4 actly two oppositely charged particles: a primary lepton, an electron (muon) for the Υ ( nS ) → e $^{±}$τ ∓ ( Υ ( nS ) → µ $^{±}$τ $^{∓}$) search, with momentum close to the beam energy E$_{B}$ = √$_{s/}$$_{2, and a secondary charged lepton or charged}$ pion from the τ decay. Here and in the following all quantities are defined in the CM frame unless otherwise specified. If the τ decays to leptons, we require that the primary and τ -daughter leptons are of different flavor. If the τ decays to hadrons, we require one or two additional neutral pions from this decay. These requirements on the identified particle types are necessary to suppress Bhabha and µ -pair backgrounds. Thus we define four signal chan- nels, consisting of leptonic and hadronic τ decay modes for the Υ ( nS ) → e $^{±}$τ ∓ and Υ ( nS ) → µ $^{±}$τ ∓ searches, hereafter referred to as the leptonic and hadronic eτ and µτ channels. The main source of background to our events comes from τ -pair production, for which the final state particles are the same as for the signal. There is a background contribution to the eτ channels from Bhabha events in which one of the electrons is misidentified, and to the µτ channels from µ -pair events in which one of the muons is misidentified or decays in flight, or an elec- tron is generated in a material interaction. An additional background consisting of events with multiple pions and possible additional photons (‘ π -hadron background’), in which a charged pion is misidentified as a lepton and the remaining particles pass the selection criteria for the τ decay products, contributes to the hadronic eτ and µτ channels. new physics (NP) [4–7]. Many extensions to the SM, in- cluding supersymmetry and models with leptoquarks or compositeness, predict an enhancement in the rates for these processes at levels close to experimental sensitiv- ity. There have been considerable efforts in searches for CLFV in decays of particles such as µ and τ leptons and B and K mesons, but CLFV in the Υ sector remains rel- atively unexplored [8]. By using unitarity considerations, limits on CLFV τ branching fractions [9] have been used to place indirect limits on CLFV Υ branching fractions at the O (10 − $^{3}$) level [10]. In this letter we describe a search for CLFV Υ decays, which is a thousand times more sen- sitive than these indirect limits, using data collected with the B A B AR detector at the PEP-II B factory at SLAC Na- tional Accelerator Laboratory. Since these decays are in general mediated by new particles produced off-shell in loops, their measurement probes mass scales up to the TeV scale, far exceeding the e $^{+}$e − center-of-mass (CM) collision energy √$_{s}$ = M$_{Υ}$$_{(}$$_{nS}$$_{)}$ = O (10 GeV) [11]. There- fore, this analysis provides a NP probe which is comple- mentary to direct searches ongoing at the Tevatron and to be performed at the Large Hadron Collider. Assuming that the partial widths for CLFV Υ decays are comparable at the Υ (2 S ), Υ (3 S ) and Υ (4 S ) reso- nances, the branching fractions for rare decays of the narrow Υ ( nS ) resonances (henceforth n ≡ 2 , 3) are en- hanced by approximately Γ$_{Υ}$$_{(4}$$_{S}$$_{)}$ / Γ$_{Υ}$$_{(}$$_{nS}$$_{)}$ = O (10 $^{3}$) [12] with respect to those of the Υ (4 S ). We search for the CLFV decays Υ ( nS ) → e $^{±}$τ ∓ and Υ ( nS ) → µ $^{±}$τ ∓ [13], while the decay Υ ( nS ) → e $^{±}$µ∓ is constrained by uni- tarity considerations to be less than O (10 − $^{8}$) [10]. No signal is expected in data collected at the Υ (4 S ) since the CLFV branching fractions are strongly suppressed, or in data collected away from the Υ resonances, since this data contains very few Υ decays. We search for CLFV in a sample of (98 . 6 ± 0 . 9) × 10 6 Υ (2 S ) decays and (116 . 7 ± 1 . 2) × 10 6 Υ (3 S ) decays corresponding to integrated luminosities of 13.6 fb − 1 and 26.8 fb − $^{1}$, respec- tively. Data collected at the Υ (4 S ) after the upgrade of the muon detector system (77.7 fb − $^{1}$) and data collected 30 MeV below the Υ (2 S ) and Υ (3 S ) resonances (off-peak data corresponding to 2.6 fb − 1 and 1.3 fb − $^{1}$, respectively) constitute control samples that are used to validate the fit procedure. An additional data control sample col- lected at the Υ (3 S ) resonance (1.2 fb − $^{1}$) is used in a preliminary unblinded analysis to validate the analysis procedure and to ensure agreement between data and events simulated using Monte Carlo (MC) techniques. Simulated background processes consisting of continuum QED events [14, 15] and generic Υ ( nS ) decays, as well as signal Υ ( nS ) → ℓ $^{±}$τ ∓ ( ℓ ≡ e, µ ) decays [16], are pro- duced and analyzed to optimize the fit procedure. The Geant4 [17] software is used to simulate the interac- tions of particles traversing the B A B AR detector, which is described in detail elsewhere [18]. The signature for Υ ( nS ) → ℓ $^{±}$τ ∓ events consists of ex- Particle identification is performed using a multivari- ate analysis [19] which uses measurements from all of the detector subsystems. An electron selector and a muon veto, combined with the requirement that the particle falls within the angular acceptance of the electromag- netic calorimeter (EMC), are used to identify electrons. A muon selector and an electron veto are used to iden- tify muons, while a charged pion selector, an electron In order to reduce background, we first apply require- ments common to all the decay modes and then a channel specific selection. All events are required to have ex- actly two tracks of opposite charge, both consistent with originating from the primary interaction point and with opening angle greater than 90 $^{◦}$. To suppress Bhabha and µ -pair backgrounds, we require that M$_{vis}$$_{/}$√$_{s <}$ 0 . 95, where M$_{vis}$ is the invariant mass of the sum of the 4- vectors of the two charged particles and of all photon candidates in the event. To ensure that the missing mo- mentum is not pointing toward the holes in the detec- tor near the beamline, we require that cos( θ lab $_{miss}$) < 0 . 9 and cos( θ CM $_{miss}$) > − 0 . 9, where θ lab miss ( θ CM $_{miss}$) is the po- lar angle of the missing momentum in the lab (CM) frame. To suppress two-photon processes, we require that ( p 1 + p $_{2}$)$_{⊥}$ / $_{(}$√$_{s}$ − | p 1 | − | p 2 | ) > 0 . 2, where p 1 and p 2 are the momenta of the two charged particles and ⊥ indi- cates the transverse component with respect to the beam axis.