Malcolm John

Abstract:

The substructure of jets in quantum chromodynamics (QCD) has garnered significant attention with the advent of infrared- and collinear-safe clustering algorithms and observables. A key question emerging from these studies is how in-jet emissions at soft and hard energy scales, across collinear and wide angles relative to the emitter, differ with the mass of the emitting parton. The Lund jet plane (LJP) is a perturbatively well-defined substructure observable that maps the radiation pattern of jets onto a plane, visually distinguishing emissions with different kinematic properties. Comparing LJP for jets containing hadrons of low versus high mass enables the testing of QCD splitting functions from first-principles calculations across both soft and hard regimes and at different radiation angles. This article presents the first measurement of the LJP for light-quark-enriched and beauty-initiated jets at a center-of-mass energy of 13 TeV at LHCb. This marks the first direct observation of the dead-cone effect in beauty-quark jets, measured in the collinear region of the LJP.

Abstract:

The lifetimes of the Ωc0 and Ξc0 baryons are measured using a pp collision dataset collected by the LHCb experiment, corresponding to an integrated luminosity of 9 fb−1. The charm baryons are produced in the fully reconstructed decay chains Ωb−→Ωc0→pK−K−π+π− and Ξb−→Ξc0→pK−K−π+π−. The measurement uses topologically and kinematically similar B− → D0(→ K−K+π−π+)π− decays for normalisation. The measured lifetimes areτΩc0=276.3±19.4stat±1.8syst±0.7τD0fs, τΞc0=149.2±2.5stat±0.9syst±0.4τD0fs, where the first uncertainty is statistical, the second systematic and the third due to the uncertainty of the D0 lifetime. These results are consistent with previous measurements performed by the LHCb experiment.

Abstract:

The ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{D}_s^{-}{K}^{+}{K}^{-}$ decay is observed for the first time using the data sample from proton-proton collisions recorded at a center-of-mass energy of 13 TeV with the LHCb detector, corresponding to an integrated luminosity of $6{\mathrm{fb}}^{-1}$ . The ratio of branching fraction to that of ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{D}_s^{-}$ decays is measured as $0.0141\pm 0.0019\pm 0.0012$ , where the first uncertainty is statistical and the second systematic. A search for hidden-charm pentaquarks with strangeness is performed in the ${\mathrm{\Lambda }}_c^{+}{D}_s^{-}$ system. No evidence is found, and upper limits on the production ratio of $P_{c\overline{c}s}(4338{)}^0$ and $P_{c\overline{c}s}(4459{)}^0$ pentaquarks relative to the ${\mathrm{\Lambda }}_c^{+}{D}_s^{-}$ final state are set at the 95% confidence level as 0.12 and 0.20, respectively.

Abstract:

A sample of $pp$ collision data, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$ and collected by the LHCb experiment during LHC Run 2, is used to measure the ratio of the lifetime of the ${\mathrm{\Xi }}_b^0$ baryon to that of the ${\mathrm{\Lambda }}_b^0$ baryon, $r_{\tau }\equiv {\tau }_{{\mathrm{\Xi }}_b^0}/{\tau }_{{\mathrm{\Lambda }}_b^0}$ . The value $r_{\tau }^{\mathrm{Run}2}=1.004\pm 0.009\pm 0.006$ is obtained, where the first uncertainty is statistical and the second systematic. This value is averaged with the corresponding value from Run 1 to obtain $r_{\tau }=1.004\pm 0.008\pm 0.005$ . Multiplying by the known value of the ${\mathrm{\Lambda }}_b^0$ lifetime yields ${\tau }_{{\mathrm{\Xi }}_b^0}=1.475\pm 0.012\pm 0.008\pm 0.009\mathrm{ps}$ , where the last uncertainty is due to the limited knowledge of the ${\mathrm{\Lambda }}_b^0$ lifetime. This measurement improves the precision of the current world average of the ${\mathrm{\Xi }}_b^0$ lifetime by about a factor of two, and is in good agreement with the most recent theoretical predictions.

Abstract:

The LHCb collaboration presents a novel suite of heavy-flavour jet substructure measurements at forward rapidity in proton-proton collisions at a centre-of-mass energy of s = 13 TeV. The jet mass is a perturbatively calculable probe of the virtuality of hard-scattered quarks and gluons, connecting small-distance quantum chromodynamics (QCD) with long-distance experimental measurement. The jet mass is dominated by nonperturbative corrections at small values, presenting an excellent test of QCD across a broad range of jet energies. Measuring heavy-flavour jet mass with a theoretically unambiguous flavour definition for the first time probes the gluon splitting mechanism for heavy-flavour production and offers tests of perturbative QCD at new levels of theoretical precision. Utilising the soft drop jet-grooming technique to access the perturbative jet core further enhances constraints on first-principles theory. Measurements of the jet mass for jets containing fully reconstructed B ± hadrons are reported with and without grooming. The unique phase space instrumented by LHCb offers a different quark-gluon fraction than at midrapidity. These results offer unparalleled tests of quark flavour and mass dependence in QCD and provide a baseline for future studies of heavy-flavour jet quenching in heavy-ion collisions.