Measurement of sin2θW (MW) using e+e pairs in the Z-boson region from pp collisions at √s = 1.96 TeV
05 Sept 2013

Updated: 17 Oct 2013


W. Sakumoto, A. Bodek, and J.-Y. Han
University of Rochester



Introduction

Drell-Yan lepton pairs are produced in the process pp → γ*/Z + X, with the subsequent decay of the γ*/Z into lepton pairs. The angular distribution of decay electrons is used to measure the electroweak-mixing sin2θW. The measurement uses 2.1 fb-1 of CDF Run II data in the e+e channel, where the pairs are in the mass range of 66—116 GeV/c2. In the standard model, the Drell-Yan process at tree level consists of two parton level diagrams: qq → γ* or Z → e+e. Fermions (f) couple to the virtual photon via vector coupling, Qfγμ, where Qf is the fermion charge (in units of e). The fermion coupling to a Z boson has both vector (V) and axial (A) couplings: gVfγμ + gAfγμγ5. The Born couplings are:

where T3f is the third component of the fermion weak isospin. These couplings affect the electron angular distributions.

The angular distribution of the electrons is analyzed in the Collins-Soper (CS) rest frame of the ee-pair. The distribution depends on the polarization state of the γ*/Z boson. QCD radiation from the colliding quarks changes the polarization state. The polar angle of the e is denoted as θ, and the azimuthal angle as φ. In the CS frame, the placement of the z axis is approximately along the direction of the incoming quark. When the ee-pair transverse momentum, PT is zero, the CS and lab coordinates are the same. Thus, the polar angle θ is similar to the one used in e+e collisions. The angular distribution integrated over φ is

where A0 and A4 are called angular distribution coefficients, and they are ratios of γ*/Z cross sections. A0 appears at NLO with QCD radiation, so it is zero at PT=0. The A4 coefficient is parity violating, non-zero at PT=0, and induces an asymmetry in the cosθ distribution. It is a consequence of vector and axial current interference, and there are two sources: γ*-Z interference and Z self interference. The Z self interference includes gV, so at tree level, the A4 contribution from it is proportional to where the Qe term is from the electron-Z (e-Z) vertex, Qe is the electron charge, the Qq term is from the quark-Z (q-Z) vertex, and Qq is the quark charge. As sin2θW ∼ 0.223, the lepton factor is about 0.1, and it is 0.4 for u-quarks and 0.7 for d-quarks. Thus A4 is most sensitive to the electron-Z couplings. The γ*-Z interference depends on gA (T3) so it is like a dilution to the sin2θW sensitivity.

The method used to extract sin2θW has two components: the measurement of A4 and calculations of A4 for various input values of sin2θW. This analysis measures A4 for electron pairs over the mass range of 66‐116 GeV. There are no restrictions on the pair rapidity or transverse momentum. The measurement is denoted as A4. The calculation of A4 uses LEP-like implementation of electroweak radiative corrections.

Analysis

The A4 measurement is derived from our 2.1 fb-1 measurement of the electron angular distribution coefficients as a function of the ee-pair transverse momentum [Phys. Rev. Lett. 106, 241801 (2011)]. Analysis details are available on the CDF EWK web page (under W/Z Cross Sections and Asymmetries). The 2.1 fb-1 dataset is understood well and the simulation of the data is tuned well. The simulation modeling is data driven. The modeling starts with the Pythia 6.2 simulation of pp → γ*/Z (→ ee) + X (at tree level) followed by parton showering. PHOTOS 2.0 adds final state QED radiation to charged particle vertices. This is then followed by the full CDF detector simulation. All data efficiencies, both global and ηdet dependent, as well as time-dependent, are measured and incorporated into the simulation. Both the physics and detector models are tuned (iteratively) so that the simulated, reconstructed event distributions match the data. The measured PT dependent angular distibution coefficients are incorporated in the physics model for the A4 analysis.

