Probing the relaxed relaxion and Higgs portal scenarios with XENON1T scintillation and ionization data

Abstract

We study the recent XENON1T excess in the context of solar scalar, specifically in the framework of Higgs portal and the relaxion model. We show that ${m}_{\ensuremath{\phi}}=1.9\text{ }\text{ }\mathrm{keV}$ and ${g}_{\ensuremath{\phi}e}=2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ can explain the observed excess in science run 1 (SR1) analysis in the 1--7 keV range. In the minimal scenarios we consider, the best-fit parameters are in tension with stellar cooling bounds. Despite this fact, the excess represents an example bringing attention to two interesting effects of general relevance. First, the scalar-Higgs mixing angle reproducing the excess, $\mathrm{sin}\ensuremath{\theta}\ensuremath{\simeq}{10}^{\ensuremath{-}8}$, is intriguingly close to the maximum value of mixing angle for the technical naturalness of the scalar mass. While finding a parameter value very close to its theoretical limit may naively seem an unlikely coincidence, we demonstrate that there exists a class of models which generically saturate the mixing naturalness bound. Secondly, 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. Finally, we present two nonminimal models that demonstrate how the cooling bounds can be relaxed as a result of high density effects.