Search for resonant $WZ$ production in the fully leptonic final state in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search for a heavy resonance decaying into $WZ$ in the fully leptonic channel (electrons and muons) is performed. It is based on proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 36.1 fb$^{-1}$. No significant excess is observed over the Standard Model predictions and limits are set on the production cross section times branching ratio of a heavy vector particle produced either in quark-antiquark fusion or through vector-boson fusion. Constraints are also obtained on the mass and couplings of a singly charged Higgs boson, in the Georgi-Machacek model, produced through vector-boson fusion.


Introduction
Searches for diboson resonances provide an essential test of theories of electroweak symmetry breaking beyond the Standard Model (BSM).Vector resonances are predicted in various BSM scenarios, such as in Grand Unified theories [1], Little Higgs models [2], and Composite Higgs models [3,4], unitarized Electroweak Chiral Lagrangian models [5], as well as in theories with extra dimensions [6][7][8].In addition, new scalar diboson resonances result from models with an extended Higgs sector [9,10].This Letter reports on a search for a W Z resonance in the fully leptonic decay channel ν ( = e or µ), produced either by quark-antiquark (q q) fusion or by vector-boson fusion (VBF).The proton-proton collision data were collected by the ATLAS detector [11] at the Large Hadron Collider (LHC) at a centre-of-mass energy √ s = 13 TeV.
Parameterized Lagrangians [12][13][14] incorporating a heavy vector triplet (HVT) permit the interpretation of searches for vector resonances in a generic way.Here, the simplified phenomenological Lagrangian of Ref. [13] is used.The coupling of the new heavy vector resonance, V, to the Higgs boson and the Standard Model (SM) gauge bosons is parameterized by g V c H and to the fermions via the combination (g 2 /g V )c F , where g is the SM SU(2) gauge coupling.The parameter g V represents the typical strength of the vector-boson interaction, while the parameters c H and c F are expected to be of the order of unity in most models.The vector-boson scattering process is only sensitive to the gauge boson coupling and, in this case, the benchmark model used to interpret the results assumes no coupling of the heavy vector resonance to fermions.
The Georgi-Machacek model (GM) [15,16] is used as a benchmark for a singly charged scalar resonance.
The model extends the Higgs sector by including one real and one complex triplet, while preserving custodial symmetry, ensuring that the parameter ρ = M 2 W /(M 2 Z cos 2 θ W ) = 1 at tree level.It is less experimentally constrained [17,18] than other models with higher isospin representations, such as Little Higgs models or Left-Right symmetric models [19].A parameter sin θ H , representing the mixing of the vacuum expectation values, determines the contribution of the triplets to the masses of the W and Z bosons.The ten physical scalar states are organized into different custodial multiplets: a fiveplet (H ++  5 , H + 5 , H 0 5 , H − 5 , H −− 5 ) which is fermiophobic but couples to W Z, a triplet, and two singlets, one of which is identified as the 125 GeV SM Higgs boson.Assuming that the triplet states are heavier than the fiveplet scalars, H 5 can only be produced by vector-boson fusion and the cross section is proportional to sin 2 θ H .The singly charged members of this fiveplet are the object of the present search in the VBF channel.For both models the resonance width is below 4% and lower than the experimental resolution on the resonance width, in nearly all the parameter space explored in the present analysis.
The present analysis extends the search for resonant W Z production beyond that in Run 1 pp collision data at

