> LHCb for example does not use simulated background. That depends on the analysis. If you're looking at a partially-reconstructed decay (common in semileptoni

> LHCb for example does not use simulated background. That depends on the analysis. If you're looking at a partially-reconstructed decay (common in semileptonics) then you can't rely on the regular trick of choosing a sideband sample to work as your combinatorial background. Also, it's very common to model specific misidentified or partially-reconstructed backgrounds using simulation. > In any case, the more complex your model gets (number of variables) the exponentially more simulated Monte Carlo events you need to fill that multidimensional space. I understand this argument if you're trying to model the efficiency in nD space (through splines, histograms, moments etc), but that's usually done when you're fitting to n variables, e.g. in an amplitude fit. If you just want the efficiency of a cut on the score from an MVA algorithm, I don't think it matters. What definitely matters is that the behaviour on simulation reproduces that of real signal as faithfully as possible.