Several new physics models including versions of supersymmetry (SUSY) characterized by $R$-parity violation (RPV) or with additional hidden sectors predict the production of events with top quarks, low missing transverse momentum, and many additional quarks or gluons. The results of a search for top squarks decaying to two top quarks and six additional light-flavor quarks or gluons are reported. The search employs a novel machine learning method for background estimation from control samples in data using decorrelated discriminators. The search is performed using events with 0, 1, or 2 electrons or muons in conjunction with at least six jets. No requirement is placed on the magnitude of the missing transverse momentum. The result is based on a sample of proton-proton collisions at $\sqrt{s}$ = 13 TeV corresponding to 138 fb$^{-1}$ of integrated luminosity collected with the CMS detector at the LHC in 2016$-$2018. The data are used to determine upper limits on the top squark pair production cross section in the frameworks of RPV and stealth SUSY. Models with top squark masses less than 700 (930) GeV are excluded at 95% confidence level for RPV (stealth) SUSY scenarios.

A search for the production of a single top quark in association with invisible particles is performed using proton-proton collision data collected with the CMS detector at the LHC at $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. In this search, a flavor-changing neutral current produces a single top quark or antiquark and an invisible state nonresonantly. The invisible state consists of a hypothetical spin-1 particle acting as a new mediator and decaying to two spin-1/2 dark matter candidates. The analysis searches for events in which the top quark or antiquark decays hadronically. No significant excess of events compatible with that signature is observed. Exclusion limits at 95% confidence level are placed on the masses of the spin-1 mediator and the dark matter candidates, and are compared to constraints from the dark matter relic density measurements. In a vector (axial-vector) coupling scenario, masses of the spin-1 mediator are excluded up to 1.85 (1.85) TeV with an expectation of 2.0 (2.0) TeV, whereas masses of the dark matter candidates are excluded up to 0.75 (0.55) TeV with an expectation of 0.85 (0.65) TeV.

A search for the pair production of the lightest supersymmetric partner of the top quark, the top squark ($\tilde{\mathrm{t}}_1$), is presented. The search targets the four-body decay of the $\tilde{\mathrm{t}}_1$, which is preferred when the mass difference between the top squark and the lightest supersymmetric particle is smaller than the mass of the W boson. This decay mode consists of a bottom quark, two other fermions, and the lightest neutralino ($\tilde{\chi}^0_1$), which is assumed to be the lightest supersymmetric particle. The data correspond to an integrated luminosity of 138 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV collected by the CMS experiment at the CERN LHC. Events are selected using the presence of a high-momentum jet, an electron or muon with low transverse momentum, and a significant missing transverse momentum. The signal is selected based on a multivariate approach that is optimized for the difference between $m(\tilde{\mathrm{t}}_1)$ and $m(\tilde{\chi}^0_1)$. The contribution from leading background processes is estimated from data. No significant excess is observed above the expectation from standard model processes. The results of this search exclude top squarks at 95% confidence level for masses up to 480 and 700 GeV for $m(\tilde{\mathrm{t}}_1) - m(\tilde{\chi}^0_1$) = 10 and 80 GeV, respectively.

Incoherent J/$ψ$ photoproduction in heavy ion ultraperipheral collisions (UPCs) provides a sensitive probe of localized, fluctuating gluonic structures within heavy nuclei. This study reports the first measurement of the photon-nucleon center-of-mass energy ($W_{γ\mathrm{N}}$) dependence of this process in PbPb UPCs at a nucleon-nucleon center-of-mass energy of 5.02 TeV, using 1.52 nb$^{-1}$ of data recorded by the CMS experiment. The measurement covers a wide $W_{γ\mathrm{N}}$ range of $\approx$ 40-400 GeV, probing gluons carrying a fraction $x$ of nucleon momentum down to an unexplored region of 6.5 $\times$ 10$^{-5}$. Compared to baseline predictions neglecting nuclear effects, the measured cross sections exhibit significantly greater suppression at lower $x$. Additionally, the ratio of incoherent to coherent photoproduction is found to be constant across the probed $W_{γ\mathrm{N}}$ and $x$ range, disfavoring the establishment of the black disk limit. This study provides critical insights into the $x$-dependent evolution of fluctuating gluonic structures within nuclei and calls for further advancements in theoretical models incorporating nuclear shadowing and gluon saturation.

