(Upper panel) The $\pi^{\pm}$K$^{\mp}$ invariant mass distribution in $|y|$$< $0.5 for the bin 0.4 $<  \pt < $ 0.5 GeV/$c$ (left) and 0.9 $<  \pt < $ 1.0 GeV/$c$ (right), in pp collisions at 7 TeV. The background shape estimated using unlike-sign pairs from different events (event mixing) and like-sign pairs from the same event are shown as open red squares and full green squares, respectively (Lower panel). The $\pi^{\pm}$K$^{\mp}$ invariant mass distribution after like-sign background subtraction for 0.4 $<  \pt < $0.5 GeV/$c$ (left) and 0.9 $<  \pt < $ 1.0 GeV/$c$ (right). The solid curve is the result of the fit by Eq. 1, the dashed line describes the residual background.