Equivalent Neutrinos, Light WIMPs, and the Chimera of Dark Radiation

Abstract

According to conventional wisdom, in the standard model (SM) of particle physics and cosmology the effective number of neutrinos is Neff=3 (more precisely, 3.046). In extensions of the standard model allowing for the presence of DeltaNnu equivalent neutrinos (or dark radiation), Neff is generally >3. The canonical results are reconsidered here, revealing that a measurement of Neff>3 can be consistent with DeltaNnu=0 (dark radiation without dark radiation). Conversely, a measurement consistent with Neff=3 is not inconsistent with the presence of dark radiation (DeltaNnu>0). In particular, if there is a light WIMP that annihilates to photons after the SM neutrinos have decoupled, the photons are heated beyond their usual heating from e+- annihilation, reducing the late time ratio of neutrino and photon temperatures (and number densities), leading to Neff<3. This opens the window for one or more equivalent neutrinos, including sterile neutrinos, to be consistent with Neff=3. By reducing the neutrino number density at present, this allows for more massive neutrinos, relaxing the current constraints on the sum of the neutrino masses. In contrast, if the light WIMP only couples to the SM neutrinos and not to the photons, its late time annihilation heats the neutrinos but not the photons, resulting in Neff>3 even in the absence of equivalent neutrinos or dark radiation. A measurement of Neff>3 is thus no guarantee of the presence of equivalent neutrinos or dark radiation. In the presence of light WIMPs and/or equivalent neutrinos there are degeneracies among the light WIMP mass and its nature (fermion or boson, as well as its couplings to neutrinos or photons), the number and nature (fermion or boson) of the equivalent neutrinos, and their decoupling temperature (the strength of their interactions with the SM particles). There's more to a measurement of Neff than meets the eye.

revised version, more figures, more references, 21 pages, 13 figures

References in corpus (6)

• The WIMPless Miracle: Dark Matter Particles without Weak-scale Masses or Weak Interactions
• A Natural Supersymmetric Model with MeV Dark Matter
• Constraints on Light Dark Matter From Core-Collapse Supernovae
• MeV Dark Matter and Small Scale Structure
• Sterile Neutrinos and Light Dark Matter Save Each Other
• Constraining Fundamental Physics with Future CMB Experiments

Cited by in corpus (63)

• The Phenomenology of Right Handed Neutrinos
• The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics
• A Lower Bound on the Mass of Cold Thermal Dark Matter from Planck
• Constraints on Large-Scale Dark Acoustic Oscillations from Cosmology
• Refined Bounds on MeV-scale Thermal Dark Sectors from BBN and the CMB
• BBN And The CMB Constrain Neutrino Coupled Light WIMPs
• Constraining Dark Matter-Neutrino Interactions using the CMB and Large-Scale Structure
• Dark Matter Annihilation to Neutrinos
• BBN And The CMB Constrain Light, Electromagnetically Coupled WIMPs
• Reverse Direct Detection: Cosmic Ray Scattering With Light Dark Matter
• Combined explanations of B-physics anomalies: the sterile neutrino solution
• Dark matter-neutrino interactions through the lens of their cosmological implications
• Lepton-Number-Charged Scalars and Neutrino Beamstrahlung
• Supernova Constraints on MeV Dark Sectors from e+ e- Annihilations
• Thermal Dark Matter Below an MeV
• BBN constraints on the annihilation of MeV-scale dark matter
• WIMP and SIMP Dark Matter from the Spontaneous Breaking of a Global Group
• White Dwarfs constrain Dark Forces
• A robust deuterium abundance; Re-measurement of the z=3.256 absorption system towards the quasar PKS1937-1009
• A Thermal Neutrino Portal to Sub-MeV Dark Matter
• Ruling out the light WIMP explanation of the galactic 511 keV line
• Precise WIMP Dark Matter Abundance and Standard Model Thermodynamics
• Combining Planck with Large Scale Structure gives strong neutrino mass constraint
• A precise deuterium abundance: Re-measurement of the z=3.572 absorption system towards the quasar PKS1937-101
• Joint CMB and BBN Constraints on Light Dark Sectors with Dark Radiation
• Dark Photon Decay Beyond The Euler-Heisenberg Limit
• Relic Neutrino Freeze-out: Dependence on Natural Constants
• Dark Sector Equilibration During Nucleosynthesis
• Probing Decoupling in Dark Sectors with the Cosmic Microwave Background
• CosmoBit: A GAMBIT module for computing cosmological observables and likelihoods
• Pseudo-Dirac neutrinos via mirror-world and depletion of UHE neutrinos
• Cosmological Implications of Light Sterile Neutrinos produced after the QCD Phase Transition
• Constraining neutrino properties with a Euclid-like galaxy cluster survey
• Direct Detection of MeV-Scale Dark Matter Utilizing Germanium Internal Amplification for the Charge Created by the Ionization of Impurities
• Dark matter production from Goldstone boson interactions and implications for direct searches and dark radiation
• What does cosmology tell us about the mass of thermal-relic dark matter?
• Towards a full description of MeV dark matter decoupling: a self-consistent determination of relic abundance and $N_{\mathrm{eff}}$
• Gravitational Radiation from hyperbolic encounters in the presence of dark matter
• New observational limits on dark radiation in brane-world cosmology
• Probing interactions within the dark matter sector via extra radiation contributions
• A Rich Tapestry: Supersymmetric Axions, Dark Radiation, and Inflationary Reheating
• Limits on Brane-World and Particle Dark Radiation from Big Bang Nucleosynthesis and the CMB
• A short survey of matter-antimatter evolution in the primordial universe
• The Impact of the Charge Barrier Height on Germanium (Ge) Detectors with Amorphous-Ge Contacts for Light Dark Matter Searches
• Conversion of Gravitons into Dark Photons in Cosmological Dark Magnetic Fields
• Higgs inflation and cosmological electroweak phase transition with N scalars in the post-Higgs era
• Cosmological and astrophysical signatures of dark matter annihilations into pseudo-Goldstone bosons
• Quark-Gluon Plasma as the Possible Source of Cosmological Dark Radiation
• Anisotropic Scalar Field Dark Energy with a Disformally Coupled Yang-Mills Field
• Light Dark Matter and Dark Radiation
• Daughters mimic sterile neutrinos (almost!) perfectly
• The Darkness of Spin-0 Dark Radiation
• Dark Photon as Fractional Cosmic Neutrino Masquerader
• Limit on the dark matter mass from its interaction with photons
• Equality scale-based and sound horizon-based analysis of the Hubble tension
• Cosmology with Independently Varying Neutrino Temperature and Number
• Asymmetric dark matter and effective number of neutrinos
• Primordial neutrinos fade to gray: constraints from cosmological observables
• Interacting light thermal-relic dark matter: self-consistent cosmological bounds
• Observation of Time-Dependent Internal Charge Amplification in a Planar Germanium Detector at Cryogenic Temperature
• A Design of Hadronic Calorimeter for DarkSHINE Experiment
• Probing Dark Sector Particles Coupling to Neutrinos with Double Beta Decay
• Quarks to Cosmos: Particles and Plasma in Cosmological evolution