We discuss the option of having polarised protons in the HERA ep-collider at DESY. We give an overview of the physics potential, with special emphasis on the polarised structure function g[ and the polarised gluon density G(x) of the proton.
TECHNICAL
ASPECTS
The ep-collider HERA has been operational in collider mode (27.5 GeV electrons on 820 GeV protons) since summer 1992 with the experiments H1 and ZEUS, delivering plenty of new results on deep inelastic scattering and photoproduction, in previously inaccessible kinematic regions. This has in particular led to important progress in the understanding of the proton structure at low x ~ 10 -2 and low Q~. For the first time also very high Q2 of O ~ 10000 GeV 2 could be accessed in deep inelastic scattering (DIS). Due to the Sokolov-Ternov effect the electron beam gets transversely polarised in the machine to about 60%. Spin rotators can flip transverse into longitudinal polarisation for physics studies. The fixed target experiment HERMES uses a polarised gas target to study polarised ep collisions at x/~ = 7 GeV. The natural extension of this program is to have a polarised proton beam to study the spin in the proton at a centre of mass system (cms) energy of x/~ = 300 GeV. Polarisation of the proton beam at HERA is technically more involved than for the electron beam, since protons do not' polarise naturally in a storage ring. Polarised protons have to be generated with a high current polarised source (recent progress on polarised sources is reviewed in [4]), accelerated from low energies while maintaining their polarisation during the whole chain of pre-accelerators. Studies on this are ongoing, and so far the results look very encouraging . Based on these studies, we assume that HERA can be operated with polarised electron and proton beams, each polarised to about 70°~, and a luminosity of 200 to 500 pb -l integrated over several years can be reached. Precision studies of polarised observables require also a good knowledge on the absolute polarisation of proton and electron beams. Various designs for proton polarimeters have been proposed and are reviewed in . The goal is to reach an accuracy of 5% on the measurement of the proton polarisation. Apart from protons, it is very interesting to have data on polarised neutrons. Deuterium is not a viable option for HERA, due to problems in rotating the transverse polarisation into a longitudinal one in the interaction regions, but 3He seems to be a good candidate. The physics prospects of a polarised HERA collider were investigated for the first time in a working group of the 1996 "Future Physics at HERA" workshop [1]. In a dedicated workshop "Physics with polarized protons at HERA", which was held at DESY in 1997, promising topics were studied in more detail, e.g. detector effects were included using parameters corresponding to tile present detectors H1 and ZEUS, arid new