Universality and m X cut effects in B → X s ℓ + ℓ −

The most precise comparison between theory and experiment for the $B\ensuremath{\rightarrow}{X}_{s}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ rate is in the low ${q}^{2}$ region, but the hadronic uncertainties associated with an experimentally required cut on ${m}_{X}$ potentially spoil the search for new physics in these decays. We show that a 10%\char21{}30% reduction of $\mathrm{d}\ensuremath{\Gamma}(B\ensuremath{\rightarrow}{X}_{s}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}})/\mathrm{d}{q}^{2}$ due to the ${m}_{X}$ cut can be accurately computed using the $B\ensuremath{\rightarrow}{X}_{s}\ensuremath{\gamma}$ shape function. The effect is universal for all short distance contributions in the limit ${m}_{X}^{2}\ensuremath{\ll}{m}_{B}^{2}$, and this universality is spoiled neither by realistic values of the ${m}_{X}$ cut nor by ${\ensuremath{\alpha}}_{s}$ corrections. Both the differential decay rate and forward-backward asymmetry with an ${m}_{X}$ cut are computed.