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A new technique is developed to identify dielectrons ( $e^{+}{e}^{-}$ ) with Lorentz boost ${\gamma }_{\mathrm{L}}>20$ that produce one single merged cluster in the electromagnetic calorimeter of the CMS detector. The identification uses two multivariate models: one for the case where both electron tracks are reconstructed, and another where only one of the tracks is reconstructed. The efficiency is determined using proton-proton collision data collected at a center-of-mass energy of 13 TeV. Boosted $J/\psi$ mesons decaying into $e^{+}{e}^{-}$ pairs are used to estimate the efficiency of the model with two tracks, yielding an overall efficiency of 80%. The $Z\to {\mu }^{+}{\mu }^{-}\gamma$ events, where the photon converts into a collimated dielectron, are used for the model with a single track, yielding an efficiency of about 60%. A dedicated energy correction for dielectron candidates is also developed using $B^{\pm }\to J/\psi K^{\pm }\to e^{+}{e}^{-}{K}^{\pm }$ data. 

A search for Higgs boson ( $H$ ) production in association with two vector bosons ( $V=W$ , $Z$ ) via vector-boson scattering (VBS) is presented using proton-proton collision data collected at $\sqrt{s}=13\mathrm{TeV}$ by the CMS experiment, corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$ . Events containing two forward jets consistent with VBS, a large-radius jet from the decay of a boosted $H$ to a pair of $b$ quarks, and zero, one, or two charged leptons coming from $V$ decays are selected. The process is excluded at 95% confidence level for observed (expected) values of the $VVHH$ coupling modifier ${\kappa }_{2V}$ outside the interval $0.40<{\kappa }_{2V}<1.60$ ( $0.34<{\kappa }_{2V}<1.66$ ), assuming standard model values for all other couplings, thus establishing a novel probe of the $VVHH$ interaction. Constraints are also set on the individual ${\kappa }_{2W}$ and ${\kappa }_{2Z}$ coupling modifiers and on the allowed region in the ${\kappa }_{2W}\text{−}{\kappa }_{2Z}$ plane. 

Evidence of the production of two $Z$ bosons and a photon in proton-proton collisions is reported for the first time in CMS. The analysis uses data collected by the CMS experiment between 2016 and 2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$ . Evidence for the process $pp\to ZZ\gamma \to 4\ell \gamma$ ( $\ell =e$ , $\mu$ ), with an observed (expected) significance of 3.7 (3.1) standard deviations in a fiducial region defined by $p_{\mathrm{T}}^{\gamma }>20\mathrm{GeV}$ , $\left|{\eta }^{\gamma }\right|<2.4$ , $\mathrm{\Delta }R(\ell ,\gamma )>0.5$ , $m_Z$ between 60 and 120 GeV, and the invariant mass of either of the two $Z$ bosons combined with the photon ( $m_{Z_{\gamma }}$ ) larger than 100 GeV, is reported. The measured (predicted) fiducial cross section is ${60}_{-22}^{+27}\mathrm{ab}$ ( $47.56\pm 0.04\mathrm{ab}$ ). Additionally, the inclusive production of $pp\to 4\ell \gamma$ is studied by removing the $m_{Z_{\gamma }}$ requirement to include final state radiation where one $Z$ boson decays to $2\ell \gamma$ , yielding an observed (expected) significance of 5.0 (4.2) standard deviations and a measured (predicted) fiducial cross section of ${156}_{-35}^{+39}\mathrm{ab}$ ( $99.97\pm 0.09\mathrm{ab}$ ). 

The existence of a new spin-zero particle with a mass below the electroweak scale is predicted by several theoretical models. Searches for resonant production of photon pairs at the LHC are able to probe these models. We present a search for a narrow resonance produced through gluon fusion that decays into a pair of photons with an invariant mass between 10 and 70 GeV, using a proton-proton collision dataset from the CMS experiment. This dataset, corresponding to an integrated luminosity of $54.4{\mathrm{fb}}^{-1}$ , was recorded in 2018 at a center-of-mass energy of 13 TeV using a newly introduced diphoton trigger that enabled exploration of the low-mass diphoton spectrum. No significant excess above the expected background is observed. Upper limits are set on the product of the gluon fusion production cross section and the branching fraction of the diphoton decay of a narrow resonance. An interpretation of these limits within an effective field theory framework for axionlike particles is also provided. 

