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SPIN Physics at GSI

SPIN Physics at GSI. Frank Rathmann Institut für Kernphysik Forschungszentrum Jülich. Outline. WHY? Physics Case HOW? Polarized Antiprotons WHERE? FAIR Project at Darmstadt WHAT? Transversity Measurement WHEN? Time Schedule Conclusion. Central Physics Issue.

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SPIN Physics at GSI

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  1. SPIN Physics at GSI Frank Rathmann Institut für Kernphysik Forschungszentrum Jülich

  2. Outline WHY?Physics Case HOW?Polarized Antiprotons WHERE?FAIR Project at Darmstadt WHAT? Transversity Measurement WHEN?Time Schedule Conclusion

  3. Central Physics Issue Transversity distribution of the nucleon: • last leading-twist missing piece of the QCD description of the partonic structure of the nucleon • directly accessible uniquely via the double transverse spin asymmetry ATT in the Drell-Yan production of lepton pairs • theoretical expectations for ATT in DY, 30-40% • transversely polarized antiprotons • transversely polarized proton target • definitive observation of h1q(x,Q2) of the proton for the valence quarks

  4. Leading Twist Distribution Functions R L L R L R R L quark quark’ 1/2 1/2 1/2 1/2 1/2 1/2 1/2 1/2 proton proton’ R L 1/2 -1/2     -     Probabilistic interpretation in helicity base: + f1(x) q(x) spin averaged (well known) - Dq(x) helicity diff. (known) g1(x) No probabilistic interpretation in the helicity base (off diagonal) h1(x) u = 1/2(uR + uL) u = 1/2(uR - uL) Transversity base dq(x) helicity flip (unknown)

  5. Transversity Properties: • Probes relativistic nature of quarks • No gluon analog for spin-1/2 nucleon • Different evolution than • Sensitive to valence quark polarization Chiral-odd: requires another chiral-odd partner epe’hX ppl+l-X Indirect Measurement: Convolution of with unknown fragment. fct. Impossible in DIS Direct Measurement

  6. M invariant Mass of lepton pair Transversity in Drell-Yan processes Polarized Antiproton Beam → Polarized Proton Target (both transversely polarized) l+ Q2=M2 l- Q QT p p QL

  7. Other Topics • Single-Spin Asymmetries • Electromagnetic Form Factors • Hard Scattering Effects • Soft Scattering • Low-t Physics • Total Cross Section • pbar-p interaction

  8. Proton Electromagnetic Formfactors • Measurement of relative phases of magnetic and electric FF in the time-like region • Possible only via SSA in the annihilation pp → e+e- • Double-spin asymmetry • independent GE-Gm separation • test of Rosenbluth separation in the time-like region S. Brodsky et al., Phys. Rev. D69 (2004)

  9. Study onset of Perturbative QCD p (GeV/c) • High Energy • small t: Reggeon Exchange • large t: perturbative QCD • Pure Meson Land • Meson exchange • ∆ excitation • NN potential models • Transition Region • Uncharted Territory • Huge Spin-Effects in pp elastic scattering • large t: non- and perturbative QCD

  10. pp elastic scattering from ZGS Spin-dependence at large-P (90°cm): Hard scattering takes place only with spins . T=10.85 GeV Similar studies in pp elastic scattering D.G. Crabb et al., PRL 41, 1257 (1978)

  11. Outline WHY? Physics Case HOW?Polarized Antiprotons WHERE? FAIR Project at Darmstadt WHAT? Transversity Measurements WHEN? Time Schedule Conclusion

  12. For initially equally populated spin states:  (m=+½) and  (m=-½) transverse case: longitudinal case: Spin Filter Method P beam polarization Q target polarization k || beam direction σtot = σ0 + σ·P·Q + σ||·(P·k)(Q·k) Time dependence of P and I

  13. 1992 Filter Test at HD-TSR with protons

  14. Experimental Results from Filter Test Results Experimental Setup T=23 MeV F. Rathmann. et al., PRL 71, 1379 (1993) Low energy pp scattering 1<0  tot+<tot-

  15. Puzzle from FILTEX Test Observed polarization build-up: dP/dt = ± (1.24 ± 0.06) x 10-2 h-1 • Expectedbuild-up: P(t)=tanh(t/τpol), • 1/τpol=σ1Qdtf=2.4x10-2 h-1 •  about factor 2 larger! σ1 = 122 mb (pp phase shifts) Q = 0.83 ± 0.03 dt = (5.6 ± 0.3) x 1013cm-2 f = 1.177 MHz Three distinct effects: • Selective removal through scattering beyond Ψacc=4.4 mradσR=83 mb • Small angle scattering of target protons into ring acceptanceσS=52 mb • Spin transfer from polarized electrons of the target atoms to the stored protons σEM=70 mb (-) Horowitz & Meyer, PRL 72, 3981 (1994) H.O. Meyer, PRE 50, 1485 (1994)

