A search for long-lived heavy neutral leptons (HNLs) is presented, which considers the hadronic final state and coupling scenarios involving all three lepton generations in the 2-20 GeV HNL mass range for the first time. Events comprising two leptons (electrons or muons) and jets are analyzed in a data sample of proton-proton collisions, recorded with the CMS experiment at the CERN LHC at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. A novel jet tagger, based on a deep neural network, has been developed to identify jets from an HNL decay using various features of the jet and its constituent particles. The network output can be used as a powerful discriminating tool to probe a broad range of HNL lifetimes and masses. Contributions from background processes are determined from data. No excess of events in data over the expected background is observed. Upper limits on the HNL production cross section are derived as functions of the HNL mass and the three coupling strengths $V_{\ell\mathrm{N}}$ to each lepton generation $\ell$ and presented as exclusion limits in the coupling-mass plane, as lower limits on the HNL lifetime, and on the HNL mass. In this search, the most stringent limit on the coupling strength is obtained for pure muon coupling scenarios; values of $\lvert V_{\mu\mathrm{N}}\rvert^{2}$$\gt $ 5 (4) $\times$ 10$^{-7}$ are excluded for Dirac (Majorana) HNLs with a mass of 10 GeV at a confidence level of 95% that correspond to proper decay lengths of 17 (10) mm.

A search for W' bosons decaying to a top and a bottom quark in final states including an electron or a muon is performed with the CMS detector at the LHC. The analyzed data correspond to an integrated luminosity of 138 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 Tev. Good agreement with the standard model expectation is observed and no evidence for the existence of the W' boson is found over the mass range examined. The largest observed deviation from the standard model expectation is found for a W' boson mass ($m_\mathrm{W'}$) hypothesis of 3.8 TeV with a relative decay width of 1%, with a local (global) significance of 2.6 (2.0) standard deviations. Upper limits on the production cross sections of W' bosons decaying to a top and a bottom quark are set. Left- and right-handed W' bosons with $m_\mathrm{W'}$ below 3.9 and 4.3 TeV, respectively, are excluded at the 95% confidence level, under the assumption that the new particle has a narrow decay width. Limits are also set for relative decay widths up to 30%.

The first measurement of pseudorapidity and azimuthal angle distributions relative to the momentum vector of a Z boson for low transverse momentum ($p_\mathrm{T}$) charged hadrons in lead-lead (PbPb) collisions is presented. By studying the hadrons produced in an event with a high-$p_\mathrm{T}$ Z boson (40 $\lt$$p_\mathrm{T}$$\lt$ 350 GeV), the analysis probes how the quark-gluon plasma (QGP) medium created in these collisions affects the parton recoiling opposite to the Z boson. Utilizing PbPb data at a nucleon-nucleon center-of-mass energy $\sqrt{s_{_\mathrm{NN}}}$ = 5.02 TeV from 2018 with an integrated luminosity of 1.67 nb$^{-1}$ and proton-proton (pp) data at the same energy from 2017 with 301 pb$^{-1}$, the distributions are examined in bins of charged-hadron $p_\mathrm{T}$. A significant modification of the distributions for charged hadrons in the range 1$\lt$$p_\mathrm{T}$$\lt$ 2 GeV in PbPb collisions is observed when compared to reference measurements from pp collisions. The data provide new information about the correlation between hard and soft particles in heavy ion collisions, which can be used to test predictions of various jet quenching models. The results are consistent with expectations of a hydrodynamic wake created when the QGP is depleted of energy by the parton propagating through it. Based on comparisons of PbPb data with pp references and predictions from theoretical models, this Letter presents the first evidence of medium-recoil and medium-hole effects caused by a hard probe.

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.

A search is presented for a heavy scalar ($H$) or pseudo-scalar ($A$) predicted by the two-Higgs-doublet models, where the $H/A$ is produced in association with a top-quark pair ($t\bar{t}H/A$), and with the $H/A$ decaying into a $t\bar{t}$ pair. Events are selected requiring exactly one or two opposite-charge electrons or muons. Data-driven corrections are applied to improve the modelling of the $t\bar{t}$+jets background in the regime with high jet and $b$-jet multiplicities. These include a novel multi-dimensional kinematic reweighting based on a neural network trained using data and simulations. An $H/A$-mass parameterised graph neural network is trained to optimise the signal-to-background discrimination. In combination with the previous search performed by the ATLAS Collaboration in the multilepton final state, the observed upper limits on the $t\bar{t}H/A \rightarrow t\bar{t}t\bar{t}$ production cross-section at 95% confidence level range between 14 fb and 5.0 fb for an $H/A$ with mass between 400 GeV and 1000 GeV, respectively. Assuming that both the $H$ and $A$ contribute to the $t\bar{t}t\bar{t}$ cross-section, $\tanβ$ values below 1.7 or 0.7 are excluded for a mass of 400 GeV or 1000 GeV, respectively. The results are also used to constrain a model predicting the pair production of a colour-octet scalar, with the scalar decaying into a $t\bar{t}$ pair.

