Probing the relaxed relaxion and Higgs-portal with S1 & S2

TL;DR

This paper investigates whether a scalar particle with specific properties can explain the XeT excess observed in SR1, exploring relaxion and Higgs-portal models, and discusses implications for stellar cooling and phase transitions.

Contribution

It demonstrates that a scalar with particular mass and coupling can account for the XeT excess and analyzes the naturalness, stellar bounds, and phase transition effects in relaxion and Higgs-portal frameworks.

Findings

A scalar with m_phi=1.9 keV and g_{phi e}=2.4×10^{-14} explains the XeT excess.

Stellar cooling bounds are in tension with the minimal scenarios considered.

Large density regions may trigger phase transitions affecting scalar mass and stellar cooling.

Abstract

We study the recent \XeT excess in context of solar scalar, specifically in the framework of Higgs-portal and the relaxion model. We show that $m_\phi = 1.9\,\keV$ and $g_{\phi e}=2.4\times 10^{-14}$ can explain the observed excess in science run 1 (SR1) analysis in the 1-7 keV range. When translated into the scalar-Higgs mixing angle, the corresponding mixing angle $\sin\theta = 10^{-8}$ is intriguingly close to the maximum value of mixing angle for the technical naturalness of the scalar mass. Unlike the solar axion model, the excess favors a massive scalar field because of its softer spectrum. In the minimal scenarios we consider, the best fit parameters are in tension with stellar cooling bounds. We discuss a possibility that a large density of red giant stars may trigger a phase transition, resulting in a local scalar mass increase suppressing the stellar cooling. For the particular case of minimal relaxion scenarios, we find that such type of chameleon effects is automatically present but they can not ease the cooling bounds. They are however capable of triggering a catastrophic phase transition in the entire universe. Following this observation we derive a new set of bounds on the relaxed-relaxion parameter space.