5 veto and a muon veto are used to identify charged pions. The particle misidentification efficiencies are O (10 − $^{6}$) ( µ → e ), O (10 − $^{5}$) ( e → µ ) and O (10 − $^{1}$) ( ℓ → π ), where x y indicates that a particle of type x is misidenti- fied as a particle of type y . A photon candidate must deposit at least 50 MeV in the EMC and have a shower profile consistent with that expected from an electromag- netic shower. All pairs of photons with an invariant mass between 0.11 GeV and 0.16 GeV are selected as neutral pion candidates. The channel specific selection classifies events into one of the four signal channels. The momentum of the pri- mary lepton normalized to the beam energy is required to satisfy x ≡ | p 1 | /E$_{B}$ > 0 . 75. For the hadronic τ -decay channels, the momentum of the τ -daughter charged pion is required to satisfy | p 2 | /E$_{B}$ < 0 . 8. Since these τ de- cays to hadronic final states are dominated by the de- cays τ ± → ρ $^{±}$ν$_{τ}$ and τ ± → a ± 1 ν$_{τ}$ , the masses of the π $^{±}$π 0 and π $^{±}$π $^{0}$π0 systems are required to be consistent with the masses m$_{ρ}$ = 0 . 77 GeV and m$_{a}$$_{1}$ = 1 . 26 GeV, respectively, where the requirement on the π $^{±}$π $^{0}$π0 sys- tem mass is included only if there are two neutral pi- ons in the event. In order to suppress Bhabha events in which an electron is misidentified as a muon, for the leptonic eτ channel the τ -daughter muon is required to penetrate deeply into the muon detector. In order to suppress µ -pair events in which the tracks are back-to- back, for the leptonic µτ channel the tracks are required to satisfy Δ φ < 172 $^{◦}$, where Δ φ is the difference between the track azimuthal angles. After including all selection requirements, typical signal efficiencies determined from MC are (4 − 6)% [20], including the τ decay branching fractions. The typical number of events passing the se- lection criteria is (10 − 15) × 10 3 for Υ (2 S ) data and (20 − 30) × 10 3 for Υ (3 S ) data, depending on the signal channel. These yields are consistent with background expectations from MC simulations. After selection an unbinned extended maximum likeli- hood fit is performed to the distribution of the discrim- inant variable x . The signal peaks at x ≈ 0 . 97, while the τ -pair background x distribution is smooth and ap- proaches zero as x → x$_{MAX}$ , where x$_{MAX}$ ≈ 0 . 97 is the effective kinematic endpoint for the lepton momentum in the decay τ ± → ℓ ± ¯ ν$_{ℓ}$ν$_{τ}$ , boosted into the Υ ( nS ) rest- frame. The x distributions for the Bhabha/ µ -pair back- grounds have a peaking component near x = 1, about (2.5-3) σ$_{x}$ above the signal peak, where σ$_{x}$ ≈ 0 . 01 denotes the detector x resolution. The x distribution for the π - hadron background is smooth and falls off sharply near x = x$_{MAX}$ . Probability density functions (PDFs) for signal, τ -pair, Bhabha/ µ -pair and π -hadron backgrounds are determined as discussed below, and a PDF consist- ing of the sum of these components weighted by their yields is fitted to the data for each signal channel, with the yields of the components allowed to vary in the fit. The PDFs for signal and Bhabha/ µ -pair backgrounds To validate the fit procedure, we perform fits to data control samples in order to verify that signal yields con- are extracted from fits to the x distributions of MC events. The signal is modeled by a modified Gaussian with low- and high-energy tails, hereafter referred to as a double Crystal Ball [21] function, which peaks near x = 0 . 97. The Bhabha and µ -pair backgrounds have a threshold component truncating near x = 1, which is modeled by an ARGUS distribution [22], and a peaking component near x = 1, which is modeled by a Gaus- sian function. The π -hadron PDF is determined from data by modifying the selection to require that the pri- mary lepton is instead identified as a charged pion. The resulting binned x distribution is scaled by the proba- bility for pions to be misreconstructed as charged lep- tons, as measured in data, to yield a binned PDF for the π -hadron background. The yield of this component is fixed in the maximum likelihood fit and an uncertainty of 10% is assessed. The τ -pair background is modeled by the convolution of a polynomial, which vanishes above the kinematic endpoint x$_{MAX}$ , and a detector resolution function. The detector resolution function is modeled by a double Crystal Ball function whose shape is extracted from τ -pair MC events. Since the signal peaks in the region near the kinematic endpoint of the τ -pair back- ground x distribution, the signal yield depends strongly on x$_{MAX}$ , which must therefore be extracted from data. The value of this parameter is extracted from fits to the Υ (4 S ) data control sample and corrected for differences in the decay kinematics at the Υ (4 S ) vs. Υ ( nS ) res- onances. The polynomial shape parameters, which are not strongly correlated with the signal yield, are allowed to vary in the fits to Υ ( nS ) data. FIG. 1: Maximum likelihood fit results for the leptonic eτ channel in Υ (3 S ) data. The red dotted line represents the signal PDF, the green dashed line represents the sum of all background PDFs and the solid blue line represents the sum of these components. The inset shows a close-up of the region 0 . 95 < x < 1 . 02. The top plot shows the normalized residuals (data − fit) /σ data (pull). 0.75 0.8 0.85 0.9 0.95 1 1 10 2 10$^{3}$ 10 0.95 1 0 5 10 15 20 0.75 0.8 0.85 0.9 0.95 1 -2 0 2 /ndf=52.4/49) 2 (  e (3S) leptonic  x Events/0.005 Pull