Search for Supersymmetry via Associated Production of Charginos and Neutralinos in Final States with Three Leptons

A search for associated production of charginos and neutralinos is performed using data recorded with the D0 detector at a ppbar center-of-mass energy of 1.96 TeV at the Fermilab Tevatron Collider. This analysis considers final states with missing transverse energy and three leptons, of which at least two are electrons or muons. No evidence for supersymmetry is found in a dataset corresponding to an integrated luminosity of 320 pb-1. Limits on the product of the production cross section and leptonic branching fraction are set. For the minimal supergravity model, a chargino lower mass limit of 117 GeV at the 95% C.L. is derived in regions of parameter space with enhanced leptonic branching fractions.

(Dated: April 18, 2005) A search for associated production of charginos and neutralinos is performed using data recorded with the DØ detector at a pp center-of-mass energy of 1.96 TeV at the Fermilab Tevatron Collider. This analysis considers final states with missing transverse energy and three leptons, of which at least two are electrons or muons. No evidence for supersymmetry is found in a dataset corresponding to an integrated luminosity of 320 pb −1 . Limits on the product of the production cross section and leptonic branching fraction are set. For the minimal supergravity model, a chargino lower mass limit of 117 GeV at the 95% C.L. is derived in regions of parameter space with enhanced leptonic branching fractions. PACS numbers: 14.80.Ly,13.85.Rm,12.60.Jv Supersymmetry (SUSY) predicts the existence of a new particle for each of the standard model particles, differing by half a unit in spin but otherwise sharing the same quantum numbers. No supersymmetric particles have been observed so far, and it is therefore generally assumed that they are heavier than their standard model partners. Experiments at the CERN LEP Collider have set lower limits on the masses of SUSY particles, excluding in particular charginos with masses below 103.5 GeV as well as sleptons with masses below about 95 GeV [1] in the framework of the minimal supersymmetric model. Due to its high center-of-mass energy of 1.96 TeV, the Tevatron pp collider may produce SUSY particles with masses above these limits. A search for SUSY can be performed via the associated production of charginos and neutralinos. The lightest charginoχ ± 1 and the secondlightest neutralinoχ 0 2 are assumed to decay via exchange of vector bosons or sleptons into the lightest neutralino and standard model fermions. Assuming conservation of R-parity, the lightest neutralino is stable and can only be detected indirectly.
This Letter reports on a search for pp →χ ± 1χ 0 2 in final states with missing transverse energy and three charged leptons (e, µ or τ ), of which at least two are electrons or muons. The analysis is based on a dataset recorded with the DØ detector between March 2002 and July 2004, corresponding to an integrated luminosity of 320 pb −1 . Previous searches in this channel have been performed by the CDF and DØ collaborations with Tevatron Run I data [2].
The DØ detector consists of a central tracking system surrounded by a liquid-argon sampling calorimeter and a system of muon detectors [3]. Charged particles are reconstructed using multiple layers of silicon detectors as well as eight double layers of scintillating fibers in the 2 T magnetic field of a superconducting solenoid. The DØ calorimeter provides hermetic coverage up to pseudorapidities |η| ≈ 4 in a semi-projective tower geometry with longitudinal segmentation. After passing through the calorimeter, muons are detected in three layers of tracking detectors and scintillation counters.
Events containing electrons or muons are selected for offline analysis by a real-time three-stage trigger system. A set of single and dilepton triggers has been used to tag the presence of electrons and muons based on their characteristic energy deposits in the calorimeter, the presence of high-momentum tracks in the tracking system, and hits in the muon detectors. SUSY and standard model processes are modeled using the pythia [4] Monte Carlo (MC) generator and a detailed simulation of the detector geometry and response based on geant [5]. Multiple interactions per crossing as well as pile-up of signals in the calorimeter have been simulated. The MC events are then processed using the same reconstruction and analysis programs that are used for the data. The background predictions are normalized using cross-section calculations at next-to-leading order (NLO) and next-to-NLO (for Drell-Yan production) with T and p ℓ2 T are electron and muon p T , respectively. b Opposite-sign muons only. c Using p ℓ2 T instead of p ℓ3 T .
CTEQ6.1M [6] parton distribution functions (PDFs). Background from multijet production is estimated from data. For this, samples dominated by multijet background have been defined that are identical to the search sample except for reversed lepton identification requirements. These samples are normalized at an early stage of the selection in a region of phase space dominated by multijet production.
