The first observation of single top quark production in association with a W and a Z boson in proton-proton collisions is reported. The analysis uses data at center-of-mass energies of 13 and 13.6 TeV recorded with the CMS detector at the CERN LHC, corresponding to a total integrated luminosity of 200 fb$^{-1}$. Events with three or four charged leptons, which can be electrons or muons, are selected. Advanced machine-learning algorithms and improved reconstruction methods, compared to an earlier analysis, result in an unprecedented sensitivity to tWZ production. The measured cross sections for tWZ production are 248 $\pm$ 52 fb and 244 $\pm$ 74 fb for $\sqrt{s}$ =13 and 13.6 TeV, respectively. The signal is established with a statistical significance of 5.8 standard deviations, with 3.5 expected, compared to the background-only hypothesis.

A search for the violation of the charge-parity ($CP$) symmetry in the production of top quarks in association with Z bosons is presented, using events with at least three charged leptons and additional jets. The search is performed in a sample of proton-proton collision data collected by the CMS experiment at the CERN LHC in 2016-2018 at a center-of-mass energy of 13 TeV and in 2022 at 13.6 TeV, corresponding to a total integrated luminosity of 173 fb$^{-1}$. For the first time in this final state, observables that are odd under the $CP$ transformation are employed. Also for the first time, physics-informed machine-learning techniques are used to construct these observables. While for standard model (SM) processes the distributions of these observables are predicted to be symmetric around zero, $CP$-violating modifications of the SM would introduce asymmetries. Two $CP$-odd operators $\mathcal{O}_\text{tW}^\text{I}$ and $\mathcal{O}_\text{tZ}^\text{I}$ in the SM effective field theory are considered that may modify the interactions between top quarks and electroweak bosons. The obtained results are consistent with the SM prediction within two standard deviations, and exclusion limits on the associated Wilson coefficients of $-$2.7 $\lt$$c_\text{tW}^\text{I}$$\lt$ 2.5 and $-$0.2 $\lt$$c_\text{tZ}^\text{I}$$\lt$ 2.0 are set at 95% confidence level. The largest discrepancy is observed in $c_\text{tZ}^\text{I}$ where data is consistent with positive values, with an observed local significance with respect to the SM hypothesis of 2.5 standard deviations, when only linear terms are considered.

Inclusive and differential cross section measurements of top quark pair ($\mathrm{t\bar{t}}$) production in association with a photon ($γ$) are performed as a function of lepton, photon, top quark, and $\mathrm{t\bar{t}}$ kinematic observables, using data from proton-proton collisions at $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. Events containing two leptons (electrons or muons) and a photon in the final state are considered. The fiducial cross section of $\mathrm{t\bar{t}}γ$ is measured to be 137 $\pm$ 8 fb, in a phase space including events with a high momentum, isolated photon. The fiducial cross section of $\mathrm{t\bar{t}}γ$ is also measured to be 56 $\pm$ 5 fb when considering only events where the photon is emitted in the production part of the process. Both measurements are in agreement with the theoretical predictions, of 126 $\pm$ 19 fb and 57 $\pm$ 5 fb, respectively. Differential measurements are performed at the particle and parton levels. Additionally, inclusive and differential ratios between the cross sections of $\mathrm{t\bar{t}}γ$ and $\mathrm{t\bar{t}}$ production are measured. The inclusive ratio is found to be 0.0133 $\pm$ 0.0005, in agreement with the standard model prediction of 0.0127 $\pm$ 0.0008. The top quark charge asymmetry in $\mathrm{t\bar{t}}γ$ production is also measured to be $-$0.012 $\pm$ 0.042, compatible with both the standard model prediction and with no asymmetry.

A search for pseudoscalar or scalar bosons decaying to a top quark pair ($\mathrm{t\bar{t}}$) in final states with one or two charged leptons is presented. The analyzed proton-proton collision data was recorded at $\sqrt{s}$ = 13 TeV by the CMS experiment at the CERN LHC and corresponds to an integrated luminosity of 138 fb$^{-1}$. The invariant mass $m_\mathrm{t\bar{t}}$ of the reconstructed $\mathrm{t\bar{t}}$ system and variables sensitive to its spin and parity are used to discriminate against the standard model $\mathrm{t\bar{t}}$ background. Interference between pseudoscalar or scalar boson production and the standard model $\mathrm{t\bar{t}}$ continuum is included, leading to peak-dip structures in the $m_\mathrm{t\bar{t}}$ distribution. An excess of the data above the background prediction, based on perturbative quantum chromodynamics (QCD) calculations, is observed near the kinematic $\mathrm{t\bar{t}}$ production threshold, while good agreement is found for high $m_\mathrm{t\bar{t}}$. The data are consistent with the background prediction if the contribution from the production of a color-singlet ${}^1\mathrm{S}_0^{[1]}$$\mathrm{t\bar{t}}$ quasi-bound state $η_\mathrm{t}$, predicted by nonrelativistic QCD, is added. Upper limits at 95% confidence level are set on the coupling between the pseudoscalar or scalar bosons and the top quark for boson masses in the range 365$-$1000 GeV, relative widths between 0.5 and 25%, and two background scenarios with or without $η_\mathrm{t}$ contribution.

