Since the initial data taking of the CERN LHC, the CMS experiment has undergone substantial upgrades and improvements. This paper discusses the CMS detector as it is configured for the third data-taking period of the CERN LHC, Run 3, which started in 2022. The entire silicon pixel tracking detector was replaced. A new powering system for the superconducting solenoid was installed. The electronics of the hadron calorimeter was upgraded. All the muon electronic systems were upgraded, and new muon detector stations were added, including a gas electron multiplier detector. The precision proton spectrometer was upgraded. The dedicated luminosity detectors and the beam loss monitor were refurbished. Substantial improvements to the trigger, data acquisition, software, and computing systems were also implemented, including a new hybrid CPU/GPU farm for the high-level trigger.

Solid state photodetectors like silicon photomultipliers (SiPMs) are playing an important role in several fields of medical imaging, life sciences and high energy physics. They are able to sense optical photons with a single photon detection time precision below 100 ps, making them ideal candidates to read the photons generated by fast scintillators in time of flight positron emission tomography (TOF-PET). By implementing novel high-frequency readout electronics, it is possible to perform a completely new evaluation of the best timing performance achievable with state-of-the-art analog-SiPMs and scintillation materials. The intrinsic SiPM single photon time resolution (SPTR) was measured with Ketek, HPK, FBK, SensL and Broadcom devices. Also, the best achieved coincidence time resolution (CTR) for these devices was measured with LSO:Ce:Ca of mm³ and mm³ size crystals. The intrinsic SPTR for all devices ranges between 70 ps and 135 ps FWHM when illuminating the entire mm² or mm² area. The obtained CTR with LSO:Ce:Ca of mm³ size ranges between 58 ps and 76 ps FWHM for the SiPMs evaluated. Bismuth Germanate (BGO), read out with state of-the-art NUV-HD SiPMs from FBK, achieved a CTR of 158 ps and 277 ps FWHM for mm³ and mm³ crystals, respectively. Other BGO geometries yielded 167 3 ps FWHM for mm³ and 235 5 ps FWHM for mm³ also coupled with Meltmount (n = 1.582) and wrapped in Teflon. Additionally, the average number of Cherenkov photons produced by BGO in each 511 keV event was measured to be 17 3 photons. Based on this measurement, we predict the limits of BGO for ultrafast timing in TOF-PET with Monte Carlo simulations. Plastic scintillators (BC422, BC418), BaF₂, GAGG:Ce codoped with Mg and CsI:undoped were also tested for TOF performance. Indeed, BC422 can achieve a CTR of 35 2 ps FWHM using only Compton interactions in the detector with a maximum deposited energy of 340 keV. BaF₂ with its fast cross-luminescence enables a CTR of 51 5 ps FWHM when coupled to VUV-HD SiPMs from FBK, with only ∼22% photon detection efficiency (PDE). We summarize the measured CTR of the various scintillators and discuss their intrinsic timing performance.

A search for narrow and broad resonances with masses greater than 1.8 TeV decaying to a pair of jets is presented. The search uses proton-proton collision data at $$ \sqrt{s} $$ s = 13 TeV collected at the LHC, corresponding to an integrated luminosity of 137 fb ⁻¹. The background arising from standard model processes is predicted with the fit method used in previous publications and with a new method. The dijet invariant mass spectrum is well described by both data-driven methods, and no significant evidence for the production of new particles is observed. Model independent upper limits are reported on the production cross sections of narrow resonances, and broad resonances with widths up to 55% of the resonance mass. Limits are presented on the masses of narrow resonances from various models: string resonances, scalar diquarks, axigluons, colorons, excited quarks, color-octet scalars, W′ and Z′ bosons, Randall-Sundrum gravitons, and dark matter mediators. The limits on narrow resonances are improved by 200 to 800 GeV relative to those reported in previous CMS dijet resonance searches. The limits on dark matter mediators are presented as a function of the resonance mass and width, and on the associated coupling strength as a function of the mediator mass. These limits exclude at 95% confidence level a dark matter mediator with a mass of 1.8 TeV and width 1% of its mass or higher, up to one with a mass of 4.8 TeV and a width 45% of its mass or higher.

Results are reported from a search for supersymmetric particles in the final state with multiple jets and large missing transverse momentum. The search uses a sample of proton-proton collisions at $$ \sqrt{s} $$ s = 13 TeV collected with the CMS detector in 2016–2018, corresponding to an integrated luminosity of 137 fb ⁻¹, representing essentially the full LHC Run 2 data sample. The analysis is performed in a four-dimensional search region defined in terms of the number of jets, the number of tagged bottom quark jets, the scalar sum of jet transverse momenta, and the magnitude of the vector sum of jet transverse momenta. No significant excess in the event yield is observed relative to the expected background contributions from standard model processes. Limits on the pair production of gluinos and squarks are obtained in the framework of simplified models for supersymmetric particle production and decay processes. Assuming the lightest supersymmetric particle to be a neutralino, lower limits on the gluino mass as large as 2000 to 2310 GeV are obtained at 95% confidence level, while lower limits on the squark mass as large as 1190 to 1630 GeV are obtained, depending on the production scenario.