Gregory-Laflamme-type instability of boson strings and related phases in D=5 Kaluza-Klein theory

We add an $S^1$ extra dimension (size $L$) to the well known $D=4$ static, spherically symmetric $Q$-balls and boson stars. We show that the resulting uniform horizonless boson strings possess a static zero-mode for a critical value of $L$. This is at the threshold of a Gregory-Laflamme instability of these objects, occurring for larger values of $L$.The non-linear continuation of the zero-mode yields $D=5$ non-uniform boson strings. In addition, there are also intrinsic $D=5$ solutions describing localized boson stars on the $S^1$, supported by the contribution of the scalar field Kaluza-Klein modes. Basic properties of all three types of aforementioned solutions are discussed, together with their phase space.