The traditional quark model accounts for the existence of baryons, such as protons and neutrons, which consist of three quarks, as well as mesons, composed of a quark-antiquark pair. Only recently has substantial evidence started to accumulate for exotic states composed of four or five quarks and antiquarks. The exact nature of their internal structure remains uncertain. This paper reports the first measurement of quantum numbers of the recently discovered family of three all-charm tetraquarks, using data collected by the CMS experiment at the Large Hadron Collider from 2016 to 2018. The angular analysis techniques developed for the discovery and characterization of the Higgs boson have been applied to the new exotic states. Here we show that the quantum numbers for parity $P$ and charge conjugation $C$ symmetries are found to be +1. The spin $J$ of these exotic states is consistent with 2$\hbar$, while 0$\hbar$ and 1$\hbar$ are excluded at 95% and 99% confidence level, respectively. The $J^{PC} = 2^{++}$ assignment implies particular configurations of constituent spins and orbital angular momenta, which constrain the possible internal structure of these tetraquarks.

A search for signatures of a dark analog to quantum chromodynamics is performed. The analysis targets long-lived dark mesons that decay into standard-model particles, with a high branching fraction of the dark mesons decaying into muons. The dark mesons are formed by the hadronisation of dark partons, which are produced by a decay of the Higgs boson. The search is performed using a data set corresponding to an integrated luminosity of 41.6 fb$^{-1}$, which was collected in proton-proton collisions at $\sqrt{s}$ = 13 TeV by the CMS experiment at the CERN LHC in 2018 using non-prompt muon triggers. The search is based on resonant muon pair signatures. Machine-learning techniques are employed in the analysis, utilising boosted decision trees to discriminate between signal and background. No significant excess is observed above the standard model expectation. Upper limits on the branching fraction of the Higgs boson decaying to dark partons are determined to be as low as 10$^{-4}$ at 95% confidence level, surpassing and extending the existing limits on models with dark $\tildeω$ mesons for mean proper decay lengths of less than 500 mm and for $\tildeω$ masses down to 0.3 GeV. First limits are set for extended dark-shower models with two dark flavours that contain dark photons, probing their masses down to 0.33 GeV.

A search for pairs of light neutral pseudoscalar bosons (A) resulting from the decay of a Higgs boson is performed. The search is conducted using LHC proton-proton collision data at $\sqrt{s}$ = 13 TeV, collected with the CMS detector in 2016$-$2018 and corresponding to an integrated luminosity of 138 fb$^{-1}$. The A boson decays into a highly collimated electron-positron pair. A novel multivariate algorithm using tracks and calorimeter information is developed to identify these distinctive signatures, and events are selected with two such merged electron-positron pairs. No significant excess above the standard model background predictions is observed. Upper limits on the branching fraction for H $\to$ AA $\to$ 4e are set at 95% confidence level, for masses between 10 and 100 MeV and proper decay lengths below 100 $μ$m, reaching branching fraction sensitivities as low as 10$^{-5}$. This is the first search for Higgs boson decays to four electrons via light pseudoscalars at the LHC. It significantly improves the experimental sensitivity to axion-like particles with masses below 100 MeV.

A measurement of the Z$γ$ production cross section in proton-proton collisions at a center-of-mass energy of 13.6 TeV is presented. Data corresponding to an integrated luminosity of 34.8 fb$^{-1}$, collected by the CMS experiment at the LHC in 2022 are used. Events with an oppositely charged pair of muons or electrons, with an invariant mass corresponding to a Z boson, together with an isolated photon are selected. The measured fiducial cross section for the combined electron and muon channels is 1.896 $\pm$ 0.033 (stat) $\pm$ 0.05 (syst) $\pm$ 0.006 (theo) pb, in agreement with the standard model prediction of 1.922 $\pm$ 0.094 pb. Constraints on neutral triple gauge couplings generated by dimension-8 operators in a recently proposed effective field theory framework are determined for the first time.

A search for heavy resonances decaying into a Higgs boson (H) or a Z boson and a photon ($γ$), with the H or Z bosons decaying to a bottom quark-antiquark pair ($\mathrm{b\bar{b}}$) is presented. The analysis is performed using proton-proton collision data at $\sqrt{s}$ = 13 TeV collected by the CMS experiment at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 138 fb$^{-1}$. The analyzed events contain a photon and a massive large-radius jet originating from a Lorentz-boosted $\mathrm{b\bar{b}}$ system. An advanced transformer-based algorithm classifies jets according to their substructure and quark flavors, forming a tagger that identifies jets as candidates from H/Z $\to$$\mathrm{b\bar{b}}$ decays. A set of parametric functions is used to fit the photon-jet invariant mass spectrum and to extract potential signals. No significant excess is observed above the standard model expectations. The results set upper limits at 95% confidence level on the product of the cross section and the branching fraction for spin-1 H$γ$ resonances and spin-0 Z$γ$ resonances, below 0.1 and 0.3 fb, respectively, representing the most stringent limits to date.

A search for flavor violating decays of the Z boson to charged leptons is performed using data from proton-proton collisions at $\sqrt{s}$ = 13 TeV collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 138 fb$^{-1}$. Each of the decays Z $\to$ e$μ$, Z $\to$ e$τ$, and Z $\to$$μτ$ is considered. The data are consistent with the backgrounds expected from standard model processes. For the Z $\to$ e$μ$ channel the observed (expected) 95% confidence level upper limit on the branching fraction is 1.9 (2.0) $\times$ 10$^{-7}$, which is the most stringent direct limit to date on this process; the corresponding limits for the Z $\to$ e$τ$ and Z $\to$ $μτ$ channels are 13.8 (11.4) $\times$ 10$^{-6}$ and 12.0 (5.3) $\times$ 10$^{-6}$, respectively. Additionally, the e$μ$ final state is used to search for lepton flavor violating decays of Z' resonances in the mass range from 110 to 500 GeV. No significant excess is observed above the predicted background levels.