Disentangling geometry & growth in cosmic neutrino mass constraints: a more physical split and diagnostic

Recent cosmological data hint at a neutrino mass sum below the minimum value allowed by the neutrino mass oscillation experiments, with a preference for the opposite of massive neutrinos (“negative” neutrino masses). Since massive neutrinos impact the expansion history of the Universe, which in turn modifies the growth of structure, we ask whether such a preference originates from the observed geometry or structure growth, and whether the two agree. In this paper, we introduce a more physical split of $\sum m_{\nu }$ into parameters $\sum m_{\nu }^{geo}$ and $\sum m_{\nu }^{growth}$ based on their physical effects, with extension to negative values. We split the expansion rate at the Boltzmann-code level: $\sum m_{\nu }^{geo}$ sets $H(z)$ for distance computation, and $\sum m_{\nu }^{growth}$ sets $H(z)$ for perturbation evolution. Combining Planck primary CMB, Planck+ACT+SPT CMB lensing and DESI BAO data, we find similar sensitivity and “negative” preference from geometry and growth with $\sum m_{\nu }^{geo}=-0.07\pm 0.10$ eV (68% C.L.) and $\sum m_{\nu }^{growth}=-0.13_{-0.12}^{+0.10}$ eV (68% C.L.). The difference of the two is a null test of the $\mathrm{\Lambda }$CDM model, passed by current data at $0.4\sigma$ level. Forecasts on Advanced SO CMB and DESI DR3 BAO show that $0.058$ eV would be rejected at $3.0\sigma$ from the geometry channel alone, if the negative mean of $\sum m_{\nu }^{geo}=-0.07$ eV persists. The two channels can also provide a powerful new diagnostic between possible origins of the neutrino mass tension. With future CMB and BAO data, our Fisher forecasts show that an incorrect dark energy model displaces $\sum m_{\nu }^{geo}$ from $0.058$ eV by $3.1$-$4.2\sigma$ and $\sum m_{\nu }^{growth}$ by $1.1$-$1.3\sigma$, while an underestimated reionization optical depth reverses the pattern, $2.7$-$3.6\sigma$ in $\sum m_{\nu }^{growth}$ against at most $0.23\sigma$ in $\sum m_{\nu }^{geo}$. (Abridged)