LHCb

Abstract:

A search for $CP$ violation with an entangled system of ${\mathrm{\Xi }}^{-}{\overline{\mathrm{\Xi }}}^{+}$ pairs is performed, using $(10,087\pm 44)\times {10}^{6}J/\psi$ events collected with the BESIII experiment. A nine-dimensional helicity amplitude is used to fit $e^{+}{e}^{-}\to J/\psi \to {\mathrm{\Xi }}^{-}{\overline{\mathrm{\Xi }}}^{+}$ and its subsequent decays. The ${\mathrm{\Xi }}^{-}$ and ${\overline{\mathrm{\Xi }}}^{+}$ decay parameters are determined with higher precision compared to the best results reported so far. Furthermore, the strong phase difference, $({\delta }_P-{\delta }_S)=(0.3\pm 1.2\pm 0.2)\times {10}^{-2}\mathrm{rad}$ , and the weak phase difference, $({\xi }_P-{\xi }_S)=(-0.2\pm 1.2\pm 0.1)\times {10}^{-2}\mathrm{rad}$ , are directly determined. These are the most precise measurements to date. The $CP$ asymmetry observables $A_{CP}^{\mathrm{\Xi }}=(-7.8\pm 4.8\pm 0.8)\times {10}^{-3}$ and $\mathrm{\Delta }{\varphi }_{CP}^{\mathrm{\Xi }}=(0.6\pm 5.1\pm 0.2)\times {10}^{-3}\mathrm{rad}$ are determined, which are consistent with $CP$ conservation. In addition, independent measurements of the $\mathrm{\Lambda }$ decay parameter and $CP$ asymmetry $A_{CP}^{\mathrm{\Lambda }}=(-2.9\pm 4.3\pm 0.7)\times {10}^{-3}$ are also obtained, which are in agreement with the previous measurements, but with much improved precision.

Abstract:

The TORCH time-of-flight detector is part of a proposed upgrade of the LHCb experiment, foreseen for the high-luminosity phase of the LHC. The TORCH detector provides particle identification of hadrons in the sub-10 GeV/c momentum range, exploiting the prompt production of Cherenkov photons in an array of fused-silica plates. Photons are propagated to the periphery of the detector via total internal reflection, where they are focused by a cylindrical mirror onto an array of fast-timing MCP-PMT photon detectors. In order to achieve the design goals of TORCH, individual photons must be timed to 70ps precision or better. The development of the MCP-PMTs, the mechanical design and assembly strategy of a full-scale TORCH detector module, plus its system-level validation in a test beam are described. A validation of timing references, the optical integrity across glue joints and the readout integration are presented.