LHCb

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.

Abstract:

The ${\mathrm{\Xi }}_c^{+}{\pi }^{-}{\pi }^{+}$ spectrum is investigated using proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$ , collected by the LHCb experiment during 2016–2018. Four states are observed with high significance, and their masses and widths are measured to be $m[{\mathrm{\Xi }}_c(2815{)}^{+}]=2816.65\pm 0.03\pm 0.03\pm 0.23\mathrm{MeV}$ , $\mathrm{\Gamma }[{\mathrm{\Xi }}_c(2815{)}^{+}]=2.07\pm 0.08\pm 0.12\mathrm{MeV}$ , $m[{\mathrm{\Xi }}_c(2923{)}^{+}]=2922.8\pm 0.3\pm 0.5\pm 0.2\mathrm{MeV}$ , $\mathrm{\Gamma }[{\mathrm{\Xi }}_c(2923{)}^{+}]=5.3\pm 0.9\pm 1.4\mathrm{MeV}$ , $m[{\mathrm{\Xi }}_c(2970{)}^{+}]=2968.6\pm 0.5\pm 0.5\pm 0.2\mathrm{MeV}$ , $\mathrm{\Gamma }[{\mathrm{\Xi }}_c(2970{)}^{+}]=31.7\pm 1.7\pm 1.9\mathrm{MeV}$ , $[{\mathrm{\Xi }}_c(3080{)}^{+}]=3076.8\pm 0.7\pm 1.3\pm 0.2\mathrm{MeV}$ , $\mathrm{\Gamma }[{\mathrm{\Xi }}_c(3080{)}^{+}]=6.8\pm 2.3\pm 0.9\mathrm{MeV}$ , where the uncertainties are statistical, systematic, and due to the limited precision on the ${\mathrm{\Xi }}_c^{+}$ mass, respectively. The ${\mathrm{\Xi }}_c(2923{)}^{+}$ baryon is observed for the first time, and is consistent with being the isospin partner of the previously observed ${\mathrm{\Xi }}_c(2923{)}^0$ state. Most of the measured parameters are more precise than existing world averages.

Abstract:

A search for the doubly-charmed-baryon decay Ξcc++→Ξc0π+π+ is performed using proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 5.4 fb−1. A significant structure consistent with the Ξcc++ baryon is observed in the Ξc0π+π+ invariant-mass spectrum. Using the Ξcc++→Λc+K−π+π+ decay as the normalisation channel, the branching fraction ratio, BΞcc++→Ξc0π+π+BΞcc++→Λc+K−π+π+, is measured to be 1.37 ± 0.18 (stat) ± 0.09 (syst) ± 0.35 (ext). This measurement provides critical input for testing QCD factorisation methods in the weak decays of doubly-heavy baryons, particularly in quantifying nonperturbative effects such as final-state interactions and resonance contributions to the hadronisation process.

Abstract:

The elliptic and triangular flow of charged particles are measured using two-particle angular correlations in pPb collisions in the pseudorapidity range 2.0 < |η| < 4.8. The data sample was collected by the LHCb experiment in 2016 at a centre-of-mass energy per nucleon pair of sNN=8.16 TeV, containing in total approximately 1.5 billion collision events. Non-flow contributions are obtained in low-multiplicity collisions and subtracted to extract the flow harmonics. The results are presented as a function of event multiplicity and hadron transverse momentum. Comparisons with a full (3+1)D dynamic model indicate that it overestimates the measured elliptic flow. A comparison between the forward and backward regions reveals no significant differences in flow parameters, suggesting that final-state effects may dominate over initial-state effects in the origin of flow in small systems.