118 Search Results

The polarization of τ leptons is measured using leptonic and hadronic τ lepton decays in Z → τ$$^{+}$$τ$$^{−}$$ events in proton-proton collisions at $$ \sqrt{s} $$ = 13 TeV recorded by CMS at the CERN LHC with an integrated luminosity of 36.3 fb$$^{−1}$$. The measured τ$$^{−}$$ lepton polarization at the Z boson mass pole is $$ {\mathcal{P}}_{\tau}\left(\textrm{Z}\right) $$ = −0.144 ± 0.006 (stat) ± 0.014 (syst) = −0.144 ± 0.015, in good agreement with the measurement of the τ lepton asymmetry parameter of A$$_{τ}$$ = 0.1439 ± 0.0043 = $$ -{\mathcal{P}}_{\tau}\left(\textrm{Z}\right) $$ at LEP. The τ lepton polarization depends onmore » the ratio of the vector to axial-vector couplings of the τ leptons in the neutral current expression, and thus on the effective weak mixing angle sin$$^{2} {\theta}_{\textrm{W}}^{\textrm{eff}} $$, independently of the Z boson production mechanism. The obtained value sin$$^{2} {\theta}_{\textrm{W}}^{\textrm{eff}} $$ = 0.2319 ± 0.0008(stat) ± 0.0018(syst) = 0.2319 ± 0.0019 is in good agreement with measurements at e$$^{+}$$e$$^{−}$$ colliders.[graphic not available: see fulltext]« less

The strange quark content of the proton is probed through the measurement of the production cross section for a W boson and a charm (c) quark in proton–proton collisions at a center-of-mass energy of 13$$\,\text {Te}\hspace{-.08em}\text {V}$$. The analysis uses a data sample corresponding to a total integrated luminosity of 138$$\,\text {fb}^{-1}$$ collected with the CMS detector at the LHC. The W bosons are identified through their leptonic decays to an electron or a muon, and a neutrino. Charm jets are tagged using the presence of a muon or a secondary vertex inside the jet. The $$\hbox {W}+\hbox {c}$$ production crossmore » section and the cross section ratio $$R_\textrm{c}^{\pm }= \sigma ({\hbox {W}}^{+}+\bar{\text {c}})/\sigma (\hbox {W}^{-}+{\textrm{c}})$$ are measured inclusively and differentially as functions of the transverse momentum and the pseudorapidity of the lepton originating from the W boson decay. The precision of the measurements is improved with respect to previous studies, reaching 1% in $$R_\textrm{c}^{\pm }= 0.950 \pm 0.005\,\text {(stat)} \pm 0.010 \,\text {(syst)} $$. The measurements are compared with theoretical predictions up to next-to-next-to-leading order in perturbative quantum chromodynamics.« less

A search for new physics in top quark production with additional final-state leptons is performed using data collected by the CMS experiment in proton-proton collisions at $$ \sqrt{s} $$ = 13 TeV at the LHC during 2016–2018. The data set corresponds to an integrated luminosity of 138 fb$$^{−1}$$. Using the framework of effective field theory (EFT), potential new physics effects are parametrized in terms of 26 dimension-six EFT operators. The impacts of EFT operators are incorporated through the event-level reweighting of Monte Carlo simulations, which allows for detector-level predictions. The events are divided into several categories based on lepton multiplicity,more » total lepton charge, jet multiplicity, and b-tagged jet multiplicity. Kinematic variables corresponding to the transverse momentum (p$$_{T}$$) of the leading pair of leptons and/or jets as well as the p$$_{T}$$ of on-shell Z bosons are used to extract the 95% confidence intervals of the 26 Wilson coefficients corresponding to these EFT operators. No significant deviation with respect to the standard model prediction is found.[graphic not available: see fulltext]« less

A search for direct production of low-mass dimuon resonances is performed using $$ \sqrt{s} $$ = 13 TeV proton-proton collision data collected by the CMS experiment during the 2017–2018 operation of the CERN LHC with an integrated luminosity of 96.6 fb$$^{−1}$$. The search exploits a dedicated high-rate trigger stream that records events with two muons with transverse momenta as low as 3 GeV but does not include the full event information. The search is performed by looking for narrow peaks in the dimuon mass spectrum in the ranges of 1.1–2.6 GeV and 4.2–7.9 GeV. No significant excess of events abovemore » the expectation from the standard model background is observed. Model-independent limits on production rates of dimuon resonances within the experimental fiducial acceptance are set. Competitive or world’s best limits are set at 90% confidence level for a minimal dark photon model and for a scenario with two Higgs doublets and an extra complex scalar singlet (2HDM+S). Values of the squared kinetic mixing coefficient ε$$^{2}$$ in the dark photon model above 10$$^{−6}$$ are excluded over most of the mass range of the search. In the 2HDM+S, values of the mixing angle sin(θ$$_{H}$$) above 0.08 are excluded over most of the mass range of the search with a fixed ratio of the Higgs doublets vacuum expectation tan β = 0.5.[graphic not available: see fulltext]« less