ATLAS detector
The ATLAS detector at the LHC has a cylindrical geometry with a near 4π coverage in solid angle. 1 The inner detector (ID), consisting of silicon pixel, silicon microstrip and transition radiation detectors, is surrounded by a thin superconducting solenoid providing a 2 T axial magnetic field.It allows precise reconstruction of tracks from charged particles and measurement of their momenta up to a pseudorapidity of |η| = 2.5.High-granularity lead/liquid-argon (LAr) sampling electromagnetic and steel/scintillator-tile hadron calorimeters, at larger radius, provide energy measurements in the central pseudorapidity range |η| < 1.7.In the endcap and forward regions, LAr calorimeters for both the EM and hadronic energy measurements extend the region of angular acceptance up to |η| = 4.9.Outside the calorimeters, the muon spectrometer incorporates multiple layers of trigger and tracking chambers in a magnetic field produced by a system of superconducting toroid magnets, enabling an independent precise measurement of muon track momenta for |η| < 2.7.The ATLAS trigger system consists of a hardware-based level-1 trigger was set to 1 for Model A and to 3 for Model B. For both models, the parameter c F is assumed to be the same for all types of fermions.Simulated signal samples for the HVT benchmark Model A were generated for masses of vector resonances ranging from 250 GeV to 3 TeV with M G _ MC@NLO 2.2.2 [44], using the model file provided by the authors in Ref. [45] with the NNPDF23LO [46] PDF set.They are hadronized with P 8.186.For interpretation in terms of Model B, the Model A cross sections are simply scaled.This is justified since the width remains well below the experimental resolution and the angular distributions are the same for both models.
For the VBF production channel, HVT samples were generated with g V = 1 for masses ranging from 250 GeV to 2 TeV.The coupling parameter c H was set to 1 and all other couplings of the heavy triplet, including c F , were set to 0 in order to maximize the VBF contribution.A dijet invariant mass of at least 150 GeV was required during event generation.
For the GM signal samples, pp → H ± 5 → W ± Z j j were produced with M G _ MC@NLO 2.2.2 for the mass range 200 to 900 GeV in the H 5 -plane defined in [47], compatible with present limits [18,48], using GMCALC [49] and with sin θ H = 0.5.They were produced at leading order, but normalized to next-to-leading order according to Ref. [9], where the cross sections and widths, which scale as sin 2 θ H , are also given.For these samples, a minimum p T of 15 GeV (10 GeV) for the jets (leptons) was required during event generation and the pseudorapidity must be in the range |η| < 5 for jets and |η| < 2.7 for leptons.
The background sources in this analysis include processes with two or more electroweak gauge bosons, namely VV and VVV as well as processes with top quarks, such as t t, t tV, single top and t Z, and processes with gauge bosons produced in association with jets or photons (Z + j and Zγ ).MC simulation is used to estimate the contribution from background processes with three or more prompt leptons while data-driven techniques are used for the case of background processes with at least one misidentified or non-prompt lepton.Simulated events are used for cross checks and to assess the systematic uncertainties in these backgrounds.
The dominant W Z SM background process (QCD W Z) was modelled using S 2.2.2 [50] at nextto-leading order (NLO), and includes hard-scattering, parton shower, hadronization and the underlying events.Up to three additional partons generated at tree level were merged with the parton shower.In order to estimate an uncertainty due to the parton shower modelling, two alternative W Z samples were produced using P -B 2 interfaced with P 8.186 and H ++ respectively.A sample of the purely electroweak process W Z j j → ν j j (labelled W Z j j) with a matrix-element b-quark veto (at zero order in α s ) was generated separately with S 2.2.2.Contributions from W Z jb → ν bj (labelled W Z bj) are included in the t Z sample described below.To estimate an uncertainty due to the parton shower modelling an alternative M +P 8 sample was produced.This M sample includes b-quarks in the initial state and was split to provide a sample without (with) a b-quark in the final state to model the W Z j j (t Z + W Z bj) background.
Samples of q q → Z Z → 4 or q q → Z Z → νν were generated by P -B 2 [51] at NLO, interfaced to P 8.186 and normalized to NNLO by K-factors evaluated in Ref. [52].The gg → Z Z and tribosons were generated with S 2.1.1.The t tV and t Z processes were generated at LO using M _ MC@NLO, interfaced with P 8.186 (t tV) and P 6.428 (t Z).The t tV samples were normalized to NLO predictions [9].
Finally samples of SM backgrounds with at least one misidentified or non-prompt lepton, including Zγ, W γ, Drell-Yan Z → , W → ν as well as top-pair and single-top were generated to assist in the fake/non-prompt lepton background estimate.Events with Zγ and W γ in the final state were generated with S 2.1.1.Drell-Yan Z → , W → ν as well as top-pair and single-top production channels were generated with P -B 2 and hadronized with P .To avoid double counting the Zγ events, Z events produced by the Drell-Yan process with a photon from final-state radiation with p T > 10 GeV were removed.The parton shower for processes with top quarks was modelled with P 6.428.M _ MC@NLO and P 8.186 were used for background processes involving a pair of top quarks accompanied by a W boson or by a pair of charged leptons.The Z and single-top cross sections were normalized to NNLO by K-factors evaluated in Ref. [52,53].

