Neutrino magnetic moments meet precision Neff measurements

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Right-handed neutrinos (RHNs) provide a natural portal to a dark sector accommodating dark matter (DM). In this work, we consider that the dark sector is connected to the standard model only via RHNs and ask how DM can be produced from RHNs. Our framework concentrates on a rather simple and generic interaction that couples RHNs to a pair of dark particles. Depending on whether RHNs are light or heavy in comparison to the dark sector and also on whether one or both of them are in the freeze-in/out regime, there are many distinct scenarios resulting in rather different results. We conduct a comprehensive and systematic study of all possible scenarios in this paper. For illustration, we apply our generic results to the type-I seesaw model with the dark sector extension, addressing whether and when DM in this model can be in the freeze-in or freeze-out regime. Some observational consequences in this framework are also discussed.

Section: Case Ef In the Light ν R Limitmentioning

confidence: 84%

Dark matter produced from right-handed neutrinos

2023

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“…where m ν is the neutrino mass, e is the electric charge unit, G F is the Fermi constant, and µ B = e/(2m e ) is the Bohr magneton, with m e the electron mass. So far, neutrino magnetic moments have proven too small to be detectable, but numerous constraints have been derived from neutrino experiments [3,4] as well as from astrophysical [5][6][7][8][9][10] and cosmological arguments [11][12][13][14][15][16][17]. While these constraints still fall several orders of magnitude short of probing magnetic moments as small as in eq.…”

Section: Jcap03(2024)043mentioning

confidence: 99%

Magnetic moments of astrophysical neutrinos

Kopp,

Opferkuch,

Wang

2024

We study the impact of neutrino magnetic moments on astrophysical neutrinos, in particular supernova neutrinos and ultra-high energy neutrinos from extragalactic sources. We show that magnetic moment-induced conversion of Dirac neutrinos from left-handed states into unobservable right-handed singlet states can substantially change the flux and flavour composition of these neutrinos at Earth. Notably, neutrinos from a supernova's neutronisation burst, whose flux can be predicted with 𝒪(10%) accuracy, offer a discovery reach to neutrino magnetic moments ∼ few × 10-13 μB , up to one order of magnitude below current limits. For high-energy neutrinos from distant sources, for which no robust flux prediction exists, we show how the flavour composition at Earth can be used as a handle to establish the presence of non-negligible magnetic moments, potentially down to few× 10-17 μB if the measurement can be performed on neutrinos from a single source. In both cases, the sensitivity strongly depends on the galactic (intergalactic) magnetic field profiles along the line of sight. Therefore, while a discovery is possible down to very small values of the magnetic moment, the absence of a discovery does not imply an equally strong limit. We also comment on the dependence of our results on the right-handed neutrino mass, paying special attention to the transition from coherent deflection by a classical magnetic field to incoherent scattering on individual scattering targets. Finally, we show that a measurement of Standard Model Dirac neutrino magnetic moments, of order 10-19 μB , could be possible under rather optimistic, but not completely outrageous, assumptions using flavour ratios of high-energy astrophysical neutrinos.

“…Therefore, the collision rate in the Boltzmann eq. (3.1) should be further multiplied by the spin degrees of freedom g χ = 2 [56] so as to obtain a collision term without spin average. For a nonthermal DM in the freeze-in paradigm, we expect f χ f eq χ so that f χ can be neglected in the determination of the DM relic density.…”

Section: Boltzmann Equationmentioning

confidence: 99%

“…For most situations, the thermal corrections to the scattering processes are significant only when there are IR singularities or resonance. For example, the IR singularity is known in neutrino and electron chirality-flipping processes at finite temperatures [56,[62][63][64], and the resonant effect from thermal corrections is also known in neutrino oscillations at finite temperature and density [65,66].…”

Section: Double Counting and Resonant Enhancementmentioning

confidence: 99%

Dark matter freeze-in via a light fermion mediator: forbidden decay and scattering

2023

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The connection between a hidden nonthermal sector and a thermal plasma can be established by a light thermal fermion mediator. When the fermion mediator is much lighter than the hidden species, kinematically forbidden decay of the mediator can be opened at finite temperatures to produce the hidden species. Unlike bosons having quartic couplings, renormalizable forbidden fermion decay generically shares the same order of couplings with the scattering. We present a dedicated investigation into the freeze-in dark matter production via a thermal fermion mediator. We demonstrate that the plasma-induced decay rate differs from that calculated via the tree-level amplitude, but the former can be obtained from the latter via constant rescaling. Furthermore, we find that the relative effect of the forbidden decay and the scattering on the dark matter relic density can be simply estimated via the thermal coupling between the plasma and the mediator. Applying to different thermal interactions, we show that the forbidden decay contribution can reach the level of 4%- 45% for a thermal coupling at 0.1- 1.