Increasing Neff with particles in thermal equilibrium with neutrinos

Boehm, Celine; Dolan, Matthew J.; McCabe, Christopher

doi:10.48550/arxiv.1207.0497

Cited by 63 publications

(95 citation statements)

References 56 publications

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“…Also, our bounds will be weakened to the extent that the dark matter couples to both the electron-photon plasma and to neutrinos. In fact, in the extreme opposite limit (coupling to neutrinos only), the χ χ annihilation heats the neutrinos instead of the photons, increasing N ef f and providing better agreement with current observations [48].…”

supporting

confidence: 74%

Limits on MeV dark matter from the effective number of neutrinos

Scherrer

2013

Phys. Rev. D

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Thermal dark matter that couples more strongly to electrons and photons than to neutrinos will heat the electron-photon plasma relative to the neutrino background if it becomes nonrelativistic after the neutrinos decouple from the thermal background. This results in a reduction in N ef f below the standard-model value, a result strongly disfavored by current CMB observations. Taking conservative lower bounds on N ef f and on the decoupling temperature of the neutrinos, we derive a bound on the dark matter particle mass of mχ > 3 − 9 MeV, depending on the spin and statistics of the particle. For p-wave annihilation, our limit on the dark matter particle mass is stronger than the limit derived from distortions to the CMB fluctuation spectrum produced by annihilations near the epoch of recombination.Roughly 20−25% of the total energy content of the universe is in the form of non-baryonic dark matter. While a dark matter particle mass in the GeV range is often assumed, there has also been interest in masses in the MeV range. Dark matter with a mass in this range was invoked to explain the 511 keV γ-rays observed by INTE-GRAL [1], and to explain the cosmic γ-ray background at 1 − 20 MeV [2]. Supersymmetric models with MeV dark matter have been proposed [3], and MeV dark matter can arise in the context of the WIMPless dark matter model [4]. MeV dark matter can have interesting effects on large-scale structure [5].We note here that a thermal MeV dark matter particle that couples more strongly to electrons and photons than to neutrinos will heat the electron-photon plasma when it becomes nonrelativistic before its abundance freezes out. If this occurs after the neutrinos decouple from the thermal background, then the ratio of the neutrino temperature to the photon temperature will be reduced, a process similar to the heating that occurs when the electron-positron pairs become nonrelativistic. The final result is a decrease in the effective number of neutrino degrees of freedom. This effect was first explored by Kolb et al. [6] and more recently by Serpico and Raffelt [7] in the context of primordial nucleosynthesis. Recent CMB observations [8][9][10] place severe lower bounds on N ef f , allowing us to constrain this process. (See also the earlier work of Ref. [11], which examined heating of the photons relative to the neutrinos from decaying particles).At recombination, the energy density in relativistic particles includes photons, whose temperature, T γ , and therefore energy density is extremely well-measured, and a neutrino background with temperature T ν = (4/11) 1/3 T γ . The theoretical prediction for the effective number of neutrinos (assuming slight reheating of the neutrinos from early e + e − annihilation) is N ef f = 3.046 [12,13]. The neutrino density cannot be measured directly, but it can be inferred from measurements of the CMB. −0.68 (95% CL). Clearly, the data favor values of N ef f larger than the standard-model theoretical prediction, rather than smaller.The extent of the heating from dark matter a...

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supporting

confidence: 74%

Limits on MeV dark matter from the effective number of neutrinos

Scherrer

2013

Phys. Rev. D

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“…The discussion in § V considered the effects of a "truly weak" light WIMP, a particle that couples only to the SM neutrinos but not to the other SM particles (in particular, it does not couple to the photons and the e ± pairs) [23,24,37]. Before the SM neutrinos decouple (T γ ≥ T νd ), T ν = T γ .…”

Section: Discussionmentioning

confidence: 99%

“…As a novel alternative to the case discussed above in § IV, consider the consequences of a "truly weak" light WIMP that couples only to the standard model neutrinos, but not to the photons or the e ± pairs [23,24,37]. Assume there are no equivalent neutrinos (∆N ν = 0).…”

Section: Dark Radiation Without Dark Radiation: "Truly Weak" Light Wimpsmentioning

confidence: 99%

Equivalent neutrinos, light WIMPs, and the chimera of dark radiation

Steigman

2013

Phys. Rev. D

143

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According to conventional wisdom, in the standard model (SM) of particle physics and cosmology the "effective number of neutrinos" measured in the late Universe is N eff = 3 (more precisely, 3.046). For extensions of the standard model allowing for the presence of ∆Nν "equivalent neutrinos" (or "dark radiation"), it is generally the case that N eff > 3. These canonical results are reconsidered, demonstrating that a measurement of N eff > 3 can be consistent with ∆Nν = 0 ("dark radiation without dark radiation"). Conversely, a measurement consistent with N eff = 3 is not inconsistent with the presence of dark radiation (∆Nν > 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 N eff < 3. This opens the window for one or more equivalent neutrinos, including "sterile neutrinos", to be consistent with N eff = 3. By reducing the neutrino number density in the present Universe, this also allows for more massive neutrinos, relaxing the current constraints on the sum of the neutrino masses. In contrast, if the light WIMP couples only to the SM neutrinos and not to the photons and e ± pairs, its late time annihilation heats the neutrinos but not the photons, resulting in N eff > 3 even in the absence of equivalent neutrinos or dark radiation. A measurement of N eff > 3 is no guarantee of the presence of equivalent neutrinos or dark radiation. In the presence of a light WIMP 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 and 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). As the analysis here reveals, there's more to a measurement of N eff than meets the eye. * steigman.1@asc.ohio-state.edu 2

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“…At what time the daughter becomes non-relativistic (7) depends on its mass only indirectly. The dependence is on the mass hierarchy to its mother as given by (44). Interestingly, a heavier daughter with an energy density above the tight HDM bound from a decay with τ t eq becomes non-relativistic at a time before photons finally decouple, t nr 2 < t dpl γ .…”

Section: Hot Dark Matter Constraint and An Opportunitymentioning

confidence: 99%

Dark radiation from particle decay: cosmological constraints and opportunities

Hasenkamp¹,

Kersten²

2013

J. Cosmol. Astropart. Phys.

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We study particle decay as the origin of dark radiation. After elaborating general properties and useful parametrisations we provide model-independent and easy-touse constraints from nucleosynthesis, the cosmic microwave background and structure formation. Bounds on branching ratios and mass hierarchies depend in a unique way on the time of decay. We demonstrate their power to exclude well-motivated scenarios taking the example of the lightest ordinary sparticle decaying into the gravitino. We point out signatures and opportunities in cosmological observations and structure formation. For example, if there are two dark decay modes, dark radiation and the observed dark matter with adjustable free-streaming can originate from the same decaying particle, solving small-scale problems of structure formation. Hot dark matter mimicking a neutrino mass scale as deduced from cosmological observations can arise and possibly be distinguished after a discovery. Our results can be used as a guideline for model building.

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Increasing Neff with particles in thermal equilibrium with neutrinos

Cited by 63 publications

References 56 publications

Limits on MeV dark matter from the effective number of neutrinos

Limits on MeV dark matter from the effective number of neutrinos

Equivalent neutrinos, light WIMPs, and the chimera of dark radiation

Dark radiation from particle decay: cosmological constraints and opportunities

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