Branched covers of quasipositive links and L-spaces

Let $L$ be a oriented link such that ${\mathrm{\Sigma }}_n(L)$, the $n$-fold cyclic cover of $S^3$ branched over $L$, is an L-space for some $n\ge 2$. We show that if either $L$ is a strongly quasipositive link other than one with Alexander polynomial a multiple of $(t-1{)}^{2g(L)+(\mathrm{\mid }L\mathrm{\mid }-1)}$, or $L$ is a quasipositive link other than one with Alexander polynomial divisible by $(t-1)^{2g_4(L) + (|L|-1)}$,thenthereisaninteger$n(L)$, determined by the Alexander polynomial of $L$ in the first case and the Alexander polynomial of $L$ and the smooth $4$-genus of $L$, $g_4(L)$, in the second, such that $n\le n(L)$. If $K$ is a strongly quasipositive knot with monic Alexander polynomial such as an L-space knot, we show that ${\mathrm{\Sigma }}_n(K)$ is not an L-space for $n\ge 6$, and that the Alexander polynomial of $K$ is a non-trivial product of cyclotomic polynomials if ${\mathrm{\Sigma }}_n(K)$ is an L-space for some $n=2,3,4,5$. Our results allow us to calculate the smooth and topological 4-ball genera of, for instance, quasi-alternating quasipositive links. They also allow us to classify strongly quasipositive alternating links and $3$-strand pretzel links.