The observation of the production of four top quarks in proton-proton collisions is reported, based on a data sample collected by the CMS experiment at a center-of-mass energy of 13 TeV in 2016–2018 at the CERN LHC and corresponding to an integrated luminosity of 138 ${\text{fb}}^{-1}$ . Events with two same-sign, three, or four charged leptons (electrons and muons) and additional jets are analyzed. Compared to previous results in these channels, updated identification techniques for charged leptons and jets originating from the hadronization of b quarks, as well as a revised multivariate analysismore » strategy to distinguish the signal process from the main backgrounds, lead to an improved expected signal significance of 4.9 standard deviations above the background-only hypothesis. Four top quark production is observed with a significance of 5.6 standard deviations, and its cross section is measured to be $17.7{}_{-3.5}^{+3.7}\text{(stat)}{}_{-1.9}^{+2.3}\text{(syst)}\text{fb}$ , in agreement with the available standard model predictions.« less

A search for new physics in final states consisting of at least one photon, multiple jets, and large missing transverse momentum is presented, using proton-proton collision events at a center-of-mass energy of 13 TeV. The data correspond to an integrated luminosity of 137 fb$$^{−1}$$, recorded by the CMS experiment at the CERN LHC from 2016 to 2018. The events are divided into mutually exclusive bins characterized by the missing transverse momentum, the number of jets, the number of b-tagged jets, and jets consistent with the presence of hadronically decaying W, Z, or Higgs bosons. The observed data are found tomore » be consistent with the prediction from standard model processes. The results are interpreted in the context of simplified models of pair production of supersymmetric particles via strong and electroweak interactions. Depending on the details of the signal models, gluinos and squarks of masses up to 2.35 and 1.43 TeV, respectively, and electroweakinos of masses up to 1.23 TeV are excluded at 95% confidence level.[graphic not available: see fulltext]« less

We present an observation of photon-photon production of $$\tau$$ lepton pairs in ultraperipheral lead-lead collisions. The measurement is based on a data sample with an integrated luminosity of 404 $$\mu$$b$$^{-1}$$ collected by the CMS experiment at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The $$\gamma\gamma$$$$\to$$$$\tau^+\tau^-$$ process is observed for $$\tau\tau$$ events with a muon and three charged hadrons in the final state. The measured fiducial cross section is $$\sigma(\gamma\gamma$$$$\to$$$$\tau^+\tau^-)$$ = 4.8 $$\pm$$ 0.6 (stat) $$\pm$$ 0.5 (syst) $$\mu$$b, in agreement with leading-order QED predictions. Using $$\sigma(\gamma\gamma$$$$\to$$$$\tau^+\tau^-)$$, we estimate a model-dependent value of the anomalous magnetic moment of the $$\tau$$ leptonmore » of $$a_\tau$$ = 0.001 $$^{+0.055}_{-0.089}$$.« less

The first search is presented for vector-like leptons (VLLs) in the context of the “4321 model”, an ultraviolet-complete model with the potential to explain existing B physics measurements that are in tension with standard model predictions. The analyzed data, corresponding to an integrated luminosity of 96.5 ${\text{fb}}^{-1}$ , were recorded in 2017 and 2018 with the CMS detector at the LHC in proton-proton collisions at $\sqrt{s}=13\text{TeV}$ . Final states with ≥3 b -tagged jets and two third-generation leptons (ττ, $\mathrm{\tau }{more »}_{}$ ν τ , or ${\mathrm{\nu }}_{\mathrm{\tau }}{\mathrm{\nu }}_{\mathrm{\tau }}$ ) are considered. Upper limits are derived on the VLL production cross section in the VLL mass range 500–1050 GeV. The maximum likelihood fit prefers the presence of signal at the level of 2.8 standard deviations, for a representative VLL mass point of 600 GeV. As a consequence, the observed upper limits are approximately double the expected limits.« less

The Precision Proton Spectrometer (PPS) of the CMS and TOTEM experiments collected 107.7 fb$$^{-1}$$ in proton-proton (pp) collisions at the LHC at 13 TeV (Run 2). This paper describes the key features of the PPS alignment and optics calibrations, the proton reconstruction procedure, as well as the detector efficiency and the performance of the PPS simulation. The reconstruction and simulation are validated using a sample of (semi)exclusive dilepton events. The performance of PPS has proven the feasibility of continuously operating a near-beam proton spectrometer at a high luminosity hadron collider.

A data sample containing top quark pairs ($$\mathrm{t\bar{t}}$$) produced in association with a Lorentz-boosted Z or Higgs boson is used to search for signs of new physics using effective field theory. The data correspond to an integrated luminosity of 138 fb$$^{-1}$$ of proton-proton collisions produced at a center-of-mass energy of 13 TeV at the LHC and collected by the CMS experiment. Selected events contain a single lepton and hadronic jets, including two identified with the decay of bottom quarks, plus an additional large-radius jet with high transverse momentum identified as a Z or Higgs boson decaying to a bottom quarkmore » pair. Machine learning techniques are employed to discriminate between $$\mathrm{t\bar{t}}$$Z or $$\mathrm{t\bar{t}}$$H events and events from background processes, which are dominated by $$\mathrm{t\bar{t}}$$ + jets production. No indications of new physics are observed. The signal strengths of boosted $$\mathrm{t\bar{t}}$$Z and $$\mathrm{t\bar{t}}$$H production are measured, and upper limits are placed on the $$\mathrm{t\bar{t}}$$Z and $$\mathrm{t\bar{t}}$$H differential cross sections as functions of the Z or Higgs boson transverse momentum. The effects of new physics are probed using a framework in which the standard model is considered to be the low-energy effective field theory of a higher energy scale theory. Eight possible dimension-six operators are added to the standard model Lagrangian and their corresponding coefficients are constrained via fits to the data.« less