Measuring the Gluon Helicity Distribution at a Polarized

Measuring the Gluon Helicity Distribution at a Polarized Electron-Proton Collider Christine Aidala UMass Amherst APS April Meeting 2007 Jacksonville, FL C. Aidala, APS April Meeting April 15, 2007

Proton Structure • One of the most common, stable components of everyday matter • Fundamental object in QCD Proton • “If we understand the proton, we understand everything. ” – F. Wilczek • But we still don’t understand the proton! C. Aidala, APS April Meeting April 15, 2007 2

Proton Structure • Complex linear momentum structure – Depends on energy scale at which probed – Now well measured over a wide range in x, Q 2 • Can be described in terms of structure functions • Or in terms of parton distribution functions (pdf’s) – f(x): Probability of finding a quark of flavor f carrying momentum fraction x of the proton momentum • Complex angular momentum structure! • Discovered in late ’ 80’s by EMC experiment at CERN that quark spin contribution to proton spin only 20 -30%! – “Spin crisis” – Rest from gluon spin and orbital angular momentum C. Aidala, APS April Meeting April 15, 2007 3

World Data on F 2 p Structure Function Next-to-Leading-Order (NLO) perturbative QCD (DGLAP) fits Electromagnetic probes of DIS don’t interact directly with gluons. Obtain gluon distribution via Bjorken scaling violations. Note sharp rise of gluon contribution below x~0. 1. Gluons measured to carry ~50% of proton’s linear C. Aidala, APS April Meeting 4 April 15, 2007 momentum!

World Data on g 1 p Polarized Structure Function Unpolarized Very limited kinematic region currently measured by fixed-target experiments. Extremely poor constraint on gluon helicity distribution from Polarized scaling violations! [Add x. Dg(x) figure? Which? ] Polarized electron-proton collider could provide kinematic coverage necessary! C. Aidala, APS April Meeting April 15, 2007 5

World Data on F 2 p Projected Data on g 1 p A. Bruell 5 fb-1 Meeting 6 EIC makes it possible! Region of existing g 1 C. p Aidala, data. APS April. An April 15, 2007

g from g 1 at the EIC A. Bruell 5 fb-1 GRSV std ( g > 0) GRSV g = 0 GRSV g = +g GRSV g = -g Note that positive g leads to negatively divergent g 1 at low x, negative g to positively divergent g 1 at low x. Excellent discrimination with EIC for lower Q 2 bins. C. Aidala, APS April Meeting April 15, 2007 7

Polarized Gluon Distribution via Charm Production Very clean process ! c D mesons LO QCD: asymmetry in D production directly proportional to G/G C. Aidala, APS April Meeting April 15, 2007 8

Polarized Gluon Distribution via Charm Production: A First Study for EIC Precise determination of G/G for 0. 003 < xg < 0. 4 at common Q 2 of 10 Ge. V 2 D Kp 10 fb-1 2. 5 fb-1 RHIC SPIN A. Bruell C. Aidala, APS April Meeting April 15, 2007 9

Summary • Proton a fundamental object in QCD. Decades of studies have revealed a rich linear momentum structure. Much remains to be understood of the proton’s spin structure! • Polarized electron-proton collider would open up new kinematic regime and allow deeper understanding of proton spin structure, including greatly improved measurement of gluon spin contribution. • Studies underway for two alternate EIC facilities, one at RHIC (BNL), the other at CEBAF (JLab) • More info available at http: //www. bnl. gov/eic C. Aidala, APS April Meeting April 15, 2007 10

Extra C. Aidala, APS April Meeting April 15, 2007 11

To Add? • Add one-slide intro to EIC—e. RHIC and ELIC designs, kinematic coverage, basic (minimum? ) machine parameters. Cite also website. • More details on charm • Comments on RHIC spin program C. Aidala, APS April Meeting April 15, 2007 12

Polarized Parton Distribution Functions PRD 74: 014015 (2006) up quarks down quarks EMC, SMC at CERN E 142 to E 155 at SLAC HERMES at DESY PHENIX at RHIC gluon • Polarized pdf--the difference in probability between scattering off of a parton with one spin state vs. the other sea quarks C. Aidala, APS April Meeting April 15, 2007 – Function of x. Bjorken, the momentum fraction of the proton carried by the parton 13

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Comparison to Other Facilities Q 2 vs. x Luminosity vs. CM Energy C. Aidala, APS April Meeting April 15, 2007 15

Future: Polarized Gluon Distribution from RHIC C. Aidala, APS April Meeting April 15, 2007 16

