More is Different: Multi-Axion Dynamics, String Bundles, and a Solution to the QCD Axion Domain Wall Problem

by Fuminobu Takahashi (Tohoku U.)

Thursday 4 Sept 2025, 16:00 → 17:30 Asia/Seoul

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Title: More is Different: Multi-Axion Dynamics, String Bundles, and a Solution to the QCD Axion Domain Wall Problem

Speaker: Fuminobu Takahashi (Tohoku U.)

Abstract: The formation and evolution of topological defects can be radically altered by multiple axions with kinetic and mass mixings. We examine the resulting string wall network in generic multi-axion models and discover that high-tension domain walls often persist, resurrecting the cosmological domain wall problem, even when the vacuum is unique. In certain regimes, the network collapses into string bundles: composite objects in which several global strings are bound together by domain walls.
This dynamics also provides a minimal solution to the QCD axion domain wall problem. Introducing an additional massless (or sufficiently light) axion coupled to gluons causes the QCD axion domain walls to be bounded by the new axion strings and convert them into string bundles. These string bundles couple to photons and remain stable or long-lived, producing distinctive signatures: anisotropic cosmic birefringence, a stochastic gravitational wave background, and characteristic CMB temperature and polarization anisotropies.

 

Zoom link: 

https://kias-re-kr.zoom.us/j/87949017008?pwd=aKTH6YmKIIaPwn1ZYTdBc6NtImurX9.1

Meeting ID: 879 4901 7008
Passcode: 191624