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sideband fit for all events passing selection. The fit resid- uals, plotted in Figure 2, show that the exponential fit is an adequate model of the background shape with the present level of statistics. The statistical uncertainty on the fit results in a ≈ 5% uncertainty on the background expectation. 

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From the sideband fit, a total of 219 ± 10 events are expected in the signal region from the fit and 206 are ob- served. Since the data in the signal region are consistent with the expected background, we set a 95% C.L. upper limit on the σ ( p ¯ p → W ) × BR ( W ± → π $^{±}$γ ). We divide the signal region into four 2.5 GeV bins. The limits cal- 

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culated for each bin are then combined assuming that uncertainties across the bins are 100% correlated. This provides a gain in sensitivity by using information about the shape of the expected W ± → π $^{±}$γ mass peak. 

For the normalization measurement in the W ± → e $^{±}$ν channel we select events which have a central ( | η | < 1 . 2) electron with E$_{T}$ > 25 GeV and missing transverse en- ergy ̸ E$_{T}$ > 25 GeV [15]. The electron reconstruction and identification algorithms used in this analysis are dis- cussed in [10]. In addition, a standalone measurement of electron energy in the CES helps to identify electron candidates that radiate a significant fraction of their en- 

