The experiment's goal was to investigate the quark–gluon structure of matter by examining how a nucleon's constituents affect its spin.
[4] HERA's longitudinally polarized electron beam was used to explore the spin structure of nucleons.
For this purpose, the electrons were scattered at energies of 27.5 GeV at an internal gas target.
This target and the detector itself were designed especially with a view to spin-polarized physics.
The unpolarized gas feed system (UGFS) was used for non-polarized "end of fill" or heavy-target running before the polarized target was removed in 2003.