The Data

The dataset is identical to the angular distribution coefficient measurement, but is summarized below. The electron candidates used in this analysis can be in either the CDF central (C) or the forward end plug (P) calorimeters. The central and plug calorimeters cover the range, |ηdet|<1.1 and 1.1<|ηdet|<3.5 respectively. Each electron is required to have an associated track and pass standard CDF electron selection and fiducial cuts. Three electron-pair topology categories are used with the following kinematic selections.

  1. Central-central (CC)
  2. Central-plug (CP)
  3. Plug-plug (PP)
For the plug-plug topology, both electrons are required to be in the same end-plug calorimeter. The number of events with 66<Mee<116 GeV/c2 passing all cuts for the CC, CP, and PP topologies are 51951, 63752, and 22469, respectively. Because of the track requirement on each electron, the backgrounds are small. The QCD background, derived from the data, is 0.3%. EWK backgrounds from WW, ZZ, WZ, ttbar, W+jets, and Z → ττ are also small: 0.2%. The EWK background rate is estimated with Monte Carlo (MC) samples. These backgrounds are subtracted from the data. The background level over cosθ does not vary drastically.

The determination of cosθ requires the identification of the e and this done with the track associated with the electrons. Charge misidentification is very small for central electrons. However, it is large for plug electrons at large |ηeta|. So for the PP topology, |cosθ|, which is indepenent of charge, is used.
This plot shows the tuned data-vs-simulation cosθ distribution for the combined CC+CP topologies. The black crosses are the data and the blue are the simulated data.
This plot shows the tuned data-vs-simulation cosθ distribution for the PP topology. The black crosses are the data and the blue are the simulated data.

The QED FSR added by PHOTOS and PYTHIA are checked with the ZGrad2 Drell-Yan simulation which has an exact order αem calculation. PHOTOS and PYTHIA provides a good model for QED FSR in regions that are important for electron reconstruction. The cosθ resolution estimated by the simulation has negligible bias and a narrow rms of 0.01 but with non-Gaussian tails. Charge misidentification in the CC and CP topologies has a negligible bias.

PHOTOS simulates QED FSR to αem2 in leading-logarithmic accuracy using a fragmentation function approach. To compare QED FSR added by PHOTOS and PYTHIA against the ZGrad2 calculation, the ee-γ system is first boosted to its CM frame to mimimize distortions due to QCD radiation. ZGrad2 only produces 0 or 1 real photons. As PHOTOS and PYTHIA adds multiple photons, these photons are clustered into a single object by summing up their momentum vectors prior to their comparison with ZGrad2. For PYTHIA+PHOTOS to ZGrad2 comparisons, the minimum photon energy is 0.5 GeV (ZGrad2 threshold).
Photon energy comparison between ZGrad2 and PHOTOS based on the absolute rates. The PHOTOS plus PYTHIA total cross section is normalized to the ZGrad2 total cross section.
The angle between the photon (cluster) and the nearest e+ or e is defined as β.
cosβ separation between the photon (cluster) and the nearest e+ or e. The ZGrad2 and PHOTOS total cross section is normalized over 0<cosβ<0.8, so this is a shape comparison for 0<cosβ<1.

The A4 Measurement

The tuned simulation physics model which includes the previously measured angular distribution coefficients, is used to calculate A4. This is a fully corrected cross section weighted average. The blue simulated data histograms in the cosθ plots above is the result of the tuned simulation physics model. As a cross check, A4 is also determined with a second method. The cosθ data distributions shown above are fit with same technique used in the PT binned measurement of A4. The result is consistent with the physics model calculation. The second method is used for uncertainty estimates: statistical fit uncertainty, and systematic uncertainties from the background and electron efficiency measurments. The result is

The systematic from the electron efficiency measurements is negligible.

QCD Calculations

Electroweak radiative corrections

QCD, QED, and weak radiative corrections can be organized to be separately gauge invariant, and corrections for each can be applied separately and independently. QED radiative corrections (with real photons) are not included in the calculation of A4. They are included in the simulation physics model, so QED radiative effects are removed from the measurement of A4.

