SEMINAR 2024

Abstract

Breaking of discrete parity symmetry at high energy scale gives rise to Z₂ domain walls (DW). The metastability of such walls can make them relatively long lived and contradict standard cosmology. We consider two classes of theories with similar underlying features, the left right symmetric theories and two Higgs doublet models. As a first step, domains form at a high energy scale during Z₂ parity breaking. In the second step, these domains further decompose into subdomains due to Z₂ symmetry breaking in two Higgs doublet models closer to the electroweak scale. We show that after this two step formation of domains and subdomains, a QCD instanton induced energy difference can remove both types of domain walls at around the same time successfully. The removal occurs purely as the result of a chance event taking place with probability very close to 0.25. We then investigate the gravitational waves arising from the collapse of such domain walls and show that the peak frequency of these waves lies in the 10^-7–10^-6 Hz band, corresponding to annihilation temperatures of 1–10 GeV. The recent NANOGrav results rule out our DW collapse model for higher values of parity breaking scale above 10⁷ GeV. Our DW collapse model with parity breaking scales below 10⁷ GeV remains consistent with the current NANOGrav results and has a good chance of being seriously tested in future pulsar timing array based experiments.