Measurement of the τ\tau lepton polarization in Z boson decays in proton-proton collisions at s\sqrt{s} = 13 TeV

International audienceThe polarization of $\tau$ leptons is measured using leptonic and hadronic $\tau$ lepton decays in Z $\to\tau^+\tau^-$ 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 $\tau^-$ polarization at the Z boson mass pole is $\mathcal{P}_{\tau}$(Z) = $-$0.144$\pm$0.006 (stat) $\pm$ 0.014 (syst) = $-$0.144$\pm$0.015, in good agreement with the measurement of the $\tau$ lepton asymmetry parameter of $A_{\tau}$ = 0.1439$\pm$0.0043 = $-\mathcal{P}_{\tau}$(Z) at LEP. The $\tau$ polarization depends on the ratio of the vector to axial-vector couplings of the $\tau$ leptons in the neutral current expression, and thus on the effective weak mixing angle $\sin^{2}\theta_\mathrm{W}^{\text{eff}}$, independently of the Z boson production mechanism. The obtained value $\sin^{2}\theta_\mathrm{W}^{\text{eff}}$ = 0.2319$\pm$0.0008 (stat) $\pm$ 0.0018 (syst) = 0.2319$\pm$0.0019 is in good agreement with measurements at e$^+$e$^-$ colliders

Search for long-lived particles decaying in the CMS muon detectors in proton-proton collisions at s\sqrt{s} = 13 TeV

International audienceA search for long-lived particles (LLPs) decaying in the CMS muon detectors is presented. A data sample of proton-proton collisions at $\sqrt{s}$ = 13 TeV corresponding to an integrated luminosity of 138 fb$^{-1}$ recorded at the LHC in 2016-2018, is used. The decays of LLPs are reconstructed as high multiplicity clusters of hits in the muon detectors. In the context of twin Higgs models, the search is sensitive to LLP masses from 0.4 to 55 GeV and a broad range of LLP decay modes, including decays to hadrons, $\tau$ leptons, electrons, or photons. No excess of events above the standard model background is observed. The most stringent limits to date from LHC data are set on the branching fraction of the Higgs boson decay to a pair of LLPs with masses below 10 GeV. This search also provides the best limits for various intervals of LLP proper decay length and mass. Finally, this search sets the first limits at the LHC on a dark quantum chromodynamic sector whose particles couple to the Higgs boson through gluon, Higgs boson, photon, vector, and dark-photon portals, and is sensitive to branching fractions of the Higgs boson to dark quarks as low as 2$\times$10$^{-3}$

Search for flavor changing neutral current interactions of the top quark in final states with a photon and additional jets in proton-proton collisions at s\sqrt{s} = 13 TeV

International audienceA search for the production of a top quark in association with a photon and additional jets via flavor changing neutral current interactions is presented. The analysis uses proton-proton collision data recorded by the CMS detector at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. The search is performed by looking for processes where a single top quark is produced in association with a photon, or a pair of top quarks where one of the top quarks decays into a photon and an up or charm quark. Events with an electron or a muon, a photon, one or more jets, and missing transverse momentum are selected. Multivariate analysis techniques are used to discriminate signal and standard model background processes. No significant deviation is observed over the predicted background. Observed (expected) upper limits are set on the branching fractions of top quark decays: $\mathcal{B}$(t$\to$u$\gamma$) $\lt$ 0.95$\times$10$^{-5}$ (1.20$\times$10$^{-5}$) and $\mathcal{B}$(t$\to$c$\gamma$) $\lt$ 1.51$\times$10$^{-5}$ (1.54$\times$10$^{-5}$) at 95% confidence level, assuming a single nonzero coupling at a time. The obtained limit for $\mathcal{B}$(t$\to$u$\gamma$) is similar to the current best limit, while the limit for $\mathcal{B}$(t$\to$c$\gamma$) is significantly tighter than previous results

Search for the lepton flavor violating τ→\tau \to 3μ\mu decay in proton-proton collisions at s\sqrt{s} = 13 TeV

