1. Peter Paul 12/16/06e-A Collider concept1 Brief History of the e-A Collider Concept Peter Paul BNL/SBU

2. Peter Paul 12/16/06e-A Collider concept2 The Early Discussions in Europe In the mid 1990’s the European nuclear community was searching for the next nuclear facility (DESY/GSI/NUPECC) GSI had not yet developed a viable concept. There was great interest in a high luminosity fixed target electron accelerator (Super CEBAF) Eventually interest centered on upgrading HERA which was running toward the end of its scientific program. It still lasted until 2007 with a luminosity upgrade. A number of meetings took place in 1995/96 to discuss parton physics at low x. Many members present here participated HERA demonstrated gluon saturation. But DESY was aiming to build TESLA and it was not clear that management wanted another accelerator option.

3. Peter Paul 12/16/06e-A Collider concept3 Starting point HERA: a special machine PETRA Ring receives electrons at 7.5 GeV, injects them into HERA at 14 GeV where they are accelerated to 27.5 GeV. Protons are accelerated in PETRA to 40 GeV and then in HERA to 920 GeV Its magnets have a cycle time of ~20 minutes! PETRA is needed to accelerate both electrons and protons L ~ 10 29 cm -2 s -1

4. Peter Paul 12/16/06e-A Collider concept4 Important Meetings in 1999 at DESY May 25/26,1999 at DESY HERA e-A Collider workshop identified the need for a high luminosity L ~ 10 33 cm -2 s -1 e-A collider. Lots of discussion. The DESY machine experts quickly investigated in detail what improvements would have to be done to the HERA ring to achieve this. The result was published in Sept. 1999. (www.desy.de/heraeA/)www.desy.de/heraeA/ The result was that Luminosity for heavy ions fell off sharply with A because of serious intra-beam scattering problems BNL people at the meeting realized quickly that RHIC would be a much more suitable heavy ion ring for an e-A collider because it was designed to minimize intra-beam scattering problem

5. Peter Paul 12/16/06e-A Collider concept5 Arrival of the Color Glass Condensate The study of saturated gluon matter was widely considered a worthwhile goal at all of these conferences. But the introduction of the color glass condensate defined a grand and intuitively exciting experimental goal for partonic matter. The aim for very low x favored electron scattering at very high energies or from heavy nuclei. Question: Why is there still so much skepticism about the CGC? Is this a repeat similar to the longtime skepticism about the QGP?

6. Peter Paul 12/16/06e-A Collider concept6 The e-A Collider probes CGC for less cost The statements at right taken from Raju’s talk to BNL PAC Q s 2 is parton (i.e. gluon) density. It seems to scale like A 1/3 which does not gain much from p to Au. However, HERA data have shown that  = 0.3 = 1/3 Thus an increase from p to AU downscales x by factor 200 for same gluon density.

7. Peter Paul 12/16/06e-A Collider concept7 The concept of an e-A collider moves forward in the U.S. The initial meeting at BNLin November 1999 and at Yale in Spring 2000 got things rolling (much thanks to Vernon Hughes). Next meetings followed at BNL and MIT from 2000 on. A set of machine concepts were developed at BNL and Jefferson Laboratory. Then the first White Paper was put together in 2002 involving many people present here. A Long Range Plan exercise in 2002 endorsed the exciting and broad physics potential. Since then the e-A Collider designs have been greatly refined.

8. Peter Paul 12/16/06e-A Collider concept8. http://www.agsrhichome.bnl.gov/eRHIC/ Physics Requirements To provide electron-proton and electron-ion collisions Energy ranges: 2-10 GeV polarized e - or 10 GeV polarized e + 26-250 GeV polarized protons or 100 GeV/u Au Luminosities: > 10 33 cm -2 s -1 region for e-p > 10 31 cm -2 s -1 region for e-Au >70% polarization degree for both lepton and proton beams Longitudinal polarization in the collision point

9. Peter Paul 12/16/06e-A Collider concept9 So why did HERA never build an electron - Heavy Ion collider? HERA had many limitations: The machine has a very small momentum aperture: Could not exceed 2 x 10 32 cm -2 s -1 for e-p, and less for e-A. Slow ramping time (20 min versus RHIC 2 min) could not handle intra-beam scattering for heavy ions  low luminosity. While protons could be injected above the transition energy, heavy ions would have to be injected below. German Government had decided to use PETRA for a high x- ray energy Synchrotron ring. Thus HERA would need a new electron and proton injector. The TESLA e beam and a new heavy ion ring would have been a powerful collider, but TESLA became FEL. GSI developed and received approval for the FAIR project

10. Peter Paul 12/16/06e-A Collider concept10 Linac EBIS Booster AGS RHIC IP#4- optional IP#12 - main IP#10 - optional Electron cooling For multiple passes: vertical separation of the arcs Ø1.22 km Linac-Ring Design based on 5-20 + GeV ERL Polarized electrons are generated in a gun, accelerated, put into collision(s), decelerated and dumped inside superconducting energy recovery linac (ERL). No beam-beam limitation for electron beam (the beam is used once!). No prohibited energy areas for the polarization. No spin rotators needed No trade-offs between detector length and luminosity e - -p luminosity up to 10 34 cm -2 s -1 Polarized positrons are possible only with additional ring

11. Peter Paul 12/16/06e-A Collider concept11 Great advantages of linac-ring option Polarized electrons are generated in a gun, accelerated once ore more, put into collision(s), decelerated and dumped inside superconducting energy recovery linac (ERL). No beam-beam limitation for electron beam (the beam is used once!). Very high intensity beam with energy recovery. Electron beam energy many MW! No prohibited energy areas for the polarization. No spin rotators needed No trade-offs between detector length and luminosity e - -p luminosity up to 10 34 cm -2 s -1 Polarized positrons are possible only with additional ring Cost of ring-ring and linac-ring versions about the same.

12. Peter Paul 12/16/06e-A Collider concept12 Effort in Europe continues TESLA + HERA  THERA in 2001: E e = 250 to 800 GeV; E p = 920 MeV But TESLA was a relatively low intensity machine Now LHeC: based on LHC presented in January 2006 at Orsay meeting 70 GeV electron beam with 50 MW beam power! 7 TeV proton beam L = 10 33 cm -2 s -1 Linac-Ring version

13. Peter Paul 12/16/06e-A Collider concept13

14. Peter Paul 12/16/06e-A Collider concept14 SUMMARY The scientific merits of a high-luminosity e-p and e-A facility seem accepted on both sides of he Atlantic. The machine design has largely matured and optimized: Luminosities similar to fixed targets can be achieved. The U.S. can build such a facility for about $650 Million (including one detector). If the U.S. community does not declare its strong interest in such a facility soon, will the center of gravity of this science eventually move to CERN?