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.
The accurate identification of charged hadrons, distinguishing between pions, kaons, and protons, stands as a pivotal aspect in the physics analyses conducted at LHCb. Playing a crucial role in these measurements, the Ring Imaging Cherenkov (RICH) system offers outstanding Particle Identification (PID) capabilities across a wide momentum spectrum, ranging from 2.6 to 100 GeV/$c$. LHCb is planning an Upgrade II in Run 5 and 6 to exploit the full potential of Hi-Lumi LHC and collect ∼ 300 fb$^{−2}$, by operating at instantaneous luminosity 1.0 – 1.5 x 10 $^{34}$ cm$^{−2}$s$^{−1}$. The RICH system will be fully upgraded to cope with such challenging conditions. The objective of the study is to examine different layouts of the RICH system, together with different types of photon detectors and compare the resulting performance to identify the best combination for the future of the experiment.