Journal
JOURNAL OF HIGH ENERGY PHYSICS
Volume -, Issue 3, Pages -Publisher
SPRINGER
DOI: 10.1007/JHEP03(2019)071
Keywords
Cosmology of Theories beyond the SM; Beyond Standard Model; Neutrino Physics
Categories
Funding
- US Department of Energy [DE-AC02-07CH11359]
- European Union Horizon 2020 research and innovation programme under the Marie Skodowska-Curie grant agreement Elusives ITN [674896]
- European Union Horizon 2020 research and innovation programme under the Marie Skodowska-Curie grant agreement InvisiblesPlus RISE [690575]
- Spanish Agencia Estatal de Investigacion [FPA2015-65929-P]
- IFT Centro de Excelencia Severo Ochoa [SEV-2016-0597]
- Red Consolider MultiDark [FPA2017-90566-REDC]
- European Research Council under the European Union's Horizon 2020 program (ERC Grant) [648680 DARKHORIZONS]
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In this paper, we explore in detail the cosmological implications of an abelian L - L gauge extension of the Standard Model featuring a light and weakly coupled Z. Such a scenario is motivated by the longstanding approximate to 4 sigma discrepancy between the measured and predicted values of the muon's anomalous magnetic moment, (g - 2), 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 approximate to 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 g (-) similar or equal to (3 - 8) x 10(-4), such a particle can also explain the observed (g - 2) anomaly. In the second region, the Z is very light (mZ approximate to 1eV to MeV) and very weakly coupled (g (-) approximate to 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, N-eff similar or equal to 0.2.
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