Higher order intercommutations in Cosmic String Collisions

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 $β= m^2_{scalar} / m^2_{gauge}$) 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 $\sim 0.98 c$ at $β= 1$ to $\sim 0.86 c$ for $β= 64$. Furthermore, we observe a qualitative change in the process leading to the second intercommutation: if $β\geq 16$ it is mediated by a loop expanding from the collision point whereas if $1 < β\leq 8$ 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 $β\geq 16$. 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 $β\approx 1$ regime.

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