Gravity-Aligned Vertical Electron-Positron Linear Collider
A gravity-aligned vertical electron-positron linear collider is proposed as a concept to break the cost-and-scale impasse that confronts conventional horizontal Higgs factories. The accelerating axis is oriented along the local gravitational field inside a single deep vertical shaft. Positrons are injected downward from the surface while electrons are accelerated upward from the bottom, the two beams meeting at an interaction point located deep underground. Targeting a staged center-of-mass energy in the Higgs factory range (250-500 GeV), this concept assumes high-gradient, high-efficiency, normal-conducting accelerating structures operated at cryogenic temperature together with a two-beam power-delivery scheme. It can be designed to reuse the existing KEK/SuperKEKB accelerator complex at the KEK Tsukuba Campus. We discuss the advantages of the vertical orientation, namely a small surface footprint, intrinsic azimuthal symmetry of the gravity load, deep vibration isolation of the collision point, gravity-assisted cryogenics, a natural vertical alignment reference, a cosmically quiet deep interaction point, an opportunity for on-site electricity self-sufficiency, and a unique opportunity in social/industrial evaluation, as well as the principal disadvantages and challenges, including the required accelerating gradient and the construction of a kilometers-deep shaft. Using only publicly available information, we argue that excavating a shaft with a depth of ~3 km is feasible at a geologically favorable site. This paper is deliberately concept-level and is intended to motivate technological innovation, feasibility studies, and the exploration of possible new channels for Beyond-Standard-Model searches through vertical collisions.