Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment

TL;DR

This ATLAS study measures the polarization of W bosons produced with high transverse momentum in 7 TeV pp collisions by analyzing the transverse-plane decay angle cosθ2D in W→ℓν events (ℓ = e, μ). Helicity fractions f0 and fL − fR are extracted from templates corresponding to longitudinal, left-, and right-handed states, using data-driven background control and two W pT bins (35–50 GeV and >50 GeV). The analysis employs mc@nlo and powheg templates, with corrections to relate cosθ2D to the underlying cosθ3D distributions, and accounts for detector effects and systematic uncertainties, including jet and electroweak backgrounds, energy scales, PDFs, and generator choices. In the high-pT^W regime, the data yield f0 = 0.127 ± 0.030 (stat) ± 0.108 (syst) and fL − fR = 0.252 ± 0.017 (stat) ± 0.030 (syst), consistent with Standard Model expectations within uncertainties and in agreement with MC predictions. The results validate the modeling of W polarization at high pT and demonstrate ATLAS's capability to probe electroweak production dynamics through lepton angular distributions.

Abstract

This paper describes an analysis of the angular distribution of W->enu and W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with the ATLAS detector at the LHC in 2010, corresponding to an integrated luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and the missing transverse energy, the W decay angular distribution projected onto the transverse plane is obtained and analysed in terms of helicity fractions f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw > 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour, are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017 +/- 0.030, where the first uncertainties are statistical, and the second include all systematic effects.

Figures (8)

• Figure 1: Cosine of the helicity angle of the lepton from $W$ decay at generator-level for positive charge (left) and negative charge (right). Solid lines are without selection, dashed lines are after all acceptance plus $m_{\mathrm{T}}^W$ cuts except the $\eta_{\mathrm{\ell}}$ cuts and dotted lines are after all acceptance plus $m_{\mathrm{T}}^W$ cuts. "All events" distributions are normalised to unity.
• Figure 2: Representation of $\cos \theta_{\rm 2D}$ as a function of $\cos \theta_{\rm 3D}$ in events where the $W$ transverse momentum is greater than 50 Ge V, for (a) positive and (b) negative leptons. Events are simulated with mc@nlo after applying the acceptance and $m_{\mathrm{T}}^W$ cuts, as defined in Section \ref{['sect:selection']}.
• Figure 3: The $\cos \theta_{\rm 2D}$ distributions for 35 $<$$p_{\mathrm{T}}^W$$<$ 50 Ge V. The data (dots) are compared to the distributions from powheg (dashed line), mc@nlo (solid line), and for unpolarised $W$ bosons (dotted line) in the muon (top) and electron (bottom) channel, split by charge. The bottom parts of each plot represent the ratio of data, powheg and unpolarised distributions to mc@nlo.
• Figure 4: The $\cos \theta_{\rm 2D}$ distributions for $p_{\mathrm{T}}^W$$>$ 50 Ge V. The data (dots) are compared to the distributions from powheg (dashed line), mc@nlo (solid line), and for unpolarised $W$ bosons (dotted line) in the muon (top) and electron (bottom) channel, split by charge. The bottom parts of each plot represent the ratio of data, powheg and unpolarised distributions to mc@nlo.
• Figure 5: Computed values of $f_{\mathrm 0}$ (top) and $f_{\mathrm L} - f_{\mathrm R}$ (bottom) using fits with Equation \ref{['eqn:fracdef']} to mc@nlo samples in ($|y_W|$, $p_{\mathrm{T}}^W$) bins, split by charge. These values are used to calculate the weights needed to create helicity templates.