Ratio of the $\pT$-differential production cross section of muons from heavy-flavour hadron decays in $3.7 < y < 4$ to that in $2.5 < y < 2.8$ in pp collisions at $\sqrt s$ = 5.02 TeV. Statistical uncertainties (bars) and systematic uncertainties (boxes) are shown. The ratio is compared with FONLL predictions . See the text for details.