International audienceA search for the lepton flavor violating $\tau \to$ 3$\mu$ decay is performed using proton-proton collision events at a center-of-mass energy of 13 TeV collected by the CMS experiment at the LHC in 2017-2018, corresponding to an integrated luminosity of 97.7 fb$^{-1}$. Tau leptons produced in both heavy-flavor hadron and W boson decays are exploited in the analysis. No evidence for the decay is observed. The results of this search are combined with an earlier null result based on data collected in 2016 to obtain a total integrated luminosity of 131 fb$^{-1}$. The observed (expected) upper limits on the branching fraction $\mathcal{B}$($\tau \to$ 3$\mu$) at confidence levels of 90 and 95% are 2.9$\times$10$^{-8}$ (2.4$\times$10$^{-8}$) and 3.6$\times$10$^{-8}$ (3.0$\times$10$^{-8}$), respectively

Search for charged-lepton flavor violation in the production and decay of top quarks using trilepton final states in proton-proton collisions at s\sqrt{s} = 13 TeV

International audienceA search is performed for charged-lepton flavor violating processes in top quark (t) production and decay. The data were collected by the CMS experiment from proton-proton collisions at a center-of-mass energy of 13 TeV and correspond to an integrated luminosity of 138 fb$^{-1}$. The selected events are required to contain one opposite-sign electron-muon pair, a third charged lepton (electron or muon), and at least one jet of which no more than one is associated with a bottom quark. Boosted decision trees are used to distinguish signal from background, exploiting differences in the kinematics of the final states particles. The data are consistent with the standard model expectation. Upper limits at 95% confidence level are placed in the context of effective field theory on the Wilson coefficients, which range between 0.024-0.424 TeV$^{-2}$ depending on the flavor of the associated light quark and the Lorentz structure of the interaction. These limits are converted to upper limits on branching fractions involving up (charm) quarks, t$\to$e$\mu$u (t$\to$e$\mu$c), of 0.032 (0.498)$\times$10$^{-6}$, 0.022 (0.369)$\times$10$^{-6}$, and 0.012 (0.216)$\times$10$^{-6}$ for tensor-like, vector-like, and scalar-like interactions, respectively

The CMS Statistical Analysis and Combination Tool: COMBINE

International audienceThis paper describes the COMBINE software package used for statistical analyses by the CMS Collaboration. The package, originally designed to perform searches for a Higgs boson and the combined analysis of those searches, has evolved to become the statistical analysis tool presently used in the majority of measurements and searches performed by the CMS Collaboration. It is not specific to the CMS experiment, and this paper is intended to serve as a reference for users outside of the CMS Collaboration, providing an outline of the most salient features and capabilities. Readers are provided with the possibility to run COMBINE and reproduce examples provided in this paper using a publicly available container image. Since the package is constantly evolving to meet the demands of ever-increasing data sets and analysis sophistication, this paper cannot cover all details of COMBINE. However, the online documentation referenced within this paper provides an up-to-date and complete user guide

Search for production of a single vector-like quark decaying to tH or tZ in the all-hadronic final state in pp collisions at s\sqrt{s} = 13 TeV

International audienceA search for electroweak production of a single vector-like T quark in association with a bottom (b) quark in the all-hadronic decay channel is presented. This search uses proton-proton collision data at $\sqrt{s}$ = 13 TeV collected by the CMS experiment at the CERN LHC during 2016-2018, corresponding to an integrated luminosity of 138 fb$^{-1}$ The T quark is assumed to have charge 2/3 and decay to a top (t) quark and a Higgs (H) or Z boson. Event kinematics and the presence of jets containing b hadrons are used to reconstruct the hadronic decays of the t quark and H or Z boson. No significant deviation from the standard model prediction is observed in the data. The 95% confidence level upper limits on the product of the production cross section and branching fraction of a T quark produced in association with a b quark and decaying via tH or tZ range from 1260 to 68 fb for T quark masses of 600-1200 GeV

Searches for violation of Lorentz invariance in tt‾ \mathrm{t} \overline{\mathrm{t}} production using dilepton events in proton-proton collisions at s= \sqrt{s}= 13 TeV

