[Paper Review] New inflationary probes of axion dark matter

Xingang Chen, JiJi Fan+1 more|arXiv (Cornell University)|Mar 6, 2023

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper introduces new cosmological observables—correlated clock signals in curvature and isocurvature spectra, and mixed non-Gaussianities involving both modes—to probe light axions during inflation. Despite strong CMB constraints on axion isocurvature, these novel signals remain potentially observable and could significantly constrain the inflationary Hubble scale independently of tensor modes.

ABSTRACT

If a light axion is present during inflation and becomes part of dark matter afterwards, its quantum fluctuations contribute to dark matter isocurvature. In this article, we introduce a whole new suite of cosmological observables for axion isocurvature, which could help test the presence of axions, as well as its coupling to the inflaton and other heavy spectator fields during inflation such as the radial mode of the Peccei-Quinn field. They include correlated clock signals in the curvature and isocurvature spectra, and mixed cosmological-collider non-Gaussianities involving both curvature and isocurvature fluctuations with shapes and running unconstrained by the current data analyses. Taking into account of the existing strong constraints on axion isocurvature fluctuations from the CMB, these novel signals could still be sizable and potentially observable. In some models, the signals, if observed, could even help us significantly narrow down the range of the inflationary Hubble scale, a crucial parameter difficult to be determined in general, independent of the tensor mode.

Motivation & Objective

• To identify new cosmological observables sensitive to light axions during inflation, beyond standard isocurvature constraints.
• To explore how axion couplings to the inflaton and heavy spectator fields (e.g., radial mode of PQ field) generate novel, potentially observable signals.
• To investigate whether mixed non-Gaussianities and correlated clock signals in curvature and isocurvature spectra can test axion presence and couplings.
• To assess whether these signals can constrain the inflationary Hubble scale independently of tensor mode measurements.
• To evaluate the viability of these signals under existing CMB bounds on axion isocurvature perturbations.

Proposed method

• Utilizes cosmological collider physics to study on-shell production of heavy spectator particles during inflation, generating non-analytical momentum and angular dependence in primordial correlators.
• Analyzes three- and higher-point correlation functions (bispectra) involving both curvature and isocurvature fluctuations, focusing on mixed types like $icc$ and $iii$.
• Applies effective field theory techniques to compute loop-induced signals, including gauge and Higgs boson loops, with explicit power counting and loop integration for $m_A/H \lesssim 10$.
• Introduces a mixing mechanism between adiabatic and isocurvature modes via Higgs-inflaton or Higgs-axion mixing, leading to non-zero correlation $A_{is}$ and mixing angle $\cos\Delta$.
• Estimates signal amplitudes using $f^{\rm NL}_{\rm NL}$ parameters and normalization factors, incorporating suppression factors from $\beta \lesssim 0.04$, $m_A/H \lesssim 10$, and $\sqrt{\beta}$ scaling.
• Considers scenarios with both Higgs-inflaton and Higgs-axion mixing to reduce suppression in mixed-spectrum signals.

Experimental results

Research questions

• RQ1 Can new cosmological observables beyond standard isocurvature constraints detect light axions during inflation?
• RQ2 What types of non-Gaussianity signals arise from axion couplings to the inflaton and heavy spectator fields?
• RQ3 How do clock signals and mixed non-Gaussianities in curvature and isocurvature spectra provide new probes of axion physics?
• RQ4 To what extent can these signals constrain the inflationary Hubble scale independently of tensor modes?
• RQ5 How do suppression factors from $\beta$, $m_A/H$, and coupling strengths affect the observability of these signals?

Key findings

• Correlated clock signals in curvature and isocurvature spectra emerge from axion couplings to the inflaton and heavy spectator fields, offering a new class of observable signals.
• Mixed non-Gaussianities of type $icc$ and $iii$ are generated via gauge and Higgs boson loops, with amplitudes suppressed by $\mathcal{O}(10^{-2})$ relative to $ccc$ type, but still potentially observable.
• The $icc$ bispectrum signal is estimated to be suppressed by $\sim \sqrt{\beta} \cdot H / \Lambda_f \cdot u \dot{\phi}_0 / m_A^2$, with $\beta \lesssim 0.04$ and $m_A/H \lesssim 10$.
• Mixed two-point correlations $A_{is}$ between curvature and isocurvature modes are generated via loop effects, with $\cos\Delta \sim A_s H / (4 m_A z) \times e^{\tilde{p}}$, highly suppressed by $m_A^{-2}$.
• Enhanced $iii$-type bispectra can arise when both Higgs-inflaton and Higgs-axion mixing are present, reducing suppression from $H/m_A$ and improving observability.
• These signals could significantly narrow the range of the inflationary Hubble scale, providing a model-independent constraint independent of tensor mode measurements.