A: ATHENA has been designed as a general purpose detector capable of delivering the full scientific scope of the Electron-Ion Collider. Careful technology choices provide fine tracking and momentum resolution, high performance electromagnetic and hadronic calorimetry, hadron identification over a wide kinematic range, and near-complete hermeticity.This article describes the detector design and its expected performance in the most relevant physics channels. It includes an evaluation of detector technology choices, the technical challenges to realizing the detector and the R&D required to meet those challenges.

We present predictions and postdictions for a wide variety of hard jet-substructure observables using a multi-stage model within the JETSCAPE framework. The details of the multi-stage model and the various parameter choices are described in Ref. [1]. A novel feature of this model is the presence of two stages of jet modification: a high virtuality phase (modeled using MATTER), where coherence effects diminish medium-induced radiation, and a lower virtuality phase (modeled using LBT), where parton splits are fully resolved by the medium as they endure multiple scattering induced energy loss. Energy loss calculations are carried out on event-by-event viscous fluid dynamic backgrounds constrained by experimental data. The uniformed and consistent descriptions of multiple experimental observables demonstrate the essential role of coherence effects and the multi-stage modeling of the jet evolution. Using the best choice of parameters from Ref.[1], and with no further tuning, we present calculations for the medium modified jet fragmentation function, the groomed

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