1. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 1 Saturation, Low-x, and QCD at RHIC, Part 4 Future Perspectives (near and long term) T. Hallman Villaggio Guglielmo, Italy

2. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 2 Overview of forward-tagged proton program The physics program of tagged forward protons with STAR at RHIC in addition to the elastic scattering will: 1.Study standard both elastic and inelastic hadron diffraction and its spin dependence in unexplored t and  s range. 2.Study the structure of color singlet exchange in the non-perturbative regime of QCD. 3.Search for central production of light and massive systems in double Pomeron exchange process - glueballs, hybrids. 4.Search for an Odderon - an eigenstate of CGC. There is a great potential for important discoveries beginning with the next RHIC run (November 2007)

3. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 3 PQCD Picture Gluon Ladders Gluonic Exchanges In terms of QCD, Pomeron exchange consists of the exchange of a color singlet combination of gluons. Triggering on forward protons at high energies predominantly selects such exchanges and hence interactions mediated by gluon matter.

4. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 4 A new QCD exploration at RHIC Forward Tagged Proton Studies in STAR Polarized proton-proton Elastic Scattering (p  p   pp) The instrumentation used will be the STAR detector and the Roman Pots of the pp2pp experiment The pp2pp Roman Pot detectors will be used to tag very forward protons, thus selecting processes in which the proton stays intact, and the exchange has quantum numbers of the vacuum,  so called Pomeron (IP) exchange. (For these events, the probability of measuring reactions where colorless gluonic matter dominates the exchange is enhanced). The use of polarized proton beams, unique at RHIC, will allow exploring unknown spin dependence of diffraction including both elastic and inelastic processes. As the entire energy range of this study has been inaccessible to proton-proton (elastic) scattering in the past, all results will be new results.

5. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 5 Spin averaged observables Measurement of the differential pp cross section d  /dt over extended t-range will include the region at lower |t| that is particularly sensitive to the  parameter: measurements will include: 1) the  -parameter; 2) the nuclear slope parameter b in a combined fit to the differential cross section, and 3) the total cross section  tot. Interest in such studies: an asymptotic difference observed between the differential and total cross sections for pp could indicate an Odderon contribution to the scattering amplitude. (absence of an Odderon contribution would lead to identical cross sections); measurement of the total cross section,  tot, at the highest possible energy will also probe the prevalent assumption that the cross sections for pp and p  p scattering are asymptotically identical. With the expected 20  10 6 elastic events the estimated error on the slope parameter is  b=0.31 (GeV/c) -2 and that for the ratio of the real to imaginary part,  =0.01. This is comparable to the precision for existing measurements from pp and p  p data. The  tot = 2- 3 mb. A new QCD exploration at RHIC

6. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 6 Spin dependent observables By measuring spin related asymmetries one will be able to determine elastic scattering at the amplitude level. The availability of longitudinal polarization will allow measuring A LL in addition to A NN, A SS, and A N resulting in a significant improvement of the scientific reach of these measurements. Physics motivation to measure A N : possibility of the rise with energy of the spin-flip to spin- nonflip amplitudes ratio ( it may occur that the small contribution from hadronic spin-flip to the spin single-spin asymmetry observed with a polarized jet target at 100 GeV/c could be increased at  s=200 GeV). Reaching small |t|-values will allow measurement of the single spin analyzing power A N close to its maximum at |t| =0.0024 (GeV/c) 2, where A max = 0.04, at  s = 200 GeV. The A N and its t-dependence in the covered range are sensitive to a possible contribution of the single spin-flip amplitude,  5, from the interference between the hadronic spin-flip amplitude with the electromagnetic non-flip amplitude. An additional contribution of the hypothetical Odderon to the pp scattering amplitude can be probed by measuring the double spin-flip asymmetry, A NN. (A NN is thought to be sensitive to contributions of the real and imaginary parts to the double spin-flip amplitude,  2, in the range 0.003 < |t| < 0.010 (GeV/c) 2 ). A new QCD exploration at RHIC

7. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 7 Central Production in Double Pomeron Exchange In the double Pomeron exchange process each proton “emits” a Pomeron and the two Pomerons interact producing a massive system M X where M X =      c (  b ), qq(jets), H(Higgs boson), gg(glueballs) The massive system could form resonances. We expect that because of the constraints provided by the double Pomeron interaction, glueballs, hybrids, and other states coupling preferentially to gluons, will be produced with much reduced backgrounds compared to standard hadronic production processes. For each proton vertex one has t four-momentum transfer  p/p M X invariant mass

8. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 8 Central production of glueballs The production of glueballs may be enhanced in the central region in the process pp  pM X p.The crucial argument here is that the pattern of resonances produced in the central region, where both forward protons are tagged, depends on the vector difference of the transverse momentum of the final state protons  k T1,  k T2, with dP T  k T1  k T2 . The so-called dP T filter argument is that when dP T is large (   QCD )  qbar-q states are prominent and when dP T is small, the surviving resonances include glueball candidates. The geometrical acceptance of the proposed setup for both SDD and DPE processes, generating protons with t and  uniformly distributed in the regions 0.003< |t|< 0.04 and 0.005 <  < 0.05 respectively. In the proposed research, large data samples of diffractive states can be obtained and analyzed as a function of diffractive mass and t (d 2 σ/dM X 2 dt) for central production. As noted above, when dP T between two out going protons is small, the fraction of glueball candidates is enhanced; when the dP T is big the qqbar states are enhanced. In quantifying the threshold between these regimes, the WA102 experiment [54] found "small" to be dP T 0.5 GeV. With the expected luminosity one can collect about 450,000 triggered DPE events during the three days of running, with 40 hrs of useful beam time. The search for glueballs remains an experimental question.. A new QCD exploration at RHIC

9. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 9 Central Production in DPE at RHIC Glueball production at in DPE process at RHIC is complementary to GLUEX and PANDA Sparse spectrum! New I=0 mesons starting with 0  1.6 GeV 0 , 2  2.3 - 2.5 GeV No J PC -exotic glueballs until 2  4 GeV The glueball spectrum from an anisotropic lattice study Colin Morningstar, Mike Peardon Phys. Rev. D60 (1999) 034509

10. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 10 WA102 F(1500)         dP T > 0.5 GeV. Strong qq f 1 (1285), weak G? f 0 (1500) dP T < 0.2 GeV. Very weak qq f 1 (1285), stronger G? f 0 (1500) 0.2 GeV < dP T < 0.5 GeV. f 0 (1500) f 1 (1285) F.E.Close and A.Kirk, PLB397, 333 (1997).

11. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 11 Polarized Proton Collisions in RHIC

12. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 12 PP2PP Setup - tag forward protons Phys. Lett. B 579 (2004) 245-250 Phys. Lett. B 632 (2006) 167-172 Phys. Lett. B 647 (2007) 98-103 NIM A535 (2004) 415 - 420 The pp2pp Roman pots mounted on the outgoing beams, down stream from the STAR detector as shown. To achieve full azimuthal coverage for elastic events, at each location one Roman Pot station is horizontal and one vertical. RHIC accelerator magnets are used for momentum analysis resulting in forward proton taggers installed in the warm straight section between Q3 and Q4 magnets. The pp2pp Roman Pots are a moveable detector system allowing approach to the beam as closely as possible, thus extending the t and x ranges to the lowest values.

13. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 13 Roman Pot Stations at RHIC to IR Roman Pot below the beam Roman Pot above the beam

14. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 14 Reconstruction of the Proton Momentum Loss   Accelerator transport 1.Need to measure vector at the detection point, hence two RPs are needed on each side of STAR. 2.For a proton, which scatters with  and  we have: For M X = 2 GeV,  = 0.01 Because Because  and  are small special focusing is needed

15. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 15 STAR Detector - measure recoil system M x TPC: -1.0 <  < 1.0 FTPC: 2.8 <  < 3.8 FPD: |  |  3.8 (p+p) |  |  4.0 (p+p, d+Au)STAR

16. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 16 Resonance Signal in p+p and Au+Au collisions from STAR K(892)   (1520) p+p Au+Au  (1385) p+p Au+Au    (1020) p+p Au+Au p+p

17. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 17 Leading particle spectra Charged hadron p T distributions measured up to 12 GeV in Au+Au, d+Au and p+p reference

18. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 18 Run 2008 - Phase I Projections Elastic scattering: 1.100% acceptance for elastic scattering for 0.003 < |t| < 0.024; 2.20×10 6 elastic events:  b=0.31 (GeV/c)-2,  r=0.01,  tot = 2-3 mb; 3.In four t subintervals we shall have 5×10 6 events in each resulting in corresponding errors  A n =0.0017,  A nn =  A ss =0.003. DPE process: Luminosity few ×10 29 cm -2 sec -1 : 4.5  10 5 DPE events with full proton momentum reconstruction; (5-7)  10 6 DPE events with tagged protons - good size data sample for this physics. Need to reach small t and  values to measure small masses of interest  large  *=20m, special optics and beam scraping are needed. Hence a dedicated three-day run is planned

19. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 19 In summary the physics program with tagged forward protons at STAR will: Study elastic scattering and its spin dependence in unexplored t and  s range; Study the structure of color singlet exchange in the non-perturbative regime of QCD; Search for diffractive production of light and massive systems in double Pomeron exchange process; Search for new physics, including glueballs and Odderon. The proposed studies will add to the world body of knowledge concerning QCD in the non-perturbative regime where calculations are not easy and one has to be guided by measurements. In this way the proposed program extends the physics reach of both STAR and RHIC. A new QCD exploration at RHIC

20. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 20 The not-too-distant RHIC future? Beyond presently planned STAR / PHENIX Upgrades

21. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 21 A Fundamental Test of Universality: Transverse Spin Drell Yan at RHIC vs Sivers Asymmetry in Deep Inelastic Scattering Important test at RHIC of recent fundamental QCD predictions for the Sivers effect, demonstrating… attractive vs repulsive color charge forces Possible access to quark orbital angular momentum Latest development from DIS, first direct evidences of Sivers and Collins effects (recent new results at DIS 2007 in Műnchen, April 2007, HERMES, Diefenthaler et al.) Requires very high luminosity (RHIC II) Both STAR and PHENIX can make important, exciting, measurements Discussion available at http://spin.riken.bnl.gov/rsc/

22. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 22

23. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 23

24. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 24 Simple QED example: DIS: attractive Drell-Yan: repulsive Same in QCD: As a result: Attractive vs Repulsive Sivers Effects Unique Prediction of Gauge Theory !

25. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 25

26. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 26 0.1 0.2 0.3 x Sivers Amplitude 0 0 Experiment SIDIS vs Drell Yan: Sivers| DIS = − Sivers| DY *** Probes QCD attraction and QCD repulsion *** HERMES Sivers ResultsRHIC II Drell Yan Projections Markus Diefenthaler DIS Workshop Munich, April 2007

27. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 27 Luminosity Projection to 2016 (Collider Accelerator Department)

28. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 28 The more distant (but still foreseeable) RHIC future?

29. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 29 eRHIC 250 GeV protons, 100 GeV/n ions ELIC (JLab) 225 GeV protons, 100 GeV/n ions 9 GeV electrons, positrons Peak lumi (e+p) ~10 35 cm -2 s -1 Candidate EIC designs Electron Cooling Snake IR 10(20) GeV electrons Peak lumi (e+p) = 2.6x10 33 cm -2 s -1 10 GeV electrons+positrons Peak lumi (e+p) = 0.47x10 33 cm -2 s -1

30. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 30 Explore the new QCD frontier: strong color fields in nuclei - How do the gluons contribute to the structure of the nucleon / 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?

31. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 31 The e-A physics EIC physics agenda Broad theme: precision studies of emergent features of QCD What are the momentum distributions of quarks, anti-quarks and gluons in the nucleon? How are the symmetries of QCD manifest or broken in the nucleon? How are quarks and gluons distributed spatially in the nucleon? What is the nature of glue at high density? –How do strong fields appear in hadronic or nuclear wave functions at high energies? –What are the appropriate degrees of freedom? –How do they respond to external probes or scattering? –Is this response universal (ep, pp, eA, pA, AA)?

32. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 32 Kinematic reach of eRHIC and ELIC for e-A studies Region of gluon saturation

33. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 33 eRHIC physics example: F 2 (x,Q 2 ) (sea quarks generated by glue) one of the first EIC measurements nDS, EKS, FGS: pQCD models with different amounts of shadowing EIC can distinguish pQCD and saturation model expectations

34. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 34 50% of momentum carried by gluons, but still a lot we don’t know Longitudinal Structure Function F L includes systematic uncertainties

35. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 35 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

36. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 36 GPDs and Transverse Gluon Imaging Deep exclusive measurements in ep / eA with an EIC: diffractive:transverse gluon imagingJ/y, r o, g (DVCS) non-diffractive:quark spin/flavor structurep, K, r +, … [ 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?

37. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 37 Long Term Strategic View eRHIC: ~$700M [FY07$] RHIC II: $95M [FY07$] 2007201520202010 RHIC physics runs RHIC II physics runs eRHIC physics runs Detector Upgrades: $35M EBIS: $20M CD0 e beam + new detector e-cooling of ion beams Legend: R&D Construction Multiple small projects CD0: DOE Critical Decision, mission need QCD Collider Laboratory LHC

38. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 38 Summary Saturation, Low-x, and QCD are a vibrant, integral component of the ongoing and future RHIC physics program Lots of new data just around the bend (in the next 1-2 years) Lots of potential for major discoveries, now and in the future.