Electron-Ion Collider Taking us to the next QCD Frontier

QCD Evolution 2014 Workshop at Santa Fe, NM (May 12 16, 2014) Electron-Ion Collider Taking us to the next QCD Frontier Jianwei Qiu Brookhaven National Laboratory Acknowledgement: Much of the physics presented here are based on the work of EIC White Paper Writing Committee put together by BNL and JLab managements,

q A giant Microscope Electron-Ion Collider (EIC) To see quarks and gluons (dark!) q A sharpest CT To cat-scan nucleons and nuclei

EIC: the world wide interest HERA@DESY LHeC@CERN erhic@bnl MEIC@JLab HIAF@CAS ENC@GSI E CM (GeV) 320 800-1300 45-175 12-140 12 à 65 14 proton x min 1 x 10-5 5 x 10-7 3 x 10-5 5 x 10-5 7 x10-3 à 3x10-4 5 x 10-3 ion p p to Pb p to U p to Pb p to U p to ~ 40 Ca polarization - - p, 3 He p, d, 3 He ( 6 Li) p, d, 3 He p,d L [cm -2 s -1 ] 2 x 10 31 10 33 10 33-34 10 33-34 10 32-33 à 10 35 10 32 IP 2 1 2+ 2+ 1 1 Year 1992-2007 2022 (?) 2022 Post-12 GeV 2019 à 2030 upgrade to FAIR The past Possible future

EIC: the world wide interest HERA@DESY LHeC@CERN erhic@bnl MEIC@JLab HIAF@CAS ENC@GSI E CM (GeV) 320 800-1300 30-145 12-140 12 à 65 14 proton x min 1 x 10-5 5 x 10-7 3 x 10-5 5 x 10-5 7 x10-3 à 3x10-4 5 x 10-3 ion p p to Pb p to U p to Pb p to U p to ~ 40 Ca polarization - - p, 3 He p, d, 3 He ( 6 Li) p, d, 3 He p,d L [cm -2 s -1 ] 2 x 10 31 10 33 10 33-34 10 33-34 10 32-33 à 10 35 10 32 IP 2 1 2+ 2+ 1 1 Year 1992-2007 2022 (?) 2022 Post-12 GeV 2019 à 2030 upgrade to FAIR High Energy Medium Energy Low Energy

U.S. - based EIC q NSAC 2007 Long-Range Plan: An Electron-Ion Collider (EIC) with polarized beams has been embraced by the U.S. nuclear science community as embodying the vision for reaching the next QCD frontier. EIC would provide unique capabilities for the study of QCD well beyond those available at existing facilities worldwide and complementary to those planned for the next generation of accelerators in Europe and Asia. q NSAC Facilities Subcommittee (2013): The Subcommittee ranks an EIC as Absolutely Central in its ability to contribute to world-leading science in the next decade. q NSAC NEXT Long-Range Process: Officially started! Final report due on October 15, 2015 q EIC Users Meeting @ Stony Brook University, June 24-27, 2014 http://skipper.physics.sunysb.edu/~eicug/meetings/sbu.html

arxiv:1212.1701 U.S.-based EICs the White Paper Appointed by S. Vigdor (BNL) and R. McKeown (Jlab)

U.S.-based EICs the Physics q Explore and image the spin and 3D structure of the nucleon Need a machine capable of exploring how do quarks and gluons move and distribute inside a hadron? q Discover the role of gluons in structure and dynamics Need a machine capable of exploring QCD dynamics of gluons with quantum occupation number near ONE q Understand the evergence of hadrons from color charge Need a vertex detector at a femtometer scale the nuclei

U.S.-based EICs the Measurement Key questions: Key measurements: u How are the sea quarks and gluons, and their spins, distributed in space and momentum inside the nucleon? u Where does the saturation of gluon densities set in? u How does the nuclear environment affect the distribution of quarks and gluons and their interactions in nuclei? u Inclusive Deep-Inelastic Scattering (DIS) u Semi-inclusive DIS with one or two of the particles in the final-state u Exclusive DIS DVCS, u Diffractive DIS rapidity gap,