For the first time at LHC energies, the forward rapidity gap spectra from proton-lead collisions for both proton and lead dissociation processes are presented. The analysis is performed over 10.4 units of pseudorapidity at a center-of-mass energy per nucleon pair of $\sqrt{s_\mathrm{NN}}$ = 8.16 TeV, almost 300 times higher than in previous measurements of diffractive production in proton-nucleus collisions. For lead dissociation processes, which correspond to the pomeron-lead event topology, the EPOS-LHC generator predictions are a factor of two below the data, but the model gives a reasonable description of the rapidity gap spectrum shape. For the pomeron-proton topology, the EPOS-LHC, QGSJET II, and HIJING predictions are all at least a factor of five lower than the data. The latter effect might be explained by a significant contribution of ultra-peripheral photoproduction events mimicking the signature of diffractive processes. These data may be of significant help in understanding the high energy limit of quantum chromodynamics and for modeling cosmic ray air showers.

The first observation of electroweak production of a photon in association with two forward jets in proton-proton collisions is presented. The measurement uses data recorded by the CMS experiment at the LHC during 2016$-$2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. The analysis is performed in a region enriched in photon production via vector boson fusion, with a requirement on the transverse momentum of the photon to exceed 200 GeV. The cross section is measured to be 202$^{+36}_{-32}$ fb, at a significance with respect to the null hypothesis that exceeds five standard deviations. This is in agreement with the standard model prediction of 177$^{+13}_{-12}$ fb. Differential cross sections are measured as a function of various observables. Limits are set on dimension-6 effective field theory operators that contribute to the WW$γ$ interaction. The observed 95% confidence intervals for the corresponding Warsaw basis Wilson coefficients $c_\mathrm{W}$ and $c_\mathrm{HWB}$ are [$-$0.11, 0.16] and [$-$1.6, 1.5], respectively.

A search for the rare decay D$^0$$\to$$μ^+μ^-$ is reported using proton-proton collision events at $\sqrt{s}$ = 13.6 TeV collected by the CMS detector in 2022$-$2023, corresponding to an integrated luminosity of 64.5 fb$^{-1}$. This is the first analysis to use a newly developed inclusive dimuon trigger, expanding the scope of the CMS flavor physics program. The search uses D$^0$ mesons obtained from D$^{*+}$$\to$ D$^0π^+$ decays. No significant excess is observed. A limit on the branching fraction of $\mathcal{B}$(D$^0$$\to$$μ^+μ^-$) $\lt$ 2.4 $\times$ 10$^{-9}$ at 95% confidence level is set. This is the most stringent upper limit set on any flavor changing neutral current decay in the charm sector.

This Letter reports the first measurement of photonuclear D$^0$ meson production in ultraperipheral heavy ion collisions. The study is performed using lead-lead collision data, with an integrated luminosity of 1.38 nb$^{-1}$, collected by the CMS experiment at a nucleon-nucleon center-of-mass energy of 5.36 TeV. Photonuclear events, where one of the colliding nuclei breaks up and the other remains intact, are selected based on breakup neutron emissions and by requiring no particle activity in a large rapidity interval in the direction of the photon-emitting nucleus. The D$^0$ mesons are reconstructed via the D$^0$$\to$ K$^-$$π^+$ decay channel, with the cross section measured as a function of D$^0$ meson transverse momentum and rapidity. The results are compared with next-to-leading-order perturbative QCD calculations that employ recent parametrizations of the lead nuclear parton distribution functions, as well as with predictions based on the color glass condensate framework. This measurement is the first photonuclear collision study characterizing parton distribution functions of lead nuclei for parton fractional momenta $x$ (relative to the nucleon) ranging approximately from a few 10$^{-4}$ to 10$^{-2}$ for different hard energy scale $Q^2$ selections.

This paper presents a measurement of fiducial and differential cross-sections for $W^{+}W^{-}$ production in proton-proton collisions at $\sqrt{s}=13$ TeV with the ATLAS experiment at the Large Hadron Collider using a dataset corresponding to an integrated luminosity of 139 fb$^{-1}$. Events with exactly one electron, one muon and no hadronic jets are studied. The fiducial region in which the measurements are performed is inspired by searches for the electroweak production of supersymmetric charginos decaying to two-lepton final states. The selected events have moderate values of missing transverse momentum and the `stransverse mass' variable $m_{\textrm{T2}}$, which is widely used in searches for supersymmetry at the LHC. The ranges of these variables are chosen so that the acceptance is enhanced for direct $W^{+}W^{-}$ production and suppressed for production via top quarks, which is treated as a background. The fiducial cross-section and particle-level differential cross-sections for six variables are measured and compared with two theoretical SM predictions from perturbative QCD calculations.