Diboson productions and aTGCs search at LHC

The ATLAS and CMS collaborations have measured the production cross-sections of dibosons (WW, WZ, ZZ, Wγ, Zγ) in leptonic decay final states with collision data produced at the LHC with both √ s = 7 TeV and √ s = 8 TeV. Based on these data, limits on anomalous triple-gauge-couplings are also derived.


Introduction
The Standard Model (SM) electroweak theory is based on the S U(2) L ⊗U(1) Y gauge groups, in which the gauge symmetry is spontaneously broken by the Higgs mechanism.The studies of diboson productions at LHC help to test the theory at TeV scale, improve the understanding of background processes in Higgs searches and also offer great opportunities to search for new physics at the high energy frontier.
At leading order, dibosons are produced mainly through q q annihilation, while gg fusion contributes typically less than 10% [3].The diboson productions of WW, WZ, ZZ and W/Z + γ are studied in leptonic decay final states.Diboson production cross-sections are known to next-to-leading order (NLO) in QCD, and the electroweak radiation corrections to the cross-sections are important at high √ s [4]; besides, the leptonic decay final states offer clean signature and enable the possibility of precision measurements.These cross-section measurements therefore provide crucial and sensitive test of both QCD and electroweak theory at the TeV scale.
Many new physics models predict resonance particles that can decay to boson pairs.Some of these models are based on the non-SM implementation of electroweak symmetry breaking; for instance, there are little Higgs [5], technicolor [6], extended gauge models [7] and Randall-Sundrum graviton [8] models.These new particles can be searched in either fully leptonic or semileptonic decay final states of diboson events, and the high energy of the LHC offers the greatest potential.A direct search for new resonances is not covered in this paper, instead an indirect approach is carried out by searching for anomalous triple-gauge-couplings (aTGCs).In the SM, triple-gaugecouplings (TGCs) are completely determined by gauge theory; and any deviation from the SM couplings indicates existence of new physics beyond the SM.aTGCs depend strongly on the center of mass energy of the colliding a e-mail: Yusheng.Wu@cern.chbosons; therefore, the LHC offers a great opportunity for measuring stringent limits.Besides, aTGCs largely manifest themselves in high momentum or high mass regions, the sensitivity can be further enlarged by proper binning of kinematics distributions.
The reported analysis results are based on the pp collision data collected by the ATLAS [1] and CMS detectors [2] during the years of 2011 and 2012.The integrated luminosities of analyzed data are slightly different among various diboson channels; but the maximum amount of data in use corresponds to about 5 fb −1 at both √ s = 7 TeV and √ s = 8 TeV.The detectors were operating amazingly well during the data taking periods; the detectors operated at a fraction of above 95% operational channels and data taking efficiencies were higher than 93%.The following sections will describe both the results of cross-section measurements and aTGCs search.

Cross-section measurement 2.1 Overview
As an overview, Figure 1 shows the measured production cross-sections of various diboson processes and their comparisons with SM NLO predictions from the ATLAS and CMS experiments [9, 10].The measurements are consistent with theoretical predictions.

W/Z + γ
In this channel, events with a leptonic decay boson (W or Z) and a photon are selected.Major background processes include W/Z + jets and γ + jets, where jet-induced photons or jet-faking leptons are selected.In order to suppress these backgrounds, events with low boson mass or low E miss T are rejected, and in addition photons are required to be separated from leptons.The total systematic uncertainty for the measured cross-section is about 7 − 9%; the major ones come from photon reconstruction and the background estimation.The cross-sections are measured

WW
WW events are selected to have two high p T isolated leptons and large E miss T .Backgrounds consist of t t, Z + jets, W + jets and other diboson processes.The signal is purified by removing events within Z mass windows, rejecting events with one or more jets and discarding low p T (transverse momentum of two lepton system) region.The dominant source of uncertainty is the jet-veto uncertainty, and the total systematic uncertainty is around 8%. Measured cross-sections are in agreement with theory, which are presented in Table 1 [13,14].

WZ
Three leptons plus large E miss T make this signature very clean, but there are still background events contributing from ZZ, Z + jets and t t.An on-shell Z and a W boson with large transverse mass are required in these events in order to further suppress the contamination.The systematic uncertainties mainly come from lepton and E miss T reconstructions, which turns out to be around 5%.The measured cross-sections are shown in Table 2 [15, 16].

WW/WZ → ν j
In this analysis, the semi-leptonic decays of diboson are studied.The signal events have one lepton, large E miss T and two separated jets, where the jets are from hadronic W or Z boson decay.The irreducible W + jets background is subtract using template fit method, and the combined cross-section of WW + WZ is derived as shown in Table 3 [17, 18], where the large systematic uncertainty is driven by uncertainties in the background estimation.

ZZ
The ZZ production cross-section is measured in two decay modes: ZZ → 4 and ZZ → νν.In the four lepton case, the signature is very clean with background contribution of less than 2% after requiring two on-shell Z bosons for the signal selection.The measured cross-sections in both ATLAS and CMS are shown in Table 4 [19,20].In the two lepton plus E miss T channel, there is large background contamination from Z + jets, t t and other diboson processes.Apart from selecting an on-shell Z, events are further required to have zero jets, large E miss T in the Z transverse momentum direction.The measured cross-section in AT-LAS at √ s = 7 TeV is 5.4 +1.3 −1.2 (stats.)+1.4 −1.0 (syst.)± 0.2(lumi.)pb [21], which is consistent with SM NLO prediction.

Figure 5 .
Figure 5.The 95% CI of aTGCs parameters in ZZγ and Zγγ vertices derived from Zγ production in ATLAS (left two, from Figure 4 of [11]) and CMS (right two, from public twiki [12]).
EPJ Web of Conferences DOI: 10.1051/ C Owned by the authors, published by EDP Sciences, 2013 This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2 0 , which .permits unrestricted use, distributi and reproduction in any medium, provided the original work is properly cited.on,Articleavailable at http://www.epj-conferences.orgor http://dx.doi.org/10.1051/epjconf/20134914006

Table 2 .
The measured WZ cross-sections in ATLAS and CMS (referenced from Section 6.1 of [15] and Section 5.4 of [16]).

Table 3 .
The measured WW + WZ cross-sections in ATLAS and CMS through semi-leptonic decay channels (referenced from Section 7 of [17] and Page 5 of [18]).

Table 4 .
The measured ZZ cross-sections in ATLAS and CMS through fully leptonic decay channels (referenced from Section 6 of [19] and Section 7 of [20]).