1. Explore the new QCD frontier: strong color fields in nuclei - How do the gluons contribute to the structure of the nucleus? - What are the properties of high density gluon matter? - How do fast quarks or gluons interact as they traverse nuclear matter? Precisely image the sea-quarks and gluons in the nucleon - How do the gluons and sea-quarks contribute to the spin structure of the nucleon? - What is the spatial distribution of the gluons and sea quarks in the nucleon? - How do hadronic final-states form in QCD? How do we understand the visible matter in our universe in terms of the fundamental quarks and gluons of QCD? - How do the gluons contribute to the structure of the nucleon?

2. 50% of momentum carried by gluons … but still unknowns in our knowledge g S S xS > xg ??? A low Q 2 puzzle …

3. Parameterize as = -  lnF 2 /  lnx Hadron-hadron scattering energy dependence Observe transition from partons to hadrons (gluon clumps?) in data at a distance scale  0.3 fm ?? 50% of momentum carried by gluons … but still unknowns in our knowledge

4. Longitudinal Structure Function F L Experimentally can be determined directly IF VARIABLE ENERGIES! Highly sensitive to effects of gluon + 12-GeV data + EIC alone (includes systematic uncertainties)

5. The Spin Structure of the Nucleon We know from lepton scattering experiments over the last three decades that: quark contribution ΔΣ ≈ 0.3 gluon contribution ΔG ≈ 1 ± 1  u > 0,  d < 0,  s ~ 0 measured anti-quark polarizations are consistent with zero ½ = ½  +  G + L q + L g Proton helicity sum rule:  G: a “quotable” property of the proton (like mass, momentum contrib., …)

6. The Quest for  G First approach: use scaling violations of world g 1 spin structure function measurements Not enough range in x and Q 2 g 1 =  q e q 2  f q (x,Q 2 )

7. PHENIX, STAR   The Quest for  G   HERMES, COMPASS

8. World Data on F 2 p World Data on g 1 p  50% of momentum carried by gluons  20% of proton spin carried by quark spin The dream is to produce a similar plot for x  g(x) vs x

9. World Data on F 2 p World Data on g 1 p The dream is to produce a similar EIC plot for g 1 (x,Q 2 ) over similar x and Q 2 range An EIC makes it possible! Region of existing g 1 p data

10. The Gluon Contribution to the Proton Spin at small x Superb sensitivity to  g at small x!

11. Projected data on  g/g with an EIC, via  + p  D 0 + X K - +  + Access to  g/g is also possible from the g 1 p measurements through the QCD evolution, and from di-jet measurements. RHIC-Spin The Gluon Contribution to the Proton Spin Advantage: measurements directly at fixed Q 2 ~ 10 GeV 2 scale! Uncertainties in x  g smaller than 0.01 Measure 90% of  G (@ Q 2 = 10 GeV 2 )  g/g

12. RHIC-Spin region Precisely image the sea quarks Spin-Flavor Decomposition of the Light Quark Sea | p = + + + … > u u d u u u u d u u d d d Many models predict  u > 0,  d < 0 No competition foreseen!

13. GPDs and Transverse Gluon Imaging Deep exclusive measurements in ep/eA with an EIC: diffractive:transverse gluon imagingJ/ ,  o,  (DVCS) non-diffractive:quark spin/flavor structure , K,  +, … [ or J/ , ,  0 , K,  +, … ] Describe correlation of longitudinal momentum and transverse position of quarks/gluons  Transverse quark/gluon imaging of nucleon (“tomography”) Are gluons uniformly distributed in nuclear matter or are there small clumps of glue?

14. GPDs and Transverse Gluon Imaging gives transverse size of quark (parton) with longitud. momentum fraction x EIC: 1) x < 0.1: gluons! x < 0.1x ~ 0.3x ~ 0.8 Fourier transform in momentum transfer x ~ 0.001 2)  ~ 0  the “take out” and “put back” gluons act coherently. 2)  ~ 0 x -  x +   d

15. GPDs and Transverse Gluon Imaging Two-gluon exchange dominant for J/ , ,  production at large energies  sensitive to gluon distribution squared! LO factorization ~ color dipole picture  access to gluon spatial distribution in nuclei: see eA! Measurements at DESY of diffractive channels ( J/ , , ,  ) confirmed the applicability of QCD factorization: t-slopes universal at high Q 2 flavor relations  :  Unique access to transverse gluon imaging at EIC! Fit with d  /dt = e -Bt

16. GPDs and Transverse Gluon Imaging k k'k' ** q q'q'  pp'p' e A Major new direction in Nuclear Science aimed at the 3-D mapping of the quark structure of the nucleon. Simplest process: Deep-Virtual Compton Scattering Simultaneous measurements over large range in x, Q 2, t at EIC! At small x (large W):  ~ G(x,Q 2 ) 2

17. GPDs and Transverse Gluon Imaging Goal: Transverse gluon imaging of nucleon over wide range of x: 0.001 < x < 0.1 Requires: - Q 2 ~ 10-20 GeV 2 to facilitate interpretation - Wide Q 2, W 2 (x) range - Sufficient luminosity to do differential measurements in Q 2, W 2, t Q 2 = 10 GeV 2 projected data Simultaneous data at other Q 2 -values EIC enables gluon imaging! Scaled from 2 to 16 wks. EIC (16 weeks)

18. The Future of Fragmentation p h x TMD  D Current Fragmentation h q Can we understand the physical mechanism of fragmentation and how do we calculate it quantitatively? Target Fragmentation p M h  d  h ~  q f q (x)  D f h (z) d  h ~  q  M h/p q (x,z) QCD Evolution Correlate at EIC

19. Proposed EIC recommendation for the Galveston meeting A high luminosity Electron-Ion Collider (EIC) is the highest priority of the QCD community for new construction after the JLab 12 GeV and RHIC II luminosity upgrades. EIC will address compelling physics questions essential for understanding the fundamental structure of matter: - Explore the new QCD frontier: strong color fields in nuclei; - Precisely image the sea-quarks and gluons to determine the spin, flavor and spatial structure of the nucleon. This goal requires that R&D resources be allocated for expeditious development of collider and detector design.

