20 Search Results

The mass of the top quark is measured in 36.3 fb^-1 of LHC proton–proton collision data collected with the CMS detector at $$\sqrt{s}=13\,\text {Te}\hspace{-.08em}\text {V}$$. The measurement uses a sample of top quark pair candidate events containing one isolated electron or muon and at least four jets in the final state. For each event, the mass is reconstructed from a kinematic fit of the decay products to a top quark pair hypothesis. A profile likelihood method is applied using up to four observables per event to extract the top quark mass. The top quark mass is measured to be 171.77more » ± 0.37 GeV. This approach significantly improves the precision over previous measurements.« less

A generic search is presented for the associated production of a Z boson or a photon with an additional unspecified massive particle X, pp → pp + Z/γ + X, in proton-tagged events from proton–proton collisions at $$\sqrt{s}$$ = 13 TeV, recorded in 2017 with the CMS detector and the CMS-TOTEM precision proton spectrometer. The missing mass spectrum is analysed in the 600–1600 GeV range and a fit is performed to search for possible deviations from the background expectation. No significant excess in data with respect to the background predictions has been observed. Model-independent upper limits on the visible promore » duction cross section of pp → pp + Z/γ + X are set« less

Multijet events at large transverse momentum ($$p$$_T) are measured at $$\sqrt{s}$$ = 13 TeV using data recorded with the CMS detector at the LHC, corresponding to an integrated luminosity of 36.3 fb^-1. The multiplicity of jets with $$p$$_T > 50 GeV that are produced in association with a high-$$p$$_T dijet system is measured in various ranges of the $$p$$_T of the jet with the highest transverse momentum and as a function of the azimuthal angle difference Δ$$\phi$$_1,2 between the two highest $$p$$_T jets in the dijet system. The differential production cross sections are measured as a function of the transversemore » momenta of the four highest $$p$$_T jets. The measurements are compared with leading and next-to-leading order matrix element calculations supplemented with simula tions of parton shower, hadronization, and multiparton interactions. In addition, the measurements are compared with next-to-leading order matrix element calculations combined with transverse-momentum dependent parton densities and transverse-momentum dependent parton shower.« less

The production of Z bosons associated with jets is measured in pp collisions at $$\sqrt{s}=13\,\text {TeV}$$ with data recorded with the CMS experiment at the LHC corresponding to an integrated luminosity of 36.3 fb^-1. The multiplicity of jets with transverse momentum $$p$$_T > 30 GeV is measured for different regions of the Z boson’s $$p$$_T (Z ), from lower than 10 GeV to higher than 100 GeV. The azimuthal correlation $$Δ\phi$$ between the Z boson and the leading jet, as well as the correlations between the two leading jets are measured in three regions of $$p$$_T(Z ). The measurements aremore » compared with several predictions at leading and next-to-leading orders, interfaced with parton showers. Predictions based on transverse-momentum dependent parton distributions and corresponding parton showers give a good description of the measurement in the regions where multiple parton interactions and higher jet multiplicities are not important. The effects of multiple parton interactions are shown to be important to correctly describe the measured spectra in the low $$p$$_T(Z ) regions.« less

The double differential cross sections of the Drell–Yan lepton pair ($$\ell^+\ell^-$$, dielectron or dimuon) production are measured as functions of the invariant mass m$$_{\ell\ell}$$, transverse momentum p_T($$\ell\ell$$), and $$\varphi^{*}_{\eta}$$. The observable, derived from angular measurements of the leptons and highly correlated with p_T($$\ell\ell$$), is used to probe the low-p_T region in a complementary way. Dilepton masses up to 1 TeV are investigated. Additionally, a measurement is performed requiring at least one jet in the final state. To benefit from partial cancellation of the systematic uncertainty, the ratios of the differential cross sections for various m$$_{\ell\ell}$$ ranges to those in themore » Z mass peak interval are presented. The collected data correspond to an integrated luminosity of 36.3 fb^-1 of proton–proton collisions recorded with the CMS detector at the LHC at a centre-of-mass energy of 13 TeV. Measurements are compared with predictions based on perturbative quantum chromodynamics, including soft-gluon resummation.« less

