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Measurement of sin2θefflept using e+e pairs from γ*/Z bosons produced in pp¯ collisions at a center-of-momentum energy of 1.96 TeV

T. Aaltonen et al. (CDF Collaboration)
Phys. Rev. D 93, 112016 – Published 28 June 2016; Erratum Phys. Rev. D 95, 119901 (2017)
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Abstract

At the Fermilab Tevatron proton-antiproton (pp¯) collider, Drell-Yan lepton pairs are produced in the process pp¯e+e+X through an intermediate γ*/Z boson. The forward-backward asymmetry in the polar-angle distribution of the e as a function of the e+e-pair mass is used to obtain sin2θefflept, the effective leptonic determination of the electroweak-mixing parameter sin2θW. The measurement sample, recorded by the Collider Detector at Fermilab (CDF), corresponds to 9.4fb1 of integrated luminosity from pp¯ collisions at a center-of-momentum energy of 1.96 TeV, and is the full CDF Run II data set. The value of sin2θefflept is found to be 0.23248±0.00053. The combination with the previous CDF measurement based on μ+μ pairs yields sin2θefflept=0.23221±0.00046. This result, when interpreted within the specified context of the standard model assuming sin2θW=1MW2/MZ2 and that the W- and Z-boson masses are on-shell, yields sin2θW=0.22400±0.00045, or equivalently a W-boson mass of 80.328±0.024GeV/c2.

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  • Received 9 May 2016
  • Corrected 25 May 2017

DOI:https://doi.org/10.1103/PhysRevD.93.112016

© 2016 American Physical Society

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Particles & Fields

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25 May 2017

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Vol. 93, Iss. 11 — 1 June 2016

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  • Figure 1
    Figure 1

    Representation of the Collins-Soper coordinate axes (x, z) in the lepton-pair rest frame, along with the laboratory z axis (zlab). The three axes are in the plane formed by the proton (PA) and antiproton (PB) momentum vectors in the rest frame. The z axis is the angular bisector of PA and PB. The y axis is along the direction of PB×PA, and the x axis is in the direction away from the transverse component of PA+PB.

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  • Figure 2
    Figure 2

    Typical dependence of Afb as a function of the lepton-pair invariant mass M. The label u+d denotes the overall asymmetry, and the labels u and d denote the contribution to the overall asymmetry from quarks with charges 2/3 and 1/3, respectively. The contribution of quarks categorized by the u or d label is (σq+σq)/σ, where q=u or d, σ+() their forward (backward) cross section, and σ the total cross section from all quarks. The vertical line is at M=MZ.

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  • Figure 3
    Figure 3

    Invariant ee-mass distribution for opposite-charged CC events prior to the calibration and background subtractions. The crosses are the data, and the solid histogram is the simulation.

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  • Figure 4
    Figure 4

    Invariant ee-mass distribution for CP events prior to the calibration and background subtractions. The crosses are the data, and the solid histogram is the simulation.

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  • Figure 5
    Figure 5

    Corrections to the global energy scales as functions of the calibration period for the data. The central calorimeter corrections are the crosses (blue), and for the plug calorimeter, the histogram (red) gives the corrections.

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  • Figure 6
    Figure 6

    Corrections to the energy scales as functions of the calorimeter η-tower ring index for the CP-topology data. The corrections from time periods 0 to 17 are represented by the histograms (blue), and those from time periods 18 to 38 by the crosses (red). The central calorimeter region is index 0 to 9. Index 21 towers are about 23 cm from the beamline.

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  • Figure 7
    Figure 7

    Logarithmically binned mass distributions for oppositely charged ee-pair candidates in CC-topology events. The (black) crosses are the data, the (red) histogram overlapping the data is the sum of all components, the (green) histogram concentrated at lower masses is the Zτ+τ component, and the (cyan) histogram in the middle with the Z peak is the diboson component. The remaining broad distributions, from top to bottom, are QCD (magenta), W+jets (blue), and tt¯ (purple). The comparison of the data with the sum of the components yields a χ2 of 56 for 50 bins.

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  • Figure 8
    Figure 8

    Logarithmically binned mass distributions for CP-topology electron-pair candidates. The (black) crosses are the data, the (red) histogram overlapping the data is the sum of all components, the (green) histogram concentrated at lower masses is the Zτ+τ component, and the (cyan) histogram in the middle with the Z peak is the diboson component. The remaining broad distributions, from top to bottom are: QCD (magenta), W+jets (blue), and tt¯ (purple). The comparison of the data with the sum of the components yields a χ2 of 50 for 50 bins.

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  • Figure 9
    Figure 9

    Logarithmically binned mass distributions for PP-topology ee-pair candidates. The (black) crosses are the data, the (red) histogram overlapping the data is the sum of all components, the (green) histogram concentrated at lower masses is the Zτ+τ component, and the (cyan) histogram in the middle with the Z peak is the diboson component. The remaining broad distributions, from top to bottom are QCD (magenta), W+jets (blue), and tt¯ (purple). The comparison of the data with the sum of the components yields a χ2 of 69 for 50 bins.

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  • Figure 10
    Figure 10

    Distributions of cosϑ in the Collins-Soper frame for dielectrons with 66<M<116GeV/c2. The crosses are the background-subtracted data, and the histograms are simulated data. The upper pair of crosses and histogram is from the combination of the CC and CP topologies, and the lower pair is the contribution from the CC topology only.

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  • Figure 11
    Figure 11

    Distribution of φ in the Collins-Soper frame for CC- and CP-topology dielectrons with 66<M<116GeV/c2. The crosses are the background-subtracted data, and the solid histogram is the simulation.