Reconstructed objects
Events are required to have at least one primary vertex with at least two associated tracks, each with transverse momentum p T > 0.4 GeV.If there is more than one vertex reconstructed in the event, the one with the largest track p 2 T is chosen as the hard-scatter primary vertex and is subsequently used for the reconstruction of electrons, muons, jets and missing transverse momentum.
Electron candidates are reconstructed from energy deposits in the EM calorimeter which are matched to a well-reconstructed ID track originating from the primary vertex.The electron identification is based on a likelihood evaluated from a multivariate discriminant.They are categorized as satisfying the medium or the tight reconstruction quality requirements, as defined in Ref. [54].Only electrons with transverse energy E T > 25 GeV in the pseudorapidity range |η| < 2.47 are considered in this analysis.The candidate electrons are required to pass an isolation condition: an upper value of the scalar sum of the transverse momentum of the tracks with p T > 0.4 GeV in a cone of size ∆R = min(0.2,10 GeV/E T ) around the electron, excluding the track of the electron itself, is chosen such that the efficiency is constant at 99% for electrons in Z → ee events.For tight electrons, an isolation requirement is imposed, based on calorimeter as well as track variables, which varies as a function of transverse energy and yields an efficiency between 95% and 99% for electrons with p T in the range 25-60 GeV.For a pair of electrons sharing the same ID-track, the electron with higher cluster E T is kept.
Muons are reconstructed by combining tracks from the inner detector with tracks from the muon spectrometer.They are required to satisfy medium or tight quality requirements, as defined in Ref. [55].Only muons with p T > 25 GeV and |η| < 2.7 are considered in this analysis.Isolation requirements are also applied to all muons, based on the ratio p varcone T /p µ T , where p varcone T is the scalar sum of the transverse momenta of the tracks with p T > 1 GeV in a cone of size ∆R = min(10 GeV/p µ T , 0.3) around the muon, excluding the muon track itself.This isolation gives 99% efficiency, independently of η or p µ T , in Z → µµ samples.
Electron and muon candidates are required to originate from the primary vertex.Thus, the significance of the track's transverse impact parameter calculated relative to the beam line, |d 0 /σ d 0 |, must be less than three for muons and less than five for electrons, and the longitudinal impact parameter, z 0 (the difference between the value of z of the point on the track at which d 0 is defined and the longitudinal position of the primary vertex), is required to satisfy |z 0 • sin(θ)| < 0.5 mm.
Jets are reconstructed from clusters of energy deposition in the calorimeter [56] using the anti-k t algorithm [57] with a radius parameter R = 0.4.Events with jets arising from detector noise or other non-collision sources are discarded [58].This search considers jets with p T > 30 GeV in the range |η| < 4.5.Furthermore, to mitigate the pile-up contamination, a jet vertex tagger [59], based on information about tracks associated with the primary vertex and pile-up vertices, is applied to jets with p T < 60 GeV and |η| < 2.4.The selected working point provides at least 92% efficiency.The energy of each jet is calibrated and corrected for detector effects using a combination of simulated events and in situ methods in 13 TeV data [60].
As lepton and jet candidates can be reconstructed from the same detector information, a procedure to resolve overlap ambiguities is applied.If an electron and a muon share the same ID track, the muon is selected.Reconstructed jets which overlap with electrons or muons in a cone of size ∆R = 0.2 are removed.
Jets containing b-hadrons are identified as b-jets by the MV2c10 b-tagging algorithm [61], which uses information such as track impact-parameter significances and positions of explicitly reconstructed secondary decay vertices.A working point corresponding to 85% b-tagging efficiency on a sample of t t events is chosen [62], with a light-flavour jet rejection factor of about 34 and a c-jet rejection of about 3. Correction factors are applied to the simulated event samples to compensate for differences between data and simulation in the b-tagging efficiency for b-jets, c-jets and light-flavour jets.
The missing transverse momentum, ì p miss T , and its magnitude E miss T , are calculated from the imbalance in the sum of visible transverse momenta of reconstructed physics objects: electrons, muons and jets, as well as a "soft" term reconstructed from tracks compatible with the primary vertex and not associated with any of those objects [63, 64].

