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 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.
