Measurement of WZ production in proton-proton collisions at sqrt(s) = 7 TeV with the ATLAS detector

TL;DR

This study measures WZ production in pp collisions at sqrt(s)=7 TeV with the ATLAS detector using ~4.6 fb^-1. It reports both fiducial and total cross-sections that agree with SM predictions and uses the Z boson pT spectrum to constrain anomalous triple gauge couplings, finding no evidence of new physics. The analysis employs data-driven background estimations, a four-channel leptonic final-state combination, and Bayesian unfolding to deliver normalized differential cross-sections in pT^Z and m_WZ. The results yield competitive limits on TGCs and provide high-precision, unfolded cross-sections that test SM diboson production and gauge structure.

Abstract

A study of WZ production in proton-proton collisions at sqrt(s) = 7 TeV is presented using data corresponding to an integrated luminosity of 4.6 fb^-1 collected with the ATLAS detector at the Large Hadron Collider in 2011. In total, 317 candidates, with a background expectation of 68+/-10 events, are observed in double-leptonic decay final states with electrons, muons and missing transverse momentum. The total cross-section is determined to be sigma_WZ(tot) = 19.0+1.4/-1.3(stat.)+/-0.9(syst.)+/-0.4(lumi.) pb, consistent with the Standard Model expectation of 17.6+1.1/-1.0 pb. Limits on anomalous triple gauge boson couplings are derived using the transverse momentum spectrum of Z bosons in the selected events. The cross section is also presented as a function of Z boson transverse momentum and diboson invariant mass.

Figures (7)

• Figure 1: The SM tree-level Feynman diagrams for $W^{\pm}Z$ production through the $s$-, $t$-, and $u$-channel exchanges in $q\bar{q}'$ interactions at hadron colliders.
• Figure 2: (a) Dilepton invariant mass $m_{\ell\ell}$ of the $Z$ candidate in the events that pass all event selection criteria except for the $m_{\ell\ell}$ cut. (b) Transverse momentum $p_{\mathrm{T}}$ of the lepton attributed to the $W$ candidate. (c) Missing transverse momentum $E_{\mathrm{T}}^{\mathrm{miss}}$ of the trilepton events. (d) Transverse mass $M_{\mathrm{T}} \xspace ^W$ of the $W$ candidates. Samples shown in (b), (c), and (d) are the candidate events remaining before the cut on the variable displayed. The stacked histograms are expectations from simulation for $WZ$, $ZZ$, and $Z$+$\gamma$. For $Z$+jets and $t\bar{t}$, the expected shape is taken from simulation but the normalization is taken from the data-driven estimates. The rightmost bins include overflows.
• Figure 3: Distributions of the $W^{\pm}Z$ candidates after all selection. (a) Transverse momentum $p_{\mathrm{T}}^Z$ of the $Z$ boson. (b) Invariant mass $m_{W\!Z}$ of the $W^{\pm}Z$ pair. The shaded bands indicate the total statistical and systematic uncertainties of the prediction. For $Z$+jets and $t\bar{t}$, the expected shape is taken from simulation but the normalization is taken from the data-driven estimates. The rightmost bins include overflow.
• Figure 4: Transverse momentum $p_{\mathrm{T}}^Z$ of the $Z$ boson in $W^{\pm}Z$ candidate events. Data are shown together with expected background and signal events, assuming the Standard Model. Expected events in the case of anomalous TGC without form factor are also shown for illustration. The last bin is shortened for display purposes.
• Figure 5: 95% confidence intervals for anomalous TGCs from ATLAS (this work), CDF CDFlimits, and D0 D0limits. Integrated luminosity, centre-of-mass energy and cut-off $\Lambda$ for each experiment are shown.