# Cosmology with a very light L μ − L τ gauge boson

Escudero, Miguel (0000 0001 2322 6764, Department of Physics, King’s College London, Strand, London, WC2R 2LS, U.K.) (0000 0001 2178 9889, Instituto de Física Corpuscular (IFIC), CSIC-Universitat de València, Valencia, Spain) ; Hooper, Dan (0000 0001 0675 0679, Fermi National Accelerator Laboratory, Batavia, IL, U.S.A.) (0000 0004 1936 7822, Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL, U.S.A.) (0000 0004 1936 7822, Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL, U.S.A.) ; Krnjaic, Gordan (0000 0001 0675 0679, Fermi National Accelerator Laboratory, Batavia, IL, U.S.A.) ; Pierre, Mathias (0000 0001 2171 2558, Laboratoire de Physique Théorique (UMR8627), CNRS, Univ. Paris-Sud, Université Paris-Saclay, 91405, Orsay, France) (0000 0004 0561 6576, Instituto de Física Teórica (IFT) UAM-CSIC, Campus de Cantoblanco, 28049, Madrid, Spain) (0000000119578126, Departamento de Física Teórica, Universidad Autonoma de Madrid (UAM), Campus de Cantoblanco, 28049, Madrid, Spain)

14 March 2019

Abstract: In this paper, we explore in detail the cosmological implications of an abelian L μ − L τ 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 g μ − 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 ( m Z ′ ${m}_{Z^{\prime }}$ ∼ 1eV to MeV) and very weakly coupled ( g μ − 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, Δ N eff ≃ 0 . 2.

Published in: JHEP 1903 (2019) 071