Abstract
The majoron, a neutrinophilic pseudo-Goldstone boson conventionally arising in the context of neutrino mass models, can damp neutrino free-streaming and inject additional energy density into neutrinos prior to recombination. The combination of these effects for an eV-scale mass majoron has been shown to ameliorate the outstanding H_0 tension, however only if one introduces additional dark radiation at the level of Delta N_{mathrm{eff}} sim 0.5. We show here that models of low-scale leptogenesis can naturally source this dark radiation by generating a primordial population of majorons from the decays of GeV-scale sterile neutrinos in the early Universe. Using a posterior predictive distribution conditioned on Planck2018+BAO data, we show that the value of H_0 observed by the SH_0ES collaboration is expected to occur at the level of sim 10% in the primordial majoron cosmology (to be compared with sim 0.1% in the case of Lambda CDM). This insight provides an intriguing connection between the neutrino mass mechanism, the baryon asymmetry of the Universe, and the discrepant measurements of H_0.
Highlights
Background evolutionIn order to describe the background evolution of majorons and neutrinos in the early Universe we once again use the formalism developed in [112,117]
This motivated the authors’ previous study [54], where it was shown that a majoron with a mass mφ ∼ eV arising from a the spontaneous breaking of a lepton number symmetry at scales vL ∼ O(1) TeV could reduce the Hubble tension to the ∼ 2σ level, only if additional dark radiation was present at the level of Neff ∼ 0.5
We have found that so long as the Higgs’ portal coupling is sufficiently small (|λφH | 10−7) so as to avoid thermalizing the new scalars at high temperatures and that the lepton number phase transition occurs at T > 104 − 106 GeV, ARS leptogenesis in the singlet majoron model will proceed as normal
Summary
A large number of potential solutions have been proposed which typically fall into one of two categories: those which modify the Universe at late times (z 1) and those which modify the dynamics and evolution near recombination (103 z 105) – we refer the reader to [25,26,27,28,29,30,31,32] and to [33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49] for several recent proposals of each type, and to [50] for a recent comprehensive review of models. [54] (see [55] for a short summary), the authors illustrated that a light majoron, naturally arising in neutrino mass models from the spontaneous breaking of a global lepton number symmetry [56,57,58,59], could partially counteract the effect of additional dark radiation (i.e. Neff ), pushing the inferred value of H0 to larger values while maintaining a. We verify explicitly that symmetry breaking scales vL ∼ (0.01−1) TeV required to resolve the Hubble tension can be made fully consistent with conventional ARS leptogenesis, so long as the Higgs mixing is small enough so as to avoid thermalizing the scalar responsible for breaking lepton number, and that the lepton number phase transition occurs at T > 104−106 GeV. Diagonalizing the neutrino mass matrix in the limit m D MN yields light active Majorana neutrinos with masses of the order:
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