Neutrino Physics from the Cosmic Microwave Background and Large Scale Structure

TL;DR

Cosmology provides a powerful probe of neutrino properties through the cosmic neutrino background, with current data constraining the total neutrino mass and the effective number of relativistic species. The paper outlines a multi-probe forecast showing that Stage-IV CMB polarization experiments (CMB-S4) combined with large-scale structure surveys (eBOSS/DESI, LSST/Euclid) can measure $ sum m_ u$ to ~16 meV and $N_{ m eff}$ to ~0.020, enabling a high-significance detection of neutrino mass and tests of the standard cosmology with potential discovery of new light degrees of freedom. A normal mass hierarchy could be decisively distinguished from an inverted hierarchy, given the minimum sum of ~58 meV; deviations in $N_{ m eff}$ would indicate sterile neutrinos or dark radiation. The study also discusses systematics, multi-probe data combinations, and the detector and computing infrastructure required for CMB-S4 to realize these gains.

Abstract

This is a report on the status and prospects of the quantification of neutrino properties through the cosmological neutrino background for the Cosmic Frontier of the Division of Particles and Fields Community Summer Study long-term planning exercise. Experiments planned and underway are prepared to study the cosmological neutrino background in detail via its influence on distance-redshift relations and the growth of structure. The program for the next decade described in this document, including upcoming spectroscopic galaxy surveys eBOSS and DESI and a new Stage-IV CMB polarization experiment CMB-S4, will achieve sigma(sum m_nu) = 16 meV and sigma(N_eff) = 0.020. Such a mass measurement will produce a high significance detection of non-zero sum m_nu, whose lower bound derived from atmospheric and solar neutrino oscillation data is about 58 meV. If neutrinos have a minimal normal mass hierarchy, this measurement will definitively rule out the inverted neutrino mass hierarchy, shedding light on one of the most puzzling aspects of the Standard Model of particle physics --- the origin of mass. This precise a measurement of N_eff will allow for high sensitivity to any light and dark degrees of freedom produced in the big bang and a precision test of the standard cosmological model prediction that N_eff = 3.046.

Figures (8)

• Figure 0-1: Fractional change in the matter density power spectrum as a function of comoving wavenumber $k$ for different values of $\sum m_\nu$. Neutrino mass suppresses the power spectrum due to free streaming below the matter-radiation equality scale. The shape of the suppression is highly characteristic and precision observations over a range of scales can measure the sum of neutrino masses (here assumed all to be in a single mass eigenstate). Also shown are the approximate ranges of experimental sensitivity in the power spectrum for representative probes: the cosmic microwave background (CMB), galaxy surveys (Gal.), weak lensing of galaxies (WL), and the Lyman-alpha forest (Ly$\alpha$). The CMB lensing power spectrum involves (an integral over) this same power spectrum, and so is also sensitive to neutrino mass.
• Figure 0-2: Shown are the current constraints and forecast sensitivity of cosmology to the neutrino mass in relation to the neutrino mass hierarchy. In the case of an "inverted hierarchy," with an example case marked as a diamond in the upper curve, future combined cosmological constraints would have a very high-significance detection, with $1\sigma$ error shown as a blue band. In the case of a normal neutrino mass hierarchy with an example case marked as diamond on the lower curve, future cosmology would detect the lowest $\sum m_\nu$ at a level of $\sim 4 \sigma$. Also shown is the sensitivity from future long baseline neutrino experiments as the pink shaded band, which should be sensitive to the neutrino hierarchy at least at $3 \sigma$Adams:2013ita.
• Figure 0-3: Forecasted $1 \sigma$ and $2 \sigma$ constraints in the $\Sigma m_\nu - \Omega_m h^2$ plane showing the synergy of an experiment like DESI and a Stage-IV CMB lensing experiment. For contrast, a combination of $\it Planck$ data with the lensing information removed and DESI are shown in the red contours. The blue contours show the constraint generated by the CMB lensing experiment, corresponding to a $24$ meV constraint on massive neutrinos. The black contours show the result of adding only DESI BAO information, resulting in a 16 meV constraint. This can be compared to the case where no Stage-IV CMB lensing information but all galaxy clustering information is used, yielding a 24 meV constraint. The combination of a Stage-IV CMB experiment and BAO information from DESI should allow a robust measurement of the sum of the neutrino masses.
• Figure 0-4: The same as Figure \ref{['fig:Mnu-Omh2']}, but showing forecasts in the $\Sigma m_\nu$ - $N_{\rm eff}$ plane for a model including the effective number of neutrino species as a free parameter. A Stage-IV CMB experiment will not be able to distinguish between the standard model value of $N_{\rm eff}=3.046$ and the integer value of $3$ at high statistical significance, but it will indicate a preference for one over the other at the $\sim 2~\sigma$ level.
• Figure 0-5: The effect of massive neutrinos on the CMB lensing potential power spectrum $C_L^{\Phi \Phi}$. The fractional change in $C_L^{\Phi \Phi}$ for a given value of $\sum m_\nu$ is shown relative to the case for zero neutrino mass. Projected constraints on $C_L^{\Phi \Phi}$ for a Stage-IV CMB experiment are shown for $\sum m_\nu = 100 \ \mathrm{meV}$. Here we have approximated all of the neutrino mass to be in one mass eigenstate and fixed the total matter density $\Omega_m h^2$ and $H_0$. The $1\sigma$ constraint for $\sum m_\nu$ is approximately 45 meV for lensing alone and drops to 16 meV when combined with other probes.