Prime Decomposition and the Iwasawa mu-invariant

For $\mathrm{\Gamma }={\mathbb{Z}}_p$, Iwasawa was the first to construct $\mathrm{\Gamma }$-extensions over number fields with arbitrarily large $\mu$-invariants. In this work, we investigate other uniform pro-$p$ groups which are realizable as Galois groups of towers of number fields with arbitrarily large $\mu$-invariant. For instance, we prove that this is the case if $p$ is a regular prime and $\mathrm{\Gamma }$ is a uniform pro-$p$ group admitting a fixed-point-free automorphism of odd order dividing $p-1$. Both in Iwasawa’s work, and in the present one, the size of the $\mu$-invariant appears to be intimately related to the existence of primes that split completely in the tower.