Weak radiative corrections are based on ZFITTER 6.43's e+e → Z → qq amplitude form factors. The ZFITTER form factors are finite and gauge invariant (and so potentially measureable). Thus photon corrections that involve massive gauge bosons are included in the Z amplitude form factors for gauge invariance. This includes photon propagator W-loop corrections, and photon vertex corrections containing Z propagators. Thus, internal QED radiative corrections to the photon propagator are only from fermion loops, and this correction is another form factor. All ZFITTER form factors, including the photon propagator form factor, are complex valued, and at least functions of the s Mandelstam variable.

ZFITTER uses the on-shell renormalization scheme for its form factors. All particle masses are on shell, and sin2θW = 1 − MW2/MZ2 to all orders of perturbation theory. As the Z mass is well known, sin2θW and MW are equivalent, ie, the determination of one implies the other. ZFITTER is used to calculate tables of form factors in sin2θW and the Mandalstam variable s (ie, Mee). The total decay width of the Z is also calculated for the given sin2θW. The Z-boson propagator is the LEP standard, [s − (MZ2 − i sΓZ/MZ)].

The ZFITTER interface that calculates e+e cross sections and asymmetries implement complex-valued form factors, including the one for the photon correction. The real part of the photon correction is often absorbed into αem and this is called the running αem. The imaginary part of the photon correction is not negligible. ZGrad2 also implements weak form factors but with a different implementation that ZFITTER. The effective sin2θW from both calculations are very similar. However, ZGrad2 only uses the real valued running αem for the photon amplitude. This is an issue as it affects γ*-Z interference, which is significant away from the Z pole.
LO calculation of Afb(M), where M=√s (ee-pair mass). The vertical line is at M=MZ, and this is where γ*-Z interference is zero. The intercept with the Afb curve is the level of the sin2θW "signal" from Z self interference.
The A4 and Afb are proportional to each other: Afb(M) = 38A4(M).

Drell-Yan QCD calculations

The ZFITTER complex-valued form factors are incorporated into QCD calculations for an enhanced Born approximation (EBA) to the electroweak couplings. For the form factors, √s is assigned the mass of the ee-pair. This done for both LO and NLO QCD calculations of the Drell-Yan process. Operationally, the QCD related portion of matrix elements are unchanged, and only the electroweak coupling portions need to be appropriately modified. Two NLO calculations are used in the calculations of A4 for input values of sin2θW.

Both ResBos and Powheg-Box have PT2 distributions that are finite at PT=0. A simple QCD LO, or tree calculation of the Drell-Yan process is used as a basesline reference for the higher order QCD calculations. The CT10 NLO PDFs are used in all QCD calculations. In addition, the ZFITTER form factors are implemented as simple look-up tables which are pre-calculated. The EBA form factors modifies the Born gA and gV couplings: where ρ and κ are s-dependent, complex valued, and fermion charge and weak iso-spin dependent form factors. The combination, κ sin2θW is called an effective sin2θW. The mesured A4 is directly related to this combination, and in particular, most sensitive to the one at the lepton vertex. The κ form factor is different for the e-Z and q-Z vertex. Its reference value, for comparisons with LEP measurements, is the real part at √s = MZ. That is, its value at the Z pole.

ZFITTER form factor EBA calculations of A4 for various values of input sin2θW.
The different calculations are similar to the baseline tree (LO) calculation. Differences can be seen in a ratio plot.
R4 = A4(NLO) / A4(tree), where NLO are the ResBos and Powheg-Box calculations, and tree is the simple LO calculation. The Powheg-Box LO matrix element calculation with parton showering is also shown.
Higher order QCD radiation appears to induce a small reduction in A4 relative to the baseline tree level calculation. The A4 angular coefficient is similar to the other coefficients in that the coefficients vary slowly at low boson PT and large changes occur at larger PT.