New sets of parameter tunes for two of the colour reconnection models, quantum chromodynamics-inspired and gluon-move, implemented in the PYTHIA 8 event generator, are obtained based on the default CMS PYTHIA 8 underlying-event tune, CP5. Measurements sensitive to the underlying event performed by the CMS experiment at centre-of-mass energies $$\sqrt{s}=$$7 and 13 TeV, and by the CDF experiment at 1.96 TeV are used to constrain the parameters of colour reconnection models and multiple-parton interactions simultaneously. The new colour reconnection tunes are compared with various measurements at 1.96, 7, 8, and 13 TeV including measurements of the underlying-event, strange-particle multiplicities, jetmore » substructure observables, jet shapes, and colour flow in top quark pair ($$t\bar{t}$$) events. The new tunes are also used to estimate the uncertainty related to colour reconnection modelling in the top quark mass measurement using the decay products of $$t\bar{t}$$ events in the semileptonic channel at 13 TeV .« less

A search is reported for pairs of light Higgs bosons (H₁) produced in supersymmetric cascade decays in final states with small missing transverse momentum. A data set of LHC pp collisions collected with the CMS detector at $$\sqrt{s}$$ = 13 TeV and corresponding to an integrated luminosity of 138 fb^-1 is used. The search targets events where both H₁ bosons decay into $$\text{b}\bar{\text{b}}$$ pairs that are reconstructed as large-radius jets using substructure techniques. No evidence is found for an excess of events beyond the background expectations of the standard model (SM). Results from the search are interpreted in the next-to-minimalmore » supersymmetric extension of the SM, where a “singlino” of small mass leads to squark and gluino cascade decays that can predominantly end in a highly Lorentz-boosted singlet-like H₁ and a singlino-like neutralino of small transverse momentum. Upper limits are set on the product of the squark or gluino pair production cross section and the square of the branching fraction of the $$\text{b}\bar{\text{b}}$$ in a benchmark model containing almost mass-degenerate gluinos and light-flavour squarks. Under the assumption of an SM-like H₁ → $$\text{b}\bar{\text{b}}$$ branching fraction, H₁ bosons with masses in the range 40–120 GeV arising from the decays of squarks or gluinos with a mass of 1200–2500 GeV are excluded at 95% confidence level.« less

A measurement of the jet mass distribution in hadronic decays of Lorentz-boosted top quarks is presented. The measurement is performed in the lepton + jets channel of top quark pair production ($$\hbox {t}\overline{\hbox {t}}$$) events, where the lepton is an electron or muon. The products of the hadronic top quark decay are reconstructed using a single large-radius jet with transverse momentum greater than 400 GeV. The data were collected with the CMS detector at the LHC in proton-proton collisions and correspond to an integrated luminosity of 138 fb^-1. The differential $$\hbox {t}\overline{\hbox {t}}$$ production cross section as a function ofmore » the jet mass is unfolded to the particle level and is used to extract the top quark mass. The jet mass scale is calibrated using the hadronic W boson decay within the large-radius jet. The uncertainties in the modelling of the final state radiation are reduced by studying angular correlations in the jet substructure. These developments lead to a significant increase in precision, and a top quark mass of 173.06 ± 0.84 GeV.« less

Protons consist of three valence quarks, two up-quarks and one down-quark, held together by gluons and a sea of quark-antiquark pairs. Collectively, quarks and gluons are referred to as partons. In a proton-proton collision, typically only one parton of each proton undergoes a hard scattering – referred to as single-parton scattering – leaving the remainder of each proton only slightly disturbed. Here, we report the study of double- and triple-parton scatterings through the simultaneous production of three J/ψ mesons, which consist of a charm quark-antiquark pair, in proton-proton collisions recorded with the CMS experiment at the Large Hadron Collider. Wemore » observed this process – reconstructed through the decays of J/ψ mesons into pairs of oppositely charged muons – with a statistical significance above five standard deviations. We measured the inclusive fiducial cross-section to be 272$$^{+141}_{-104}$$(stat) ± 17 (syst)fb, and compared it to theoretical expectations for triple-J/ψ meson production in single-, double- and triple-parton scattering scenarios. Assuming factorization of multiple hard-scattering probabilities in terms of single-parton scattering cross-sections, double- and triple-parton scattering are the dominant contributions for the measured process.« less

Measurements of the associated production of a W boson and a charm (c) quark in proton–proton collisions at a centre-of-mass energy of 8TeV are reported. The analysis uses a data sample corresponding to a total integrated luminosity of 19.7 fb^-1 collected by 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 quark jets are selected using distinctive signatures of charm hadron decays. The product of the cross section and branching fraction σ(pp → W + c + X)β(W →ℓv), where ℓ = e ormore » μ, and the cross section ratio σ(pp → W⁺ + $$\bar{c}$$ + X)/σ(pp → W^- + c + X) are measured in a fiducial volume and differentially as functions of the pseudorapidity and of the transverse momentum of the lepton from the W boson decay. The results are compared with theoretical predictions. The impact of these measurements on the determination of the strange quark distribution is assessed.« less