Event selection
Preselection criteria are first applied to all the event samples.The presence of three prompt leptons is required, two of which will be associated with the Z boson, and are required to satisfy the medium quality requirement (Section 4).The Z boson candidate is reconstructed from the two leptons of same flavour and opposite charge, whose invariant mass is closest to the on-shell mass m Z , and in the range |m − m Z | < 20 GeV.The third lepton, associated with the W boson decay, is required to satisfy the tight quality criteria to enhance the background rejection.To ensure that the trigger efficiency is well determined, at least one of the candidate leptons is required to have p T > 27 GeV.
To suppress background processes with at least four prompt leptons, events with a fourth lepton candidate satisfying looser selection criteria are rejected.For this looser selection, the requirement on the minimum p T of the leptons is lowered to p T > 7 GeV and medium identification requirements are used for both the electrons and muons.
Since there is a neutrino in the signal events, E miss T > 25 GeV is also required.The third lepton and the missing transverse momentum are assumed to result from the W boson decay.The longitudinal momentum p ν z of the neutrino is calculated by requiring that the invariant mass of the lepton-neutrino system be equal to the W mass.The solution results in a quadratic equation which leads to two possible solutions.If they are real, the one with the smaller |p ν z | is chosen since it was found to provide a better agreement with the truth.Otherwise, the real part is chosen.The invariant mass, m W Z , of the W Z resonance candidate is then reconstructed using the chosen solution for p ν z along with the four-momenta of the three charged leptons.
The selected events are then separated into two categories targeting different production mechanisms: VBF and q q.The VBF category contains events with two or more jets with p T > 30 GeV which fail the b-tagging requirements described in Section 4. The dijet pair defined by the two highest-p T jets in Figure 1: The signal selection acceptance times efficiency (A× ), defined as the ratio of the number of MC signal events in the VBF category to the number of generated signal events, is presented as a function of the resonance mass.Figure 1 the event must also have large η separation (∆η j j > 3.5) and an invariant m j j above 500 GeV.If more than two jets are found in an event, the two highest-p T jets are considered.By imposing a b-jet veto, backgrounds containing one or more top quarks, including t t, t t + W/Z, and t Z are suppressed.
The net acceptance times efficiency (A× ) of the selection, relative to signal events generated for H ± 5 and HVT models in the VBF category is shown in Figure 1.The generation of the GM Model H ± 5 events had the following requirements: p T (jets) > 15 GeV, p T (leptons) > 10 GeV, |η|(jets) < 5 and |η|(leptons) < 2.7.Decays of W bosons into all lepton flavours, and of Z bosons into e + e − and µ + µ − , were simulated.The Z → τ + τ − decays give a negligible contribution and were not included in the simulation, but the A× shown was scaled to include all decays.For the HVT VBF samples, m j j > 150 GeV was required at generator level.Decays of W and Z bosons into all flavours of leptons were included.For HVT and H ± 5 the A× falls in the range 2-8% and 3-12% respectively for resonance masses ranging between 200 and 900 GeV, the difference being due in part to the generator level selection and in part to the different angular distributions of the final products.
The remaining events are assigned to the q q category signal region.For this category, the W and Z bosons from a resonance produced in the s-channel with m W Z larger than 250 GeV are expected to have transverse momenta close to 50% of its mass.The requirements p W T /m W Z > 0.35 and p Z T /m W Z > 0.35 enhance the sensitivity to the signal.The overall selection efficiency relative to generated event increases from about 15% to 25% for resonances masses ranging from 500 GeV to 3 TeV as illustrated in Figure 2. Decays of W and Z bosons into all flavours of leptons are included at event generation.The A× values decrease for resonance masses above approximately 2 TeV due to the colinearity of electrons from the Z → ee decays which spoils the isolation.
Figure 2: The signal selection acceptance times efficiency (A× ), defined as the ratio of the number of MC signal events in the q q category to the number of generated signal events, as a function of the HVT resonance mass.The error bars represent the total statistical and systematic uncertainties.

Background estimation
The dominant background in the resonance search is the SM production of W Z. Its normalization and shape are estimated from MC and validated in dedicated validation regions by comparing the data and MC distributions.Events in the validation regions are selected in exactly the same way as those in their corresponding signal categories except for the following requirements.The VBF W Z validation region is defined by inverting the requirements on the dijet variables: 100 < m j j < 500 GeV and ∆η j j < 3.5.The W Z q q validation region requires the events to have p Z T /m W Z < 0.35 or p W T /m W Z < 0.35.These validation regions are dominated by the W Z contribution, with a purity higher than 80%.For the benchmark models with parameters given in Section 3, the signal contamination in the q q (VBF) validation region is below 5% (1%) for resonance masses above 500 GeV.The reconstructed m W Z mass in the validation regions is shown in Figure 3, where good agreement of data with the background prediction is observed.
Events from Z+jets, Zγ, W γ, t t, single top or WW where jets or photons were misidentified as leptons (here called fake/non-prompt leptons), can also satisfy the selection criteria.The distribution shapes and number of fake/non-prompt lepton events are estimated for both the q q and VBF categories by a data-driven method using a global matrix which exploits differences in object characteristics between real and fake/non-prompt leptons on a statistical basis.Details of the method, here referred to as "Matrix Method", can be found in Ref. [65].
Other backgrounds include t tV, Z Z, t Z, W Z bj and triple boson production.They are estimated by Monte Carlo simulation (Section 3).The t Z, W Z bj and VVV backgrounds are added as a single contribution, here called t Z+VVV.

