CERN Accelerating science

During Runs 5 and 6, the LHCb experiment at CERN will operate at a luminosity up to 1.5 x 10$^{34}$ cm$^{-2s^-1}$, requiring substantial upgrades to its Electromagnetic Calorimeter (ECAL) to handle high radiation doses and achieve time resolutions of few tens of picoseconds mitigating pile-up effects. The detector under development is a Spaghetti Calorimeter (SpaCal) composed of scintillating fibres (polystyrene or garnet crystals) in a dense absorber (lead or tungsten). Ongoing investigations are focused on the photodetectors (PMTs) selection and their impact on the overall timing performance. Simulation studies of a lead-polystyrene module show that fast PMTs result in worse time resolutions due to the longitudinal showers' fluctuations, which introduce a bias in the time stamps defined by the Constant Fraction Discriminator (CFD) algorithm. A correction procedure has been developed to remove such bias, improving the time resolution by few tens of picoseconds. Additionally, a correlation between signal rise time and shower depth has been observed. Data from a test beam campaign conducted at the CERN SPS in June 2024 have been analysed to measure the timing resolution of two tungsten-polystyrene SpaCal prototypes, comparing four PMT models and two fibre types. By exploiting a rise-time-based correction procedure, time resolutions below 20 ps at high energies have been reached, with the fastest PMTs undergoing larger corrections, as expected from simulations.

A measurement of the CKM angle $\gamma$ is performed in $B^{\pm} \to D K^*(892)^{\pm}$ decays at the LHCb experiment, where $D$ represents a superposition of $D^0$ and $\overline{D}{}^0$ states. Using the dataset collected during Run 1 and Run 2, this analysis represents a comprehensive study of this channel, with the $D$ meson reconstructed in two-body final states $K^{\pm}\pi^{\mp}$, $K^+K^-$ and $\pi^+\pi^-$; four-body final states $K^{\pm}\pi^{\mp}\pi^{\pm}\pi^{\mp}$ and $\pi^+\pi^-\pi^+\pi^-$; and three-body final states $K^0_{S} \pi^+\pi^-$ and $K^0_{S} K^+ K^-$. This measurement constitutes the first observation of the suppressed $B^{\pm} \to [\pi^+K^-]_D K^{*\pm}$ and $B^{\pm} \to [\pi^+K^-\pi^+\pi^-]_D K^{*\pm}$ decays. The combined result gives $\gamma=(63\pm 13)^\circ$.

The LHCb RICH detector will undergo a significant upgrade during LS3 as part of an approved enhancement program to introduce fast-timing information. The upgrade will address the challenges posed by increased particle multiplicity and high occupancy anticipated for the LHC HL phase. Integrating sub-100 ps timing information is crucial for maintaining excellent particle identification (PID) performance. In the RICH detector, Cherenkov photons from a track arrive nearly simultaneously at the detector plane, allowing precise hit time prediction. Determining the primary vertex time (PV T$_0$) is key to accurately predicting the time of arrival of photons on the photodetector plane. By integrating time information in the RICH reconstruction, a software time gate can be applied around the predicted time per track to enhance signal-to-background ratio and PID performance. This contribution describes the integration of fast-timing information into the RICH detector, focusing on a novel method to estimate PV T$_0$ using only RICH information.