  16. Spin Transfer from Electrons to Protons Horowitz & Meyer, PRL 72, 3981 (1994) H.O. Meyer, PRE 50, 1485 (1994) α fine structure constant λp=(g-2)/2=1.793 anomalous magnetic moment me, mp rest masses p cm momentum a0 Bohr radius C02=2πη/[exp(2πη)-1] Coulomb wave function η=zα/ν Coulomb parameter (negative for antiprotons) v relative lab. velocity between p and e z beam charge number

  17. σEM|| (mb) Atomic Electrons 600 500 Pure Electrons 400 300 200 100 T (MeV) 1 10 100 Exploitation of Spin Transfer PAX will employ spin-transfer from polarized electrons of the target to antiprotons (QED Process:calculable) Hydrogen gas target: ①+② in strong field (300 mT) Pe=0.993 Pz=0.007

  18. B Dedicated Antiproton Polarizer (AP) Injection Siberian Snake HESR AP 440 m e-cooler e-cooler Internal Experiment 150 m Extraction ABS Polarization Buildup in AP parallel to measurement in ESR Polarizer Target db=ψacc·β·2dt=dt(ψacc) lb=40 cm (=2·β) df=1 cm, lf=15 cm β=0.2 m q=1.5·1017 s-1 T=100 K Longitudinal Q (300 mT)

  19. ψacc(mrad) 40 10 8 6 beam lilfetime τbeam (h) 30 25 4 20 2 10 10 100 1000 T (MeV) Beam lifetimes in the AP Beam Lifetime Coulomb Loss Total Hadronic

  20. Ψacc=50 mrad EM only 0.4 40 30 0.3 20 Beam Polarization P(2·τbeam) 10 0.2 5 0.1 0 1 10 100 T (MeV) Beam Polarization Filter Test: T = 23 MeV Ψacc= 4.4 mrad Buildup in HESR (800 MeV)

  21. I/I0 0.8 Beam Polarization 0.6 0.4 0.2 0 2 6 4 t/τbeam Polarization Buildup: Optimum Interaction Time statistical error of a double polarization observable (ATT) Measuring time t to achieve a certain error δATT ~ FOM = P2·I (N ~ I) Optimimum time for Polarization Buildup given by maximum of FOM(t) tfilter = 2·τbeam

  22. Optimum Beam Energies for Buildup in AP ψacc= 50 mrad FOM AP Space charge limit 15 40 mrad 10 F. Rathmann et al., physics/0410067 (2004) 5 30 mrad 20 mrad 10 mrad T (MeV) 10 100 1

  23. Space-Charge Limitation in the AP Before filtering starts: Nreal = 107 s-1 · 2τbeam Nind. 1013 1012 1011 ψacc= 50 mrad 40 mrad 1010 30 mrad Nreal 20 mrad 10 mrad 109 10 mrad 10 100 T (MeV) 1

  24. B Transfer from AP to HESR and Accumulation Injection Siberian Snake HESR AP 440 m e-cooler e-cooler Internal Experiment 150 m Extraction ABS Polarizer Target

  25. 10 mrad 20 mrad 30 mrad 40 mrad 50 mrad 8·1010 6·1010 4·1010 2·1010 0 20 40 60 80 t (h) Accumulation of Polarized Beam in HESR PIT: dt=7.2·1014 atoms/cm2 τHESR=11.5 h Number accumulated in equilibrium independentof acceptance Npbar No Depolarization in HESR during energy change

  26. Longitudinal Field (B=335 mT) Dz PT = 0.845 ± 0.028 Dzz HERMES H PT = 0.795  0.033 HERMES Performance of Polarized Internal Targets HERMES: Stored Positrons PINTEX: Stored Protons H Fast reorientation in a weak field (x,y,z) Transverse Field (B=297 mT) Targets work very reliably (many months of stable operation)

  27. dt = areal density frev = revolution frequency Npbar = number of pbar stored in HESR L= dtxfrevxNpbar Estimated Luminosity for Double Polarization Polarized Internal Target in HESR In equilibrium: Qtarget = 0.85 Pbeam = 0.3 σtot(15 GeV) = 50 mb (factor >70 in measuring time for ATT with respect to beam extracted on solid target)

  28. σEM|| (mb) Atomic Electrons 600 500 Pure Electrons 400 300 200 100 T (MeV) 1 10 100 How about a Pure Polarized Electron Target? Maxiumum σEM|| for electrons at rest: (675 mb ,Topt = 6.2 MeV): Gainfactor ~15 over atomic e- in a PIT • Density of an Electron-Cooler fed by 1 mA DC polarized electrons: • Ie=6.2·1015 e/s • A=1 cm2 • l=5 m • dt = Ie·l·(β·c·A)-1 = 5.2·108 cm-2 • Electron target density by factor ~106 smaller, • no match for a PIT (>1014 cm-2)

  29. Outline WHY? Physics Case HOW? Polarized Antiprotons WHERE?FAIR Project at Darmstadt WHAT? Transversity Measurement WHEN? Time Schedule Conclusion