Selection criteria are optimized to obtain the best average expected limit assuming that no signal will be observed. Limits are calculated at the 95% C.L. using the modified frequentist approach [7]. The optimization of selection cuts is based on signals inspired by minimal supergravity (mSUGRA) [8] withχ ± 1 ,χ 0 2 and slepton masses in the range 110-130 GeV. Due to the large production cross section and leptonic branching fraction via slepton exchange, this mass range is of particular interest for a search in the trilepton channel. In the following discussion of the selection, as a representative example, a signal is used with common scalar mass m 0 = 84 GeV, common fermion mass m 1/2 = 176 GeV, ratio of Higgs vacuum expectation values tan β = 3, Higgs mass parameter µ > 0, and no slepton mixing, which corresponds to a chargino mass of 110 GeV and σ × BR(3ℓ) = 0.265 pb.
Four different selections are defined depending on the lepton content of the final state: two electrons plus lepton (eeℓ selection); two muons plus lepton (µµℓ); two muons of the same charge (µ ± µ ± ); and one electron, one muon plus lepton (eµℓ). The selection criteria are summarized in Table I and are discussed in more detail below.
Isolated electrons are identified based on their characteristic energy deposition in the calorimeter, including the fraction of energy deposited in the electromagnetic portion of the calorimeter and their transverse shower profile inside a cone of radius ∆R = (∆φ) 2 + (∆η) 2 < 0.4 around the direction of the electron. In addition it is required that a track points to the energy deposition in the calorimeter and that its momentum and the calorimeter energy are consistent with the same electron energy. Remaining backgrounds from jets and photon conversions are suppressed based on the track activity within ∆R = 0.4 around the track direction and by requiring the track associated with electron candidates to have associated hits in the innermost layers of the silicon detector.
Muons are reconstructed by finding tracks that point to patterns of hits in the muon system. Non-isolated muons from backgrounds with heavy-flavor jets are rejected by requiring the sum of track p T inside a cone with ∆R = 0.5 around the muon direction to be less than 4 GeV (loose muons) or less than 2.5 GeV (tight muons). Tight muons are also required to have less than 2.5 GeV deposited in the calorimeter in a hollow cone 0.1 < ∆R < 0.4 around the muon direction.
Electron and muon reconstruction efficiencies have been measured using leptonic Z boson decays collected by single-lepton triggers. The electron and muon trigger efficiencies have been measured in data and translate to an event trigger efficiency close to 100% (85%) for signal events passing offline analysis requirements in the eeℓ, eµℓ (µ ± µ ± , µµℓ) selections.
Each selection requires two identified leptons with minimum transverse momenta p ℓ1 T and p ℓ2 T , using one loose muon for the eµℓ, one loose and one tight muon for the µµℓ and two tight muons for the µ ± µ ± selection. Further selection cuts exploiting the difference in event kinematics and topology are applied as summarized in Table I. Di-electron and di-muon backgrounds from Drell-Yan and Z boson production as well as multijet background are suppressed using cuts on the invariant dilepton mass m ℓℓ as well as the azimuthal opening angle ∆φ ℓℓ . As illustrated in Fig. 1, a large fraction of these events can be rejected by removing events containing lep- tons that are back-to-back in azimuthal angle as well as events with m ℓℓ close to the Z boson mass.
A further reduction in dilepton and multijet backgrounds can be achieved by requiring missing transverse energy E T in an event. This is calculated as the vectorial sum of energy depositions in calorimeter cells and then adjusted using energy response corrections for reconstructed electrons, muons and jets. Jets are defined using an iterative seed-based cone algorithm, clustering calorimeter energy within ∆R = 0.5. The jet energy calibration has been determined from transverse momentum balance in photon plus jet events. For background events, an imbalance in transverse energy can be generated by mismeasurements of jet or lepton energies. Therefore, events in which the E T direction is aligned with the lepton are removed using a cut on the minimum transverse mass m min T = min(m ℓ1, ET T , m ℓ2, ET T ) as shown in Fig. 2. In addition, events are rejected if they contain jets with transverse energies above 15 GeV and have a small significance Sig( E T ), which is defined by normalizing the E T to σ(E j T E T ), a measure of the jet energy resolution projected onto the E T direction: Most of the remaining background from tt production can be rejected by removing events with large H T , defined as the scalar sum of the transverse energies of all jets with E T > 15 GeV. The presence of the third lepton in signal events can be used for further separation from the background by requiring events to have a third, isolated and well-measured track originating from the same vertex as the two identified leptons. To maximize signal yield, no additional lepton identification cuts are applied. The track (calorimeter) isolation conditions for this third track have been designed to be efficient for all lepton flavors, including hadronic decays of τ leptons, by allowing for tracks (energy deposits) inside an inner cone of ∆R < 0.1 (∆R < 0.2). The distribution of the transverse momentum p ℓ3 T of the isolated track is shown in Fig. 2 for the eeℓ selection. Except for WZ events, a third track in background events generally originates from the underlying event or jets, and therefore tends to have very low transverse momentum. WZ events are suppressed by removing events where the third track and one of the identified leptons have an invariant mass m ℓ 1,2 ℓ 3 consistent with the Z boson mass M Z . For the µ ± µ ± selection, backgrounds are low enough such that the requirement of a third track is not needed. Instead, background from WZ → µ ± νµ ± µ ∓ is removed by vetoing events containing opposite-sign muons with an invariant mass close to M Z . For the µµℓ selection on the other hand, a significant amount of multijet background remains; this is reduced by requiring that the vectorial sum |Σ pT | of E T and muon transverse momenta balances the transverse momentum of the third track.
