ePIC

ePIC. The **Electron–Ion Collider** (**EIC**) is a next-generation nuclear physics facility under construction at Brookhampton National Laboratory, designed to probe the fundamental structure of protons, neutrons, and atomic nuclei. As its primary, general-purpose detector, the ePIC (**Electron–Proton/Ion Collider**) spectrometer is engineered to capture the complex particle showers produced in high-energy collisions between polarized electron beams and polarized beams of protons or heavier ions. This sophisticated apparatus is central to the scientific mission of the EIC, aiming to provide unprecedented three-dimensional imaging of the quark and gluon distributions inside nucleons and to explore the emergent properties of dense gluonic matter.

Overview

The ePIC detector is a hermetic, multi-purpose spectrometer optimized for the challenging environment of the Electron–Ion Collider, which is being built by leveraging the existing infrastructure of the Relativistic Heavy Ion Collider (RHIC). Its design philosophy emphasizes high precision, full angular coverage, and the ability to handle high interaction rates, enabling a comprehensive exploration of quantum chromodynamics (QCD) over a wide kinematic range. The international collaboration behind the detector involves hundreds of scientists from institutions worldwide, including lead roles from Jefferson Lab, Brookhaven National Laboratory, and contributions from CERN, DESY, and numerous universities. As the cornerstone instrument for the first phase of the EIC physics program, ePIC is tasked with making precise measurements of processes like deep inelastic scattering and semi-inclusive deep inelastic scattering to unravel the spin and spatial structure of hadrons.

Scientific goals and design

The primary scientific goals driving the ePIC design are the precise determination of the proton's spin structure, the tomographic imaging of parton distributions in three dimensions, and the study of the gluon saturation regime expected in high-energy collisions with heavy ions. To achieve these, the detector employs a design with a 1.5 T solenoid magnet and is structured in a forward, central, and backward configuration relative to the ion beam direction, ensuring nearly 4π steradian coverage. Key design requirements include excellent vertex resolution provided by silicon trackers, high-performance particle identification via time-of-flight detectors and ring-imaging Cherenkov counters, and precise energy measurement from electromagnetic and hadronic calorimeters. This integrated approach allows ePIC to reconstruct the full final state of collisions, identifying particles like pions, kaons, and protons over a wide momentum range and separating the signatures of scattered electrons from hadronic backgrounds.

Detector components

The ePIC detector comprises several major subsystems arranged in a barrel and endcap geometry. The innermost tracking system consists of silicon pixel and strip detectors surrounding the interaction point, providing precise vertex determination and track reconstruction. Outside the tracker, a combination of a time projection chamber and drift chambers measures particle momenta in the magnetic field. Particle identification in the central region is achieved with a time-of-flight wall and a detector for internally reflected Cherenkov light (DIRC), while the forward region utilizes a ring-imaging Cherenkov detector and a gas electron multiplier-based tracker. Electromagnetic calorimetry is provided by lead tungstate crystals and silicon–tungsten sampling calorimeters, and hadronic energy is measured with steel-scintillator sampling calorimeters. The entire system is supported by a sophisticated trigger and data acquisition system designed to manage the high collision rates.

Physics program

The physics program enabled by ePIC is vast, focusing on fundamental questions in strong interaction physics. A central pillar is the detailed mapping of generalized parton distributions and transverse momentum dependent parton distribution functions through measurements of exclusive processes like deeply virtual Compton scattering and semi-inclusive hadron production. The experiment will precisely measure the contribution of gluon spin to the proton's spin and search for signs of color glass condensate in electron–nucleus collisions. Additional programs include studies of hadronization and cold nuclear matter effects, precision tests of quantum electrodynamics and electroweak theory at high energies, and searches for physics beyond the Standard Model through rare processes. The ability to collide polarized electrons with polarized protons or light ions, as well as a wide range of nuclei from deuterium to uranium, provides a unique laboratory for exploring the interplay between partonic and nuclear degrees of freedom.

Collaboration and timeline

The ePIC project is managed by a large international collaboration, the **ePIC Collaboration**, which includes physicists, engineers, and technicians from over 150 institutions across more than 30 countries, including the United States, Germany, China, and India. The project leadership involves key personnel from Brookhaven National Laboratory, Jefferson Lab, and Lawrence Berkeley National Laboratory. The timeline for ePIC is closely tied to the construction of the Electron–Ion Collider accelerator; detector design and prototyping began in earnest following the United States Department of Energy's approval of the EIC project in 2020. Major subsystem reviews and integration planning are ongoing, with technical design reports completed for critical components. Installation of the detector is scheduled to commence in the late 2020s, with first collisions and the beginning of the physics data-taking program anticipated for the early 2030s, marking the start of a new era in nuclear physics research.