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Abstract:

A search for the lepton-flavour-violating decay τ− → μ−μ+μ− is carried out using data collected by the LHCb experiment between 2016 and 2018 in proton-proton collisions at the LHC at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb−1. An upper limit of 1.9 (2.3) × 10−8 is set at the 90% (95%) confidence level on the branching fraction of the decay.

Abstract:

A precision measurement of the $W$ -boson production cross section is performed using the $W→\mathrm{μ}\mathrm{ν}$ decay channel, based on a sample of proton-proton collision data collected by the LHCb experiment at $\sqrt{s}=13\mathrm{TeV}$ and corresponding to an integrated luminosity of $5.1{\mathrm{fb}}^{−1}$ . The cross section is measured for muons with transverse momentum between 25 and 55 GeV and pseudorapidity between 2.0 and 4.5. The total production cross sections of $W$ bosons are measured to be ${\mathrm{σ}}_{W^{+}→{\mathrm{μ}}^{+}\mathrm{ν}}=1754.2Â\pm 1.5Â\pm 11.8Â\pm 35.1\mathrm{pb}$ ${\mathrm{σ}}_{W^{−}→{\mathrm{μ}}^{−}\stackrel{¯}{\mathrm{ν}}}=1178.1Â\pm 1.3Â\pm 9.7Â\pm 23.6\mathrm{pb}$ where uncertainties are statistical, systematic, and due to the luminosity determination, respectively. Differential distributions $\mathrm{d}{\mathrm{σ}}_{W^{Â\pm }→{\mathrm{μ}}^{Â\pm }\mathrm{ν}}/\mathrm{d}{\mathrm{η}}^{{\mathrm{μ}}^{Â\pm }}$ are also presented. The total and differential cross sections are in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics. This measurement is significantly more precise than previous results in this kinematic regime.

Abstract:

The first measurement of the polarization of charm baryons by the LHCb experiment recorded in fixed-target mode is presented. The polarization of ${\mathrm{Âì}}_c$ baryons is studied in collisions of protons, at an energy of 2.51 TeV, incident on a gaseous target of neon, at a nucleon-nucleon center-of-mass energy of 68.6 GeV. The world’s first measurement of separate-charge polarizations for ${\mathrm{Âì}}_c^{+}$ and ${\stackrel{¯}{\mathrm{Âì}}}_c^{−}$ baryons is performed, determining $P_{{\mathrm{Âì}}_c^{+}}=(24Â\pm 9Â\pm 2)\%$ , $P_{{\stackrel{¯}{\mathrm{Âì}}}_c^{−}}=(−8Â\pm 12Â\pm 3)\%$ , where the first uncertainty is statistical and the second systematic. The polarization is also measured in intervals of baryon transverse momentum and the Feynman- $x$ variable.

Abstract:

A measurement of the flavor-untagged decay-time-integrated $CP$ asymmetry in the flavor-specific decay $B_s^0→D_s^{−}{\mathrm{Ï€}}^{+}$ , $⟨A_{\text{untagged}}^sâŸ\copyright$ , is performed using proton-proton collision data collected by the LHCb experiment between 2016 and 2018 at a center-of-mass energy of 13 TeV, corresponding to a total integrated luminosity of $5.4{\mathrm{fb}}^{−1}$ . The $CP$ asymmetry is measured in two $D_s^{−}$ meson decay modes, $D_s^{−}→K^{−}{K}^{+}{\mathrm{Ï€}}^{−}$ and $D_s^{−}→{\mathrm{Ï€}}^{−}{\mathrm{Ï€}}^{+}{\mathrm{Ï€}}^{−}$ . The combined result, $⟨A_{\text{untagged}}^sâŸ\copyright =(−1.4Â\pm 5.9(\mathrm{stat})Â\pm 1.1(\mathrm{syst}))×{10}^{−3}$ , is consistent with the Standard Model expectation and provides a direct constraint on new physics in tree-level $b$ -hadron decays.