Finally, a combined cut on the product of E T and p ℓ3 T (p ℓ2 T for µ ± µ ± ) has been found to optimally reduce the remaining background, which tends to have both low E T and low p ℓ3 T . The expected number of events for background and the reference signal defined above is summarized in Table II at various stages of the selection. After all cuts, the expected background is dominated by multijet background (66% and 53% for the µµℓ and µ ± µ ± selections, respectively) and di-boson backgrounds (80% and 88% for eeℓ and eµℓ).
The estimate for expected number of background and signal events depends on numerous measurements that each introduce a systematic uncertainty: integrated luminosity (6.5%), trigger efficiencies (1-2%), lepton identification and reconstruction efficiencies (1-2%), jet energy scale calibration in signal (< 4%) and background events (7-20%), lepton and track momentum calibration (1%), detector modeling (2%), PDF uncertainties (< 4%), and modeling of multijet background (4-40%). The uncertainties quoted in Table II in addition contain the statistical uncertainty due to limited MC statistics, which is the dominant uncertainty for backgrounds from W and Z boson production.
As can be seen in Table II, the numbers of events observed in the data are in good agreement with the expectation from standard model processes at all stages of the selection. Combining all four selections, a total background of 2.93±0.54(stat)±0.57(syst) events is expected after all cuts, while 3 events are observed in the data.
Since no evidence for associated production of charginos and neutralinos is observed, an upper limit on the product of production cross section and leptonic branching fraction σ × BR(3ℓ) is extracted from this result. As mentioned above, information from the four selections is combined using the modified frequentist approach, taking into account correlated errors. The small fraction of signal events that is selected by more than one selection has been assigned to the selection with the largest signal-to-background ratio and removed from all others.
The expected and observed limits are shown in Figs. 3 and 4 as a function of chargino mass and of the difference between chargino and slepton masses, respectively. This result improves significantly the upper limit of about 1.5 pb set by the DØ Run I analysis [2]. Assuming the mSUGRA-inspired mass relation mχ± 1 ≈ mχ0 2 ≈ 2mχ0 1 as well as degenerate slepton masses ml (no slepton mixing), the limit on σ×BR(3ℓ) is a function of mχ± 1 and ml, with a relatively small dependence on the other SUSY parameters. This result can therefore be interpreted in more general SUSY scenarios, as long as the above mass relations are satisfied and R-parity is conserved. The lep- tonic branching fraction of chargino and neutralino depends on the relative contribution from the slepton-and W/Z-exchange graphs, which varies as a function of the slepton masses. W/Z exchange is dominant at large slepton masses, resulting in relatively small leptonic branching fractions (large-m 0 scenario). The leptonic branching fraction for three-body decays is maximally enhanced for ml mχ0 2 (3 ℓ-max scenario). Decays into leptons can even be dominant if sleptons are light enough that two-body decays are possible. In the latter case, one of the leptons from the neutralino decay can have a very low transverse momentum if the mass difference between neutralino and sleptons is small. In this region, only the µ ± µ ± selection remains efficient, leading to a higher limit for −6 ml − mχ0 2 < 0 GeV (see Fig. 4). In addition, theχ ± 1χ 0 2 production cross section depends on the squark masses due to the negative interference with the t-channel squark exchange. Relaxing scalar mass unification, the cross section is maximal in the limit of large squark masses (heavy-squarks scenario). The NLO prediction [9] for σ × BR(3ℓ) for these reference scenarios is shown in Figs. 3 and 4. The cross-section limit set in this analysis corresponds to a chargino mass limit of 117 GeV (132 GeV) in the 3ℓ-max (heavy-squarks) scenario, which improves on the mass limit set by chargino searches at LEP.
In summary, no evidence for supersymmetry is observed in a search for associated chargino and neutralino production in trilepton events. Upper limits on the product of cross section and leptonic branching fraction are set, which improve previous limits set with the Run I dataset. Chargino mass limits beyond the reach of LEP chargino searches are derived for several SUSY reference scenarios with enhanced leptonic branching fractions.
We thank the staffs at Fermilab and collaborating institutions, and acknowledge support from the DOE and NSF (