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Light Higgsino from Axion Dark Radiation

Kwang Sik Jeong, Fuminobu Takahashi

TL;DR

This work addresses hints of extra dark radiation by identifying the QCD axion as a natural candidate in a SUSY PQ framework where the saxion dominates the early Universe and decays to axions. A Kim–Nilles coupling generates the $\mu$-term and modulates saxion decays, enabling $\Delta N_{\rm eff} \sim 1$ without strong dependence on the PQ scale $F_a$ and imposing an upper bound on the Higgsino mass parameter $\mu$ (roughly $\mu \lesssim 300$ GeV for $m_\sigma < 2m_W$, or $\mu < m_\sigma$ otherwise). This setup suggests a light Higgsino could be within reach of the LHC/ILC even if other SUSY states are multi-TeV, connecting dark radiation, the $\mu$ problem, and collider phenomenology. The model also analyzes axino dark matter production—both thermal and non-thermal from saxion decays—highlighting viable relic-density regulators and complementary axion dark matter from misalignment. Overall, the work links cosmological hints of new relativistic species to concrete particle physics in the PQ-extended MSSM with testable Higgsino signatures at present or near-future colliders.

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.

Light Higgsino from Axion Dark Radiation

TL;DR

This work addresses hints of extra dark radiation by identifying the QCD axion as a natural candidate in a SUSY PQ framework where the saxion dominates the early Universe and decays to axions. A Kim–Nilles coupling generates the -term and modulates saxion decays, enabling without strong dependence on the PQ scale and imposing an upper bound on the Higgsino mass parameter (roughly GeV for , or otherwise). This setup suggests a light Higgsino could be within reach of the LHC/ILC even if other SUSY states are multi-TeV, connecting dark radiation, the problem, and collider phenomenology. The model also analyzes axino dark matter production—both thermal and non-thermal from saxion decays—highlighting viable relic-density regulators and complementary axion dark matter from misalignment. Overall, the work links cosmological hints of new relativistic species to concrete particle physics in the PQ-extended MSSM with testable Higgsino signatures at present or near-future colliders.

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.

Paper Structure

This paper contains 9 sections, 27 equations, 1 figure.

Table of Contents

  1. Introduction
  2. PQ Extension of the MSSM
  3. Saxion Properties
  4. Saxion Cosmology
  5. PQ Solution to the $\mu$ Problem
  6. Axion Dark Radiation
  7. Axion Dark Radiation from Saxion Decays
  8. Axino Dark Matter
  9. Conclusions

Figures (1)

  • Figure 1: The constant contours of $\Delta N_{\rm eff}$ for $|B|/m_A=0.6$ and $m_h=125$ GeV in the $(m_\sigma,\mu)$ plane. The black lines represent the contours of $\Delta N_{\rm eff}=0.5,\,1,\,1.5$ from the above, respectively. In the shadowed region, $0.4\leq \Delta N_{\rm eff}\leq 2$. We also plot constant contours of the quantity, $(c_aF_a/10^{11}{\rm GeV})\times T_\sigma=0.1,\,0.5,\,1,\,3,\,6,\,10$ GeV from the below, respectively, in red lines.