eRHIC A QCD microscope to study the glue that binds us all

1. eRHICA QCD microscope to study the glue that binds us all E.C. Aschenauer arXiv: 1212.1701 & 1108.1713

2. What do we know This is us !!! protons, neutrons electrons HERA’s discovery: Gluon density dominates at x<0.1 Proton 10-15m Binding-energy: ~109eV increase beam energy Quarks and Gluons 10-17m Proton: Quark-Masses: ~1% Mp Proton mass completely generated by QCD dynamics HOW ? Need a high resolution microscope to resolve quark and gluon structure  eRHIC

3. What we DON’T know Enhancement of QSwith A ⇒saturation regime reached at significantly lower √s in nuclei Au:~200 times smaller effectivex ! • gluons distribution cannot rise forever •  non-linear pQCDeffects provide a • natural way to tame this growth • characterized by the saturation • scale Q2s(x) • QCD: Dynamical balance between splitting • andrecombination • Which correlations constitute the dynamics • of the multi body system at low x • How does quark and gluon dynamics generate the proton spin? • It is more than the number 1/2 ℏ! • crucial interplay between the intrinsic • properties and interactions of quarks and • gluons • How do hadrons emerge from a created quark • or gluon? • Neutralization of color - hadronization • 2+1D Structure of nucleons and nuclei • How does the glue bind quarks and itself into • nucleons and nuclei? • A new regime of QCD matter • Color Glass Condensate (CGC) • Hints from HERA, RHIC and LHC • discover it unambiguously at eRHIC Asymptotic freedom Color Confinement Q2 GeV2 Probing momentum 2 GeV(1/10) fm) 200 MeV (1 fm)

4. Nucleon and Nuclei Tomography H, H, E, E (x,ξ,t) e J/Y gL* (Q2) x+ξ x-ξ ~ Proton spins are used to image the structure and function of the human body using the technique of magnetic resonance imaging. e’ Fourier Transform • Measure exclusive cross sections for • different final states (g, J/Ψ, …) • asfct. of t, x and Q2 • spatial parton distributions (GPDs) • accessto orbital angular momentum t p’/A’ p/A

5. Saturation: The golden Channel • Why is diffraction so important • Sensitive to spatial gluon distribution (t  bT) • Hot topic: • Lumpiness? • Just Wood-Saxon+nucleon g(bT) • coherent part probes “shape of black disc” • incoherent part (large t) • sensitive to “lumpiness” of the source • (fluctuations, hot spots, ...) t = Δ2/(1-x) ≈ Δ2 (for small x) • Impact: • discover unambiguously a new state of QCD matter • understand the initial conditions for AA collisions • influence on Sphaleron transition probabilities • one of the possibilities to generate CP-violation • in the early universe needed to explain the • baryon-antibaryon ratio Analogy: plane monochromatic wave incident on a circular screen of radius R possible Source distribution with bTg = 2 GeV-2 Hard diffraction at small x

6. Take Away Message • Why eRHIC now? • “all stars align”: • theory developments will allow to obtain • the answers to the critical questions • discussed • accelerator technologies allow for a high • energy, high luminosity polarized ep/eA • collider • detector technologies allow for a collider • detector with high resolution, wide • acceptance and particle ID in the • entire h range Current knowledge about Gluons in p /A eRHIC scienceprogram will profoundly impact our understanding of QCD, uniquely tiedtoafuture high energy, high luminosity, polarized ep / eA collider never been measured before & never without with eRHIC

7. BACKUP

8. Multi-parton correlations in p-space • Utilize the theoretical concepts of transverse momentum distributions (TMDs) • and un-integrated PDFs, which encode correlations • spin-orbit correlations on parton level  hydrogen atom • teach us how colors charges in QCD interact • Observable: azimuthal modulationsof6-fold differential • cross section in semi-inclusive DIS can be viewed as parton flow inside nucleus analogy: currents in oceans

9. How to see the gluons: Deep Inelastic Scattering Kinematics: Measure of resolution power Measure of inelasticity Measure of momentum fraction of struck quark Quark splits into gluon splits into quarks … Gluon splits into quarks 10-16m 10-19m higher √s increases resolution

10. Most Compelling SCIENCE Questions h g* e’ q e How are sea quarks and gluons and their spin distributed in space and momentum inside the nucleon? How does the nuclear environment affect the distribution of quarks and gluons and their interaction in nuclei? only at EIC only in eA/ep only at EIC How does the transverse spatial distribution of gluons compare to that in the nucleon? How are these quark and gluon distributions correlated with the over all nucleon properties, such as spin direction? How does matter respond to fast moving color charge passing through it? Is this response different for light and heavy quarks? What is the role of the motion of sea quarks and gluons in building the nucleon spin? Where does the saturation of gluon densities set in? Is there a simple boundary that separates the region from the more dilute quark gluon matter? If so how do the distributions of quarks and gluons change as one crosses the boundary? Does this saturation produce matter of universal properties in thenucleon and all nuclei viewed at nearly the speed of light?