Systematic uncertainties
Systematic uncertainties result from the theoretical modelling of backgrounds and from object and event reconstruction.Figure 3: Observed and expected distributions of the W Z invariant mass in (a) the q q validation region and (b) the VBF validation region.The points correspond to the data and the histograms to the expectations for the different SM processes.The uncertainty in the total background prediction, shown as bands, combines statistical, theory and systematic contributions.The last bin contains the overflow.
The uncertainty in the normalization of the S samples of SM W Z background is evaluated by taking into account the variations obtained with different PDF sets [66].The nominal set NNPDF30nloas0118 is compared with other samples generated with the CT14nnlo and MMHT2014nlo68cl PDF sets and the uncertainty is evaluated from the maximum differences.It is estimated to be below 6% in all mass bins for both the VBF and q q categories.The uncertainty associated with the choice of renormalization and factorization scales, µ R and µ F , is taken as the maximum downward and upward variation when the scales are varied independently by factors of 1/2 and 2. While these uncertainties can in principle affect the shape of the m W Z distribution, in practice the shape differences do not have a strong impact on the sensitivity of the search.The uncertainties are therefore treated as normalization uncertainties, taken to be 20% and 40% respectively.Shape systematic uncertainties associated with showering and hadronization of the QCD W Z are evaluated by comparing the P -B 2 samples interfaced with P 8.186 and H . For the electroweak W Z process the S 2.2.2 and M +P 8 predictions are compared.This uncertainty band ranges from 10% to 30% for both categories.
The uncertainties assigned to the cross sections of the other background sources consist of a contribution from PDF uncertainties and from QCD scale uncertainties.They are estimated to be 10% for Z Z, 13% for ttV, 20% for VVV and 15% for t Z.
The theoretical uncertainties in the cross section and acceptance of the simulated signal samples are evaluated in a similar way to the background.The PDF errors are taken from the NNPDF LO PDF error set, and the NNPDF set is also compared with the CTEQ6L1 and MSTW2008lo68cl sets.The different predictions from these PDF sets are taken as an extra contribution to the overall uncertainty.For both the q q and VBF categories, the uncertainties are typically below 5%.This procedure was followed for each mass point and a generator-level event selection was chosen to closely mimic the one used in the reconstruction-level analysis.Scale and PDF uncertainties are not correlated between signal and background.
An uncertainty due to the reconstruction efficiency, momentum scales and resolution of electrons and muons is evaluated by varying correction factors applied to the MC samples [55,67] within appropriate limits.
The jet energy scale and resolution uncertainties [58] are also taken into account as they affect the shape and normalization of the background distributions.The uncertainty due to b-tagging [68] is also included.
Missing transverse momentum is calculated using the preselected leptons, jets and other reconstructed objects.The uncertainties in the reconstruction of those objects are then used to evaluate the uncertainty in E miss T reconstruction.Those due to the p T scale and resolution of the soft term are also considered [63,64].
An uncertainty in the prediction of the fake/non-prompt background is also taken into account as it affects the shape and normalization of the background distributions.The total uncertainty is about 20% (27%) for the q q (VBF) category.It is slightly larger for the VBF category because of the higher statistical uncertainty derived from the Matrix Method (Section 6).
The uncertainty in the integrated luminosity is 2.1%.It is derived, following a methodology similar to the one detailed in Ref. [69], from a calibration of the luminosity scale using x-y beam-separation scans performed in August 2015 and May 2016.

