We study rational curves of degree two on a smooth sextic 4-fold and their counting invariant defined using Donaldson-Thomas theory of Calabi-Yau 4-folds. By comparing it with the corresponding Gromov-Witten invariant, we verify a conjectural relation between them proposed by the author, Maulik and Toda.
IntroductionLet X be a smooth sextic 4-fold in P 5 and M β be the moduli scheme of one dimensional stable sheaves on X with Chern character (0, 0, 0, β, 1). We are interested in the counting invariant of M β defined using Donaldson-Thomas theory of Calabi-Yau 4-folds, introduced in [1, 2]. In particular, there exists a virtual classAnd we may use insertions to define counting invariants: for a class γ ∈ H 4 (X, Z), let, where π X , π M are projections from X ×M β onto corresponding factors and ch 3 (E) is the Poincaré dual to the fundamental class of the universal sheaf E.The degree matches and we define DT 4 invariants as followsSince the definition of the virtual class involves a choice of orientation on certain (real) line bundle over M β , the invariant will also depend on that (see Sect. 2.1 for more detail). Another obvious way to enumerating curves on X is by GW theory. More specifically, for γ ∈ H 4 (X, Z), the genus 0 Gromov-Witten invariant of X iswhere ev : M 0,1 (X, β) → X is the evaluation map.In a previous work [4], the author, Maulik and Toda proposed a conjectural relation between DT 4 invariants for one dimensional stable sheaves on X and genus zero GW invariants of X (see Conjecture 2.3 for details). The main result of this note is to verify this conjecture for degree two curve class on a smooth sextic 4-fold.Theorem 0.1. (Theorem 2.4) Conjecture 2.3 is true for degree one and two classes of a smooth sextic 4-fold X ⊆ P 5 , i.e. for the line class l ∈ H 2 (X, Z) and any γ ∈ H 4 (X), we have GW 0,l (γ) = DT 4 (l | γ), GW 0,2l (γ) = DT 4 (2l | γ) + 1 4 · DT 4 (l | γ),for certain choice of orientation in defining the RHS.