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.

The CERN LHC provided proton and heavy ion collisions during its Run 2 operation period from 2015 to 2018. Proton-proton collisions reached a peak instantaneous luminosity of 2.1× 10³⁴ cm^-2s^-1, twice the initial design value, at √(s)=13 TeV. The CMS experiment records a subset of the collisions for further processing as part of its online selection of data for physics analyses, using a two-level trigger system: the Level-1 trigger, implemented in custom-designed electronics, and the high-level trigger, a streamlined version of the offline reconstruction software running on a large computer farm. This paper presents the performance of the CMS high-level trigger system during LHC Run 2 for physics objects, such as leptons, jets, and missing transverse momentum, which meet the broad needs of the CMS physics program and the challenge of the evolving LHC and detector conditions. Sophisticated algorithms that were originally used in offline reconstruction were deployed online. Highlights include a machine-learning b tagging algorithm and a reconstruction algorithm for tau leptons that decay hadronically.

A search for decays to invisible particles of Higgs bosons produced in association with a top-antitop quark pair or a vector boson, which both decay to a fully hadronic final state, has been performed using proton-proton collision data collected at $${\sqrt{s}=13\,\text {Te}\hspace{-.08em}\text {V}}$$s = 13 Te V by the CMS experiment at the LHC, corresponding to an integrated luminosity of 138 $$\,\text {fb}^{-1}$$fb - 1 . The 95% confidence level upper limit set on the branching fraction of the 125 $$\,\text {Ge}\hspace{-.08em}\text {V}$$Ge V Higgs boson to invisible particles, $${\mathcal {B}({\textrm{H}} \rightarrow \text {inv})}$$B ( H → inv ) , is 0.54 (0.39 expected), assuming standard model production cross sections. The results of this analysis are combined with previous $${\mathcal {B}({\textrm{H}} \rightarrow \text {inv})}$$B ( H → inv ) searches carried out at $${\sqrt{s}=7}$$s = 7 , 8, and 13 $$\,\text {Te}\hspace{-.08em}\text {V}$$Te V in complementary production modes. The combined upper limit at 95% confidence level on $${\mathcal {B}({\textrm{H}} \rightarrow \text {inv})}$$B ( H → inv ) is 0.15 (0.08 expected).

Measurements of the polarization and spin correlation in top quark pairs ($t\overline{t}$) are presented using events with a single electron or muon and jets in the final state. The measurements are based on proton-proton collision data from the LHC at $\sqrt{s}=13\text{ }\text{ }\mathrm{TeV}$ collected by the CMS experiment, corresponding to an integrated luminosity of $138\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$. All coefficients of the polarization vectors and the spin correlation matrix are extracted simultaneously by performing a binned likelihood fit to the data. The measurement is performed inclusively and in bins of additional observables, such as the mass of the $t\overline{t}$ system and the top quark scattering angle in the $t\overline{t}$ rest frame. The measured polarization and spin correlation are in agreement with the standard model. From the measured spin correlation, conclusions on the $t\overline{t}$ spin entanglement are drawn by applying the Peres-Horodecki criterion. The standard model predicts entangled spins for $t\overline{t}$ states at the production threshold and at high masses of the $t\overline{t}$ system. Entanglement is observed for the first time in events at high $t\overline{t}$ mass, where a large fraction of the $t\overline{t}$ decays are spacelike separated, with an expected and observed significance of above 5 standard deviations.

A search is presented for charged, long-lived supersymmetric particles in final states with one or more disappearing tracks. The search is based on data from proton-proton collisions at a center-of-mass energy of 13 TeV collected with the CMS detector at the CERN LHC between 2016 and 2018, corresponding to an integrated luminosity of $137{\mathrm{fb}}^{-1}$. The search is performed over final states characterized by varying numbers of jets, $b$-tagged jets, electrons, and muons. The length of signal-candidate tracks in the plane perpendicular to the beam axis is used to characterize the lifetimes of wino- and Higgsino-like charginos produced in the context of the minimal supersymmetric standard model. The $\mathrm{d}E/\mathrm{d}x$ energy loss of signal-candidate tracks is used to increase the sensitivity to charginos with a large mass and thus a small Lorentz boost. The observed results are found to be statistically consistent with the background-only hypothesis. Limits on the pair-production cross section of gluinos and squarks are presented in the framework of simplified models of supersymmetric particle production and decay, and for electroweakino production based on models of wino and Higgsino dark matter. The limits presented are the most stringent to date for scenarios with light third-generation squarks and a wino- or Higgsino-like dark matter candidate capable of explaining the observed dark matter relic density.