Measurements of light-by-light scattering (LbL, $γγ$$\to$$γγ$) and the Breit-Wheeler process (BW, $γγ$$\to$$\mathrm{e^+e^-}$) are reported in ultraperipheral PbPb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV. The data sample, corresponding to an integrated luminosity of 1.7 nb$^{-1}$, was collected by the CMS experiment at the CERN LHC in 2018. Events with an exclusively produced $γγ$ or $\mathrm{e^+e^-}$ pair with invariant masses $m^{γγ,\mathrm{ee}}$$>$ 5 GeV, along with other fiducial criteria, are selected. The measured BW fiducial production cross section, $σ_\text{fid}$($γγ$$\to$$\mathrm{e^+e^-}$) = 263.5 $\pm$ 1.8 (stat) $\pm$ 17.8 (syst) $μ$b, as well as the differential distributions for various kinematic observables, are in agreement with leading-order quantum electrodynamics predictions complemented with final-state photon radiation. The measured differential BW cross sections allow discriminating between different theoretical descriptions of the photon flux of the lead ion. In the LbL final state, 26 exclusive diphoton candidate events are observed compared with 12.0 $\pm$ 2.9 expected for the background. Combined with previous results, the observed significance of the LbL signal with respect to the background-only hypothesis is above five standard deviations. The measured fiducial LbL scattering cross section, $σ_\text{fid} (γγ$$\to$$γγ)$ = 107 $\pm$ 24 (stat) $\pm$ 13 (syst) nb, is in agreement with next-to-leading-order predictions. Limits on the production of axion-like particles coupled to photons are set over the mass range 5-100 GeV, including the most stringent limits to date in the range of 5-10 GeV.

The collective behavior of K$^0_\mathrm{S}$ and $\Lambda/\bar{\Lambda}$ strange hadrons is studied by measuring the elliptic azimuthal anisotropy ($v_2$) using the scalar-product and multiparticle correlation methods. Proton-lead (pPb) collisions at a nucleon-nucleon center-of-mass energy $\sqrt{s_\mathrm{NN}}$ = 8.16 TeV and lead-lead (PbPb) collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV collected by the CMS experiment at the LHC are investigated. Nonflow effects in the pPb collisions are studied by using a subevent cumulant analysis and by excluding events where a jet with transverse momentum greater than 20\GeV is present. The strange hadron $v_2$ values extracted in \pPb collisions via the four- and six-particle correlation method are found to be nearly identical, suggesting the collective behavior. Comparisons of the pPb and PbPb results for both strange hadrons and charged particles illustrate how event-by-event flow fluctuations depend on the system size.

The production cross sections of B$^0_\mathrm{s}$ and B$^+$ mesons are reported in proton-proton (pp) collisions recorded by the CMS experiment at the CERN LHC with a center-of-mass energy of 5.02 TeV. The data sample corresponds to an integrated luminosity of 302 pb$^{-1}$. The cross sections are based on measurements of the B$^0_\mathrm{s}$$\to$ J/$ψ(μ^+μ^-)ϕ$(1020) (K$^+$K$^-$) and B$^+$$\to$ J/$ψ(μ^+μ^-)$K$^+$ decay channels. Results are presented in the transverse momentum ($p_\mathrm{T}$) range 7-50 GeV/$c$ and the rapidity interval $\lvert y \rvert$$\lt$ 2.4 for the B mesons. The measured $p_\mathrm{T}$-differential cross sections of B$^+$ and B$^0_\mathrm{s}$ in pp collisions are well described by fixed-order plus next-to-leading logarithm perturbative quantum chromodynamics calculations. Using previous PbPb collision measurements at the same nucleon-nucleon center-of-mass energy, the nuclear modification factors, $R_\mathrm{AA}$, of the B mesons are determined. For $p_\mathrm{T}$$\lt$ 10 GeV/$c$, both mesons are found to be suppressed in PbPb collisions (with $R_\mathrm{AA}$ values significantly below unity), with less suppression observed for the B$^0_\mathrm{s}$ mesons. In this $p_\mathrm{T}$ range, the $R_\mathrm{AA}$ values for the B$^+$ mesons are consistent with those for inclusive charged hadrons and D$^0$ mesons. Below 10 GeV/$c$, both B$^+$ and B$^0_\mathrm{s}$ are found to be less suppressed than either inclusive charged hadrons or D$^0$ mesons, with the B$^0_\mathrm{s}$$R_\mathrm{AA}$ value consistent with unity. The $R_\mathrm{AA}$ values found for the B$^+$ and B$^0_\mathrm{s}$ are compared to theoretical calculations, providing constraints on the mechanism of bottom quark energy loss and hadronization in the quark-gluon plasma, the hot and dense matter created in ultrarelativistic heavy ion collisions.

