Dark Matter $Z^\prime$ and XENON1T Excess from $U(1)_X$ Extended Standard Model

TL;DR

This paper proposes a $U(1)_X$ extended Standard Model where a keV-scale $Z'$ gauge boson acts as dark matter, explaining the XENON1T excess and achieving the correct relic density via a two-component freeze-in mechanism.

Contribution

It introduces a novel light $Z'$ dark matter scenario within a $U(1)_X$ extension, linking it to XENON1T excess and relic density through a two-component model and freeze-in production.

Findings

$Z'$ gauge boson with keV mass explains XENON1T excess.

Two-component dark matter model achieves correct relic density.

Proposes freeze-in mechanism via Higgs-portal scalar for $Z'$ production.

Abstract

A gauged $U(1)_X$ symmetry appended to the Standard Model (SM) is particularly well-motivated since it can account for the light neutrino masses by the seesaw mechanism, explain the origin of baryon asymmetry of the universe via leptogenesis, and help implement successful cosmological inflation with the $U(1)_X$ breaking Higgs field as the inflaton. In this framework, we propose a light dark matter (DM) scenario in which the $U(1)_X$ gauge boson $Z^\prime$ behaves as a DM particle in the universe. We discuss how this scenario with $Z^\prime$ mass of a few keV and a $U(1)_X$ gauge coupling $g_X \simeq 10^{-16}$ can nicely fit the excess in the electronic recoil energy spectrum recently reported by the XENON1T collaboration. In order to reproduce the observed DM relic density in the presence of such a tiny gauge coupling, we propose an extension of the model to a two-component DM scenario. The $Z^\prime$ DM density can be comparable to the observed DM density by the freeze-in mechanism through the coupling of $Z^\prime$ boson to a partner Higgs-portal scalar DM with a large $U(1)_X$ charge.