Cosmology with a very light Lμ − Lτ gauge boson

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 ∼ 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 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 ∼ 1 eV 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, ∆N eff 0.2.

Refined bounds on MeV-scale thermal dark sectors from BBN and the CMB

“…Planck CMB observations are more stringent than BBN and set T dec The bound on the neutrino decoupling temperature is generic and can be directly mapped into a constraint on the mass and couplings of various BSM species interacting both with neutrinos and electrons. The phenomenology of such type of scenarios has been studied in detail in the context of a very light U (1) µ−τ gauge boson [83], but from Equation (18)…”

Searches for decays of new particles in the DUNE Multi-Purpose near Detector

“…This loop-induced kinetic mixing, as defined in Eq. (4.1), is related to the L α − L β gauge coupling g αβ as [63,83,89]…”

Low scale seesaw models for low scale U(1)Lμ−LτAraki1, Asai2, Sato3 et al. 2019Phys. Rev. D220170View full textAdd to dashboardCite

“…[5,6]), that the U(1) L µ −L τ gauge boson, Z µτ , with an MeV scale mass can settle the long-standing discrepancy of muon anomalous magnetic moment (g µ − 2) [7][8][9][10][11][12] without conflicting other experimental constraints. Then, this result motivated many authors to study low scale U(1) L µ −L τ extension of the SM in various contexts: for instance, it was found that an MeV scale Z µτ can relax the tension between the late time and the early time determination of the Hubble constant [13], implications for the dark matter problem were studied in Refs. [14][15][16][17], and the detectability of such a light Z µτ was discussed in Refs.…”