Measurements are presented of the single-diffractive dijet cross section and the diffractive cross section as a function of the proton fractional momentum loss ξ and the four-momentum transfer squared t. Both processes p p → p X and p p → X p , i.e. with the proton scattering to either side of the interaction point, are measured, where X includes at least two jets; the results of the two processes are averaged. The analyses are based on data collected simultaneously with the CMS and TOTEM detectors at the LHC in proton-proton collisions at s = 8 Te during a dedicated run with β ∗ = 90 m at low instantaneous luminosity and correspond to an integrated luminosity of 37.5 nb - 1 . The single-diffractive dijet cross section σ jj p X , in the kinematic region ξ < 0.1 , 0.03 < | t | < 1 Ge 2 , with at least two jets with transverse momentum p T > 40 Ge , and pseudorapidity | η | < 4.4 , is 21.7 ± 0.9 (stat) - 3.3 + 3.0 (syst) ± 0.9 (lumi) nb . The ratio of the single-diffractive to inclusive dijet yields, normalised per unit of ξ , is presented as a function of x, the longitudinal momentum fraction of the proton carried by the struck parton. The ratio in the kinematic region defined above, for x values in the range - 2.9 ≤ log 10 x ≤ - 1.6 , is R = ( σ jj p X / Δ ξ ) / σ jj = 0.025 ± 0.001 (stat) ± 0.003 (syst) , where σ jj p X and σ jj are the single-diffractive and inclusive dijet cross sections, respectively. The results are compared with predictions from models of diffractive and nondiffractive interactions. Monte Carlo predictions based on the HERA diffractive parton distribution functions agree well with the data when corrected for the effect of soft rescattering between the spectator partons.
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- časopisecké články MeSH
The first study of charm quark diffusion with respect to the jet axis in heavy ion collisions is presented. The measurement is performed using jets with p_{T}^{jet}>60 GeV/c and D^{0} mesons with p_{T}^{D}>4 GeV/c in lead-lead (Pb-Pb) and proton-proton (pp) collisions at a nucleon-nucleon center-of-mass energy of sqrt[s_{NN}]=5.02 TeV, recorded by the CMS detector at the LHC. The radial distribution of D^{0} mesons with respect to the jet axis is sensitive to the production mechanisms of the meson, as well as to the energy loss and diffusion processes undergone by its parent parton inside the strongly interacting medium produced in Pb-Pb collisions. When compared to Monte Carlo event generators, the radial distribution in pp collisions is found to be well described by pythia, while the slope of the distribution predicted by sherpa is steeper than that of the data. In Pb-Pb collisions, compared to the pp results, the D^{0} meson distribution for 4
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A measurement is presented of electroweak (EW) production of a W boson in association with two jets in proton-proton collisions at s = 13 Te . The data sample was recorded by the CMS Collaboration at the LHC and corresponds to an integrated luminosity of 35.9 fb - 1 . The measurement is performed for the ℓ ν jj final state (with ℓ ν indicating a lepton-neutrino pair, and j representing the quarks produced in the hard interaction) in a kinematic region defined by invariant mass m jj > 120 Ge and transverse momenta p T j > 25 Ge . The cross section of the process is measured in the electron and muon channels yielding σ EW ( W jj ) = 6.23 ± 0.12 (stat) ± 0.61 (syst) pb per channel, in agreement with leading-order standard model predictions. The additional hadronic activity of events in a signal-enriched region is studied, and the measurements are compared with predictions. The final state is also used to perform a search for anomalous trilinear gauge couplings. Limits on anomalous trilinear gauge couplings associated with dimension-six operators are given in the framework of an effective field theory. The corresponding 95% confidence level intervals are - 2.3 < c W W W / Λ 2 < 2.5 Te - 2 , - 8.8 < c W / Λ 2 < 16 Te - 2 , and - 45 < c B / Λ 2 < 46 Te - 2 . These results are combined with the CMS EW Zjj analysis, yielding the constraint on the c W W W coupling: - 1.8 < c W W W / Λ 2 < 2.0 Te - 2 .
