Search for: All records

A<sc>bstract</sc> Measurements of the charge-dependent two-particle angular correlation function in proton-lead (pPb) collisions at a nucleon-nucleon center-of-mass energy of$$ \sqrt{s_{\textrm{NN}}} $$ $\sqrt{s_{\mathrm{NN}}}$= 8.16 TeV and lead-lead (PbPb) collisions at$$ \sqrt{s_{\textrm{NN}}} $$ $\sqrt{s_{\mathrm{NN}}}$= 5.02 TeV are reported. The pPb and PbPb data sets correspond to integrated luminosities of 186 nb⁻¹and 0.607 nb⁻¹, respectively, and were collected using the CMS detector at the CERN LHC. The charge-dependent correlations are characterized by balance functions of same- and opposite-sign particle pairs. The balance functions, which contain information about the creation time of charged particle pairs and the development of collectivity, are studied as functions of relative pseudorapidity (∆η) and relative azimuthal angle (∆ϕ), for various multiplicity and transverse momentum (p_T) intervals. A multiplicity dependence of the balance function is observed in ∆ηand ∆ϕfor both systems. The width of the balance functions decreases towards high-multiplicity collisions in the momentum region<2 GeV, for pPb and PbPb results. Integrals of the balance functions are presented in both systems, and a mild dependence of the charge-balancing fractions on multiplicity is observed. No multiplicity dependence is observed at higher transverse momentum. The data are compared withhydjet,hijing, andamptgenerator predictions, none of which capture completely the multiplicity dependence seen in the data. The comparison of results with different center-of-mass energies suggests that the balance functions become narrower at higher energies, which is consistent with the idea of delayed hadronization and the effect of radial flow. 

A<sc>bstract</sc> Measurements of inclusive and normalized differential cross sections of the associated production of top quark-antiquark and bottom quark-antiquark pairs,$$ \textrm{t}\overline{\textrm{t}}\textrm{b}\overline{\textrm{b}} $$ $t\overline{t}b\overline{b}$, are presented. The results are based on data from proton-proton collisions collected by the CMS detector at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb⁻¹. The cross sections are measured in the lepton+jets decay channel of the top quark pair, using events containing exactly one isolated electron or muon and at least five jets. Measurements are made in four fiducial phase space regions, targeting different aspects of the$$ \textrm{t}\overline{\textrm{t}}\textrm{b}\overline{\textrm{b}} $$ $t\overline{t}b\overline{b}$process. Distributions are unfolded to the particle level through maximum likelihood fits, and compared with predictions from several event generators. The inclusive cross section measurements of this process in the fiducial phase space regions are the most precise to date. In most cases, the measured inclusive cross sections exceed the predictions with the chosen generator settings. The only exception is when using a particular choice of dynamic renormalization scale,$$ {\mu}_{\textrm{R}}=\frac{1}{2}{\prod}_{i=\textrm{t},\overline{\textrm{t}},\textrm{b},\overline{\textrm{b}}}{m}_{\textrm{T},i}^{1/4} $$ ${\mu }_R=\frac{1}{2}{\prod }_{i=t,\overline{t},b,\overline{b}}{m}_{T,i}^{1/4}$, where$$ {m}_{\textrm{T},i}^2={m}_i^2+{p}_{\textrm{T},i}^2 $$ $m_{T,i}^2=m_i^2+p_{T,i}^2$are the transverse masses of top and bottom quarks. The differential cross sections show varying degrees of compatibility with the theoretical predictions, and none of the tested generators with the chosen settings simultaneously describe all the measured distributions. 

A<sc>bstract</sc> The polarization ofτleptons is measured using leptonic and hadronicτlepton decays in Z →τ⁺τ⁻events in proton-proton collisions at$$ \sqrt{s} $$ $\sqrt{s}$= 13 TeV recorded by CMS at the CERN LHC with an integrated luminosity of 36.3 fb⁻¹. The measuredτ⁻lepton polarization at the Z boson mass pole is$$ {\mathcal{P}}_{\tau}\left(\textrm{Z}\right) $$ $P_{\tau }\left(Z\right)$= −0.144 ± 0.006 (stat) ± 0.014 (syst) = −0.144 ± 0.015, in good agreement with the measurement of theτlepton asymmetry parameter ofA_τ= 0.1439 ± 0.0043 =$$ -{\mathcal{P}}_{\tau}\left(\textrm{Z}\right) $$ $-P_{\tau }\left(Z\right)$at LEP. Theτlepton polarization depends on the ratio of the vector to axial-vector couplings of theτleptons in the neutral current expression, and thus on the effective weak mixing angle sin²$$ {\theta}_{\textrm{W}}^{\textrm{eff}} $$ ${\theta }_W^{\mathrm{eff}}$, independently of the Z boson production mechanism. The obtained value sin²$$ {\theta}_{\textrm{W}}^{\textrm{eff}} $$ ${\theta }_W^{\mathrm{eff}}$= 0.2319 ± 0.0008(stat) ± 0.0018(syst) = 0.2319 ± 0.0019 is in good agreement with measurements ate⁺e⁻colliders. 

