David Bacher

Abstract:

A time-integrated angular analysis of the decay Bs0→J/ψK¯∗8920, with J/ψ → μ+μ− and K¯∗8920→K−π+, is presented. The analysis employs a sample of proton-proton collision data collected by the LHCb experiment during 2015–2018 at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 6 fb−1. A simultaneous maximum-likelihood fit is performed to the angular distributions in bins of the K−π+ mass. This fit yields measurements of the CP-averaged polarisation fractions and CP asymmetries for the P-wave component of the K−π+ system. The longitudinal and parallel polarisation fractions are determined to be f0 = 0.534 ± 0.012 ± 0.009 and f|| = 0.211 ± 0.014 ± 0.005, respectively, where the first uncertainty is statistical and the second is systematic. The CP asymmetries are measured with 3–7% precision and are found to be consistent with zero. These measurements, along with an updated determination of the branching fraction relative to the B0 → J/ψK*0 decay, are combined with previous LHCb results, providing the most precise values for these observables to date.

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.