Results
The W Z invariant mass distribution, m W Z , obtained as the sum of all four lepton-flavor permutations, is used as the discriminating variable, with bin widths comparable to the expected resolution of a narrow resonant signal.A binned likelihood function, constructed from the Poisson probability of the sum, in each bin, of the contributions of the background and of a hypothetical signal of strength µ relative to the benchmark model, is used to set limits on the presence of a signal.The fit is performed in the signal region for the q q and VBF categories separately.The systematic uncertainties described above (Section 7) enter as nuisance parameters with Gaussian or log-normal prior distributions, in convolution with the nominal background distribution.
The effects of systematic uncertainties are studied for hypothesized signals using the signal-strength parameter µ.The list of leading sources of uncertainty in the 95% confidence level (CL) upper limit on the µ value is given in Table 1 together with their relative importance (∆µ/µ).The values are quoted separately for a hypothetical HVT signal of mass m(W ) = 800 GeV in the q q category and a GM signal of mass m(H ± 5 ) = 450 GeV in the VBF category.Apart from the statistical uncertainties in the data, the uncertainty with the largest impact on the sensitivity of the searches is related to the W Z background modelling.
The numbers of background events are extracted through a background-only fit of the data in each category.Background contributions from prompt leptons, including their shapes, are taken from MC simulations.In the case of non-prompt leptons the background shapes are taken from the Matrix Method.In the fit, the normalisation of all backgrounds are allowed to vary within their uncertainties.The post-fit Source ∆µ/µ [%] q q Category VBF Category m(W ) = 800 GeV m(H ± 5 ) = 450 GeV W Z modelling : Scale, PDF  53 52 Table 1: Impact of the dominant sources of relative uncertainties on the 95% CL upper limits of the signal-strength parameter (µ) for a hypothetical HVT signal of mass m(W ) = 800 GeV in the q q category and a GM signal of mass m(H ± 5 ) = 450 GeV in the VBF category.The effect of the statistical uncertainty on the signal and background samples is also shown.Sources of systematic uncertainty with an impact of less than 2% in both categories are not shown.
background yields are summarized in Table 2 for the q q and VBF categories.The fit constrains the SM W Z background estimate to the observed data, which reduces the total background uncertainty, pulling the modeling uncertainties by less than one standard deviation from their pre-fit values.None of the nuisance parameters are significantly pulled or constrained relative to their pre-fit values in the background-only fit.
Figure 4 shows the post-fit m W Z distribution for the q q and VBF categories.The largest difference between the observed data and the SM background prediction is in the VBF category.A local excess of events at a resonance mass of around 450 GeV can be seen in Figure 4(b).The local significances for signals of H ± 5 and of a heavy vector W are 2.9 and 3.1 standard deviations, respectively.The respective global significances calculated using the Look Elsewhere method as in Ref. [70] and evaluated up to a mass of 900 GeV, are 1.6 and 1.9 standard deviations.In the q q category the largest difference between the observed data and the SM background prediction is located around a mass of 700 GeV with a local significance of 1.2 standard deviations.
Upper limits are set on the product of the production cross section of new resonances and their decay branching ratio into W Z. Exclusion intervals are derived using the CL s method [71] in the asymptotic approximation [72].For masses higher than 900 (700) GeV in q q (VBF) category, the small number of expected events makes the asymptotic approximation imprecise and the limits are calculated using pseudo-experiments.The limit set on the signal strength µ is then translated into a limit on the signal cross section times branching ratio, σ × B(W → W Z), using the theoretical cross section and branching ratio for the given signal model.Figure 5 presents the observed and expected limits on σ × B(W → W Z) at 95% CL for the HVT model in the q q category.Masses below 2260 GeV can be excluded for Model A and 2460 GeV for Model B. For resonance masses above 2 TeV the exclusion limits become worse due to the acceptance losses at high mass.For the VBF process, the limit on σ × B(W → W Z) is shown in Figure 6.

Category q q
Observed 95% CL upper limit Expected 95% CL upper limit Observed and expected 95% CL upper limits on σ × B(W → W ± Z) for the q q production of a W boson in the HVT models as a function of its mass.The theoretical predictions for HVT Models A with g V = 1 and B with g V = 3 are also shown.Expected and observed yields in the q q and VBF signal regions.Yields and uncertainties are evaluated after a background-only fit to the data in the q q or VBF signal regions after applying all selection criteria.The uncertainty in the total background estimate is smaller than the sum in quadrature of the individual background contributions due to anti-correlations between the estimates of different background sources.
Observed and expected exclusion limits at 95% CL on σ × B(H ± 5 → W ± Z) and on the mixing parameter sin θ H of the GM Model are shown in Figure 7 as a function of m H ± 5 .The intrinsic width of the scalar resonance, for sin θ H = 0.5, is narrower than the detector resolution in the mass region explored.The shaded regions show the parameter space for which the H ± 5 width exceeds 5% and 10% of m H ± 5 .The shaded region shows where the theoretical intrinsic width of the resonance would be larger than 5% or 10% of the mass.

