David Bacher

Abstract:

This paper presents a thorough study of the local and nonlocal amplitudes in B+ → K+μ+μ− transitions through an amplitude analysis of the dimuon mass spectrum of the decay. The analysis is based on pp collision data corresponding to an integrated luminosity of 8.4 fb−1 collected by the LHCb experiment during Run1 and Run2. This measurement employs a model that describes both one-particle and two-particle nonlocal amplitudes across the entirety of the dimuon mass spectrum, enabling the determination of both short- and long-distance contributions to the decay. The compatibility of the Wilson coefficient combinations C9+C9′$$ {C}_9+{C}_9^{\prime } $$ and C10+C10′$$ {C}_{10}+{C}_{10}^{\prime } $$ with the Standard Model prediction is found to vary between 1.6 σ and 4 σ, depending on the choice of local form factors.

Abstract:

The first evidence for the decay $B^{+}\to {\pi }^{+}{e}^{+}{e}^{-}$ is reported using proton-proton collision data recorded by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$ . A signal excess with a significance of $3.2\sigma$ is observed and the branching fraction is measured to be $\mathcal{B}(B^{+}\to {\pi }^{+}{e}^{+}{e}^{-})=({2.4}_{-0.8}^{+0.9}{}_{-0.2}^{+0.4})\times {10}^{-8}$ , where the first set of uncertainties is statistical and the second is systematic. The result is consistent with the Standard Model expectation.

Abstract:

The onset of nuclear effects in light-ion collisions is studied by measuring the ratio of $D^0$ meson production between NeNe and OO collisions at a center-of-mass energy per nucleon pair of 5.36 TeV recorded by the LHCb detector. The $D^0$ meson yields are measured differentially in transverse momentum ( $p_{\mathrm{T}}$ ) for $0.5<p_{\mathrm{T}}<20\mathrm{GeV}$ in the rapidity ( $y$ ) region $2.0<y<4.5$ , and are normalized to the total number of recorded inelastic nucleus-nucleus collisions in each data sample. The resulting production ratio shows evidence of variation as a function of $p_{\mathrm{T}}$ , which is inconsistent with predictions based on nuclear modification of nucleon structure alone. This measurement is consistent with the onset of quark-gluon plasma production with increasing nucleus size in high-energy nuclear collisions.

Abstract:

The first search for the lepton-flavor violating decays $B^{+}\to {\pi }^{+}{\mu }^{\pm }{e}^{\mp }$ in proton-proton collisions is presented, using data collected by the LHCb experiment between 2011 and 2018, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$ . No significant signal is observed and an upper limit on the branching fraction is set at $\mathcal{B}(B^{+}\to {\pi }^{+}{\mu }^{\pm }{e}^{\mp })<1.8\times {10}^{-9}$ at the 90% confidence level, 2 orders of magnitude more restrictive than the most stringent upper limit to date. This is the first constraint on lepton-flavor violating $b\to d$ quark transitions at the LHC and also sets the most stringent upper limits to date on $b\to d{\mu }^{\pm }{e}^{\mp }$ transitions. Limits on left-handed and scalar scenarios beyond the standard model are also reported.

Abstract:

A measurement of the CKM angle γ using the decay channels B±→ DK± and B±→ Dπ±, where the D meson decays to D→KS0π+π−$$ D\to {K}_{\mathrm{S}}^0{\pi}^{+}{\pi}^{-} $$ or D→KS0K+K−$$ D\to {K}_{\mathrm{S}}^0{K}^{+}{K}^{-} $$, is performed with a data sample corresponding to an integrated luminosity of 5.8 fb−1, collected during 2024 by the upgraded LHCb experiment. CP violation is observed through a difference in the distributions of the Dalitz plot of the D decay between the B+ and B− mesons. The CKM angle γ is determined to be γ = (68.1 ± 6.7)°. Other parameters related to the examined B meson decay modes are also measured. This is the first measurement of the CKM angle γ using the upgraded LHCb detector.