David Bacher

Abstract:

A search for the doubly-charmed-baryon decay Ξcc++→Ξc0π+π+ is performed using proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 5.4 fb−1. A significant structure consistent with the Ξcc++ baryon is observed in the Ξc0π+π+ invariant-mass spectrum. Using the Ξcc++→Λc+K−π+π+ decay as the normalisation channel, the branching fraction ratio, BΞcc++→Ξc0π+π+BΞcc++→Λc+K−π+π+, is measured to be 1.37 ± 0.18 (stat) ± 0.09 (syst) ± 0.35 (ext). This measurement provides critical input for testing QCD factorisation methods in the weak decays of doubly-heavy baryons, particularly in quantifying nonperturbative effects such as final-state interactions and resonance contributions to the hadronisation process.

Abstract:

The elliptic and triangular flow of charged particles are measured using two-particle angular correlations in pPb collisions in the pseudorapidity range 2.0 < |η| < 4.8. The data sample was collected by the LHCb experiment in 2016 at a centre-of-mass energy per nucleon pair of sNN=8.16 TeV, containing in total approximately 1.5 billion collision events. Non-flow contributions are obtained in low-multiplicity collisions and subtracted to extract the flow harmonics. The results are presented as a function of event multiplicity and hadron transverse momentum. Comparisons with a full (3+1)D dynamic model indicate that it overestimates the measured elliptic flow. A comparison between the forward and backward regions reveals no significant differences in flow parameters, suggesting that final-state effects may dominate over initial-state effects in the origin of flow in small systems.

Abstract:

Branching fraction ratios between the decays Bs0→J/ψη′ are measured 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 measured ratios of these branching fractions areBB0→J/ψη′BB0→J/ψη=0.48±0.06±0.02±0.01, BBs0→J/ψη′BBs0→J/ψη=0.80±0.02±0.02±0.01, where the uncertainties are statistical, systematic and related to the precision of the η(′) branching fractions, respectively. They are used to constrain the η/η′ mixing angle, ϕP, and to probe the presence of a possible glueball component in the η′ meson, described by the gluonic mixing angle ϕG. The obtained results are ϕP=41.6−1.2+1.0∘, ϕG=28.1−4.0+3.9∘, where the uncertainties are statistically dominated. While the value of ϕP is compatible with existing experimental determinations and theoretical calculations, the angle ϕG differs from zero by more than four standard deviations, which points to a substantial glueball component in the η′ meson and/or unexpectedly large contributions from gluon-mediated processes in these decays. The absolute branching fractions are also measured relative to that of the well-established Bs0→J/ψϕ decay, which serves as the normalisation channel. These results supersede the previous LHCb measurements and are the most precise to date.

Abstract:

This article presents doubly differential measurements of the asymmetries in production rates between mesons containing a charm quark and those containing an anti-charm quark in proton-proton collisions at a centre-of-mass energy of s=13.6 TeV using data recorded by the LHCb experiment. The asymmetries of D0, D+ and Ds+ mesons are measured for two-dimensional intervals in transverse momentum and pseudorapidity, within the range 2.5 < pT< 25.0 GeV/c and 2.0 < η < 4.5. No significant production asymmetries are observed. Comparisons to the Pythia 8 and Herwig 7 event generators are also presented, and their agreement with the data is evaluated. These measurements constitute the first measurements of production asymmetries at this centre-of-mass energy of colliding beams, and the first measurements with the LHCb Run 3 detector.

Abstract:

The lifetimes of the Ωc0 and Ξc0 baryons are measured using a pp collision dataset collected by the LHCb experiment, corresponding to an integrated luminosity of 9 fb−1. The charm baryons are produced in the fully reconstructed decay chains Ωb−→Ωc0→pK−K−π+π− and Ξb−→Ξc0→pK−K−π+π−. The measurement uses topologically and kinematically similar B− → D0(→ K−K+π−π+)π− decays for normalisation. The measured lifetimes areτΩc0=276.3±19.4stat±1.8syst±0.7τD0fs, τΞc0=149.2±2.5stat±0.9syst±0.4τD0fs, where the first uncertainty is statistical, the second systematic and the third due to the uncertainty of the D0 lifetime. These results are consistent with previous measurements performed by the LHCb experiment.