Moduli spaces and braid monodromy types of bidouble covers of the quadric

TL;DR

This paper studies bidouble covers of the quadric surface, introducing new methods to distinguish braid monodromy factorizations and exploring their implications for the classification and symplectic properties of abc-surfaces.

Contribution

It proves that braid monodromy factorizations determine the integers a, b, c for abc-surfaces and introduces a novel method to distinguish non-stably equivalent factorizations.

Findings

Braid monodromy determines a, b, c for abc-surfaces.

New method to distinguish non-stably equivalent factorizations.

Differences in symplectic structures suggested for certain abc-surfaces.

Abstract

Bidouble covers $\pi : S \mapsto Q$ of the quadric Q are parametrized by connected families depending on four positive integers a,b,c,d. In the special case where b=d we call them abc-surfaces. Such a Galois covering $\pi$ admits a small perturbation yielding a general 4-tuple covering of Q with branch curve $\De$, and a natural Lefschetz fibration obtained from a small perturbation of the composition of $ \pi$ with the first projection. We prove a more general result implying that the braid monodromy factorization corresponding to $\De$ determines the three integers a,b,c in the case of abc-surfaces. We introduce a new method in order to distinguish factorizations which are not stably equivalent. This result is in sharp contrast with a previous result of the first and third author, showing that the mapping class group factorizations corresponding to the respective natural Lefschetz pencils are equivalent for abc-surfaces with the same values of a+c, b. This result hints at the possibility that abc-surfaces with fixed values of a+c, b, although diffeomorphic but not deformation equivalent, might be not canonically symplectomorphic.