Measurement of the electroweak production of dijets in association with a Z-boson and distributions sensitive to vector boson fusion in proton-proton collisions at sqrt(s) = 8 TeV using the ATLAS detector

TL;DR

This work reports ATLAS measurements of fiducial cross sections for electroweak Z+jj production at sqrt(s)=8 TeV using 20.3 fb^{-1}, extracting the EW component via a fit to the dijet invariant mass in a VBF-like region and observing a significance above 5σ. It also provides inclusive Zjj cross sections and unfolded differential distributions across five fiducial regions, enabling stringent tests of QCD modeling with Powheg and Sherpa, and offering data-driven constraints on strong-background shapes. The analysis yields a combined EW Zjj cross section in good agreement with SM expectations and places limits on anomalous WWZ couplings, while the differential results highlight strengths and limitations of current event generators in extreme phase-space regions. Overall, the results validate vector-boson-fusion-like topologies in hadron collisions and inform future precision studies of EW processes and gauge-coupling structure at the LHC.

Abstract

Measurements of fiducial cross sections for the electroweak production of two jets in association with a Z-boson are presented. The measurements are performed using 20.3 inverse femtobarns of proton-proton collision data collected at a centre-of-mass energy of sqrt(s)=8 TeV by the ATLAS experiment at the Large Hadron Collider. The electroweak component is extracted by a fit to the dijet invariant mass distribution in a fiducial region chosen to enhance the electroweak contribution over the dominant background in which the jets are produced via the strong interaction. The electroweak cross sections measured in two fiducial regions are in good agreement with the Standard Model expectations and the background-only hypothesis is rejected with significance above the 5 sigma level. The electroweak process includes the vector boson fusion production of a Z-boson and the data are used to place limits on anomalous triple gauge boson couplings. In addition, measurements of cross sections and differential distributions for inclusive Z-boson-plus-dijet production are performed in five fiducial regions, each with different sensitivity to the electroweak contribution. The results are corrected for detector effects and compared to predictions from the SHERPA and POWHEG event generators.

Figures (16)

• Figure 1: Representative leading-order Feynman diagrams for electroweak $Zjj$ production at the LHC: (a) vector boson fusion (b) $Z$-boson bremsstrahlung and (c) non-resonant $\ell^+ \ell^- jj$ production.
• Figure 2: Examples of leading-order Feynman diagrams for (a) strong $Zjj$ production and (b) diboson-initiated $Zjj$ production.
• Figure 3: Comparison of data and simulation in the baseline region for (a,b) the leading jet transverse momentum and rapidity, (c,d) the subleading jet transverse momentum and rapidity, (e,f) the invariant mass and rapidity span of the dijet system. The simulated samples are normalised to the cross-section predictions discussed in section \ref{['sec:mc']} and then stacked. The error bars reflect the statistical uncertainties of the data. The hatched band in the ratio reflects the total experimental systematic uncertainty on the simulation.
• Figure 4: Fiducial cross-section measurements for inclusive $Zjj$ production in the $Z \rightarrow \ell^+ \ell^-$ decay channel, compared to the Powheg prediction for strong and electroweak $Zjj$ production and the small contribution from $ZV$ production predicted by Sherpa. The (black) circles represent the data and the associated error bar is the total uncertainty in the measurement. The (red) triangles represent the theoretical prediction, the associated error bar (or hatched band in the lower plot) is the total theoretical uncertainty on the prediction.
• Figure 5: Example systematic uncertainty breakdown for the $\frac{1}{\sigma} \cdot \frac{{\rm d}\sigma}{{\rm d}|\Delta{y}|}$ distribution and the jet veto efficiency as a function of $|\Delta{y}|$ in the baseline region. The effect of MC statistics, pileup modelling and JVF modelling are combined into one uncertainty labelled 'other'.