The role of Spectator Fragments at an electron Ion collider

TL;DR

Detecting spectator fragments at an electron-ion collider is crucial for understanding diffractive processes and nuclear structure, emphasizing the importance of gamma and neutron detection for accurate measurements.

Contribution

This paper highlights the significance of spectator fragment detection in eIC experiments and discusses design considerations for efficient gamma and neutron detection.

Findings

Spectator fragment detection breaks coherence in diffractive processes.

Gamma and neutron detection are essential for accurate nuclear measurements.

Design emphasis on fragment detection improves eIC experimental capabilities.

Abstract

Efficient detection of spectator fragments is key to the main topics at an electron-ion collider (eIC). Any process which leads to emission of fragments or $\gamma$'s breaks coherence in diffractive processes. Therefore this is equivalent to non-detection of rapidity gaps in pp collisions. For example, in coherent photoproduction of vector mesons their 4-momentum transfer distribution would image the "gluon charge" in the nucleus in the same way that Hofstadter measured its charge structure using elastic scattering of $\sim$100 MeV electrons. Whereas he could measure the $\sim$4 MeV energy loss by the electron due to excitation of nuclear energy levels (Figure 1), even the energy spread of the incident beam would prevent such an inclusive selection of quasielastic events at an eIC. The only available tool is fragment detection. Since, in our example, one finds that $\sim100$ of deexcitations go through $\gamma$'s or 1 neutron, rarely to 2 neutron and never to protons(due to Coulomb barrier suppression), the eIC design should emphasize their detection.