Conclusion
A search is performed for resonant W Z production in fully leptonic final states (electrons and muons) using 36.1 fb −1 of √ s = 13 TeV pp data collected by the ATLAS experiment at the LHC during the 2015 and 2016 run periods.Two different production modes are considered using quark-antiquark annihilation and vector-boson fusion.
The data in the q q fusion category are found to be consistent with Standard Model predictions.The results are used to derive upper limits at 95% CL on the cross section times branching ratio of the phenomenological Heavy Vector Triplet benchmark Model A (Model B) with coupling constant g V = 1 (g V = 3) as a function of the resonance mass, with no evidence of heavy resonance production for masses below 2260 (2460) GeV.
In the case of the VBF production processes, limits on the production cross section times branching ratio are obtained as a function of the mass of a charged member of a heavy vector triplet or of the fiveplet scalar in the Georgi-Machacek model.The results show a local excess of events over the Standard Model expectations at a resonance mass of around 450 GeV.The local significances for signals of H ± 5 and of a heavy vector W boson are 2.9 and 3.1 standard deviations respectively.The respective global significances calculated considering the Look Elsewhere effect are 1.6 and 1.9 standard deviations respectively.
[20] ATLAS p Also at Department of Physics, The University of Michigan, Ann Arbor MI; United States of America.q Also at Department of Physics, The University of Texas at Austin, Austin TX; United States of America.r Also at Department of Physics, University of Fribourg, Fribourg; Switzerland.s Also at Dipartimento di Fisica E. Fermi, Università di Pisa, Pisa; Italy.t Also at Faculty of Physics, M.V.Lomonosov Moscow State University, Moscow; Russia.u Also at Fakultät für Mathematik und Physik, Albert-Ludwigs-Universität, Freiburg; Germany.v Also at Georgian Technical University (GTU),Tbilisi; Georgia.w Also at Giresun University, Faculty of Engineering; Turkey.
x Also at Graduate School of Science, Osaka University, Osaka; Japan.y Also at Hellenic Open University, Patras; Greece.z Also at Horia Hulubei National Institute of Physics and Nuclear Engineering; Romania.aa Also at II Physikalisches Institut, Georg-August-Universität, Göttingen; Germany.ab Also at Institucio Catalana de Recerca i Estudis Avancats, ICREA, Barcelona; Spain.ac Also at Institut de Física d'Altes Energies (IFAE), The Barcelona Institute of Science and Technology,

√ s = 8
TeV performed by the ATLAS [20] and CMS [21] collaborations.Each collaboration has combined results [22-24] from searches for heavy VV and V H resonances (V = W or Z) based on Run 1 data and on partial Run 2 data at √ s = 13 TeV in the fully hadronic (qqqq), semileptonic ( νqq, qq, ννqq), and fully leptonic ( , ν , νν) final states.More recent results from VV and V H resonance searches with data at √ s = 13 TeV have been reported in Refs.[25-31].The various decay channels generally differ in sensitivity in different mass regions.The fully leptonic channel, in spite of a lower branching ratio, is expected to be particularly sensitive to low-mass resonances as it has lower backgrounds.A recent search [32] by the CMS Collaboration for a charged Higgs boson produced by vector-boson fusion and decaying into W Z in the fully leptonic mode, using 15.2 fb −1 of data collected at √ s = 13 TeV, has yielded limits on the coupling parameter of the GM model, as a function of mass.Limits on the GM model have also been set, based on analyses of same-charge WW production by CMS [33] and opposite-charge WW production by ATLAS [34], using data at √ s = 13 TeV with an integrated luminosity of 36.1 fb −1 .
Figure 1: The signal selection acceptance times efficiency (A× ), defined as the ratio of the number of MC signal events in the VBF category to the number of generated signal events, is presented as a function of the resonance mass.Figure 1(a) corresponds to the GM Model H ± 5 while Figure 1(b) corresponds to the HVT models.The A× is shown for each decay channel and the sum of all lepton flavour combinations (inclusive).The error bars shown in each figure represent the total statistical and systematic uncertainties.

Figure 4 :
Figure4: Observed and expected distributions of the W Z invariant mass (a) in the q q and (b) in the VBF categories after applying all selection criteria.Signal predictions are overlaid, normalized to the predicted cross sections.The uncertainty in the total background prediction, shown as shaded bands, combines statistical, theory and systematic contributions.The lower panel show the ratios of the observed data to the background predictions.

Figure 5 :
Figure5: Observed and expected 95% CL upper limits on σ × B(W → W ± Z) for the q q production of a W boson in the HVT models as a function of its mass.The theoretical predictions for HVT Models A with g V = 1 and B with g V = 3 are also shown.