A<sc>bstract</sc> The production of prompt$$ {\Lambda}_{\textrm{c}}^{+} $$ ${\Lambda }_c^{+}$baryons is measured via the exclusive decay channel$$ {\Lambda}_{\textrm{c}}^{+}\to p{\textrm{K}}^{-}{\pi}^{+} $$ ${\Lambda }_c^{+}\to p{K}^{-}{\pi }^{+}$at a center-of-mass energy per nucleon pair of 5.02 TeV, using proton-proton (pp) and lead-lead (PbPb) collision data collected by the CMS experiment at the CERN LHC. The pp and PbPb data were obtained in 2017 and 2018 with integrated luminosities of 252 and 0.607 nb⁻¹, respectively. The measurements are performed within the$$ {\Lambda}_{\textrm{c}}^{+} $$ ${\Lambda }_c^{+}$rapidity interval |y|<1 with transverse momentum (p_T) ranges of 3–30 and 6–40 GeV/cfor pp and PbPb collisions, respectively. Compared to the yields in pp collisions scaled by the expected number of nucleon-nucleon interactions, the observed yields of$$ {\Lambda}_{\textrm{c}}^{+} $$ ${\Lambda }_c^{+}$withp_T>10 GeV/care strongly suppressed in PbPb collisions. The level of suppression depends significantly on the collision centrality. The$$ {\Lambda}_{\textrm{c}}^{+} $$ ${\Lambda }_c^{+}$/D⁰production ratio is similar in PbPb and pp collisions atp_T>10 GeV/c, suggesting that the coalescence process does not play a dominant role in prompt$$ {\Lambda}_{\textrm{c}}^{+} $$ ${\Lambda }_c^{+}$baryon production at higherp_T. 

A<sc>bstract</sc> A measurement of the Higgs boson (H) production via vector boson fusion (VBF) and its decay into a bottom quark-antiquark pair ($$ \textrm{b}\overline{\textrm{b}} $$ $b\overline{b}$) is presented using proton-proton collision data recorded by the CMS experiment at$$ \sqrt{s} $$ $\sqrt{s}$= 13 TeV and corresponding to an integrated luminosity of 90.8 fb⁻¹. Treating the gluon-gluon fusion process as a background and constraining its rate to the value expected in the standard model (SM) within uncertainties, the signal strength of the VBF process, defined as the ratio of the observed signal rate to that predicted by the SM, is measured to be$$ {\mu}_{\textrm{Hb}\overline{\textrm{b}}}^{\textrm{qqh}}={1.01}_{-0.46}^{+0.55} $$ ${\mu }_{\mathrm{Hb}\overline{b}}^{\mathrm{qqh}}={1.01}_{-0.46}^{+0.55}$. The VBF signal is observed with a significance of 2.4 standard deviations relative to the background prediction, while the expected significance is 2.7 standard deviations. Considering inclusive Higgs boson production and decay into bottom quarks, the signal strength is measured to be$$ {\mu}_{\textrm{Hb}\overline{\textrm{b}}}^{\textrm{incl}.}={0.99}_{-0.41}^{+0.48} $$ ${\mu }_{\mathrm{Hb}\overline{b}}^{\text{incl}.}={0.99}_{-0.41}^{+0.48}$, corresponding to an observed (expected) significance of 2.6 (2.9) standard deviations. 

A<sc>bstract</sc> A search for direct production of low-mass dimuon resonances is performed using$$ \sqrt{s} $$ $\sqrt{s}$= 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 above 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 coefficientε²in the dark photon model above 10⁻⁶are 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 tanβ= 0.5. 

A<sc>bstract</sc> 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$$ \sqrt{s} $$ $\sqrt{s}$= 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, 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 thep_Tof 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.