Malcolm John

Abstract:

A new algorithm is developed to identify the flavour of neutral B mesons at production in pp collisions by utilising all tracks from the hadronisation process. The algorithm is calibrated separately for B0 and Bs0 mesons using B0 → J/ψK+π− and Bs0→Ds−π+ decays from pp collision data collected by the LHCb experiment at a centre-of-mass energy of 13 TeV. This new algorithm improves the tagging power by 35% for B0 mesons and 20% for Bs0 mesons when compared to the combined performance of the existing LHCb flavour-tagging algorithms.

Abstract:

An amplitude analysis of the ${\mathrm{\Xi }}_c^{+}\to p{K}^{-}{\pi }^{+}$ decay together with a measurement of the ${\mathrm{\Xi }}_c^{+}$ polarization vector in semileptonic beauty-hadron decays is presented. The analysis is performed using proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$ . An amplitude model is developed and the resonance fractions as well as two- and three-body decay parameters are reported. A sizeable ${\mathrm{\Xi }}_c^{+}$ polarization is found. A large sensitivity of the ${\mathrm{\Xi }}_c^{+}\to p{K}^{-}{\pi }^{+}$ decay to the polarization is seen, making the amplitude model suitable for ${\mathrm{\Xi }}_c^{+}$ polarization measurements in other systems.

Abstract:

A study of Λb0 and Ξb0 baryon decays to the final states pKS0π− and pKS0K− is performed using pp collision data collected by the LHCb experiment, corresponding to an integrated luminosity of 9 fb−1. The decays Λb0→pKS0K− and Ξb0→pKS0K− are observed for the first time, with significances reaching eight standard deviations. The branching fractions and integrated CP asymmetries are measured for the Λb0→pKS0π−, Λb0→pKS0K−, and Ξb0→pKS0K− decays. For the decay Λb0→pKS0π−, the CP asymmetries are measured in different regions of the Dalitz plot. No evidence of CP violation is observed.

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:

The first dedicated $Z$ -boson mass measurement at the LHC with $Z\to {\mu }^{+}{\mu }^{-}$ decays is reported. The dataset uses proton-proton collisions at a center-of-mass energy of 13 TeV, recorded in 2016 by the LHCb experiment, and corresponds to an integrated luminosity of $1.7{\mathrm{fb}}^{-1}$ . A template fit to the ${\mu }^{+}{\mu }^{-}$ mass distribution yields the following result for the $Z$ -boson mass: $m_Z=91,185.7\pm 8.3\pm 3.9\mathrm{MeV}$ , where the first uncertainty is statistical and the second systematic. This result is consistent with previous measurements and predictions from global electroweak fits.