David Bacher

Abstract:

The observation of a wide peaking structure in the $B_c^{+}\gamma$ mass spectrum is reported using proton-proton collision data collected by the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV, corresponding to a total integrated luminosity of $9{\mathrm{fb}}^{-1}$ . The statistical significance over the background-only hypothesis exceeds seven standard deviations. The width of the observed structure is larger than the expectation from a single-peak hypothesis, and is well described by an effective minimal model consisting of two narrow peaks located at $6704.8\pm 5.5\pm 2.8\pm 0.3\mathrm{MeV}/c^2$ and $6752.4\pm 9.5\pm 3.1\pm 0.3\mathrm{MeV}/c^2$ . The uncertainty terms are statistical, systematic, and associated to the knowledge of the $B_c^{+}$ mass, respectively. The measured peak locations are in line with theoretical predictions for lowest excited $P$ -wave $B_c^{+}$ states, marking the first observation of orbitally excited beauty-charm mesons and providing important insights into the internal dynamics of hadrons containing two heavy quarks.

Abstract:

A search for the decay B0→ ϕϕ is made using pp collision data collected with the LHCb detector 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, and an upper limit on the branching fraction of 1.3 (1.4) × 10−8 at 90 (95)% confidence level is set. This result supersedes the previous LHCb study and improves the upper limit by a factor of two.

Abstract:

A bstract A search for the decay B 0 → ϕϕ is made using pp collision data collected with the LHCb detector 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, and an upper limit on the branching fraction of 1 . 3 (1 . 4) × 10 − 8 at 90 (95)% confidence level is set. This result supersedes the previous LHCb study and improves the upper limit by a factor of two.

Abstract:

A study of the difference in the CP asymmetries between Λ b 0 → J / ψ p π - and Λ b 0 → J / ψ pK - decays, Δ A CP , is performed using proton-proton collision data collected by the LHCb experiment in the years 2015–2018, corresponding to an integrated luminosity of 6 fb - 1 . This quantity is measured to be Δ A CP = ( 4.03 ± 1.18 ± 0.23 ) % , where the first uncertainty is statistical and the second is systematic. When combined with the previous LHCb result, a value of Δ A CP = ( 4.31 ± 1.06 ± 0.28 ) % is obtained, corresponding to a significance of 3.9 σ against the CP symmetry hypothesis. Studies of triple-product asymmetries, which provide an additional probe of CP violation, show no significant deviation from CP symmetry.

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.