Electroweak diboson production at ATLAS

This paper is giving an overview of recent ATLAS results on the production cross sections of gauge boson pairs decaying leptonically using data from pp collisions at √s = 13 TeV for ZZ and at √s = 8 TeV for W±Z and W±W± at the LHC at CERN. The cross sections are found to be in agreement with the expectations from the Standard Model within the estimated uncertainties. The production cross section measurements also allow for studies of anomalous triple and quartic gauge couplings for which 95% confidence level limits are set.


INTRODUCTION
This paper presents measurements of the diboson production cross sections in proton-proton collisions and limits on charged anomalous triple gauge couplings (aTGC) with the ATLAS experimentat the LHC. The following diboson pair final states were investigated: at center of mass energy and at For pairs a sample of integrated luminosity of 2015 data was used and for analyses the data collected in 2012 correspond to All studies used the fully leptonic decay final states with and where

CROSS SECTION MEASUREMENTS
The Z boson pair production at is measured in final states with 2 pairs of charged leptons [1]. The cross section is found by counting candidate events, subtracting the expected contribution from background events, correcting for detector effects, and dividing by the integrated luminosity.
The measured fiducial cross sections are shown in and Fig. 1 (left) along with a comparison to calculations. Overall good agreement with the predictions from the SM is found. The measured total cross section is compared to measurements at lower centerof-mass energies and to a prediction from MCFM generator with the CT14 NLO PDFs, which is calcu-lated at accuracy for the -initiated process and at accuracy for the loop-induced -initiated process and is shown in Fig. 1 (right).
The analysis of boson pairs is performed in final states [2]. Ratios of fidutial cross section to the SM NLO predictions in different decay channels are shown in Fig. 2 (left). The measured cross section is 20% larger than SM prediction and NLO accuracy. However, the SM prediction, which is at NLO accuracy in perturbative QCD, is highly sensitive to the choice of renormalization scale In addition, new perturbative effects appearing at NNLO could enhance the SM prediction compared to the NLO calculation. In Fig. 2 (right) ratio to production cross sections is compared with theoretical predictions, showing that such a ratio is sensitive to the choice of PDF set.
The signal is measured in final states with two oppositely charged isolated leptons and large missing transverse energy [3]. The combined cross section is found to be compatible with the NNLO prediction within about 1.4 standard deviations. A graphical comparison between the individual measurements and their combination is shown in Fig. 3 (left).

TRIPLE GAUGE COUPLINGS MEASUREMENTS
The non-Abelian nature of the electroweak sector of the SM predicts the self-interaction of gauge bosons in the form of triple couplings. Searches for deviations from the SM can be parameterized in terms of anomalous Triple Gauge Couplings. In Fig. 3 (right) a summary of the limits set on the different aTGC parame- The article is published in the original.
aTGC Limits at 95% CL  ters is shown along with comparisons to other experiments. Limits obtained at TeV are usually 1.5-2.5 times better than limits at TeV. Measurements of and diboson production provide complementary results and combined together they give the most stringent limits on couplings.
4. CONCLUSIONS Measurements of the production cross sections of at center of mass energy and at in fully leptonic channel have been performed with the ATLAS detector. The total production cross sections are compatible with the SM expectations within uncertainties. Limits on anoma-lous triple gauge couplings are set. With an improvement by a factor of about two compared to previously existing constraints, these are the most stringent model-independent limits on WWZ anomalous couplings to date.