A search for violation of Lorentz invariance in the production of top quark pairs ($\mathrm{t} \overline{\mathrm{t}}$) is presented. The measured normalized differential $\mathrm{t} \overline{\mathrm{t}}$ production cross section, as function of the sidereal time, is examined for potential modulations induced by Lorentz-invariance breaking operators in an effective field theory extension of the standard model (SM). The cross section is measured from collision events collected by the CMS detector at a center-of-mass-energy of 13 TeV, corresponding to an integrated luminosity of 77.8 fb$^{-1}$, and containing one electron and one muon. The results are found to be compatible with zero, in agreement with the SM, and are used to bound the Lorentz-violating couplings to be in ranges of 1-8 $\times$ 10$^{-3}$ at 68% confidence level. This is the first precision test of the isotropy in special relativity with top quarks at the LHC, restricting further the bounds on such couplings by up two orders of magnitude with respect to previous searches conducted at the Tevatron.A search for violation of Lorentz invariance in the production of top quark pairs ($\mathrm{t\bar{t}}$) is presented. The measured normalized differential $\mathrm{t\bar{t}}$ production cross section, as function of the sidereal time, is examined for potential modulations induced by Lorentz-invariance breaking operators in an effective field theory extension of the standard model (SM). The cross section is measured from collision events collected by the CMS detector at a center-of-mass-energy of 13 TeV, corresponding to an integrated luminosity of 77.8 fb$^{-1}$, and containing one electron and one muon. The results are found to be compatible with zero, in agreement with the SM, and are used to bound the Lorentz-violating couplings to be in ranges of 1 - 8 $\times$ 10$^{-3}$ at 68% confidence level. This is the first precision test of the isotropy in special relativity with top quarks at the LHC, restricting further the bounds on such couplings by up two orders of magnitude with respect to previous searches conducted at the Tevatron

Dark sector searches with the CMS experiment

Astrophysical observations provide compelling evidence for gravitationally interacting dark matter in the universe that cannot be explained by the standard model of particle physics. The extraordinary amount of data from the CERN LHC presents a unique opportunity to shed light on the nature of dark matter at unprecedented collision energies. This Report comprehensively reviews the most recent searches with the CMS experiment for particles and interactions belonging to a dark sector and for dark-sector mediators. Models with invisible massive particles are probed by searches for signatures of missing transverse momentum recoiling against visible standard model particles. Searches for mediators are also conducted via fully visible final states. The results of these searches are compared with those obtained from direct-detection experiments. Searches for alternative scenarios predicting more complex dark sectors with multiple new particles and new forces are also presented. Many of these models include long-lived particles, which could manifest themselves with striking unconventional signatures with relatively small amounts of background. Searches for such particles are discussed and their impact on dark-sector scenarios is evaluated. Many results and interpretations have been newly obtained for this Report.Astrophysical observations provide compelling evidence for gravitationally interacting dark matter in the universe that cannot be explained by the standard model of particle physics. The extraordinary amount of data from the CERN LHC presents a unique opportunity to shed light on the nature of dark matter at unprecedented collision energies. This Report comprehensively reviews the most recent searches with the CMS experiment for particles and interactions belonging to a dark sector and for dark-sector mediators. Models with invisible massive particles are probed by searches for signatures of missing transverse momentum recoiling against visible standard model particles. Searches for mediators are also conducted via fully visible final states. The results of these searches are compared with those obtained from direct-detection experiments. Searches for alternative scenarios predicting more complex dark sectors with multiple new particles and new forces are also presented. Many of these models include long-lived particles, which could manifest themselves with striking unconventional signatures with relatively small amounts of background. Searches for such particles are discussed and their impact on dark-sector scenarios is evaluated. Many results and interpretations have been newly obtained for this Report

Search for narrow trijet resonances in proton-proton collisions at s\sqrt{s} = 13 TeV

International audienceThe first search for narrow resonances decaying to three well-separated hadronic jets is presented. The search uses proton-proton collision data corresponding to an integrated luminosity of 138 fb$^{-1}$ at $\sqrt{s}$ = 13 TeV, collected at the CERN LHC. No significant deviations from the background predictions are observed between 1.75-9.00 TeV. The results provide the first mass limits on a right-handed boson Z$_{\mathrm{R}}$ decaying to three gluons, an excited quark decaying via a vector boson to three quarks, as well as updated limits on a Kaluza-Klein gluon decaying via a radion to three gluons