstract:

The production ratio of ψ(2S) to J/ψ charmonium states is presented as a function of multiplicity in proton-lead collisions at a centre-of-mass energy of sNN=8.16 TeV, for both prompt and nonprompt sources. The total luminosity recorded by the LHCb experiment corresponds to 13.6 nb−1 for pPb collisions and 20.8 nb−1 for Pbp collisions, where the first particle corresponds to the particle traveling towards the detector. Measurements are performed in the dimuon final state at forward (backward) centre-of-mass rapidity, with respect to the proton direction, 1.5 < y*< 4.0 (−5.0 < y*< −2.5) for pPb (Pbp) collisions. A multiplicity dependence of the prompt production ratio is observed in pPb collisions, whereas no dependence is found in nonprompt production, nor in either prompt or nonprompt production in Pbp collisions. These results suggest that in the Pb-going direction additional suppression mechanisms beyond comover effects may be present, possibly related to the formation of quark-gluon plasma. This highlights a transition from small to large collision systems and provides important insight into the suppression of charmonia in proton-nucleus collisions.

Abstract:

A first search at LHCb for the lepton-flavour-violating decays B0 → K*0τ±e∓ is presented. The analysis is performed using a sample of proton-proton collision data, collected with the LHCb detector at a centre-of-mass energy of 13 TeV between 2016 and 2018, corresponding to an integrated luminosity of 5.4 fb−1. No significant signal is observed, and upper limits on the branching fractions are determined to be BB0→K∗0τ−e+<5.97.1×10−6 and BB0→K∗0τ+e−<4.95.9×10−6 at the 90% (95%) confidence level. These results correspond to the current most stringent upper limits for b → sτl transitions.

Abstract:

The invariant-mass distribution for the coherent photoproduction of dipions in ultraperipheral PbPb collisions is measured using data, corresponding to an integrated luminosity of 224.6 ± 9.6μb−1, collected by the LHCb experiment in 2018 at a nucleon-nucleon centre-of-mass energy sNN=5.02 TeV. In the mass range from 400 to 1200 MeV, the results are consistent with previous experiments, with the spectrum dominated by the ρ0 meson, which interferes with a nonresonant component, together with a smaller ω meson contribution. In an extended mass range up to 2300 MeV, models previously used do not fit the data and a consistent description requires the introduction of two resonances at masses of 1350 ± 20 MeV and 1790 ± 20 MeV with widths of about 300 MeV. The cross-section for each meson is measured differentially in twelve bins of rapidity from 2.05 to 4.90. The ρ0 cross-section increases with rapidity from about 400 to 600 mb and is measured with a typical precision of 8%, while the cross-section times branching fraction for the ω, ρ′ and ρ′′, with the statistical precision of the data, do not have a pronounced rapidity dependence and are between 0.5 and 1.5mb, with uncertainties up to 30%. A large nuclear suppression is observed for the ρ0 meson compared to expectations based on photoproduction on the proton that use the impulse approximation. Significant suppression is also observed compared to that predicted by elastic scattering described in the Glauber approach, or with the addition of inelastic scattering in a Gribov-Glauber model.

Abstract:

A new algorithm is developed to identify the flavour of neutral B mesons at production in pp collisions by utilising all tracks from the hadronisation process. The algorithm is calibrated separately for B0 and Bs0 mesons using B0 → J/ψK+π− and Bs0→Ds−π+ decays from pp collision data collected by the LHCb experiment at a centre-of-mass energy of 13 TeV. This new algorithm improves the tagging power by 35% for B0 mesons and 20% for Bs0 mesons when compared to the combined performance of the existing LHCb flavour-tagging algorithms.

Abstract:

A study of Λb0 and Ξb0 baryon decays to the final states pKS0π− and pKS0K− is performed using pp collision data collected by the LHCb experiment, corresponding to an integrated luminosity of 9 fb−1. The decays Λb0→pKS0K− and Ξb0→pKS0K− are observed for the first time, with significances reaching eight standard deviations. The branching fractions and integrated CP asymmetries are measured for the Λb0→pKS0π−, Λb0→pKS0K−, and Ξb0→pKS0K− decays. For the decay Λb0→pKS0π−, the CP asymmetries are measured in different regions of the Dalitz plot. No evidence of CP violation is observed.