A measurement for inclusive 2- and 3-jet events of the azimuthal correlation between the two jets with the largest transverse momenta, Δ ϕ 12 , is presented. The measurement considers events where the two leading jets are nearly collinear ("back-to-back") in the transverse plane and is performed for several ranges of the leading jet transverse momentum. Proton-proton collision data collected with the CMS experiment at a center-of-mass energy of 13 Te and corresponding to an integrated luminosity of 35.9 fb - 1 are used. Predictions based on calculations using matrix elements at leading-order and next-to-leading-order accuracy in perturbative quantum chromodynamics supplemented with leading-log parton showers and hadronization are generally in agreement with the measurements. Discrepancies between the measurement and theoretical predictions are as large as 15%, mainly in the region 177 ∘ < Δ ϕ 12 < 180 ∘ . The 2- and 3-jet measurements are not simultaneously described by any of models.
- Klíčová slova
- Azimuthal angle, CMS, Jets, MC generators, Physics, QCD,
- Publikační typ
- časopisecké články MeSH
Exclusive ρ 770 0 photoproduction is measured for the first time in ultraperipheral pPb collisions at s NN = 5.02 Te with the CMS detector. The cross section σ ( γ p → ρ 770 0 p ) is 11.0 ± 1.4 (stat) ± 1.0 (syst) μ b at ⟨ W γ p ⟩ = 92.6 Ge for photon-proton centre-of-mass energies W γ p between 29 and 213 Ge . The differential cross section d σ / d | t | is measured in the interval 0.025 < | t | < 1 Ge 2 as a function of W γ p , where t is the squared four-momentum transfer at the proton vertex. The results are compared with previous measurements and theoretical predictions. The measured cross section σ ( γ p → ρ 770 0 p ) has a power-law dependence on the photon-proton centre-of-mass, consistent with electron-proton collision measurements performed at HERA. The W γ p dependence of the exponential slope of the differential cross section d σ / d | t | is also measured.
- Publikační typ
- časopisecké články MeSH
The exclusive photoproduction of Υ ( nS ) meson states from protons, γ p → Υ ( nS ) p (with n = 1 , 2 , 3 ), is studied in ultraperipheral p Pb collisions at a centre-of-mass energy per nucleon pair of s NN = 5.02 TeV . The measurement is performed using the Υ ( nS ) → μ + μ - decay mode, with data collected by the CMS experiment corresponding to an integrated luminosity of 32.6 nb - 1 . Differential cross sections as functions of the Υ ( nS ) transverse momentum squared p T 2 , and rapidity y, are presented. The Υ ( 1 S ) photoproduction cross section is extracted in the rapidity range | y | < 2.2 , which corresponds to photon-proton centre-of-mass energies in the range 91 < W γ p < 826 GeV . The data are compared to theoretical predictions based on perturbative quantum chromodynamics and to previous measurements.
- Klíčová slova
- CMS, UPC, Y, pPb, photoproduction,
- Publikační typ
- časopisecké články MeSH
Measurements of normalized differential cross sections as functions of the multiplicity and kinematic variables of charged-particle tracks from the underlying event in top quark and antiquark pair production are presented. The measurements are performed in proton-proton collisions at a center-of-mass energy of 13 Te , and are based on data collected by the CMS experiment at the LHC in 2016 corresponding to an integrated luminosity of 35.9 fb - 1 . Events containing one electron, one muon, and two jets from the hadronization and fragmentation of b quarks are used. These measurements characterize, for the first time, properties of the underlying event in top quark pair production and show no deviation from the universality hypothesis at energy scales typically above twice the top quark mass.
- Publikační typ
- časopisecké články MeSH
A search for the Higgs boson decaying to two oppositely charged muons is presented using data recorded by the CMS experiment at the CERN LHC in 2016 at a center-of-mass energy sqrt[s]=13 TeV, corresponding to an integrated luminosity of 35.9 fb^{-1}. Data are found to be compatible with the predicted background. For a Higgs boson with a mass of 125.09 GeV, the 95% confidence level observed (background-only expected) upper limit on the production cross section times the branching fraction to a pair of muons is found to be 3.0 (2.5) times the standard model expectation. In combination with data recorded at center-of-mass energies sqrt[s]=7 and 8 TeV, the background-only expected upper limit improves to 2.2 times the standard model value with a standard model expected significance of 1.0 standard deviation. The corresponding observed upper limit is 2.9 with an observed significance of 0.9 standard deviation. This corresponds to an observed upper limit on the standard model Higgs boson branching fraction to muons of 6.4×10^{-4} and to an observed signal strength of 1.0±1.0(stat)±0.1(syst).
