The Electron-Ion Collider (EIC) is the next-generation US-based project for QCD and nuclear science. It will collide polarized electrons with polarized protons and light ions, as well as unpolarized ions of any mass, and will provide a high luminosity over a wide range of c.m. energies.
The first EIC detector (ePIC) will support a broad science program, but having two detectors would significantly expand the capabilities of the EIC. The possibility to independently cross check results between the two detectors will greatly enhance the discovery potential. And as demonstrated by H1 and ZEUS, combining data from two detectors can also reduce the overall systematic uncertainties, which will be even more impactful at the EIC than at HERA since the luminosity will be a hundred times higher.
A second detector can also provide new capabilities that would expand the EIC program. In particular, the interaction region in which it would be located (IR8) can support ion beam optics with a second focus that would greatly improve the far-forward near-beam acceptance for low-x protons and light nuclei, and make it possible to detect almost all nuclear fragments in reactions where the nucleus breaks up, significantly enhancing the nuclear part of the EIC program.
The central detector could also provide complementary capabilities, such as a solenoid with a higher magnetic field and improved muon identification.
And if a second EIC detector would be built up to five years after ePIC, this would provide ample time for additional R&D. It could, for instance, make it possible to extend the momentum coverage for hadron identification in the barrel region.