U.S.-based EICs the Requirement Key requirements: Key measurements: u Electron identification scattered lepton u Inclusive Deep-Inelastic Scattering (DIS) u Momentum and angular resolution x, Q 2 u Particle identification, and acceptance:, π +, π -, K +, K -, p +, p -, u Rapidity coverage, t-resolution u Semi-inclusive DIS with one or two of the particles in the final-state u Exclusive DIS DVCS, u Diffractive DIS rapidity gap,

U.S.-based EICs the Machines MEIC (JLab) erhic (BNL) Pre-booster Linac Ion Sourc e MEIC collider rings IP IP Full Energy EIC Collider rings 12 GeV 11 GeV 12 GeV CEBAF AGS! ² First (might be the only) polarized electron-proton collider in the world ² First electron-nucleus (various species) collider in the world ² Both using the existing facilities

U.S.-based EICs the Coverage 10 3 Current polarized DIS data: CERN DESY JLab SLAC For e-n collisions at the EIC: Q 2 (GeV 2 ) 10 2 10 Current polarized BNL-RHIC pp data: PHENIX π 0 STAR 1-jet EIC s= 140 GeV, 0.01 y 0.95 EIC s= 45 GeV, 0.01 y 0.95 ü First polarized e-p collider ü Polarized beams: e, p, d/ 3 He ü Variable center of mass energy ü Luminosity L ep ~ 10 33-34 cm -2 s -1 HERA luminosity ~ 5x10 31 cm -2 s -1 1 10-4 10-3 10-2 10-1 1 For e-a collisions at the EIC: ü First e-a collider ü Wide range in nuclei ü Variable center of mass energy ü Luminosity per nucleon same as e-p x

Unified view of nucleon structure q Wigner distributions: 5D 3D HERMES JLab12 COMPASS JLab12 COMPASS for Valence 1D q EIC 3D imaging of sea and gluons: ² TMDs confined motion in a nucleon (semi-inclusive DIS) ² GPDs Spatial imaging of quarks and gluons (exclusive DIS)

Helicity contribution to proton spin q EIC@US the decisive measurement (1 st year of running): (Low x and wide x range at EIC) Before/after No other machine in the world can achieve this! q Solution to the proton spin puzzle: ² Precision measurement of G(x) extend to smaller x regime ² Quantify the role of orbital angular momentum contribution - GPDs

Semi-inclusive DIS q Naturally, two scales: ² high Q localized probe To see quarks and gluons ² Low p T sensitive to confining scale To see their confined motion ² Theory QCD TMD factorization q Spin-motion correlation: Best process to probe TMDs P, S T P, S T 4 spin combinations γ xp, k T γ xp, k T γ +,γ + γ 5,γ + γ α p z,k T p p p z,k T 4 spin combinations γ +,γ + γ 5,γ + γ α

Quantum correlation between hadron and parton q Sivers effect between hadron spin and parton motion: o Observed particle Sivers function Hadron spin influences parton s transverse motion q Collins effect between parton spin and hadronization: Transversity Observed particle Collins function Parton s transverse spin influence its hadronization JLab12, COMPASS, and low energy EIC for valence, EIC@US covers the sea and gluon! 16

EIC is the best for probing TMDs q Naturally, two planes: l l 1 N N AUT ( ϕh, ϕs ) = P N + N Collins Sivers = A sin( φ + φ ) + A sin( φ φ ) UT Pretzelosity + A sin(3 φ φ ) UT h h S S UT h S q Separation of TMDs: Collins A sin( φ + φ ) h H A A UT h S Sivers UT sin( φ φ ) h S UT 1 1 1T sin(3 φ φ ) h H Pretzelosity UT h S UT 1T 1 UT f D 1 Collins frag. Func. from e + e - collisions Hard, if not impossible, to separate TMDs in hadronic collisions

