Light Higgsino from Axion Dark Radiation

TL;DR

This paper proposes that the QCD axion, produced from saxion decay in supersymmetric models, can account for dark radiation, and explores the implications for Higgsino mass bounds and collider detectability.

Contribution

It demonstrates that axions from saxion decay can serve as dark radiation and constrains the Higgsino parameter space, linking dark radiation to collider phenomenology.

Findings

Higgsino mass parameter mu is bounded by saxion mass and decay channels.

Higgsino can be light enough for collider detection even with heavy other superparticles.

Axino abundance from Higgsino and saxion decays is estimated.

Abstract

The recent observations imply that there is an extra relativistic degree of freedom coined dark radiation. We argue that the QCD axion is a plausible candidate for the dark radiation, not only because of its extremely small mass, but also because in the supersymmetric extension of the Peccei-Quinn mechanism the saxion tends to dominate the Universe and decays into axions with a sizable branching fraction. We show that the Higgsino mixing parameter mu is bounded from above when the axions produced at the saxion decays constitute the dark radiation: mu \lesssim 300 GeV for a saxion lighter than 2m_W, and mu less than the saxion mass otherwise. Interestingly, the Higgsino can be light enough to be within the reach of LHC and/or ILC even when the other superparticles are heavy with mass about 1 TeV or higher. We also estimate the abundance of axino produced by the decays of Higgsino and saxion.