19 the case that ΔLog$_{10}$(S2 / S1) fluctuations are Gaussian. That is, we use the Gauss fits to the ΔLog$_{10}$(S2 / S1) spec- trum in each of the seven energy bins to determine the energy-dependent discrimination power. The results are shown in Table II and Figure 34. Additionally, the ex- pected number of background events in the WIMP accep- tance window, N$_{leak}$ , is shown, calculated on the basis of predicted rejection and background rate in the 58.6 live- days exposure (see Section VII). The observed trend of the ER rejection power with en- ergy is unexpected. If recombination fluctuations were flat at all energies, or if the band widths were dominated by binomial fluctuations from light collection, photoelec- tron emission, etc., one would expect the band widths to grow at low energies, and hence the ER rejection power would deteriorate. The opposite is observed, and is due to two factors. First, the ER and NR bands themselves diverge slightly at lower energies, as first measured in [4]. Second, the width of the ER band does not grow at lower energies but instead remains relatively constant. Background events in XENON10 are dominated by electron recoils from radioactivity of detector materials. Due to the self-shielding of LXe, the background event rate falls dramatically from the edges to the center of the target. Based on Monte Carlo simulations and unblinded The XENON10 Dark Matter Search data covered the period between October 6th 2006 and February 14th 2007. Figure 36 shows the accumulation of measurement livetime, interrupted by calibration runs. The global trigger rate was about 2.6 Hz with more than 92% live- time. A total of 74.7 live-days of data were collected. Of this, 16.3 were selected to be analyzed in order to de- fine the event selections and cuts. The remaining 58.4 live-days were embargoed in order to carry out a blind analysis. The XENON10 results from this blind analysis have been published in [11, 12], and are referred to as blinded WIMP search data in the following. FIG. 35: (Color online) Decomposition of the ER band vari- ance. The recombination fluctuations are inferred by compar- ing the expected statistical and instrumental fluctuations to the full observed band variance. FIG. 34: The ER rejection as a function of S1 for ΔLog$_{10}$(S2 / S1) < µ . The rejection improves at lower en- ergies, to better than 99.9% in the range 2–3 keVee. TABLE II: The nuclear recoil acceptance A$_{nr}$ , the efficiency ϵ$_{cut}$ of remaining nuclear recoil events after the GammaX cuts (see section VII), and the electron recoil rejection efficiency R$_{er}$ for each of the seven energy bins. The predicted number of leakage events, N$_{leak}$ , is based on R$_{er}$ and the number of background events, N$_{evt}$ , in each energy bin, for the 58.6 live-days WIMP-search data. Errors are the statistical uncertainty from the Gaussian fits on the electron recoil ΔLog$_{10}$(S2 / S1) distribution. Figure 35 shows a decomposition of the ER band vari- ance, using assumed statistical and instrumental fluctu- ations. It is quite evident that uncorrelated statistical and instrumental fluctuations cannot alone account for the observed degree of variance. Unfortunately, a model does not yet exist that successfully predicts recombina- tion fluctuations in noble liquids, and hence more mea- surements are needed to reach a better understanding of the subject. E$_{nr}$ (keV) A$_{nr}$ ϵ$_{cut}$ 1 - R$_{er}$ N$_{evt}$ N$_{leak}$ (10 − $^{3}$) 4.5 - 6.7 0.446 0.94 0.8 +0 . 7 − 0 . 4 213 0.2 +0 . 2 − 0 . 1 6.7 - 9.0 0.458 0.90 1.7 +1 6 − 0 . 9 195 0.3 +0 3 − 0 . 2 9.0 - 11.2 0.457 0.89 1.1 +0 − 0 . 5 183 0.2 +0 − 0 . 1 11.2 - 13.4 0.442 0.85 4.1 +3 6 − 2 . 0 190 0.8 +0 7 − 0 . 4 13.4 - 17.9 0.493 0.83 4.2 +1 8 − 1 . 3 332 1.4 +0 6 − 0 . 4 17.9 - 22.4 0.466 0.80 4.3 +1 7 − 1 . 2 328 1.4 +0 5 − 0 . 4 22.4 - 26.9 0.446 0.77 7.2 +2 4 − 1 . 9 374 2.7 +0 9 − 0 . 7 Total 1815 7.0 +1 4 − 1 . 0 0 2 4 6 8 10 12 14 10 −4 10 −3 10 −2 10 −1 S1 [keVee] (2.2 p.e./keV) 1-Rejection 99.0% 99.9% 0 5 10 15 20 0.005 0.01 0.015 0.02 0.025 0.03 S1 [keVee] (2.2 p.e./keVee) ER Band Variance [Log 10 (S2 / S 1 )] 2 XENON10 d ata Statistical fl ucts Instr. flucts (from activatio n lines) Inferred Rec omb flucts VII. THE XENON10 DARK MATTER SEARCH DATA A. Background reduction with fiducial volume selection