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Hi, In order to estimate the low Mx shape of Vcb background with KL, I produced the following plots: http://www.slac.stanford.edu/~denardo/mxplots/klmasslepton.ps http://www.slac.stanford.edu/~denardo/mxplots/kstruemasslep.ps They both are from true Vcb events. The first plot is the MX distribution for events with at least a true KL. The second plot is again the MX distribution after subtraction of a reconstructed Ks. If more than one Ks is reco'ed, the one with the smallest delta m is chosen. For both the distribution no selection cut is applied, apart requiring a lepton in the fiducial acceptance. I naively expected to find similar distributions. Instead, the Ks looks too much distorted towards small values of MX. Tomorrow I will change the code to add all the selected Ks (one by one) to the distribution, but I expect that this will only add few more entries but it will not change the shape. Someone has any idea on why the MX ks-subtracted distribution should look so different from KL events Mx one? (I can't exclude a bug in my code). Tomorrow I will investigate further. Guglielmo