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HERMES
is an experiment at HERA (DESY) in Hamburg
to study the spin structure of the nucleon by deep inelastic
scattering of polarised electrons or positrons at 27.5 GeV, off polarised protons and neutrons.
An important feature of the HERMES experiment is its good particle identification capabilities which allows for detailed studies of semi-inclusive physics, such as the flavour decomposition of the spin quark distributions.
HERMES makes use of two
innovative techniques:
 Longitudinally polarised electron (positron) beam in a storage ring. The polarisation is naturally in the transverse direction due to the Sokolov-Ternov effect, while spin rotators are used to rotate the spin into the longitudinal direction. In May 1994 for the first time longitudinal electron polarisation was produced in a high energy ring: values up to 70% have been reached.
Internal polarised gas targets based on a windowless storage cell inside the HERA storage ring. A storage cell increase the density of the target gas by two order of magnitude compared to a free gas jet. | 