Figure 6 :
Figure 6: Observed and expected 95% CL upper limits on σ × B(W → W ± Z) for the VBF production of a W boson in the HVT Model, with parameter c F = 0, as a function of its mass.

Figure 7 :
Figure 7: Observed and expected 95% CL upper limits on (a) σ × B(H ± 5 → W ± Z) and (b) the parameter sin θ H of the GM Model as a function of m H ± 5 .The shaded region shows where the theoretical intrinsic width of the resonance would be larger than 5% or 10% of the mass.

Table 2 :
Collaboration, Search for W Z resonances in the fully leptonic channel using pp collisions at √ s = 8 TeV with the ATLAS detector, Phys.Lett.B 737 (2014) 223, arXiv: 1406.4456[hep-ex].Searches for heavy Z Z and ZW resonances in the qq and ννqq final states in pp collisions at √ s = 13 TeV with the ATLAS detector, (2017), arXiv: 1708.09638[hep-ex].Search for heavy resonances decaying to a W or Z boson and a Higgs boson in the q q( ) b b final state in pp collisions at √ s = 13 TeV with the ATLAS detector, Phys.Lett.B 774 (2017) 494, arXiv: 1707.06958[hep-ex].Search for new resonances decaying to a W or Z boson and a Higgs boson in the + − b b, νb b, and ν νb b channels with pp collisions at √ s = 13 TeV with the ATLAS detector, Phys.Lett.B 765 (2017) 32, arXiv: 1607.05621[hep-ex].Search for charged Higgs bosons produced via vector boson fusion and decaying into a pair of W and Z bosons using pp collisions at √ s = 13 TeV, Phys.Rev. Lett.119 (2017) 141802, arXiv: 1705.02942[hep-ex].TeV, Eur.Phys.J. C 73 (2013) 2304, arXiv: 1112.6426[hep-ex].[59] ATLAS Collaboration, Tagging and suppression of pileup jets with the ATLAS detector, ATLAS-CONF-2014-018, 2014, : https://cds.cern.ch/record/1700870.[60] ATLAS Collaboration, Jet Calibration and Systematic Uncertainties for Jets Reconstructed in the ATLAS Detector at √ s = 13 TeV, ATL-PHYS-PUB-2015-015, 2015, : https://cds.cern.ch/record/2037613.[61] ATLAS Collaboration, Performance of b-Jet Identification in the ATLAS Experiment, JINST 11 (2016) P04008, arXiv: 1512.01094[hep-ex].[62] ATLAS Collaboration, Measurements of b-jet tagging efficiency with the ATLAS detector using t t events at √ s = 13 TeV, (2018), arXiv: 1805.01845[hep-ex].[63] ATLAS Collaboration, Performance of missing transverse momentum reconstruction with the ATLAS detector in the first proton-proton collisions at √ s = 13 TeV, ATL-PHYS-PUB-2015-027, 2015, : https://cds.cern.ch/record/2037904.[64] ATLAS Collaboration, Expected performance of missing transverse momentum reconstruction for the ATLAS detector at √ s = 13 TeV, ATL-PHYS-PUB-2015-023, 2015, : https://cds.cern.ch/record/2037700.[65] ATLAS Collaboration, Measurements of W ± Z production cross sections in pp collisions at √ s = 8 TeV with the ATLAS detector and limits on anomalous gauge boson self-couplings, Phys.Rev. D 93 (2016) 092004, arXiv: 1603.02151[hep-ex].Also at Centre for High Performance Computing, CSIR Campus, Rosebank, Cape Town; South Africa.c Also at CERN, Geneva; Switzerland.d Also at CPPM, Aix-Marseille Université and CNRS/IN2P3, Marseille; France.e Also at Departament de Fisica de la Universitat Autonoma de Barcelona, Barcelona; Spain.f Also at Departamento de Fisica Teorica y del Cosmos, Universidad de Granada, Granada (Spain); Also at Departement de Physique Nucléaire et Corpusculaire, Université de Genève, Geneva; Switzerland.h Also at Department of Financial and Management Engineering, University of the Aegean, Chios; Also at Department of Physics and Astronomy, University of Louisville, Louisville, KY; United States of America.j Also at Department of Physics, California State University, Fresno CA; United States of America.k Also at Department of Physics, California State University, Sacramento CA; United States of America.l Also at Department of Physics, King's College London, London; United Kingdom.m Also at Department of Physics, Nanjing University, Jiangsu; China.n Also at Department of Physics, St. Petersburg State Polytechnical University, St. Petersburg; Russia.o Also at Department of Physics, Stanford University, Stanford CA; United States of America.