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New sets of CMS underlying-event parameters ("tunes") are presented for the pythia8 event generator. These tunes use the NNPDF3.1 parton distribution functions (PDFs) at leading (LO), next-to-leading (NLO), or next-to-next-to-leading (NNLO) orders in perturbative quantum chromodynamics, and the strong coupling evolution at LO or NLO. Measurements of charged-particle multiplicity and transverse momentum densities at various hadron collision energies are fit simultaneously to determine the parameters of the tunes. Comparisons of the predictions of the new tunes are provided for observables sensitive to the event shapes at LEP, global underlying event, soft multiparton interactions, and double-parton scattering contributions. In addition, comparisons are made for observables measured in various specific processes, such as multijet, Drell-Yan, and top quark-antiquark pair production including jet substructure observables. The simulation of the underlying event provided by the new tunes is interfaced to a higher-order matrix-element calculation. For the first time, predictions from pythia8 obtained with tunes based on NLO or NNLO PDFs are shown to reliably describe minimum-bias and underlying-event data with a similar level of agreement to predictions from tunes using LO PDF sets.
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Measurements are presented of the triple-differential cross section for inclusive isolated-photon+jet events in p p collisions at s = 8 TeV as a function of photon transverse momentum ( p T γ ), photon pseudorapidity ( η γ ), and jet pseudorapidity ( η jet ). The data correspond to an integrated luminosity of 19.7 fb - 1 that probe a broad range of the available phase space, for | η γ | < 1.44 and 1.57 < | η γ | < 2.50 , | η jet | < 2.5 , 40 < p T γ < 1000 GeV , and jet transverse momentum, p T jet , > 25 GeV . The measurements are compared to next-to-leading order perturbative quantum chromodynamics calculations, which reproduce the data within uncertainties.
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A search for new physics in top quark production is performed in proton-proton collisions at 13 TeV . The data set corresponds to an integrated luminosity of 35.9 fb - 1 collected in 2016 with the CMS detector. Events with two opposite-sign isolated leptons (electrons or muons), and b quark jets in the final state are selected. The search is sensitive to new physics in top quark pair production and in single top quark production in association with a W boson. No significant deviation from the standard model expectation is observed. Results are interpreted in the framework of effective field theory and constraints on the relevant effective couplings are set, one at a time, using a dedicated multivariate analysis. This analysis differs from previous searches for new physics in the top quark sector by explicitly separating t W from t t ¯ events and exploiting the specific sensitivity of the t W process to new physics.
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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.
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The transverse momentum spectra of D^{0} mesons from b hadron decays are measured at midrapidity (|y|<1) in pp and Pb-Pb collisions at a nucleon-nucleon center of mass energy of 5.02 TeV with the CMS detector at the LHC. The D^{0} mesons from b hadron decays are distinguished from prompt D^{0} mesons by their decay topologies. In Pb-Pb collisions, the B→D^{0} yield is found to be suppressed in the measured p_{T} range from 2 to 100 GeV/c as compared to pp collisions. The suppression is weaker than that of prompt D^{0} mesons and charged hadrons for p_{T} around 10 GeV/c. While theoretical calculations incorporating partonic energy loss in the quark-gluon plasma can successfully describe the measured B→D^{0} suppression at higher p_{T}, the data show an indication of larger suppression than the model predictions in the range of 2
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A search is presented for a heavy pseudoscalar boson A decaying to a Z boson and a Higgs boson with mass of 125 GeV . In the final state considered, the Higgs boson decays to a bottom quark and antiquark, and the Z boson decays either into a pair of electrons, muons, or neutrinos. The analysis is performed using a data sample corresponding to an integrated luminosity of 35.9 fb - 1 collected in 2016 by the CMS experiment at the LHC from proton-proton collisions at a center-of-mass energy of 13 Te . The data are found to be consistent with the background expectations. Exclusion limits are set in the context of two-Higgs-doublet models in the A boson mass range between 225 and 1000 GeV .
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A search for supersymmetry is presented based on events with at least one photon, jets, and large missing transverse momentum produced in proton-proton collisions at a center-of-mass energy of 13 Te . The data correspond to an integrated luminosity of 35.9 fb - 1 and were recorded at the LHC with the CMS detector in 2016. The analysis characterizes signal-like events by categorizing the data into various signal regions based on the number of jets, the number of b -tagged jets, and the missing transverse momentum. No significant excess of events is observed with respect to the expectations from standard model processes. Limits are placed on the gluino and top squark pair production cross sections using several simplified models of supersymmetric particle production with gauge-mediated supersymmetry breaking. Depending on the model and the mass of the next-to-lightest supersymmetric particle, the production of gluinos with masses as large as 2120 Ge and the production of top squarks with masses as large as 1230 Ge are excluded at 95% confidence level.
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