20. eRHIC: (ERL-based) linac-ring design Electron energy range from 3 to 10 (20) GeV Peak luminosity of 2.6  10 33 cm -2 s -1 in electron-hadron collisions; high electron beam polarization (~80%); full polarization transparency at all energies for the electron beam; multiple electron-hadron interaction points (IPs) and detectors;  5 meter “element-free” straight section(s) for detector(s); ability to take full advantage of electron cooling of the hadron beams; easy variation of the electron bunch frequency to match it with the ion bunch frequency at different ion energies. PHENIX STAR e-cooling (RHIC II) Four e-beam passes Main ERL (2 GeV per pass)

21. eRHIC ring-ring design AGS BOOSTER TANDEMS RHIC 2 – 10 GeV e-ring e-cooling 2 -10GeV Injector LINAC Based on existing technology (+ electron cooling) Collisions at 12 o’clock interaction region 10 GeV, 0.5 A e-ring with 1/3 of RHIC circumference (similar to PEP II HER) Inject at full energy 5 – 10 GeV Polarized electrons and positrons Peak luminosity of 4.7  10 32 cm -2 s -1 in electron-hadron collisions

22. ELIC ring-ring design Electron Cooling Snake 3-9 GeV electrons 3-9 GeV positrons 30-225 GeV protons 30-100 GeV/n ions IR Visionary green-field design: “Figure-8” lepton and ion rings to ensure spin preservation and ease of spin manipulation. Peak luminosity up to ~10 35 cm -2 s -1 through short ion bunches, low β*, and high rep rate (crab crossing) Four interaction regions with  3m “element-free” straight sections. SRF ion linac concept for all ions ~ FRIB 12 GeV CEBAF accelerator, with present JLab DC polarized electron gun, serves as injector to the electron ring.

23. Detector design Main detector: learn from ZEUS (+ H1) But:low-field region around central tracker better particle identification forward-angle detectors auxiliary detectors for exclusive events auxiliary detectors for normalization

24. Detector design Main detector: Top view Hadronic calorimeter Alternate detector: Emphasize low-x, low-Q 2 diffractive physics (“HERA-III design”, MPI-Munchen) Main detector: Emphasize high-luminosity full physics program Si tracking stations EM calorimeter e p/A

25. Cost & Realization Generic R&D to advance the ongoing EIC conceptual design efforts in order to pursue the best EIC design by next LRP: $6M per year for 5 years (see next slide) “This goal requires that R&D resources be allocated for expeditious development of collider and detector design.” Realization of the three different EIC design options (eRHIC linac-ring, eRHIC ring-ring, ELIC ring-ring) were discussed earlier in the BNL and JLab Facility Reports. The cost scale of an EIC (here taken as the TPC of the 10 GeV x 250 GeV eRHIC linac-ring design) is estimated at ~700M$.

26. (pre-)R&D General: all designs require Electron Cooling $4M/year Accelerator R&D encompasses: Design Studies to Optimize Existing EIC Approaches High-Intensity Polarized Electron Source (eRHIC L-R) Energy Recovery Technology for High Energy and High Current Beams (eRHIC L-R) Development of Compact Recirculation Loop Magnets (eRHIC L-R) Design and Prototype of Multi-Cell Crab Cavities (ELIC) [required for L~10 35 cm -2 s -1 ] Simulations of Stacking Intense Ion Beams in Pre-Booster (ELIC) [required for L~10 35 cm -2 s -1 ] Simulations of Electron Cooling w/circulator ring and GHz kicker prototype (ELIC) Precision High-Energy Ion Polarimetry Polarized 3He Production (EBIS) and Acceleration $2M/year Detector R&D encompasses: Low-Angle Electron Tagging System Multi-Level Triggering System to Include Tracking and Reject Background High-Speed Data Acquisition System to Handle Small Bunch Spacing ( ELIC) Central Tracker Development cost-effective and compact high-rate tracking solution cost-effective method for hadron identification tagging systems for recoiling neutrons and heavier nuclei

27. Summary The last decade or so has seen tremendous progress in our understanding of the partonic sub-structure of nucleons and nuclei based upon: The US nuclear physics flagship facilities: RHIC and CEBAF The surprises found at HERA (H1, ZEUS, HERMES) The development of a theory framework allowing for a revolution in our understanding of the inside of hadrons … QCD Factorization, Lattice QCD, Saturation This has led to new frontiers of nuclear science: - the possibility to map the role of gluons - a new QCD regime of strong color fields The EIC presents a unique opportunity to maintain US and BNL&JLab leadership in high energy nuclear physics and precision QCD physics

28. Proposed EIC recommendation for the Galveston meeting A high luminosity Electron-Ion Collider (EIC) is the highest priority of the QCD community for new construction after the JLab 12 GeV and RHIC II luminosity upgrades. EIC will address compelling physics questions essential for understanding the fundamental structure of matter: - Explore the new QCD frontier: strong color fields in nuclei; - Precisely image the sea-quarks and gluons to determine the spin, flavor and spatial structure of the nucleon. This goal requires that R&D resources be allocated for expeditious development of collider and detector design.