ePIC Technical Note: E0–E1 Interface

This technical note describes the interface between Echelon 0 (E0), comprising the ePIC detector and DAQ system, and Echelon 1 (E1), comprising the distributed computing facilities at BNL and JLab, within the ePIC Computing Model. The note defines the architecture, requirements, and interface concepts for exchanging streaming detector data, operational state, metadata, and control information between the DAQ and downstream computing systems. It addresses raw-data transfer using time frames (TFs) and super time frames (STFs), state models and transition rules, latency requirements, information exchange, calibration and conditions databases, control interfaces, and support for AI-enabled workflows and experiment control. A reference implementation based on the ePIC streaming workflow testbed illustrates selected interface concepts, including distributed data management, workflow orchestration, fast and prompt processing, and AI-assisted operations. This document represents version 0.75 of the evolving E0–E1 interface specification. It identifies outstanding design questions and outlines a roadmap for refining interface definitions and implementation choices through prototypes and system-level validation, toward integrated streaming computing capabilities for the ePIC experiment.