EIC coverage on TMDs First, maybe only, measurement of polarized sea and gluon TMDs

Precision of TMDs @ EIC q Single transverse-spin asymmetry:

World effort on TMDs EIC FNAL BNL JPARC lepton lepton proton lepton proton SIDIS pion proton Drell-Yan antilepton BESIII electron pion Partonic scattering amplitude Fragmentation amplitude Distribution amplitude positron e e + to pions pion Test of the sign change! f q q 1T 1T h (SIDIS) (SIDIS) = = h 1 1 f (DY) (DY)

Spatial imaging of quarks and gluons q Need Form Factor of density operator: ² Exchange of a colorless object ² Localized probe ² Control of exchanging momentum q Exclusive processes DVCS: dσ dx B dq 2 dt GPDs H q (x, ξ, t, Q),E q (x, ξ, t, Q),... t-dep ξ =(P P ) n/2 F.T. of t-dep Spatial distributions JLab 12: Valence quarks EIC: Sea quarks Also for light meson production (EIC@China) 21

DVCS @ EIC

Polarized DVCS @ EIC

EIC coverage on GPDs First, maybe only, measurement of polarized sea and gluon GPDs

Spatial imaging of gluon density q Exclusive vector meson production: J/,, dσ dx B dq 2 dt ² Fourier transform of the t-dep Spatial imaging of glue density t-dep q Gluon imaging from simulation: ² Resolution ~ 1/Q or 1/M Q Only possible at the EIC Gluon radius? How spread at small-x?

An immediate consequence q Quark GPDs and its orbital contribution to proton s spin: J q = 1 2 lim t 0 dx x [H q (x, ξ, t)+e q (x, ξ, t)] = 1 2 q + L q The first meaningful constraint on quark orbital contribution to proton spin by combining the sea from the EIC and valence region from JLab 12 This could be checked by Lattice QCD L u + L d ~ 0? 26

Nuclear landscape q EMC discovery: Nuclear landscape = superposition of nucleon landscape How would/does a nucleus look if we only saw its quarks and gluons?

Nuclear landscape q EMC discovery: Nuclear landscape = superposition of nucleon landscape How would/does a nucleus look if we only saw its quarks and gluons? q Snapshot does not have a sharp depth at small x B ² Hard probe is very sharp in space: 1 Q 1fm 1 Transverse size -, longitudinal size - xp 1 Q 1fm ² Longitudinal size > Lorentz contracted nucleon: 1 xp > 2R m A or x 0.01 Hard probe can see gluons from all nucleons p at the same impact parameter!

Non-linear evolution and saturation in QCD q Gluon recombination: q Modified DGLAP: 1 ln Q 2 xg(x, Q2 ) α sn c dx π x K 2 αs N 1 c Q 2 R 2 π x x g(x,q 2 ) Saturation: All power correction terms are equally important x dx x [x g(x,q 2 )] 2 +O Q 2 2 s Q 2 Power corrections Saturation scale: Q 2 s(x) α s(q s ) πr 2 xg(x, Q 2 s) 1 x λ Counting single parton is not effective! Scattering amplitude, Effective fields,

Non-linear evolution and saturation in QCD q Gluon recombination: q Modified DGLAP: 1 ln Q 2 xg(x, Q2 ) α sn c dx π x K 2 αs N 1 c Q 2 R 2 π x x g(x,q 2 ) Saturation: All power correction terms are equally important x dx x [x g(x,q 2 )] 2 +O Q 2 2 s Q 2 Saturation scale: Q 2 s(x) α s(q s ) πr 2 xg(x, Q 2 s) 1 x λ Counting single parton is not effective! Scattering amplitude, Effective fields, q Modified BFKL BK equation: Power corrections JIMWLK - equation

An easiest measurement q EMC effect, Shadowing and Saturation: q Questions: Why nuclear structure function suppressed at small x? Will the suppression/shadowing continue fall as x decreases? Range of color correlation could impact the center of neutron

