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Search for Anomalous Electroweak Gauge Boson Self-Interactions with ATLAS at the LHC

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Search for Anomalous Electroweak Gauge Boson Self-Interactions with ATLAS at the LHC ATL-PHYS-PROC-2013-288 Search for anomalous electroweak gauge boson self-interactions with ATLAS at the LHC Philipp Angera, on behalf of the ATLAS collaboration aTechnische Universit¨atDresden Abstract The ATLAS collaboration has setp limits on anomalousp contributions to electroweak triple gauge boson couplings based on data produced at the LHC with both s = 7 TeV and s = 8 TeV.p Sensitivity to measurements of anomalous electroweak quartic gauge boson couplings with an upgraded ATLAS detector at s = 14 TeV has been estimated for vector boson scattering and triple boson production in the context of effective field theories and searches for additional resonances. Keywords: LHC, ATLAS, Standard Model, electroweak, triple gauge coupling, TGC, aTGC, quartic gauge coupling, QGC, aQGC 1. Introduction than 93% the ATLAS detector performed very well during these periods. In the Standard Model of particle physics the electromagnetic and weak interactions are described by the electroweak theory, which is based on the spontaneously broken SU(2)L ⊗ U(1)Y gauge group. All aspects of this theory are in good agreement 2. Standard Model diboson production with the current experimental data. Interactions between fermions are mediated by bosonic fields describing massless (photon γ) and massive (W- and Z-bosons) As a first step towards measuring anomalous contributions to particles, the electroweak gauge bosons. The Standard Model electroweak gauge boson vertices, the measurement of the Stan- also predicts self-interactions between these bosons. Allowed dard Model production of two gauge boson in the final state as self-interactions are charged triple and quartic gauge boson ver- performed by ATLAS is presented. Triple gauge boson produc- tices: WWZ, WWγ, WWWW, WWZZ, WWZγ and WWγγ. tion has not been studied up to now, mainly due to the small The study of these vertices aim to test the Standard Model production cross-section. at the TeV scale. Such studies are possible with ATLAS at the LHC in channels involving the production of two or three gauge An overview of the measured production cross-sections cor- bosons in the final state. An important special case is vector bo- rected for leptonic branching fractions and comparison to the- son scattering (VBS) accessible in the (inclusive) two jet bin of oretical expectations can be seen in Figure 1. No significant electroweak diboson production. VBS has never been observed deviation from the Standard Model has been observed. before but plays an important role in understanding the elec- troweak symmetry breaking. In addition, the search for signals 105 ATLAS Preliminary of new physics at the high energy frontier is possible. A direct [pb] -1 35 pb search for physics beyond the Standard Model would involve total LHC pp s = 7 TeV σ 35 pb-1 Theory 4 the test of hypothetical models like Technicolor, Little Higgs or 10 Data (L = 0.035 - 4.6 fb-1) Extended Gauge Models. Here, the results of a more general LHC pp s = 8 TeV ansatz are collected and indirect searches for anomalous triple 3 10 Theory -1 and quartic gauge couplings are presented. 5.8 fb Data (L = 5.8 - 20 fb-1) The reported results are based on data collected by the AT- 5.8 fb-1 2 LAS detector [1] during the data-taking periods 2011 and 2012. 10 1.0 fb-1 1.0 fb-1 13 fb-1 Two different center-of-mass energies of thep colliding protons 4.6 fb-1 20 fb-1 4.6 fb-1 were used by the LHC during these years, s = 7 TeV and 10 -1 p 2.1 fb s = 8 TeV. Analyses performed at both energies are pre- 4.6 fb-1 −1 sented based on different amounts ofp collected data: 35 pb to −1 −1 1 W Z tt t WW WZ Wt ZZ 4:6 fb integratedp luminosity for s = 7 TeV and 5:8 fb to 20 fb−1 for s = 8 TeV. With a fraction of operational de- tector channels above 95% and data taking efficiencies higher Figure 1: Standard Model production cross section measurements, corrected for leptonic branching fractions, compared to the corresponding theoretical ex- pectations [2]. Email address: [email protected] (Philipp Anger) Preprint submitted to Hepmad13 November 2, 2013 3. Anomalous triple gauge couplings Model independent aTGC can be introduced within an effec- tive Lagrangian: Since triple gauge couplings are completely determined by V y µ ν yµ ν the Standard Model, any deviation in terms of anomalous triple LWWV / g1 (WµνW V − WµνW V ) + gauge couplings (aTGC) would indicate the existence of new y µν λV y µ νρ s κV Wµ WνV + WρµWν V ; (2) physics. At