of flight axis. This distribution describes the decay of a spin-one particle ( W ) into a photon and a pseudoscalar pion averaged over W boson polarizations and summed over photon polarizations. The detector response is sim- ulated with a geant3 based simulation program [13]. 

The photon identification efficiency is determined us- ing MC simulated events and corrected using Z 0 → e $^{+}$e − data events, where only probe electrons with suppressed bremsstrahlung, | E/p − 1 | < 0 . 1, are selected. The method assumes that the detector response to photons is the same as that to non-radiating electrons. Compar- ing simulations of electron and photon showers in the CEM, we verified that the accuracy of this assumption is better than 1%. 

The identification efficiency for pions from the W ± → π $^{±}$γ decay is also calculated using MC simula- tion. The correction factor to this efficiency, which takes into account the effects of calorimeter shower mismodel- ing and instantaneous luminosity, is determined by com- paring properties of the reconstructed jets in photon+jet data to those in photon+jet MC. 

The total corrected acceptance times efficiency for π $^{±}$γ 

selection is A × ϵ = 0 . 0503 ± 0 . 0006( stat ) ± 0 . 0011( sys ). The uncertainty is dominated by the systematic uncer- tainty on the pion identification efficiency. 

The dominant backgrounds to this search come from photon+jet events, where the jet fragments into a sin- gle charged particle, and from multi-jet events, where one of the jets fragments into a single charged parti- cle and another is misidentified as a photon. Drell-Yan pair production and W/Z decays, especially to τ leptons, also contribute to the background at a level of ≈ 10%. The signal from the W ± → π $^{±}$γ decay would appear as a peak in the π $^{±}$γ invariant mass spectrum centered at the W boson mass with a resolution of 2.5 GeV /c $^{2}$, which includes the experimental resolution and the full width of the W boson, 2.1 GeV /c 2 [14]. We therefore define the signal region as 75 < M$_{π}$ $^{±}$γ < 85 GeV /c $^{2}$, which includes 90% of W ± → π $^{±}$γ decays. The background within the signal region is estimated by fitting the side- bands, 67 . 5 < M$_{π}$ $^{±}$γ < 75 GeV /c 2 and 85 < M$_{π}$ $^{±}$γ < 120 GeV /c $^{2}$, with an exponential function using a χ 2 mini- mization. 

Figure 1 shows the M$_{π}$ $^{±}$γ distribution as well as the 

