Search for electroweak supersymmetric particle production in final states with two leptons and missing transverse momentum with the ATLAS detector

Searches for the production of electroweak supersymmetric particles decaying into final states with exactly two isolated, oppositely-charged leptons (electrons, muons), no reconstructed jets and missing transverse momentum are performed using 20.3 fb-1 of 2012 proton-proton collision data at sqrt(s) = 8 TeV recorded with the general purpose detector ATLAS at the Large Hardon Collider. In the absence of any significant excess with respect to the prediction from Standard Model processes, the results are interpreted in the framework of simplified Supersymmetry models.


Introduction
Weak scale Supersymmetry (SUSY) is one of the best motivated extensions of the Standard Model (SM), providing a possible solution to the hierarchy problem and a viable dark matter candidate in the form of the lightest supersymmetric particle (LSP). The dominant SUSY production channels at the LHC depend on the masses of the sparticles. In scenarios where the first and second generation sfermions and gluinos are heavier than few TeVs, direct production of weak gauginos (charginos, χ ± and neutralinos,χ 0 ) as well as sleptons (˜ andν) may be the dominant SUSY process. The searches presented here target final states with two leptons and missing energy. They can appear in˜ ˜ production followed bỹ ± → ±χ0 1 decay giving rise to a pair of "same flavour" leptons, or byχ ± 1χ ∓ 1 production followed byχ ± 1 → (˜ ± ν or ±ν ) → ± νχ 0 1 decay to leptons of same or different flavours and two additional neutrinos contributing to the missing transverse momentum. In scenarios with a lightest charginoχ ± 1 heavier than the LSP, the chargino decays asχ ± 1 → W ±χ0 1 , producing an on-or off-shell W Email address: janet.dietrich@cern.ch (Janet Dietrich) 1 on behalf of the ATLAS collaboration boson. Only if theχ ± 1 andχ 0 2 are mass degenerated and co-NLSP, the directχ ± 1χ 0 2 production is followed by the decaysχ ± 1 → W ±χ0 1 andχ 0 2 → Zχ 0 1 . This final state contains two oppositely charged leptons, two hadronic jets, and missing transverse momentum from the leptonic Z boson and hadronic W boson decay, respectively.

Event selection
Seven signal regions (SRs) are designed selecting final states with two isolated leptons (electrons or muons) of opposite charge and missing transverse momentum (for details see [1]). The three SR-m T2 signal regions are optimised to provide sensitivity to sleptons either through direct production or in chargino decays, while the three SR-WW signal regions are targeting charginoand neutralino-pair production followed by on-shell W decays. For all six signal region a jet-veto (including b-tagged jets) was applied, while SR-Zjets, modelled specifically for chargino and second lightest neutralino associated production followed by hadronic W and leptonic Z decays, required two leading central light jets in the final state. Events containing one or more τ-jet candidates are rejected.

Background estimation
The main SM backgrounds for SR-m T2 and SR-WW are from WW diboson and top-pair production where two leptonically decaying W bosons result in the same final state as the SUSY signal. Another significant source of background in the same-flavour channel is WZ and ZZ production. These events are estimated by defining dedicated control regions (CR) for each background and extracting a normalization factor to be applied to the simulations in the signal regions. Other minor backgrounds such as Z + jets and Higgs production are estimated using the MC predictions. For SR-Zjets, the dominant sources of background are ZV production, where V = W or Z, and Z/γ * + jets. The former is estimated from simulation, validated using ZV-enriched control samples, and the latter is estimated by a datadriven technique ('jet smearing' method). Leptons originating from heavy-flavour decays or photon conversion or mistakenly reconstructed hadronic jets can be misidentified as signal leptons. This "fake" background is obtained in a fully data-driven way (matrix method).

Results
Fig . 1 shows the comparison between data and the SM prediction for a key kinematic variable m T2 in signal regions. No significant excesses over the SM predictions are observed. Exclusion limits at 95% confidencelevel are set on the slepton, chargino and neutralino masses within the specific scenarios considered using a CL s limit-setting procedure [2].
[GeV] ∓ 1 pair production with sleptons and sneutrinos decays in the (mχ± 1 , mχ0 1 ) plane (b). The solid band around the expected limit shows the ±1σ range. Illustrated also are the LEP limits [3] on the mass of the right-handed smuonμ R in (a) and on the mass of the chargino in (b). The blue line indicates the limit from the previous analysis with the 7 TeV data [1,3] The results are shown in Fig. 2 and 3. In each exclusion plot, the solid (dashed) lines show observed (expected) exclusion contours, including all uncertainties except for the theoretical signal cross-section uncertainty arising from the PDF and the renormalization and factorization scales. The solid band around the expected exclusion contour shows the ±1σ result where all uncertainties, except those on the signal cross-sections, are considered. In scenarios with direct sleptons decays, a common value for left-and right-handed slepton masses between 90 GeV and 325 GeV is excluded at 95% confidence level for a massless neutralino (Fig. 2 a). The sensitivity decreases as the˜ -χ 0 1 mass splitting decreases: For mχ 0 1 = 100 GeV, common left and right-handed slepton masses between 160 GeV and 310 GeV are excluded. For models with chargino-pair production, with wino-like charginos decaying into the lightest neutralino via an intermediate slepton, chargino masses between 140 GeV and 465 GeV for a massless neutralino (Fig. 2 b). The exclusion for this decay depends on the assumed slepton mass, which is chosen to be halfway between theχ ± 1 andχ 0 1 masses in this analysis and its choice minimizes (maximizes) the acceptance for small (large)χ ± 1 -χ 0 1 mass splitting. In case of chargino-pair production with W boson decays, the best sensitivity is obtained for mχ 0 1 = 0: chargino mass ranges of 100-105 GeV, 120-135 GeV and 145-160 GeV are excluded at 95% CL. For simplified-modelχ ± 1χ 0 2 production followed by W and Z decays,χ ± 1 andχ 0 2 masses between 180 GeV and 355 GeV are excluded at 95% CL for a masslessχ 0 1 . Combined with results from the relevant signal regions in the ATLAS search for electroweak SUSY production in the three-lepton final states [5] improves significantly the sensitivity: Degenerateχ ± 1 andχ 0 2 masses between 100 GeV and 415 GeV are excluded for mχ 0 1 = 0. The results can be also interpreted for phenomenological Minimal Supersymmetric Standard Models (pMSSM). Fig. 3 shows some of the 95% CL exclusion regions in the pMSSM μ − M 2 plane for (a) a scenario with right-handed sleptons with m˜ R = (mχ 0 1 + mχ 0 2 )/2 (M 1 = 250 GeV, tan β = 6) and (b) for a scenario with heavy sleptons (M 1 = 50 GeV, tan β = 10). The exclusion regions are obtained by combining the results of this analysis with those from the ATLAS three-lepton search [5].  2 )/2 with M 1 = 250 GeV, tan β = 6 (a) and with very large slepton masses with M 1 = 50 GeV, tan β = 10 (b). The areas covered by the −1σ expected limit are shown in green. The exclusion regions are obtained by combining the results of this analysis with those from the ATLAS three-lepton search [5]. The dash-dotted lines indicate the masses ofχ ± 1 andχ 0 1 . Also shown are the previously reported exclusion regions by AT-LAS and the LEP limits [3] on the mass of the chargino.

References
[2] A. L. Read, Presentation of search results: The CL s technique, J.