[Paper Review] Axionic domain walls at Pulsar Timing Arrays: QCD bias and particle friction

Simone Blasi, Alberto Mariotti+2 more|arXiv (Cornell University)|Jun 30, 2023

Cosmology and Gravitation Theories4 citations

TL;DR

This paper investigates axion-like particle (ALP) domain walls as a source of the stochastic gravitational wave background detected by Pulsar Timing Arrays (PTAs) at nHz frequencies. It shows that QCD-induced potential provides a natural bias for domain wall annihilation near the QCD crossover, and identifies a parameter region—particularly for ALP masses ≤10 GeV—where friction from gluon and pion scattering is negligible, preserving the scaling regime and making the ALP domain wall scenario viable for explaining the NANOGrav signal.

ABSTRACT

The recent results from the Pulsar Timing Array (PTA) collaborations show the first evidence for the detection of a stochastic background of gravitational waves at the nHz frequencies. This discovery has profound implications for the physics of both the late and the early Universe. In fact, together with the possible interpretation in terms of super massive black hole binaries, many sources in the early Universe can provide viable explanations as well. In this paper, we study the gravitational wave background sourced by a network of axion-like-particle (ALP) domain walls at temperatures around the QCD crossover, where the QCD-induced potential provides the necessary bias to annihilate the network. Remarkably, this implies a peak amplitude at frequencies around the sensitivity range of PTAs. We extend previous analysis by taking into account the unavoidable friction on the network stemming from the topological coupling of the ALP to QCD in terms of gluon and pion reflection off the domain walls at high and low temperatures, respectively. We identify the regions of parameter space where the network annihilates in the scaling regime ensuring compatibility with the PTA results, as well as those where friction can be important and a more detailed study around the QCD crossover is required.

Motivation & Objective

• To assess the viability of axion-like particle (ALP) domain walls as a source of the stochastic gravitational wave background (SGWB) detected by Pulsar Timing Arrays (PTAs) at nHz frequencies.
• To investigate the role of QCD-induced potential as a natural bias for ALP domain wall annihilation near the QCD crossover temperature.
• To evaluate the impact of unavoidable friction from QCD plasma scattering (gluons and pions) on the domain wall network dynamics and resulting SGWB.
• To identify regions of ALP parameter space where friction is negligible, preserving the scaling regime and compatibility with PTA data.
• To determine whether a more detailed analysis around the QCD crossover is required for regions where friction may dominate.

Proposed method

• Modeling the ALP domain wall network with a QCD-induced potential as the bias mechanism for annihilation, leveraging the anomaly coupling of ALPs to gluons.
• Calculating friction forces from gluon and pion scattering off domain walls at high and low temperatures, respectively, using effective field theory and thermal field theory techniques.
• Applying the scaling regime approximation for domain wall dynamics, with constraints from NANOGrav data on the network's energy density and annihilation temperature.
• Performing a parameter scan over ALP mass $m_a$ and coupling strength, identifying regions where friction dominates or is negligible.
• Using numerical results from previous simulations (e.g., Hiramatsu et al.) as a baseline for SGWB amplitude predictions.
• Comparing the predicted SGWB amplitude with NANOGrav observations to constrain viable parameter space, especially near the QCD crossover.

Experimental results

Research questions

• RQ1 Can axion-like particle (ALP) domain walls, biased by the QCD-induced potential, produce a stochastic gravitational wave background detectable by Pulsar Timing Arrays?
• RQ2 How does friction from QCD plasma scattering (gluons and pions) affect the dynamics of ALP domain walls near the QCD crossover temperature?
• RQ3 For which values of ALP mass and coupling is friction negligible, allowing the domain wall network to remain in the scaling regime and match PTA observations?
• RQ4 What is the extent of parameter space where friction dominates, necessitating a more refined analysis around the QCD crossover?
• RQ5 Can the observed PTA signal be explained by ALP domain walls if friction from QCD interactions is included in the dynamics?

Key findings

• The QCD-induced potential provides a natural bias for ALP domain wall annihilation near the QCD crossover temperature, making this scenario a viable explanation for the PTA-detectable SGWB.
• For ALP masses $m_a \lesssim 10\,\text{GeV}$, friction from QCD plasma scattering is negligible, preserving the scaling regime and ensuring compatibility with NANOGrav data.
• For heavier ALPs ($m_a > 10\,\text{GeV}$), friction from gluon scattering dominates at temperatures $T > 2\,\text{GeV}$, potentially disrupting the scaling regime.
• The pion pressure at low temperatures is insufficient to dominate friction, and thus cannot ensure friction dominance at annihilation, requiring further analysis.
• Regions of parameter space where domain walls would dominate the energy density before annihilation are excluded, particularly for higher bias values ($\epsilon = 0.26$ or $\epsilon = 1$).
• The intersection of the PTA-allowed signal band (from NANOGrav) and the low-friction region ($m_a \lesssim 10\,\text{GeV}$) identifies a robust and viable parameter space for the ALP domain wall interpretation of the PTA signal.