2019/01/31 by Miguel Escudero, Dan Hooper, Gordan Krnjaic +1 · 1 citation
Physics and Astronomy · #hep-ph #astro-ph.CO
paper · pdf · doi:10.1007/jhep03(2019)071
published as JHEP 1903 (2019) 071 · 13 pages + 5 appendices, 10 figures. v2: Added comment to reflect that $ΔN_{eff}$ is only modified for $m_{Z'} > {\rm eV}$ and cropped Fig. 2 accordingly. Added comment on possible cosmological energy injection bounds. Main conclusions unchanged. Added references. v3: Matches the published version, typos fixed, references added
arxiv created 2019/03/14 · arxiv updated 2019/03/15
In this paper, we explore in detail the cosmological implications of an abelian Lμ-Lτ gauge extension of the Standard Model featuring a light and weakly coupled Z'. Such a scenario is motivated by the longstanding ∼ 4 σ discrepancy between the measured and predicted values of the muon's anomalous magnetic moment, (g-2)μ, as well as the tension between late and early time determinations of the Hubble constant. If sufficiently light, the Z' population will decay to neutrinos, increasing the overall energy density of radiation and altering the expansion history of the early universe. We identify two distinct regions of parameter space in this model in which the Hubble tension can be significantly relaxed. The first of these is the previously identified region in which a ∼ 10-20 MeV Z' reaches equilibrium in the early universe and then decays, heating the neutrino population and delaying the process of neutrino decoupling. For a coupling of gμ-τ ≃ (3-8) × 10-4, such a particle can also explain the observed (g-2)μ anomaly. In the second region, the Z' is very light (mZ' ∼ 1 eV to MeV) and very weakly coupled (gμ-τ ∼ 10-13 to 10-9). In this case, the Z' population is produced through freeze-in, and decays to neutrinos after neutrino decoupling. Across large regions of parameter space, we predict a contribution to the energy density of radiation that can appreciably relax the reported Hubble tension, ΔN\rm eff ≃ 0.2.