Higher order intercommutations in Cosmic String Collisions

TL;DR

This study reveals that cosmic strings can undergo multiple intercommutations at high speeds, forming kink trains, with the process and outcomes strongly depending on the type-II regime parameter , significantly affecting their small-scale structure and gravitational wave signals.

Contribution

First numerical investigation of multiple intercommutations and kink train formation in abelian Higgs cosmic strings at high  values, highlighting new collision dynamics.

Findings

Multiple intercommutations observed at high 

Critical speed for double reconnection decreases with 

Formation of kink trains and quadruple reconnection events

Abstract

We report the first observation of multiple intercommutation (more than two successive reconnections) of cosmic strings at ultra-high collision speeds, and the formation of ``kink trains'' with up to four closely spaced left- or right-moving kinks. We performed a flat space numerical study of abelian Higgs cosmic string intercommutation in the type-II regime $\beta > 1$ (where $\beta = m^2_{scalar} / m^2_{gauge}$) up to $\beta = 64$, the highest value investigated to date. Our results confirm earlier claims that the minimum critical speed for double reconnection goes down with increasing $\beta$, from $\sim 0.98 c$ at $\beta = 1$ to $\sim 0.86 c$ for $\beta = 64$. Furthermore, we observe a qualitative change in the process leading to the second intercommutation: if $\beta \geq 16$ it is mediated by a loop expanding from the collision point whereas if $1 < \beta \leq 8 $ the previously reported ``loop'' is just an expanding blob of radiation which has no topological features and is absorbed by the strings. The multiple reconnections are observed in the loop-mediated, deep type-II regime $\beta \geq 16$. Triple reconnections appear to be quite generic for collision parameters on the boundary between single and double reconnection. For $\beta = 16$ we observe quadruple events. They result in clustering of small scale structure in the form of ``kink trains''. Our findings suggest that, due to the core interactions, the small scale structure and stochastic gravitational wave background of abelian Higgs strings in the strongly type-II regime may be quite different from what would be expected from studies of Nambu-Goto strings or of abelian Higgs strings in the $\beta \approx 1$ regime.