Higher order intercommutations in Cosmic String Collisions
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 β > 1 (where β = m2scalar / m2gauge) up to β = 64, the highest value investigated to date. Our results confirm earlier claims that the minimum critical speed for double reconnection goes down with increasing β, from 0.98 c at β = 1 to 0.86 c for β = 64. Furthermore, we observe a qualitative change in the process leading to the second intercommutation: if β ≥ 16 it is mediated by a loop expanding from the collision point whereas if 1 < β ≤ 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 β ≥ 16. Triple reconnections appear to be quite generic for collision parameters on the boundary between single and double reconnection. For β = 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 β ≈ 1 regime.