  We present a detailed analysis of the radial distribution of light-element
multiple populations (LE-MPs) in the massive and dense globular cluster M80
based on the combination of UV and optical Hubble Space Telescope data.
Surprisingly, we find that first generation stars (FG) are significantly more
centrally concentrated than extreme second generation ones (SG) out to $\sim
2.5 r_h$ from the cluster center. To understand the origin of such a peculiar
behavior, we used a set of $N$-body simulations following the long-term
dynamical evolution of LE-MPs. We find that, given the advanced dynamical state
of the cluster, the observed difference does not depend on the primordial
relative distributions of FG and SG stars. On the contrary, a difference of
$\sim 0.05-0.10 M_{\odot}$ between the average masses of the two
sub-populations is needed to account for the observed radial distributions. We
argue that such a mass difference might be the result of the higher He
abundance of SG stars (of the order of $\Delta Y\sim 0.05-0.06$) with respect
to FG. Interestingly, we find that a similar He variation is necessary to
reproduce the horizontal branch morphology of M80. These results demonstrate
that differences in mass among LE-MPs, due to different He content, should be
properly taken into account for a correct interpretation of their radial
distribution, at least in dynamically evolved systems.