the LHC, the sensitivity to aTGC is driven by - m2 channel diboson production. The corresponding tree level dia- W gram can be seen in Figure 2. In the Standard Model, final states where V denotes either Z or γ. All parameters are fixed in with Zγ, γγ and ZZ final states cannot be produced through the V the Standard Model: g1 = κV = 1; λV = 0. When search- process represented in Figure 2 since just charged triple gauge ing for physics beyond the Standard Model, the parameters are couplings (WWZ, WWγ) are allowed. allowed to change. Therefore, five free parameters are intro- duced: ∆gZ = gZ − 1, ∆κ = κ − 1 and λ . q 1 1 V V V V Diboson WZ production was used to set limits on the three Z parameters λZ, ∆κZ and ∆g1 [3]. The influence of each of these parameters on the transverse momentum of the Z candidate is [a]TGC shown in Figure 3 for three different choices of parameters. Data are shown together with expected background and signal q events, assuming Standard Model. The colored lines indicate V the possible aTGC scenarios without unitarization. All param- Figure 2: A schematic Feynman diagram of TGC in diboson production. The eters change the high energy region while keeping the low en- circle indicates possible existence of anomalous contributions. V indicates elec- ergy regime. No evidence for any of the anomalous scenarios troweak gauge bosons W, Z or γ. The final states Zγ, γγ and ZZ are not allowed is found. Therefore limits on all parameters can be set. for this diagram in the Standard Model. 160 ATLAS 3.1. Unitarity 140 Data p ∫ Ldt = 4.6 fb­1 s = 7 TeV SM WZ When adding aTGCs, unitarity can be violated at high s. 120 Bkg Events / bin σ This fact implies undefined probabilities leading to infinite stat + syst 100 λ Z = 0.05 cross-sections and therefore to unphysical results. In order to ∆κ Z = 0.57 obtain physical results, an unitarization procedure is needed. 80 ∆gZ = 0.10 1 Nevertheless, two scenarios have been used in ATLAS for 60 publishing results: 40 1. no unitarization 2. form-factor unitarization. 20 0 No unitarization introduces no model dependence and no as- 0 30 60 90 120 150 180 2000 sumptions, while the introduction of a form factor makes the pZ [GeV] T result less general. The form factor is chosen to be energy de- pendent in order to restore unitarity. A general form varies each Figure 3: Transverse momentum of the reconstructed Z candidate in data com- aTGC parameter α: pared to the sum of all backgrounds and Standard Model WZ production [3]. Possible un-unitarized aTGC contributions are stacked in addition for three dif- α ferent sets of parameters. α ! α(ˆs) = : (1) sˆ n (1 + Λ2 ) FF One dimensional 95% confidence intervals on all three pa- rameters accessible in WZ production are shown in Figure 4. Here, ΛFF is the energy scale of the form factor and n is a free parameter that is chosen differently for different final states: n = The results are shown without unitarization (ΛFF = 1) and with 2 for WW and WZ analyses, n = 3 for ZZ analysis and n = 3 unitarization using form factors with n = 2 and ΛFF = 2 TeV. or 4 for Zγ final states.s ˆ is the center-of-mass energy of the As comparison unitarized results from CDF and D0 are shown outgoing bosons. which are similar to the ones obtained by ATLAS. Observed two dimensional 95% confidence regions on all 3.2. WWZ and WWγ vertices combinations of the three parameters can be seen in Figure 9. The results are based on un-unitarized parameters. The hori- There are just two triple gauge vertices in the Standard zontal and vertical lines inside each contour correspond to the Model: WWZ and WWγ. Both are fixed by the gauge theory. limits found in the one-dimensional fit procedure. They are accessible in WW, WZ1 and Wγ production. In addition to the WZ final state, also WW [4] and Wγ [5] analyses have set limits on aTGC. While WW diboson produc- 1Throughout this note, analyses with Z in the final state refer to Z or γ∗ final tion has access to the same set of parameters as WZ, it was states. possible to set limits on the remaining two parameters λγ and 2 ATLAS ATLAS, s = 7 TeV 4.6 fb­1, Λ = ∞ ATLAS, s = 7 TeV Z f5 4.6 fb­1, Λ = 2 TeV ATLAS Z ∆g CDF, s = 1.96 TeV 1 7.1 fb­1, Λ = 2 TeV γ D0, s = 1.96 TeV f5 CMS, s = 7TeV ­1 4.1 fb , Λ = 2 TeV 5.0 fb­1, Λ = ∞ ATLAS, s = 7TeV 4.6 fb­1, Λ = ∞ λ + ­ Z W ±Z → l ±νl l ATLAS, s = 7TeV Z ­1 95% C.L. f 4.6 fb , Λ = 3 TeV 4 LEP, s = 130­209 GeV 0.7 fb­1, Λ = ∞ D0, s = 1.96TeV 1.0 fb­1, Λ = 1.2 TeV ∆κ γ Z f4 ­1 ­0.8 ­0.6 ­0.4 ­0.2 0 0.2 0.4 ­0.6 ­0.4 ­0.2 0 0.2 0.4 0.6 0.8 Figure 5: 95% confidence regions on aTGC parameters for the ZZV (V = Z; γ) vertices as obtained by the ZZ final state analysis [6].
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