Abstract. Using the discriminant modular form and the Noether formula it is possible to write the admissible self-intersection of the relative dualising sheaf of a semistable hyperelliptic curve over a number field or function field as a sum, over all places, of a certain adelic invariant χ. We provide a simple geometric interpretation for this invariant χ, based on the arithmetic of symmetric roots. We propose the conjecture that the invariant χ coincides with the invariant ϕ introduced in a recent paper by S.-W. Zhang. This conjecture is true in the genus 2 case, and we obtain a new proof of the Bogomolov conjecture for curves of genus 2 over number fields.