An easiest measurement q EMC effect, Shadowing and Saturation: Saturation in R F2 Saturation in F 2 Saturation in F 2 (A) = R F2 decreases until saturation in F 2 (D) q Questions: Why nuclear structure function suppressed at small x? Will the suppression/shadowing continue fall as x decreases? Range of color correlation could impact the center of neutron

The best signature for gluon saturation q Hard scattering with a rapidity gap: k' k q Mx gap p p' Double ratios: Diffractive over total cross section ea over ep ² Strong non-linear effect, color singlet exchange ² Factorization works in DIS, not in pp, pa, AA The factor of 2 enhancement is only for ea (no equivalent in pa!) This is a clean and unambiguous signal of saturation physics at EIC 34

Hadronization puzzle q Strong suppression of heavy flavors in AA collisions: q Emergence of hadrons: How do hadrons emerge from a created quark or gluon? How is the color of quark or gluon neutralized? q Need a femtometer detector or scope : Nucleus, a laboratory for QCD Evolution of partonic properties 35

Hadronization energy loss How hadrons emerge from quarks and gluons? q Unprecedented ν range at EIC: ν = Q2 2mx D 0 Control of and medium length! q Heavy quark energy loss: - Mass dependance of fragmentation semi-inclusive π DIS pion D 0 Need the collider energy of EIC and its control on parton kinematics

Electroweak physics at EIC q Running of weak interaction high luminosity: 0.242 0.240 APV (Cs) (pr 2012) E158 ν-dis sin 2 eff θ W (Q) 0.238 0.236 0.234 0.232 APV (Cs) APV (Cs) Moller (Jlab) MESA(Mainz) QWEAK (Jlab) SOLID (Jlab) EIC (statistical errors only) LEP LHeC 0.230 SLAC -3-2 -1 0 1 2 3 log 10 (Q [GeV]) ² Fills in the region that has never been measured ² have a real impact on testing the running of weak interaction 37

Summary q EIC is a ultimate QCD machine: 1) to discover and explore the quark/gluon structure and properties of hadrons and nuclei, 2) to search for hints and clues of color confinement, and 3) to measure the color fluctuation and color neutralization q EIC is a tomographic machine for nucleons and nuclei with a resolution better than 1/10 fm q EIC designs explore the polarization and intensity frontier, as well as the frontier of new accelerator/detector technology q Complementary designs of EIC around the world have been proposed to cover different kinematic regimes of QCD EIC@US is sitting at a sweet spot for rich QCD dynamics Thanks! 38

Color fluctuation azimuthal asymmetry q Preliminary low energy data: Hicks, KEK-JPAC2013 q Classical expectation: Contain terms in cos( pq ) and cos(2 pq ) only statistical uncertainties shown Any distribution seen in Carbon should be washed out in heavier nuclei q Surprise: Azimuthal asymmetry in transverse momentum broadening Fluctuation and v n at EIC too! See Brooks talk at EICAC

Another clean signature for gluon saturation q Strong suppression of dihadron correlation in ea: Theory Simulation 0.18 0.16 0.14 ep Q 2 = 1 GeV 2 0.2 EIC stage-ii Ldt = 10 fb -1 /A p T trigger > 2 GeV/c 1 < p T assoc < pt trigger η <4 C(Δϕ) 0.12 0.1 0.08 0.06 eca CeAu(Δϕ) 0.15 0.1 eau - nosat ϕ 12 0.04 0.02 eau 0.05 eau - sat 0 2 2.5 3 3.5 4 4.5 ² Never been measured! ² Directly probe Weizsacker-Williams (saturated) gluon distribution in a large nucleus ² A factor of 2 suppression of away-side hadron-correlation! ² No-sat: Pythia + npdf (EPS09) Δϕ 0 2 2.5 3 3.5 4 4.5 Δϕ