J-PAS: Forecasting constraints on Neutrino Masses

The large-scale structure survey J-PAS is taking data since October 2023. In this work, we present a forecast based on the Fisher matrix method to establish its sensitivity to the sum of the neutrino masses. We adapt the Fisher Galaxy Survey Code (FARO) to account for the neutrino mass under various configurations applied to galaxy clustering measurements. This approach allows us to test the sensitivity of J-PAS to the neutrino mass across different tracers, with and without non-linear corrections, and under varying sky coverage. We perform our forecast for two cosmological models: $ΛCDM + \sum m_ν$ and $w_0w_a CDM + \sum m_ν$. We combine our J-PAS forecast with Cosmic Microwave Background (CMB) data from the Planck Collaboration and Type Ia supernova (SN) data from Pantheon Plus. Our analysis shows that, for a sky coverage of 8,500 square degrees, J-PAS galaxy clustering data alone will constrain the sum of the neutrino masses to an upper limit at 95% C.L of $\sum m_ν< 0.32$ eV for the $ΛCDM + \sum m_ν$ model, and $\sum m_ν< 0.36$ eV for the $w_0w_a CDM + \sum m_ν$ model. When combined with Planck data, the upper limit improves significantly. For J-PAS+Planck at 95% C.L, we find $\sum m_ν< 0.061$ eV for the $ΛCDM + \sum m_ν$ model, and for J-PAS+Planck+Pantheon Plus, we obtain $\sum m_ν< 0.12$ eV for the $w_0w_a CDM + \sum m_ν$ model. These results demonstrate that J-PAS clustering measurements can play a crucial role in addressing challenges in the neutrino sector, including potential tensions between cosmological and terrestrial measurements of the neutrino mass, as well as in determining the mass ordering.

💡 Research Summary

This paper presents a Fisher‑matrix forecast for the upcoming J‑PAS (Javalambre Physics of the Accelerated Universe Survey) galaxy‑clustering measurements, focusing on their ability to constrain the sum of neutrino masses (∑ mν). Using the publicly available Fisher Galaxy Survey Code (FARO), the authors extend the code to incorporate massive neutrinos as a free parameter and to handle multiple tracers (LRGs, ELGs, QSOs) in a tomographic analysis. Two cosmological models are considered: the standard ΛCDM + ∑ mν and a dynamical dark‑energy model w₀wₐCDM + ∑ mν.

The methodology follows the standard Fisher formalism, assuming Gaussian likelihoods for the power‑spectrum observables. The observable vector consists of the three‑dimensional multi‑tracer power spectra Pδδab(z, μ, k) including Kaiser red‑shift‑space distortions, Gaussian redshift‑error convolution, and Alcock‑Paczynski scaling. Non‑linear scales are suppressed by an exponential cut‑off with Σ⊥(z)=0.785 D(z) Σ₀ and Σ∥(z)=0.785 D(z)(1+f(z)) Σ₀, and a conservative minimum wavenumber kmin=7×10⁻³ h Mpc⁻¹ is adopted. The covariance matrix combines signal and shot‑noise contributions, and the survey volume for each redshift bin is weighted by the sky fraction fsky corresponding to 8 500 deg².

Derivatives of the power spectrum with respect to cosmological parameters are computed analytically where possible; the remaining derivatives (especially those involving the matter power spectrum) are obtained numerically. A key innovation is the direct projection from model‑independent “P(k)‑bin” parameters to physical cosmological parameters via a transformation matrix Pαβ=∂pα/∂qβ, avoiding a two‑step Fisher calculation and reducing numerical errors.

The forecast results are as follows. For J‑PAS alone, the 95 % confidence upper limits on ∑ mν are 0.32 eV for ΛCDM + ∑ mν and 0.36 eV for w₀wₐCDM + ∑ mν. When combined with Planck 2018 temperature, polarization, and lensing data, the limits tighten dramatically: ΛCDM + ∑ mν yields ∑ mν < 0.061 eV, while w₀wₐCDM + ∑ mν gives ∑ mν < 0.10 eV (95 % CL). Adding the Pantheon Plus Type Ia supernova sample further relaxes the w₀wₐCDM bound to ∑ mν < 0.12 eV, but still provides a constraint competitive with the most stringent current cosmological limits.

The authors also explore the impact of non‑linear corrections (e.g., HALOFIT) and find that including them weakens the neutrino‑mass bound by roughly 5–10 %, without altering the overall conclusions. Convergence tests on k‑ and μ‑sampling, as well as on the number of redshift bins, confirm the stability of the Fisher matrix.

In the discussion, the paper emphasizes that J‑PAS’s multi‑tracer approach, combined with its narrow‑band photometric redshifts, yields high redshift‑precision clustering measurements that are especially sensitive to the free‑streaming suppression caused by massive neutrinos. The forecasted limits are comparable to, and in some cases better than, those projected for other upcoming surveys such as DESI or Euclid, particularly when J‑PAS data are combined with CMB information.

The results have important implications for the neutrino sector. The ΛCDM + ∑ mν bound of 0.061 eV is close to the minimum sum required by the normal hierarchy (≈ 0.06 eV) and well below the inverted‑hierarchy threshold (≈ 0.10 eV), suggesting that J‑PAS together with Planck could potentially discriminate between the two mass orderings. Moreover, the constraints are significantly tighter than those from terrestrial β‑decay experiments (e.g., KATRIN’s current limit of 0.45 eV), highlighting the complementary power of cosmology.

Finally, the authors outline future prospects. As J‑PAS continues to accumulate data over its full 8 500 deg² footprint and possibly expands to include higher‑redshift tracers, the statistical power will increase, potentially pushing the neutrino‑mass bound below 0.05 eV. Improvements in non‑linear modeling, inclusion of higher‑order statistics (bispectrum), and cross‑correlations with weak‑lensing surveys could further tighten the constraints and reduce model dependencies.

In summary, this work demonstrates that J‑PAS galaxy‑clustering measurements, especially when combined with Planck CMB data and supernova distances, will provide some of the most stringent cosmological limits on the sum of neutrino masses in the near future, offering a powerful tool to address the neutrino‑mass hierarchy problem and to test the consistency between cosmological and laboratory measurements.