The ResBos predictions of A4 for various input values of sin2θW are chosen as the default prediction for use with the measurement. It is about 0.7% below the tree predictions. The Powheg-Box calculations are similar to ResBos, but there are differences. For these differences, a systematic uncertainty of ±1% is assigned to each predicted value of A4 from ResBos.

The Powheg-Box has a large and useful variety of calculation options, so it is used for the estimation of the following systematic uncertainties: QCD mass factorization and renormalization scale uncertainties, and uncertainties inherent in the CT10 PDFs. All the QCD calculations use the running ee-pair mass as the factorization and renormalization scales. They are varied from the default running ee-pair mass by a factor of 0.5 to 2 for the uncertainty; this gives a QCD scale uncertainty of ΔA4 = ±0.0004. Update, 17 Oct 2013: For consistency, the PDF uncertainties are now calculated with the same method used for the CDF W-mass measurement [Phys. Rev. D 77, 112001 (2008)]. The 26 pairs of CT10 90% CL uncertainty PDFs are used to estimate the uncertainty from PDFs to A4, and this value is scaled down to the 68% CL by a factor of 1.645 to give the PDF uncertainty of ΔA4 = ±0.0026.

Overall, the QCD prediction uncertainties are small compared to the A4 measurement uncertainty.

A4 Uncertainty Summary

The uncertainites from the measurement (Data) and predictions (Pred) are summarized below.

Source Value
Data: Measurement±0.0079 (stat)
Data: Background normalization±0.0003 (syst)
Data: Electron efficienciesNegligible (syst)
Pred: QCD scales±0.0004 (syst)
Pred: QCD PDFs±0.0026 (syst)
Pred: Differences from ResBos±0.01 A4 (syst)

Results

(17 Oct 2013)

The inferred value of sin2θW (or MW) is obtained by taking the measurement, A4 = 0.1100 ± 0.0079 (stat) ± 0.0004 (syst), and extracting the sin2θW point corresponding to the measurement in the ResBos prediction. However, the A4 measurement is directly and most sensitive to the effective sin2θW at the lepton vertex, which is denoted as sin2θefflept. This is the reference value of the ZFITTER based effective leptonic sin2θW, or κe(s) sin2θW at √s = MZ. Thus, the more direct measurement is

where the uncertainty includes both statistical and systematic uncertainties combined in quadrature. The statistical uncertainty from the A4 measurement is ±0.0009, and the systematic uncertainty is from the prediction. The corresponding measurements from LEP/SLD [Phys. Rep. 427, 257 (2006)] and D0 [Phys. Rev. D84, 012007 (2011)] are: The Z-pole is a standard model analysis of combined LEP/SLD Z-pole results and the light quark one only includes hadronic final states with light u, d, and s quarks. The Tevatron Drell-Yan results (D0 and this measurement) are produced mainly from light quarks. The Drell-Yan sensitivity to sin2θW is mostly from the e-Z vertex, although the q-Z vertex does contribute, but to a lesser extent. The quark mix from ee collision production of light quarks is not the same. However, all results are consistent.

The inferred value of sin2θW is usually expressed as an indirect W-boson mass value.
The A4 measurement is the bold vertical line, and the lighter vertical bands are its one standard deviation uncertainties. The solid (blue) diagonal line is the standard model prediction of A4 in relation to the input MW calculated by ResBos, and the bands about it are its one standard deviation uncertainties.
The A4 measurement gives

where the uncertainties include both statistical and systematic uncertainties combined in quadrature. The systematic uncertainty is for the prediction. The measurement statistical uncertainty is ±0.045 GeV/c2.
Comparison with direct W-mass measurements from the Tevatron and LEP2, and the indirect W-mass from LEP1/SLD SM fits at the Z pole.
The direct measurement is a combination of Tevatron and LEP2 W-mass measurements [Phys. Rev. D88, 052018 (2013)], and the indirect one is a prediction from SM fits to Z-pole measurements with the top quark mass. Graphical summaries of the result and comparison with other measurements (described above) are shown below.