David Bacher

Abstract:

A measurement of the branching fraction for the decay Λ→pμ−ν¯μ is presented using pp collision data collected by the LHCb experiment at a centre-of-mass energy of 13 TeV. The analysis is based on data recorded between 2016 and 2018, corresponding to an integrated luminosity of 5.4 fb−1. The result is obtained using Λ → pμ− decays as a normalisation channel. The measured branching fraction is BΛ→pμ−ν¯μ=1.462±0.016±0.100±0.011×10−4, where the uncertainties are statistical, systematic, and due to the limited knowledge of the normalisation mode branching fraction, respectively. This result improves the precision of the branching fraction measurement by a factor of two compared to the previous best measurement and sets a more stringent bound on lepton flavour universality in s → u quark transitions. It is consistent with previous measurements, and the extracted lepton flavour universality test observable, Rμe=ΓΛ→pμ−ν¯μΓΛ→pe−ν¯e=0.175±0.012, agrees with the Standard Model prediction.

Abstract:

Charged-hadron distributions in heavy-flavor jets are measured in proton-proton collisions at a center-of-mass energy of s=13 TeV collected by the LHCb experiment. Distributions of the longitudinal momentum fraction, transverse momentum, and radial profile of charged hadrons are measured separately in beauty and charm jets. The distributions are compared to those previously measured by the LHCb collaboration in jets produced back-to-back with a Z boson, which in the forward region are primarily light-quark-initiated, to compare the hadronization mechanisms of heavy and light quarks. The observed differences between the heavy- and light-jet distributions are consistent with the heavy-quark dynamics expected to arise from the dead-cone effect, as well as with a hard fragmentation of the heavy-flavor hadron as previously measured in single-hadron fragmentation functions. This measurement provides additional constraints for the extraction of collinear and transverse-momentum-dependent heavy-flavor fragmentation functions and offers another approach to probing the mechanisms that govern heavy-flavor hadronization.

Abstract:

The first four-dimensional amplitude analysis of the $B^{+}\to \psi (2S)K_{\mathrm{S}}^0{\pi }^{+}$ decay is performed with proton-proton collision data collected by the LHCb experiment at $\sqrt{s}=13\mathrm{TeV}$ , corresponding to an integrated luminosity of $5.4\mathrm{f}{\mathrm{b}}^{-1}$ . The data cannot be fully explained by $B^{+}\to \psi (2S)K^{*+}$ contributions alone. A significantly better description of the data is obtained by adding a $T_{c\overline{c}}^{+}$ contribution decaying to $\psi (2S){\pi }^{+}$ . The properties of the $T_{c\overline{c}}^{+}$ structure are consistent with the exotic state $T_{c\overline{c}1}(4430{)}^{+}$ reported in the isospin-related ${\overline{B}}^0\to \psi (2S)K^{-}{\pi }^{+}$ decay. Effects of a possible $T_{c\overline{c}1}(4430{)}^{+}\to {\overline{D}}_1^{*}(2600{)}^0{D}^{+}$ decay mode on the $T_{c\overline{c}1}(4430{)}^{+}\to \psi (2S){\pi }^{+}$ mass distribution are investigated through a Flatté parametrization, providing constraints on the relative decay strength. A description of the $T_{c\overline{c}1}(4430{)}^{+}$ structure using the triangle singularity mechanism is studied and also found to be consistent with the data.

Abstract:

A search for long-lived heavy neutral leptons produced in B-meson decays and decaying to a μ±π∓ final state is performed with data collected by the LHCb experiment in proton-proton collisions at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5 fb−1. The results are interpreted in both lepton-number-conserving and lepton-number-violating scenarios. No significant excess is observed. Constraints are placed on the squared mixing element |UμN|2 to the active muon neutrino, under the assumption that couplings to other lepton flavours are negligible, in the mass range of 1.6–5.5 GeV.

Abstract:

The pp → W±(→ μ±νμ)X cross-sections are measured at a proton-proton centre-of-mass energy s=5.02 TeV using a dataset corresponding to an integrated luminosity of 100 pb−1 recorded by the LHCb experiment. Considering muons in the pseudorapidity range 2.2 < η < 4.4, the cross-sections are measured differentially in twelve intervals of muon transverse momentum between 28 < pT< 52 GeV. Integrated over pT, the measured cross-sections areσW+→μ+νμ=300.9±2.4±3.8±6.0pb, σW−→μ−ν¯μ=236.9±2.1±2.7±4.7pb, where the first uncertainties are statistical, the second are systematic, and the third are associated with the luminosity calibration. These integrated results are consistent with theoretical predictions. This analysis introduces a new method to determine the W-boson mass using the measured differential cross-sections corrected for detector effects. The measurement is performed on this statistically limited dataset as a proof of principle and yieldsmW=80369±130±33MeV, where the first uncertainty is experimental and the second is theoretical.