Cosmology with a very light Lμ - Lτ gauge boson
[1];
- King's College London, London (United Kingdom); CSIC-Univ. de Valencia, Valencia (Spain)
- Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Univ. of Chicago, Chicago, IL (United States)
- Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
- Univ. Paris-Saclay, Orsay (France); Instituto de Fisica (IFT) UAM-CSIC, Madrid (Spain); Univ. Autonoma de Madrid (UAM), Madrid (Spain)
In this study, we explore in detail the cosmological implications of an abelian $$L_\mu-L_\tau$$ gauge extension of the Standard Model featuring a light and weakly coupled $Z'$. Such a scenario is motivated by the longstanding $$\sim \, 4 \sigma$$ discrepancy between the measured and predicted values of the muon's anomalous magnetic moment, $$(g-2)_\mu$$, 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 $$\sim \, 10-20$$ 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 $$g_{\mu-\tau} \simeq (3-8) \times 10^{-4}$$, such a particle can also explain the observed $$(g-2)_{\mu}$$ anomaly. In the second region, the $Z'$ is very light ($$m_{Z'} \sim 1\,\text{eV}$$ to $$\text{MeV}$$) and very weakly coupled ($$g_{\mu-\tau} \sim 10^{-13}$$ 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, $$\Delta N_{\rm eff} \simeq 0.2$$.
- Research Organization:
- Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), High Energy Physics (HEP)
- Grant/Contract Number:
- AC02-07CH11359
- OSTI ID:
- 1501449
- Report Number(s):
- arXiv:1901.02010; FERMILAB-PUB-19-001-A; LPT-Orsay-18-15; IFIC-19-02; KCL-19-01, IFT-UAM/CSIC-19-7; KCL-19-01; 1712674
- Journal Information:
- Journal of High Energy Physics (Online), Vol. 2019, Issue 3; ISSN 1029-8479
- Publisher:
- Springer BerlinCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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