- Publikační typ
- časopisecké články MeSH
Measurements of inclusive isolated-photon and photon+jet production in proton-proton collisions at s = 13 TeV are presented. The analysis uses data collected by the CMS experiment in 2015, corresponding to an integrated luminosity of 2.26 fb - 1 . The cross section for inclusive isolated photon production is measured as a function of the photon transverse energy in a fiducial region. The cross section for photon+jet production is measured as a function of the photon transverse energy in the same fiducial region with identical photon requirements and with the highest transverse momentum jet. All measurements are in agreement with predictions from next-to-leading-order perturbative QCD.
- Publikační typ
- časopisecké články MeSH
Measurements of fragmentation functions for jets associated with an isolated photon are presented for the first time in pp and Pb-Pb collisions. The analysis uses data collected with the CMS detector at the CERN LHC at a nucleon-nucleon center-of-mass energy of 5.02 TeV. Fragmentation functions are obtained for jets with p_{T}^{jet}>30 GeV/c in events containing an isolated photon with p_{T}^{γ}>60 GeV/c, using charged tracks with transverse momentum p_{T}^{trk}>1 GeV/c in a cone around the jet axis. The association with an isolated photon constrains the initial p_{T} and azimuthal angle of the parton whose shower produced the jet. For central Pb-Pb collisions, modifications of the jet fragmentation functions are observed when compared to those measured in pp collisions, while no significant differences are found in the 50% most peripheral collisions. Jets in central Pb-Pb events show an excess (depletion) of low (high) p_{T} particles, with a transition around 3 GeV/c. This measurement shows for the first time the in-medium shower modifications of partons (quark dominated) with well-defined initial kinematics. It constitutes a new well-controlled reference for testing theoretical models of the parton passage through the quark-gluon plasma.
- Publikační typ
- časopisecké články MeSH
The production of a Z boson, decaying to two charged leptons, in association with jets in proton-proton collisions at a centre-of-mass energy of 13 TeV is measured. Data recorded with the CMS detector at the LHC are used that correspond to an integrated luminosity of 2.19 fb -1 . The cross section is measured as a function of the jet multiplicity and its dependence on the transverse momentum of the Z boson, the jet kinematic variables (transverse momentum and rapidity), the scalar sum of the jet momenta, which quantifies the hadronic activity, and the balance in transverse momentum between the reconstructed jet recoil and the Z boson. The measurements are compared with predictions from four different calculations. The first two merge matrix elements with different parton multiplicities in the final state and parton showering, one of which includes one-loop corrections. The third is a fixed-order calculation with next-to-next-to-leading order accuracy for the process with a Z boson and one parton in the final state. The fourth combines the fully differential next-to-next-to-leading order calculation of the process with no parton in the final state with next-to-next-to-leading logarithm resummation and parton showering.
- Klíčová slova
- CMS, Cross section, Jets, LHC, Physics, Proton, Standard model, Z boson,
- Publikační typ
- časopisecké články MeSH
The mass of the top quark is measured using a sample of t t ¯ events collected by the CMS detector using proton-proton collisions at s = 13 TeV at the CERN LHC. Events are selected with one isolated muon or electron and at least four jets from data corresponding to an integrated luminosity of 35.9 fb - 1 . For each event the mass is reconstructed from a kinematic fit of the decay products to a t t ¯ hypothesis. Using the ideogram method, the top quark mass is determined simultaneously with an overall jet energy scale factor (JSF), constrained by the mass of the W boson in q q ¯ ' decays. The measurement is calibrated on samples simulated at next-to-leading order matched to a leading-order parton shower. The top quark mass is found to be 172.25 ± 0.08 (stat+JSF) ± 0.62 (syst) GeV . The dependence of this result on the kinematic properties of the event is studied and compared to predictions of different models of t t ¯ production, and no indications of a bias in the measurements are observed.
- Publikační typ
- časopisecké články MeSH
This Letter presents the observation of the rare Z boson decay Z→ψℓ^{+}ℓ^{-}. Here, ψ represents contributions from direct J/ψ and ψ(2S)→J/ψX, ℓ^{+}ℓ^{-} is a pair of electrons or muons, and the J/ψ meson is detected via its decay to μ^{+}μ^{-}. The sample of proton-proton collision data, collected by the CMS experiment at the LHC at a center-of-mass energy of 13 TeV, corresponds to an integrated luminosity of 35.9 fb^{-1}. The signal is observed with a significance in excess of 5 standard deviations. After subtraction of the ψ(2S)→J/ψX contribution, the ratio of the branching fraction of the exclusive decay Z→J/ψℓ^{+}ℓ^{-} to the decay Z→μ^{+}μ^{-}μ^{+}μ^{-} within a fiducial phase space is measured to be B(Z→J/ψℓ^{+}ℓ^{-})/B(Z→μ^{+}μ^{-}μ^{+}μ^{-})=0.67±0.18(stat)±0.05(syst).
- Publikační typ
- časopisecké články MeSH