11. I The eA Physics program Hard Processes (pQCD) FF/coal. Hadron Transport CGC JIMWLK/BK Hydro (EoS) The Initial Conditions time Our understanding of some fundamental properties of the Glasma, sQGP and Hadron Gas depend stronglyon our knowledge of the initial state! Advantage over p(d)A: • eA experimentally much cleaner • no “spectator” background to subtract • Access to the parton kinematics through scattered lepton (x, Q2) • initial and final state effects can be disentangled cleanly • Saturation: • no alternative explanations, i.e. no hydro in eA 3 conundrums of the initial state: What is the spatial transverse distributions of nucleons and gluons? 2. How much does the spatial distribution fluctuate? Lumpiness, hot-spots etc. 3. How saturated is the initial state of the nucleus?  unambiguously see saturation

12. p+A compared to e+A Electron-Hadron: Hadron-Hadron: • Point-like probe • High precision & access to partonic • kinematics • Dominated by single photon exchange •  no direct color interaction •  preserve the properties of partons • in the nuclear wave function • Nuclei always “cold” nuclear matter • probe has structure as complex as • the “target” • More direct information/access on the • response of a nuclear medium to gluon • probe • Soft color interactions before the • collision can alter the nuclear wave fct. • and destroy universality of parton • properties (break factorization) • no direct access to parton kinematics

13. Idea: momentum transfer t conjugate to transverse position (bT) coherent part probes “shape of black disc” incoherent part (dominant at large t) sensitive to “lumpiness” of the source (fluctuations, hot spots, ...) Spatial Gluon Distribution Through Diffraction Spatial source distribution: t = Δ2/(1-x) ≈ Δ2 (for small x) ϕ, nosat Golden eA measurement for eRHIC

14. What Do We Know About xG in Nuclei? 99% of all h± have pt < 2 GeV/c “Bulk Matter”  x < 0.01 Other than in p: G(x,Q2) for nuclei is little known Key: FL (x,Q2) ~ xG(x,Q2)

15. present vseRHICkinematic coverage RHIC pp data constraining Δg(x) in approx. 0.05 < x <0.2 data plotted at xT=2pT/√S likewise for Q2 eRHIC extends x coverage by up to 2 decades (at Q2=1 GeV2) lowest x so far 4.6 x10-3COMPASS

16. g1p the way to find the Spin hep-ph:1206.6014 cross section: pQCD scaling violations 5 x 250 starts here 5 x 100 starts here world data

17. impact of eRHIC data on helicity PDFs DIS scaling violations mainly determine Δg at small x in addition, SIDIS data provide detailed flavor separation of quark sea yet, small x behavior completely unconstrained  determines x-integral, which enters proton spin sum • includes only “stage-1 data” • can be pushed to x=10-4 with • 20 x 250 GeV data dramatic reduction of uncertainties: “issues”: • (SI)DIS @ eRHIC limited by • systematic uncertainties • need to control rel. lumi, • polarimetry, detector performance, • … very well

18. Can we solve the spin sum rule ? gluon spin Dg ✔ ✔ totalquark spin DS what about the orbital angular momentum? current data w/ eRHIC data • can expect approx. 5-10% • uncertainties on ΔΣ and Δg • but need to control systematics

19. prerequisites azimuthal asymmetries in DIS What is needed to realize eRHIC program exclusive processes inclusive and semi-inclusive DIS adds their transverse momentum dependence all need √sep > 50 GeV longitudinal motionof spinning quarks and gluons adds their transverse position to access x < 10-3 where sea quarks and gluons dominate • multi-dimensional binning • to reach kT > 1 GeV • to reach |t| > 1 GeV2 experimental program to address these questions: machine & detector requirements

20. The Model Detector To Roman Pots Upstream low Q2 tagger and luminosity detector Mainly based on detector technologies as proposed in EIC-Detector R&D: https://wiki.bnl.gov/conferences/index.php/EIC_R%25D +h -h lepton beam hadron beam PID: -1<h<1: DIRC or proximity focusing Aerogel-RICH + TPC: dE/dx 1<|h|<3: RICH Lepton-ID: -3 <h< 3: e/p 1<|h|<3: in addition Hcal response & g suppression via tracking |h|>3: ECal+Hcalresponse & g suppression via tracking -5<h<5: Tracking (TPC+GEM+MAPS)