A search is performed for a new resonance $X$ decaying into a Higgs boson and a new scalar boson $Y$ ( $HY$ ) including the case where $Y$ is assumed to be a second Higgs boson ( $HH$ ), using proton-proton collision data collected at $\sqrt{s}=13\mathrm{TeV}$ , corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$ . This study performs a comprehensive exploitation of the $bbZZ$ events, encompassing the following decay topologies. One $H$ candidate is identified through its decay into a bottom quark-antiquark pair, while the other $H$ or the $Y$ candidate is selected through its decay into a pair of $Z$ bosons. One $Z$ boson is required to decay leptonically and the other, to decay into a pair of quarks or neutrinos. Events of interest are categorized based on the Lorentz boosts of the hadronically decaying $H$ and $Z$ bosons. Machine-learning-based discriminants, together with the reconstructed resonance mass, are employed across the different categories to separate signal from backgrounds, and their corresponding distributions are included in a simultaneous fit. No significant deviations from the standard model predictions are observed. Upper limits at the 95% confidence level are set on the $HY$ and $HH$ production cross sections. For $HY$ production, the upper limit on the cross section of the process $pp\to X\to HY\to bbZZ$ ranges from 500 to 5 fb, with the most stringent constraints achieved for high-mass $X$ resonances and $Y$ masses near the threshold for decay into a pair of massive vector bosons. This is comparable to the sensitivity achieved in other analyses, which focus on $H$ decays to $\gamma \gamma$ or $\tau \tau$ and $Y$ decays into a pair of bottom quarks or massive vector bosons. For resonant $HH$ production, the upper limit on the cross section of $pp\to X\to HH$ production is 1 pb for a high-mass resonance. 

A<sc>bstract</sc> A search for Higgs boson (H) production at high transverse momentum (p_T) in the WW decay channel is presented. The analysis uses proton-proton collisions at$$ \sqrt{s}=13 $$ $\sqrt{s}=13$ TeV recorded by the CMS experiment in 2016–2018, corresponding to an integrated luminosity of 138 fb⁻¹. The visible decay products of the Higgs boson are reconstructed as a single large-radius jet with one isolated lepton or none (1ℓand 0ℓ, respectively;ℓ= e,μ). The H-candidate jets are identified using an advanced transformer-based algorithm and are calibrated with the Lund jet plane reweighting technique. The 1ℓchannel is further split into gluon fusion, vector boson fusion, and associated production with hadronically decaying vector boson categories, while the 0ℓchannel considers all production processes inclusively. The measured cross section times the H→WW branching fraction relative to the standard model expectation is$$ \mu =-{0.19}_{-0.46}^{+0.48} $$ $\mu =-{0.19}_{-0.46}^{+0.48}$ , indicating no evidence of a signal above the background. This measurement represents the first dedicated study of highly Lorentz-boosted H→WW decays, complementing earlier searches for high-p_THiggs boson in other decay channels. 

A search for the pair production of heavy spin- $1/2$ or spin- $3/2$ resonances ( $t^{*}$ ) in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ is presented. Data collected with the CMS detector at the CERN LHC from 2016 to 2018 corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$ are used. The analysis targets benchmark signal scenarios where one $t^{*}$ decays into a top quark ( $t$ ) and a photon ( $\gamma$ ), and the other into a $t$ quark and a gluon ( $g$ ), i.e., $pp\to t^{*}\overline{t^{*}}\to tt\gamma g$ . All-hadronic final states from the $t$ pair decay chain are selected using jet substructure techniques. The signal is probed as a function of the $t^{*}$ candidate mass, which is reconstructed using the photon and a top quark candidate jet. No significant deviation from the background-only hypothesis is found. Observed (expected) upper limits on the signal cross section at 95% confidence level are set, excluding masses of spin- $1/2$ $t^{*}$ particles below 930 (930) GeV and spin- $3/2$ $t^{*}$ particles below 1330 (1390) GeV. This analysis marks the first search for heavy resonances in the $t\overline{t}\gamma g$ channel. Exploiting the high-energy photon to reduce the backgrounds, this search achieves sensitivity competitive with $pp\to t^{*}\overline{t^{*}}\to t\overline{t}gg$ searches for spin- $1/2$ $t^{*}$ despite the small expected $t^{*}\to t\gamma$ branching fraction. 

