Abstract
A measurement is presented of the effective leptonic weak mixing angle (sin ^2theta ^{ell }_{text {eff}}) using the forward–backward asymmetry of Drell–Yan lepton pairs (mu mu and mathrm {e}mathrm {e}) produced in proton–proton collisions at sqrt{s}=8,text {TeV} at the CMS experiment of the LHC. The data correspond to integrated luminosities of 18.8 and 19.6{{,text {fb}^{-1}}} in the dimuon and dielectron channels, respectively, containing 8.2 million dimuon and 4.9 million dielectron events. With more events and new analysis techniques, including constraints obtained on the parton distribution functions from the measured forward–backward asymmetry, the statistical and systematic uncertainties are significantly reduced relative to previous CMS measurements. The extracted value of sin ^2theta ^{ell }_{text {eff}} from the combined dilepton data is sin ^2theta ^{ell }_{text {eff}} =0.23101pm 0.00036,text {(stat)} pm 0.00018,text {(syst)} pm 0.00016,text {(theo)} pm 0.00031,text {(parton distributions in proton)}=0.23101 pm 0.00053.
Highlights
We report a measurement of the effective leptonic weak mixing angle using the forward–backward asymmetry ( AFB) in Drell–Yan qq → + − events, where stands for muon (μ) or electron (e)
To reduce the statistical uncertainties associated with the limited number of events in the signal Monte Carlo (MC) samples, which include simulation of detector response and lepton reconstruction, the generated cos θ ∗ distributions in each (m, y ) bin within the acceptance of the detector is reweighted to much larger MC samples, generated without simulating detector response or lepton reconstruction
We constructed a combined set from same number of replicas of NNPDF3.0, CT14, and MMHT2014 parton distribution functions (PDFs), and after including the data from the W charge asymmetry in the PDF reweighting, we find the combined weighted average in the dimuon channel differs from the NNPDF3.0 result by only 0.00009, and the standard deviation only increases from 0.00032 to 0.00036
Summary
We report a measurement of the effective leptonic weak mixing angle (sin θeff) using the forward–backward asymmetry ( AFB) in Drell–Yan qq → + − events, where stands for muon (μ) or electron (e). ΣF + σB where σF and σB are, respectively, the cross sections in the forward (cos θ ∗ > 0) and backward (cos θ ∗ < 0) hemispheres In this frame, θ ∗ is the angle of the − relative to the axis that bisects the angle between the direction of the quark and the reversed direction of the antiquark. The different couplings of u- and d-type quarks to EW bosons generate different AFB values in the corresponding production channels, which means that the average depends on the relative contributions of u- and d-type quarks to the total cross section. The definition of AFB in pp collisions is based on the sign of y , which relies on the fact that on average the dilepton pairs are Lorentz-boosted in the quark direction. A non-zero average AFB originates only from valencequark production channels and is diluted by events where the antiquark carries a larger momentum than the quark. The uncertainty originating from modeling of pT, is very small and included in the theoretical estimates
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