Speaker
Description
In this paper, we analyze parameter regions that can alleviate the Hubble tension in the U(1)Lµ−Lτ model with the broken lepton number U(1)L symmetry. As new particles, this model has a U(1)Lµ−Lτ gauge boson Z′ and a Majoron ϕ, which can affect the early universe and the effective number of neutrino species Neff. If Z′ and ϕ simultaneously exist in the early universe, Z′ – ϕ interaction processes such as Z′να ↔ ϕν¯β occur. The comparison of Neff between the cases with and without the Z′ – ϕ interaction processes shows that these processes make a small contribution of O(10−4) to Neff, and it does not need to be considered for the alleviation of the Hubble tension. Based on these facts, we calculated Neff for various Majoron parameters without the Z′ – ϕ interaction processes to search parameters that could alleviate the Hubble tension. As a result, we found that the U(1)Lµ−Lτ gauge boson and Majoron can alleviate the Hubble tension in some parameter regions, and there is a non-trivial synergy contribution between Z′ and ϕ. Moreover, the parameter region with a lighter mass mϕ ≲ 2 MeV and a larger coupling λ ≳ 10−8 is excluded because it predicts too large Neff, i.e. Neff ≳ 3.5. The favored and restricted regions of the Majoron parameters depend on the Z′ parameters because of the presence of the Z′contribution and synergy one.