A search for $γ$H production is performed with data from the CMS experiment at the LHC corresponding to an integrated luminosity of 138 fb$^{-1}$ at a proton-proton center-of-mass collision energy of 13 TeV. The analysis focuses on the topology of a boosted Higgs boson recoiling against a high-energy photon. The final states of H $\to$$\mathrm{b\bar{b}}$ and H $\to$ 4$\ell$ are analyzed. This study examines effective HZ$γ$ and H$γγ$ anomalous couplings within the context of an effective field theory. In this approach, the production cross section is constrained to be $σ_{γ\text{H}}$$\lt$ 16.4 fb at 95% confidence level (CL). Simultaneous constraints on four anomalous couplings involving HZ$γ$ and H$γγ$ are provided. Additionally, the production rate for H $\to$ 4$\ell$ is examined to assess potential enhancements in the Yukawa couplings between light quarks and the Higgs boson. Assuming the standard model values for the Yukawa couplings of the bottom and top quarks, the following simultaneous constraints are obtained: $κ_\text{u}$ = (0.0 $\pm$ 1.5) $\times$ 10$^{3}$, $κ_\text{d}$ = (0.0 $^{+6.7}_{-6.8}$) $\times$ 10$^{2}$, $κ_\text{s}$ = 0 $^{+30}_{-32}$, and $κ_\text{c}$ = 0.0 $^{+2.3}_{-2.8}$. This rules out the hypothesis that up- or down-type quarks in the first or second generation have the same Yukawa couplings as those in the third generation, with a CL greater than 95%.

Precision measurements of Higgs boson differential production cross sections are a key tool to probe the properties of the Higgs boson and test the standard model. New physics can affect both Higgs boson production and decay, leading to deviations from the distributions that are expected in the standard model. In this paper, combined measurements of differential spectra in a fiducial region matching the experimental selections are performed, based on analyses of four Higgs boson decay channels ($\gamma\gamma$, ZZ$^{(*)}$, WW$^{(*)}$, and $\tau\tau$) using proton-proton collision data recorded with the CMS detector at $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. The differential measurements are extrapolated to the full phase space and combined to provide the differential spectra. A measurement of the total Higgs boson production cross section is also performed using the $\gamma\gamma$ and ZZ decay channels, with a result of 53.4$^{+2.9}_{-2.9}$ (stat)$^{+1.9}_{-1.8}$ (syst) pb, consistent with the standard model prediction of 55.6 $\pm$ 2.5 pb. The fiducial measurements are used to compute limits on Higgs boson couplings using the $\kappa$-framework and the SM effective field theory.

A new algorithm is presented to discriminate reconstructed hadronic decays of tau leptons ($\tau_\mathrm{h}$) that originate from genuine tau leptons in the CMS detector against $\tau_\mathrm{h}$ candidates that originate from quark or gluon jets, electrons, or muons. The algorithm inputs information from all reconstructed particles in the vicinity of a $\tau_\mathrm{h}$ candidate and employs a deep neural network with convolutional layers to efficiently process the inputs. This algorithm leads to a significantly improved performance compared with the previously used one. For example, the efficiency for a genuine $\tau_\mathrm{h}$ to pass the discriminator against jets increases by 10-30% for a given efficiency for quark and gluon jets. Furthermore, a more efficient $\tau_\mathrm{h}$ reconstruction is introduced that incorporates additional hadronic decay modes. The superior performance of the new algorithm to discriminate against jets, electrons, and muons and the improved $\tau_\mathrm{h}$ reconstruction method are validated with LHC proton-proton collision data at $\sqrt{s} =$ 13 TeV.