Constraining the neutrino mass using a multi-tracer combination of two galaxy surveys and CMB lensing

Measuring the total neutrino mass is one of the most exciting opportunities available with next-generation cosmological data sets. We study the possibility of detecting the total neutrino mass using large-scale clustering in 21cm intensity mapping and photometric galaxy surveys, together with CMB information. We include the scale-dependent halo bias contribution due to the presence of massive neutrinos, and use a multi-tracer analysis in order to reduce cosmic variance. The multi-tracer combination of an SKAO-MID 21cm intensity map with Stage~4 CMB lensing dramatically shrinks the uncertainty on total neutrino mass to $\sigma(M_\nu) \simeq 45\,$meV, using only linear clustering information ($k_{\rm max} = 0.1\, h/$Mpc) and without a prior on optical depth. When we add to the multi-tracer the clustering information expected from LSST, the forecast is $\sigma(M_\nu) \simeq 12\,$meV.