Cosmology with a very light LμLτ gauge boson

  • Miguel EscuderoEmail author
  • Dan Hooper
  • Gordan Krnjaic
  • Mathias Pierre
Open Access
Regular Article - Theoretical Physics


In this paper, we explore in detail the cosmological implications of an abelian LμLτ gauge extension of the Standard Model featuring a light and weakly coupled Z′. Such a scenario is motivated by the longstanding ∼ 4σ 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 gμτ ≃ (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 (\( {m}_{Z^{\prime }} \) ∼ 1eV to MeV) and very weakly coupled (gμτ ∼ 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, ΔNeff ≃ 0.2.


Cosmology of Theories beyond the SM Beyond Standard Model Neutrino Physics 


Open Access

This article is distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited.


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Copyright information

© The Author(s) 2019

Authors and Affiliations

  1. 1.Department of PhysicsKing’s College LondonLondonU.K.
  2. 2.Instituto de Física Corpuscular (IFIC), CSIC-Universitat de ValènciaValenciaSpain
  3. 3.Fermi National Accelerator LaboratoryBataviaU.S.A.
  4. 4.Department of Astronomy and AstrophysicsUniversity of ChicagoChicagoU.S.A.
  5. 5.Kavli Institute for Cosmological PhysicsUniversity of ChicagoChicagoU.S.A.
  6. 6.Laboratoire de Physique Théorique (UMR8627), CNRS, Univ. Paris-Sud, Université Paris-SaclayOrsayFrance
  7. 7.Instituto de Física Teórica (IFT) UAM-CSICMadridSpain
  8. 8.Departamento de Física TeóricaUniversidad Autonoma de Madrid (UAM)MadridSpain

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