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					      Brief History of the e-A Collider
                  Concept



                      Peter Paul
                      BNL/SBU


Peter Paul 12/16/06   e-A Collider concept   1
     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.
Peter Paul 12/16/06        e-A Collider concept                       2
     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 ~ 1029 cm-2 s-1
 Peter Paul 12/16/06        e-A Collider concept          3
      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 ~ 1033 cm-2s-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/)
• 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

Peter Paul 12/16/06      e-A Collider concept                  4
  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                          Question: Why is there
experimental goal for partonic                    still so much skepticism
matter.                                           about the CGC?

The aim for very low x favored                    Is this a repeat similar to
electron scattering at very high                  the longtime skepticism
energies or from heavy nuclei.                    about the QGP?
Peter Paul 12/16/06        e-A Collider concept                                 5
  The e-A Collider probes CGC for less cost
The statements at right taken
from Raju’s talk to BNL PAC
Qs2 is parton (i.e. gluon)
density. It seems to scale like
A1/3 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.

 Peter Paul 12/16/06          e-A Collider concept   6
      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.
Peter Paul 12/16/06      e-A Collider concept                  7
             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:
              > 1033 cm-2s-1 region for e-p
              > 1031 cm-2s-1 region for e-Au
 • >70% polarization degree for both lepton and proton
 beams
 • Longitudinal polarization in the collision point




    http://www.agsrhichome.bnl.gov/eRHIC/




Peter Paul 12/16/06                                e-A Collider concept       8
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 1032 cm-2s-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

Peter Paul 12/16/06      e-A Collider concept                    9
              Linac-Ring Design based on 5-20+ GeV ERL

                                               IP#12 - main
IP#10 - optional

                       •   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
                                                                                                             Ø1.22 km
                       •   e--p luminosity up to 1034 cm-2s-1
                       •   Polarized positrons are possible only with additional ring

                                                                                            IP#4- optional




                                                RHIC
                       Booster


                                           AGS
 Peter Paul 12/16/06                          e-A Collider concept                                              10
     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 1034 cm-2s-1
• Polarized positrons are possible only with additional ring
• Cost of ring-ring and linac-ring versions about the same.


Peter Paul 12/16/06      e-A Collider concept                  11
                      Effort in Europe continues
• TESLA + HERA  THERA in
  2001:
  Ee = 250 to 800 GeV; Ep = 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 = 1033 cm-2s-1
  Linac-Ring version

Peter Paul 12/16/06           e-A Collider concept   12
Peter Paul 12/16/06   e-A Collider concept   13
                       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?

 Peter Paul 12/16/06   e-A Collider concept             14

				
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