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  • Figure 12
    Figure 12

    Invariant ee-mass distribution for opposite-charged CC events. The crosses are the background-subtracted data, and the solid histogram is the simulation. The comparison of the simulation with the data yields a χ2 of 214 for 200 bins.

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  • Figure 13
    Figure 13

    Invariant ee-mass distribution for CP events. The crosses are the background-subtracted data, and the solid histogram is the simulation. The comparison of the simulation with the data yields a χ2 of 235 for 200 bins.

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  • Figure 14
    Figure 14

    ET distribution for the CC-topology electron with the larger ET. The crosses are the background-subtracted data, and the solid histogram is the simulation.

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  • Figure 15
    Figure 15

    ET distribution for the CP-topology electron with the larger ET. The crosses are the background-subtracted data, and the solid histogram is the simulation.

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  • Figure 16
    Figure 16

    Invariant ee-mass distribution for same-charge CC events. The crosses are the background-subtracted data, and the solid histogram is the simulation.

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  • Figure 17
    Figure 17

    Rapidity distribution of electron pairs from the CC and CP topologies with 66<M<116GeV/c2. The crosses are the background-subtracted data, and the histogram is the simulation. The upper curve is the (arbitrarily normalized) shape of the underlying rapidity distribution from pythia. The measurement of Afb is restricted to be within the region |y|<1.7.

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  • Figure 18
    Figure 18

    Raw Afb measurement in bins of the electron-pair invariant mass. The vertical line is at M=MZ. Only statistical uncertainties are shown. The pythia prediction for |y|<1.7 does not include the effects of QED FSR.

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  • Figure 19
    Figure 19

    Event-weighting bias in bins of the electron-pair invariant mass. The biases are the crosses, and the uncertainties are the bin-by-bin unfolding estimates of the simulation. The superimposed histogram is the difference between the Afb calculations for the rapidity range |y|<1.7 and |y|<1.5, and the uncertainties are estimates for the PDF uncertainty.

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  • Figure 20
    Figure 20

    Fully corrected Afb for electron pairs with |y|<1.7. The measurement uncertainties are bin-by-bin unfolding estimates. The vertical line is M=MZ. The pythia calculation uses sin2θefflept=0.232. The EBA-based powheg-box calculation uses sin2θW=0.2243(sin2θefflept=0.2325) and the default PDF of NNPDF-3.0.

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  • Figure 21
    Figure 21

    Eigenvalues of the error matrix (solid histogram), and its regularization terms (dashed histogram). The horizontal line is the square of the statistical uncertainty of the Afb measurement for the mass bin containing the Z peak.

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  • Figure 22
    Figure 22

    Values of χ2 as functions of scan points in the sin2θW variable with the parabolic fit overlaid. The triangles are the comparisons of the electron-pair Afb measurement with the powheg-box NLO calculations. The Afb templates of each scan point are calculated with the default PDF of NNPDF-3.0. The solid curve is the fit of those points to the χ2 parabolic function.

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  • Figure 23
    Figure 23

    χ¯2 versus sin¯2θW parameters for the best-fit templates of the powheg-box NLO calculation for each of the NNPDF-3.0 ensemble PDFs. The Afb measurement with electron pairs covers 15 mass bins.

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  • Figure 24
    Figure 24

    AfbAfb (pythia) for |y|<1.7. The diamonds represent the measurement using electron pairs, and the uncertainties shown are the bin-by-bin unfolding estimates which are correlated. There are no suppressed measurement values. The solid bars represent the powheg-box calculation with the default NNPDF-3.0 PDFs. The dashed bars represent the resbos calculation with CTEQ6.6 PDFs. Both calculations use sin2θW=0.2243. The horizontal line represents the reference pythia calculation which uses CTEQ5L PDFs with sin2θefflept=0.232.

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  • Figure 25
    Figure 25

    The sin2θW versus sin2θefflept relationships from zfitter calculations. The default calculation is the middle line of the group. The outermost lines are for 1 standard-deviation shifts to the default value of the top-quark mass parameter (173.2±0.9) [49]; the lower line corresponds to a higher value of the top-quark mass. The lines for 1 standard-deviation variations of the Δαem(5)(MZ2) parameter are close to the default calculation and not easily distinguishable. The vertical line, an example reference value for sin2θefflept, is explained in the text.

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  • Figure 26
    Figure 26

    The χ¯2 versus sin¯2θW parameters of the μμ- and ee-channel combination. The prediction templates are calculated with powheg-box NLO and each of the NNPDF-3.0 ensemble PDFs. The μμ- and ee-channel Afb measurements contain 16 and 15 mass bins, respectively.

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  • Figure 27
    Figure 27

    Comparison of experimental measurements of sin2θefflept. The horizontal bars represent total uncertainties. The CDF μμ-channel, ee-channel, and combination results are denoted as CDF μμ9fb1 [6], CDF ee9fb1, and CDF ee+μμ9fb1, respectively. The other measurements are LEP-1 and SLD [11], CMS [9], ATLAS [8], LHCb [10], and D0 [7]. The LEP-1 and SLD Z pole result is the combination of their six measurements.

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  • Figure 28
    Figure 28

    Comparison of experimental determinations of the W-boson mass. The horizontal bars represent total uncertainties. The CDF μμ-channel, ee-channel, and combination results are denoted as CDF μμ9fb1 [6], CDF ee 9fb1, and CDF ee+μμ9fb1, respectively. The other indirect measurements are from LEP-1 and SLD [11, 12], which include the Tevatron top-quark mass measurement [49], and NuTeV [13]. The direct measurement is from the Tevatron and LEP-2 [52].

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