A hot and dense state of nuclear matter, known as the quark-gluon plasma, is created in collisions of ultrarelativistic heavy nuclei. Highly energetic quarks and gluons, collectively referred to as partons, lose energy as they travel through this matter, leading to suppressed production of particles with large transverse momenta ( $p_{\mathrm{T}}$ ). Conversely, high- $p_{\mathrm{T}}$ particle suppression has not been seen in proton-lead collisions, raising questions regarding the minimum system size required to observe parton energy loss. Oxygen-oxygen (OO) collisions examine a region of effective system size that lies between these two extreme cases. The CMS detector at the CERN LHC has been used to quantify charged-particle production in inclusive OO collisions for the first time via measurements of the nuclear modification factor ( $R_{\mathrm{AA}}$ ). The $R_{\mathrm{AA}}$ is derived by comparing particle production to expectations based on proton-proton ( $pp$ ) data and has a value of unity in the absence of nuclear effects. The data for OO and $pp$ collisions at a nucleon-nucleon center-of-mass energy $\sqrt{s_{\mathrm{NN}}}=5.36\mathrm{TeV}$ correspond to integrated luminosities of $6.1{\mathrm{nb}}^{-1}$ and $1.02{\mathrm{pb}}^{-1}$ , respectively. The $R_{\mathrm{AA}}$ is below unity with a minimum of $0.69\pm 0.04$ around $p_{\mathrm{T}}=6\mathrm{GeV}$ . The data exhibit better agreement with theoretical models incorporating parton energy loss as compared to baseline models without energy loss. 

A search for long-lived particles originating from the decay of $b$ hadrons produced in proton-proton collisions with a center-of-mass energy of 13 TeV at the LHC is presented. The analysis is performed on a dataset recorded in 2018, corresponding to an integrated luminosity of $41.6{\mathrm{fb}}^{-1}$ . Interactions of the long-lived particles in the CMS endcap muon system would create hadronic or electromagnetic showers, producing clusters of detector hits. Selected events contain at least one such high-multiplicity cluster in the muon endcaps and require the presence of a displaced muon. The most stringent upper limits to date on the branching fraction $\mathcal{B}(B\to K\mathrm{\Phi })$ , where the long-lived particle $\mathrm{\Phi }$ decays to a pair of hadrons, are obtained for $\mathrm{\Phi }$ masses of 0.3–3.0 GeV and $\mathrm{\Phi }$ mean proper decay lengths in the range of 1–500 cm. 

Abstract This paper presents a search for new physics through the process where a massive particle, X, decays into a Higgs boson and a second particle, Y. The Higgs boson subsequently decays into a bottom quark–antiquark pair, which is reconstructed as a single large-radius jet. The decay products of Yare also assumed to produce a single large-radius jet. The identification of the Yparticle is enhanced by computing the anomaly score of its candidate jet using an autoencoder, which measures deviations from typical quark- or gluon-induced jets. This allows a simultaneous search for multiple Ydecay scenarios within a single analysis. In the main benchmark process, Yis a scalar particle that decays into a Wboson pair. Two other scalar Ydecay processes are also considered as benchmarks: decays to a light quark–antiquark pair, and decays to a top quark–antiquark pair. A fourth benchmark process considers Yas a hadronically decaying top quark, arising from the decay of a vector-like quark into a top quark and a Higgs boson. Data recorded by the CMS experiment at a center-of-mass energy of 13$$\,\text {Te}\hspace{-.08em}\text {V}$$ $\text{Te}\text{V}$ in 2016–2018, corresponding to an integrated luminosity of 138$$\,\text {fb}^{-1}$$ ${\text{fb}}^{-1}$ , are analyzed. The search covers Xmasses between 1.4 and 3.0$$\,\text {Te}\hspace{-.08em}\text {V}$$ $\text{Te}\text{V}$ and Ymasses between 90 and 400$$\,\text {Ge}\hspace{-.08em}\text {V}$$ $\text{Ge}\text{V}$ , with all simulated signals produced in the narrow-width approximation. No significant excess above the standard model background expectation is observed. The most stringent upper limits to date are placed on benchmark signal cross sections for various masses of X and Y particles