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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. 

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<sc>bstract</sc> A search for a new neutral gauge boson, Z′, produced in association with one or two jets, including at least one b jet, and decaying into a pair of muons is presented. The analysis uses proton-proton collision data collected with the CMS detector at$$ \sqrt{s}=13 $$ $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of 138 fb⁻¹. No significant deviation from background expectations is observed. Upper limits at 95% confidence level on the product of cross section, branching fraction to dimuons, acceptance, and efficiency, from 0.2 to 2 fb, are set for Z′boson masses between 125 and 350 GeV. Process-dependent products of acceptance and efficiency, and model-independent limits on the signal yield are provided. These are the only results to date in the 125–200 GeV mass range and the most stringent for b quark fusion production modes in the 200–350 GeV range, complementing inclusive Z′boson searches. 

A<sc>bstract</sc> The jet mass of W bosons decaying to a quark-antiquark pair is measured in W+jets events from proton-proton collisions at a center-of-mass energy of 13 TeV. The data used were collected by the CMS experiment at the CERN LHC and correspond to an integrated luminosity of 138 fb⁻¹. Hadronic decays of W bosons with high momenta produce strongly collimated decay products due to the large Lorentz boost, and are reconstructed as single large-radius jets. These jets have a characteristic substructure that is exploited to distinguish them from the large background of quark- and gluon-initiated jets. The jet mass is computed using the soft-drop algorithm, which suppresses soft wide-angle radiation that leads to a broadening of the jet mass distribution. For the first time, unfolded measurements are presented of the double-differential W+jets cross section as a function of the jet transverse momentum and soft-drop mass. From these distributions, the W boson mass is obtained, with a value of 80.83 ± 0.55 GeV in a scheme with mass-dependent width, achieving the smallest uncertainty available today from an all-jets final state at a hadron collider. 

A<sc>bstract</sc> A search for single production of a vector-like T quark with charge 2e/3, decaying to a top quark and a neutral scalar boson is presented. The boson can be a standard model Higgs boson (H) or a new scalar boson (ϕ). In the first case, a branching fraction of 25% is assumed for the decay T → tH, while in the second case the T quark is assumed to decay exclusively to tϕ. The top quark is identified via its lepton+jets decay, and the neutral boson via its decay into a bottom quark-antiquark pair. Final states with Lorentz-boosted topologies are considered and machine-learning techniques are exploited for optimal event classification. The analysis uses data collected by the CMS experiment in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb⁻¹recorded at the CERN LHC in 2016–2018. Upper limits at 95% confidence level are set on the product of cross section and branching fraction for a T quark in a narrow-width approximation. They vary between 14.7 and 0.1 fb, for T quark masses in the range 1.3–3.0 TeV and ϕ boson masses in the range 25–250 GeV. These are the first exclusion limits set on the production of a single T quark decaying into a top quark and a new neutral scalar boson. For the decay channel into a top quark and a standard model Higgs boson, the results provide the best limits on production cross sections to date, for T quark masses above 2 TeV. 

Production fraction ratios of $B^{+}$ , $B^0$ , and $B_s^0$ mesons are measured in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ using a special dataset recorded in 2018 with high-rate triggers designed to collect an unbiased sample of $10^{10}$ $b$ hadrons with the CMS experiment at the LHC. These data allow the study of the open-charm decays of $B$ mesons ( $B_{\left(s\right)}\to \pi D_{\left(s\right)}$ ) where the $D$ meson decays into fully hadronic final states. By utilizing known branching fractions and precise theoretical calculations, production fraction ratios as functions of $B$ meson transverse momentum $\left(p_{\mathrm{T}}\right)$ and rapidity ( $y$ ) are measured using open-charm decays in the kinematic range of $8<p_{\mathrm{T}}<60\mathrm{GeV}$ and $\left|y\right|<2.25$ . In addition, the same dataset is used to measure the relative production fraction ratios with the charmonium decay channels [ $B_{\left(s\right)}\to XJ/\psi$ with $X$ indicating a $K^{+}$ , $K^{*}(892{)}^0$ , or $\varphi \left(1020\right)$ meson] where the $J/\psi$ meson decays into a pair of muons. The open-charm results are used to normalize the relative production fraction ratios obtained from the charmonium samples. Measurements of the ratios of branching fractions of $B$ meson decays to charmonium and open-charm final states are also reported, which will improve the world-average values of these ratios. Finally, we test isospin invariance in $B$ meson production in proton-proton collisions and observe that it holds within the experimental precision. 

A<sc>bstract</sc> A search for exotic Higgs boson decays H→𝒜𝒜, with 𝒜 → γγ is presented, using events with a semi-merged topology. One of the hypothetical particles, 𝒜, is assumed to decay promptly into a semi-merged diphoton system reconstructed as a single photon-like object, while the other 𝒜 decays into two resolved photons. The search is performed using proton-proton collision data collected by the CMS experiment at$$ \sqrt{s}=13 $$ $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of 138 fb⁻¹. The data agree with the standard model background expectation. Upper limits are set on the product of the Higgs boson production cross section and the branching fraction,σ(pp→H) ℬ(H→𝒜𝒜→4γ), which range from 0.264 to 0.005 pb at 95% confidence level, for 𝒜 masses in the range 1< m_𝒜<15 GeV. These limits are the most stringent to date in the 1–5 GeVm_𝒜range. 

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<sc>bstract</sc> A search is performed for dark matter produced in association with bottom quarks and a pair of electrons or muons in data collected with the CMS detector at the LHC, corresponding to 138 fb⁻¹of integrated luminosity of proton-proton collisions at a center-of-mass energy of 13 TeV. For the first time at the LHC, the associated production of a bottom quark-antiquark pair and a new heavy neutral Higgs boson (H) that subsequently decays into a leptonically decaying Z boson and a pseudoscalar (a) is explored. The latter acts as a dark matter mediator in the context of the two Higgs doublet model plus a pseudoscalar (2HDM+a). Multivariate techniques that target a wide range of mass configurations for the H and a particles are used. The observations are consistent with the expectations from standard model processes. Upper limits at 95% confidence level are set on the product of cross section and branching fraction of the new particles, ranging from 10⁻²pb for an H mass of 400 GeV to 10⁻³pb for an H mass of 2000 GeV. Constraints on the parameter space of a benchmark 2HDM+a model are derived and compared with expectations in the context of cosmological predictions. 

A<sc>bstract</sc> A measurement of the angular structure of inclusive jets and those containing a prompt D⁰meson in proton-proton collisions at the LHC at a center-of-mass energy of 5.02 TeV is presented. The data corresponding to an integrated luminosity of 301 pb⁻¹were collected by the CMS experiment in 2017. Two jet grooming algorithms, late-k_Tand soft drop, are used to study the intrajet radiation pattern using iterative Cambridge-Aachen declustering. The splitting-angle distributions of jets with transverse momentum (p_T) of around 100 GeV, obtained with these two algorithms, show that there is a shift of the distribution for jets containing a prompt D⁰meson with respect to inclusive jets. The suppression of emissions at small angles observed in the late-k_Tgrooming approach is consistent with the dead-cone effect, whereas the similar suppression for splittings selected with the soft-drop algorithm appears to be induced by gluon splitting to charm quark-antiquark pairs at large angles. The measured distributions are corrected to the particle level and can be used to constrain model predictions for the substructure of high-p_Tcharm quark jets.