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

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

C. Aidala, APS April Meeting April 15, 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, Electromagnetic probes of DIS don’t interact directly with gluons. Obtain gluon distribution via Bjorken scaling violations. World Data on F 2 p Structure Function Next-to-Leading-Order (NLO) perturbative QCD (DGLAP) fits Note sharp rise of gluon contribution below x~0.1. Gluons measured to carry ~50% of proton’s linear momentum!

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

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

C. Aidala, APS April Meeting April 15,  g from g 1 at the EIC 5 fb -1 Note that positive  g leads to negatively divergent g1 at low x, negative  g to positively divergent g1 at low x. Excellent discrimination with EIC for lower Q 2 bins. GRSV std (  g > 0) GRSV  g = 0 GRSV  g = +g GRSV  g = -g A. Bruell

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

C. Aidala, APS April Meeting April 15, Polarized Gluon Distribution via Charm Production: A First Study for EIC Precise determination of  G/G for < x g < 0.4 at common Q 2 of 10 GeV 2 RHIC SPIN DKDK 10 fb fb-1 A. Bruell

C. Aidala, APS April Meeting April 15, 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

C. Aidala, APS April Meeting April 15, Extra

C. Aidala, APS April Meeting April 15, To Add? Add one-slide intro to EIC—eRHIC 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, Polarized Parton Distribution Functions Polarized pdf--the difference in probability between scattering off of a parton with one spin state vs. the other –Function of x Bjorken, the momentum fraction of the proton carried by the parton up quarks down quarks sea quarks gluon EMC, SMC at CERN E142 to E155 at SLAC HERMES at DESY PHENIX at RHIC PRD74: (2006)

C. Aidala, APS April Meeting April 15,

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

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

C. Aidala, APS April Meeting April 15, Very demanding detector requirements ! 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 GeV) measurement of scattered electron (even at very small scattering angles) 100% efficiency

C. Aidala, APS April Meeting April 15, If: We can measure the scattered electron even at angles close to 0 0 (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 (  ) Polarized Gluon Distribution via Charm Production Precise determination of  G/G for < x g < 0.4 at common Q 2 of 10 GeV 2

C. Aidala, APS April Meeting April 15, ELIC Accelerator Design Specifications  Center-of-mass energy between 20 GeV and 90 GeV with energy asymmetry of ~10, which yields E e ~ 3 GeV on E A ~ 30 GeV up to E e ~ 9 GeV on E A ~ 225 GeV  Average Luminosity from to 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, ELIC Layout GeV protons GeV/n ions 3-9 GeV electrons 3-9 GeV positrons

C. Aidala, APS April Meeting April 15, 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 - e - colliding beams.  Present JLab DC polarized electron gun meets beam current requirements for filling the storage ring.  The 12 GeV 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 GeV CEBAF operation for fixed target program.

C. Aidala, APS April Meeting April 15, eRHIC Integrated electron-nucleon luminosity of ~ 50 fb -1 over about a decade for both highly polarized nucleon and nuclear (A = 2-208) RHIC beams.  GeV polarized protons  up to 100 GeV/n gold ions  up to 167 GeV/n polarized 3 He ions Two accelerator design options developed in parallel (2004 Zeroth- Order Design Report):  ERL-based design (“Linac-Ring”; presently most promising design): Superconducting energy recovery linac (ERL) for the polarized electron beam. Peak luminosity of 2.6  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.  Ring-Ring option: Electron storage ring for polarized electron or positron beam. Technologically more mature with peak luminosity of 0.47  cm -2 s -1.

C. Aidala, APS April Meeting April 15, ERL-based eRHIC Design  Electron energy range from 3 to 20 GeV  Peak luminosity of 2.6  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. PHENIX STAR e-cooling (RHIC II) Four e-beam passes e + storage ring 5 GeV - 1/4 RHIC circumference Main ERL (3.9 GeV per pass) 5 mm Compact recirculation loop magnets

C. Aidala, APS April Meeting April 15, Ring-Ring eRHIC Design  Based on existing technology  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 RHIC 5 – 10 GeV e-ring e-cooling (RHIC II) 5 -10GeV full energy injector