Polarized Gluon Distribution via Charm Production starting assumptions for EIC: • vertex separation of 100 m • full angular coverage (3< <177 degrees) • perfect particle identification for pions and kaons (over full momentum range) • detection of low momenta particles (p>0. 5 Ge. V) • measurement of scattered electron (even at very small scattering angles) • 100% efficiency Very demanding detector requirements ! C. Aidala, APS April Meeting April 15, 2007 17

Polarized Gluon Distribution via Charm Production Precise determination of G/G for 0. 003 < xg < 0. 4 at common Q 2 of 10 Ge. V 2 If: • We can measure the scattered electron even at angles close to 00 (determination of photon kinematics) • We can separate the primary and secondary vertex down to about 100 m • We understand the fragmentation of charm quarks ( ) • We can control the contributions of resolved photons • We can calculate higher order QCD corrections ( ) C. Aidala, APS April Meeting 18 April 15, 2007

ELIC Accelerator Design Specifications § Center-of-mass energy between 20 Ge. V and 90 Ge. V with energy asymmetry of ~10, which yields Ee ~ 3 Ge. V on EA ~ 30 Ge. V up to Ee ~ 9 Ge. V on EA ~ 225 Ge. V § Average Luminosity from 1033 to 1035 cm-2 sec-1 per Interaction Point § Ion species: § Polarized H, D, 3 He, possibly Li § Ions up to A = 208 § Longitudinal polarization of both beams in the interaction region (+Transverse polarization of ions +Spin-flip of both beams) all polarizations >70% desirable § Positron Beam desirable C. Aidala, APS April Meeting April 15, 2007 19

ELIC Layout 30 -225 Ge. V protons 30 -100 Ge. V/n ions 3 -9 Ge. V electrons 3 -9 Ge. V positrons C. Aidala, APS April Meeting April 15, 2007 20

Design Features of ELIC Directly aimed at addressing the science program: § “Figure-8” ion and lepton storage rings to ensure spin preservation and ease of spin manipulation. No spin sensitivity to energy for all species. § Short ion bunches, low β*, and high rep rate (crab crossing) to reach unprecedented luminosity. § Four interaction regions for high productivity. § Physics experiments with polarized positron beam are possible. Possibilities for e-ecolliding beams. § Present JLab DC polarized electron gun meets beam current requirements for filling the storage ring. § The 12 Ge. V CEBAF accelerator can serve as an injector to the electron ring. RF power upgrade might be required later depending on the performance of ring. § Collider operation appears compatible with simultaneous 12 Ge. V CEBAF operation for fixed target program. C. Aidala, APS April Meeting April 15, 2007 21

e. RHIC • Integrated electron-nucleon luminosity of ~ 50 fb-1 over about a decade for both highly polarized nucleon and nuclear (A = 2 -208) RHIC beams. Ø 50 -250 Ge. V polarized protons Øup to 100 Ge. V/n gold ions Øup to 167 Ge. V/n polarized 3 He ions • Two accelerator design options developed in parallel (2004 Zeroth. Order Design Report): ØERL-based design (“Linac-Ring”; presently most promising design): • Superconducting energy recovery linac (ERL) for the polarized electron beam. • Peak luminosity of 2. 6 1033 cm-2 s-1 with potential for even higher luminosities. • R&D for a high-current polarized electron source needed to achieve the design goals. ØRing-Ring option: • Electron storage ring for polarized electron or positron beam. • Technologically more mature with peak luminosity of 0. 47 1033 cm-2 s-1. C. Aidala, APS April Meeting April 15, 2007 22

ERL-based e. RHIC Design e-cooling (RHIC II) PHENIX Main ERL (3. 9 Ge. V per pass) STAR e+ storage ring Four e-beam 5 Ge. V - 1/4 RHIC circumference passes Ø Ø Ø Ø Compact recirculation loop magnets Electron energy range from 3 to 20 Ge. V Peak luminosity of 2. 6 1033 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 the ion bunch frequency at different ion energies. C. Aidala, APS April Meeting April 15, 2007 5 mm 23 5 mm

Ring-Ring e. RHIC Design 5 – 10 Ge. V e-ring 5 -10 Ge. V full energy injector RHIC e-cooling (RHIC II) Ø Based on existing technology Ø Collisions at 12 o’clock interaction region Ø 10 Ge. V, 0. 5 A e-ring with 1/3 of RHIC circumference (similar to PEP II HER) Ø Inject at full energy 5 – 10 Ge. V Ø Polarized electrons and positrons C. Aidala, APS April Meeting April 15, 2007 24