Future electron-hadron colliders

Outstanding research potential of electron-hadron colliders (EHC) was clearly demonstrated by first - and the only - electron-proton collider HERA (DESY, Germany) [1,2]. Physics data from HERA revealed new previously unknown facets of Quantum Chromo-Dynamics (QCD). EHC is an ultimate microscope probing QCD in its natural environment, i.e. inside the hadrons. In contrast with hadrons, electrons are elementary particles with known initial state. Hence, scattering electrons from hadrons provides a clearest pass to their secrets. It turns EHC into an ultimate machine for high precision QCD studies and opens access to rich physics with a great discovery potential: solving proton spin puzzle, observing gluon saturation[1] or physics beyond standard model [4]. Access to this physics requires high-energy high-luminosity EHCs and a wide reach in the center-of-mass (CM) energies. This paper gives a brief overview of four proposed electron-hadron colliders: ENC at GSI (Darmstadt, Germany), ELIC/MEIC at TJNAF (Newport News, VA, USA), eRHIC at BNL (Upton, NY, USA) and LHeC at CERN, Geneva, Switzerland). I want to thanks A. Lehrach (Juelich); BNLs' eRHIC team J. Beebe-Wang, I. Ben-Zvi, X. Chang, Y. Hao, A. Jain, D. Kayran, G. Mahler, W. Meng, B. Parker, V. Ptitsyn, T. Roser, J. Skaritka, D. Trbojevic, N. Tsoupas, J. Tuozzolo, G. Wang; and O. Bruening, H. Burkhardt, R. Tomas, M. Klein, J. Jowett and F. Zimmermann from LHeC team for the information. Y. Zhang from TJNAF provided six slides on ELIC/MEIC with specific request not to comment on their designs. Hence, in this paper I copy ELIC/MEIC parameters and descriptions from his material without any checks for self-consistency. This is neither a criticism nor an endorsement of the ELIC/MEIC design.