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Constraining Models of Neutrino Mass and Neutrino Interactions with the Planck Satellite

Alexander Friedland, Kathryn M. Zurek, Sergei Bashinsky

TL;DR

This work analyzes how neutrino interactions with a light scalar can suppress neutrino free-streaming and introduce extra relativistic degrees of freedom, using cosmological data to constrain such scenarios. It introduces a parameterization in terms of $N_{FS}$ and $N_{coupled}$ and forecasts Planck's ability to detect or rule out self-coupled neutrino fluids, as well as to measure $N_ u^{eff}$ with high precision. Through MCMC and Fisher analyses, the authors show Planck will dramatically tighten bounds on neutrino–scalar couplings, potentially excluding a single coupled neutrino at $\sim4.2\sigma$ and probing MaVaN parameter space down to $m_\phi \sim 10^{-8}$ eV, with implications for neutrino dark energy and late-time mass generation. The results highlight the power of CMB observations to probe beyond-Standard-Model neutrino physics and complement terrestrial constraints, offering a path to testing TeV-scale neutrino mass-generation mechanisms.

Abstract

In several classes of particle physics models -- ranging from the classical Majoron models, to the more recent scenarios of late neutrino masses or Mass-Varying Neutrinos -- one or more of the neutrinos are postulated to couple to a new light scalar field. As a result of this coupling, neutrinos in the early universe instead of streaming freely could form a self-coupled fluid, with potentially observable signatures in the Cosmic Microwave Background and the large scale structure of the universe. We re-examine the constraints on this scenario from the presently available cosmological data and investigate the sensitivity expected from the Planck satellite. In the first case, we find that the sensitivity strongly depends on which piece of data is used. The SDSS Main sample data, combined with WMAP and other data, disfavors the scenario of three coupled neutrinos at about the 3.5$σ$ confidence level, but also favors a high number of freely streaming neutrinos, with the best fit at 5.2. If the matter power spectrum is instead taken from the SDSS Large Red Galaxy sample, best fit point has 2.5 freely streaming neutrinos, but the scenario with three coupled neutrinos becomes allowed at $2σ$. In contrast, Planck alone will exclude even a single self-coupled neutrino at the $4.2σ$ confidence level, and will determine the total radiation at CMB epoch to $ΔN_ν^{eff} = ^{+0.5}_{-0.3}$ ($1σ$ errors). We investigate the robustness of this result with respect to the details of Planck's detector. This sensitivity to neutrino free-streaming implies that Planck will be capable of probing a large region of the Mass-Varying Neutrino parameter space. Planck may also be sensitive to a scale of neutrino mass generation as high as 1 TeV.

Constraining Models of Neutrino Mass and Neutrino Interactions with the Planck Satellite

TL;DR

This work analyzes how neutrino interactions with a light scalar can suppress neutrino free-streaming and introduce extra relativistic degrees of freedom, using cosmological data to constrain such scenarios. It introduces a parameterization in terms of and and forecasts Planck's ability to detect or rule out self-coupled neutrino fluids, as well as to measure with high precision. Through MCMC and Fisher analyses, the authors show Planck will dramatically tighten bounds on neutrino–scalar couplings, potentially excluding a single coupled neutrino at and probing MaVaN parameter space down to eV, with implications for neutrino dark energy and late-time mass generation. The results highlight the power of CMB observations to probe beyond-Standard-Model neutrino physics and complement terrestrial constraints, offering a path to testing TeV-scale neutrino mass-generation mechanisms.

Abstract

In several classes of particle physics models -- ranging from the classical Majoron models, to the more recent scenarios of late neutrino masses or Mass-Varying Neutrinos -- one or more of the neutrinos are postulated to couple to a new light scalar field. As a result of this coupling, neutrinos in the early universe instead of streaming freely could form a self-coupled fluid, with potentially observable signatures in the Cosmic Microwave Background and the large scale structure of the universe. We re-examine the constraints on this scenario from the presently available cosmological data and investigate the sensitivity expected from the Planck satellite. In the first case, we find that the sensitivity strongly depends on which piece of data is used. The SDSS Main sample data, combined with WMAP and other data, disfavors the scenario of three coupled neutrinos at about the 3.5 confidence level, but also favors a high number of freely streaming neutrinos, with the best fit at 5.2. If the matter power spectrum is instead taken from the SDSS Large Red Galaxy sample, best fit point has 2.5 freely streaming neutrinos, but the scenario with three coupled neutrinos becomes allowed at . In contrast, Planck alone will exclude even a single self-coupled neutrino at the confidence level, and will determine the total radiation at CMB epoch to ( errors). We investigate the robustness of this result with respect to the details of Planck's detector. This sensitivity to neutrino free-streaming implies that Planck will be capable of probing a large region of the Mass-Varying Neutrino parameter space. Planck may also be sensitive to a scale of neutrino mass generation as high as 1 TeV.

Paper Structure

This paper contains 21 sections, 41 equations, 16 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Models of Neutrino-Scalar Interactions
  3. Tightly coupled neutrinos: modified evolution equations.
  4. Numerical results
  5. Sensitivity of the current data
  6. Literature overview
  7. WMAP3 alone
  8. WMAP3 plus large scale structure (2dF, SDSS), HST, SN Ia.
  9. Adding the Lyman-$\alpha$ data.
  10. Sensitivity of Planck
  11. Planck only
  12. Planck plus other cosmological data
  13. Discussion of the results and further numerical investigations.
  14. The role of other cosmological parameters
  15. The compensation mechanism and the nature of Planck's sensitivity
  16. ...and 6 more sections

Figures (16)

  • Figure 1: Effect of neutrino free-streaming on the CMB multipole spectrum. The thickest curve is the spectrum with the best fit parameters from WMAP3; the other curves (from bottom to top) correspond to 1, 2, and 3 strongly coupled neutrinos (keeping the total number of neutrinos fixed at 3).
  • Figure 2: Effect of extra neutrinos on the CMB multipole spectrum. The central black curve is the spectrum with the best fit parameters from WMAP3, the top green curve the spectrum with 7 freely streaming neutrinos, and the lowermost magenta curve results when the total number of freely streaming neutrinos is 7, but $z_{eq}$ is kept fixed by varying $h^2$.
  • Figure 3: The shift of the best-fit parameters as a result of neutrino coupling. The dashed-dotted curves correspond to the scenario of coupled neutrinos, while the solid ones refer to the standard (free streaming) neutrinos. In both cases, the fits are to the datasets from WMAP3, SDSS, 2dF, HST, and SN Ia.
  • Figure 4: The combined reach of all available data (excluding Lyman-$\alpha$) on the number of free-streaming neutrinos, $N_{FS}$, and the number of coupled neutrinos, $N_{coupled}$: WMAP3 + SDSS (LRG) + SDSS (Main) + 2dF + HST + SNIa. The solid contours indicate 1 and 2$\sigma$ C.L.
  • Figure 5: Sensitivity of WMAP3 + SDSS (LRG) + 2dF + HST + SNIa. The removal of the SDSS Main sample lowers the best fit point for $N_{FS}$ and signficantly weakens the constraints on a scenario with $N_{coupled} = 3$ and $N_{FS} = 0$.
  • ...and 11 more figures