  30. NEW Facility • An“International Accelerator Facility for Beams of Ions and Antiprotons”: • Top priority of German hadron and nuclear physics community (KHuK-report of 9/2002) and NuPECC • Favourable evaluation by highest German science • committee (“Wissenschaftsrat” in 2002) • Funding decision from German government in • 2/2003 – staging and at least 25% foreign funding • to be build at GSI Darmstadt; • should be finished in > 2011 (depending on start) • FAIR • (Facility for Antiproton and Ion Research)

  31. Facilty for Antiproton and Ion Research (GSI, Darmstadt, Germany) • Proton linac (injector) • 2 synchrotons (30 GeV p) • A number of storage rings •  Parallel beams operation

  32. FAIR – Prospects and Challenges • FAIR is a facility, which will serve a large part of the nuclear physics community (and beyond): • Nuclear structure  Radioactive beams • Dense Matter  Relativistic ion beams • Hadronic Matter  Antiprotons, (polarized) • Atomic physics • Plasma physics • FAIR will need a significant fraction of the available man-power and money in the years to come: • 1 G€  10 000 man-years = 100 “man” for 100 years • or (1000 x 10) • FAIR will have a long lead-time (construction, no physics) •  staging (3 phases)

  33. The FAIR project at GSI SIS100/300 50 MeV Proton Linac HESR: High Energy Storage Ring: PANDA (and PAX) CR-Complex FLAIR: (Facility for very Low energy Anti-protons and fully stripped Ions) NESR

  34. The Antiproton Facility HESR (High Energy Storage Ring) • Length 442 m • Bρ = 50 Tm • N = 5 x 1010 antiprotons High luminosity mode • Luminosity = 2 x 1032 cm-2s-1 • Δp/p ~ 10-4 (stochastic-cooling) High resolution mode • Δp/p ~ 10-5 (8 MV HE e-cooling) • Luminosity = 1031 cm-2s-1 SIS100/300 HESR Super FRS CR Gas Target and Pellet Target: cooling power determines thickness NESR Antiproton Production Target Beam Cooling: e- and/or stochastic 2MV prototype e-cooling at COSY • Antiproton production similar to CERN • Production rate107/sec at 30 GeV • T = 1.5 - 15 GeV/c(22 GeV)

  35. HESR AP The New Polarization Facility • Conceptual Design Report for FAIR did not include Spin Physics: • Jan. ’04: 2 Letters of Intent for Spin Physics • ASSIA (R. Bertini) • PAX (P. Lenisa, FR) • WE NEED MORE COLLABORATORS! 210 collaborators 25 institutions

  36. LoI‘s for Spin Physics at FAIR SIS100/300 External:ASSIA Extracted beam on PET (Compass-like) Internal:PAX in HESR Polarized antiprotons + PIT

  37. Evaluation by QCD Program Advisory Committee (July 2004) • STI Report: • Your LoI has convinced the QCD-PAC • that Polarization must be included into the design of FAIR from the beginning, and • that the presently proposed scheme is not optimized as to the physics. You […] are invited and encouraged to design a world-class facility with unequalled degree of polarization of antiprotons. • Common Report: • […] The PAC considers the spin physics of extreme interest and the building of an antiproton polarized beam as a unique possibility for the FAIR Project. • […] The unique physics opportunities, made possible with polarized antiproton beams and/or polarized target are extremely exciting, especially in double spin measurements. • […] It would be very unfortunate if decisions about the facility, made now, later preclude the science.

  38. Outline WHY? Physics Case HOW? Polarized Antiprotons WHERE? FAIR Project at Darmstadt WHAT? Transversity Measurement at PAX • Rates • Angular Distribution • Background • Detector Concept WHEN? Time Schedule Conclusion

  39. M invariant Mass of lepton pair Transversity in Drell-Yan processes at PAX Polarized Antiproton Beam → Polarized Proton Target (both transversely polarized) l+ Q2=M2 l- Q QT p p QL

  40. ATT for PAX kinematic conditions RHIC: τ=x1x2=M2/s~10-3 → Exploration of the sea quark content(polarizations small!) ATT very small (~ 1 %) PAX: M2~10 GeV2, s~30-50 GeV2, τ=x1x2=M2/s~0.2-0.3 →Exploration ofvalence quarks(h1q(x,Q2) large) 0.3 ATT/aTT > 0.3 Models predict |h1u|>>|h1d| 0.25 0.15 T=15 GeV (s=5.7 GeV) T=22 GeV (s=6.7 GeV) 0.10 Anselmino et al. PLB 594,97 (2004) Main contribution to Drell-Yan events at PAXfrom x1~x2~τ: deduction of x-dependence of h1u(x,M2) 0 0.6 0.4 0.2 xF=x1-x2 xF=x1-x2 Similar predictions by Efremov et al., Eur. Phys. J. C35, 207 (2004)

  41. l+ Q2=M2 l- Q QT p p QL 1) Count rate estimate. Signal Estimate Polarized Antiproton Beam → Polarized Proton Target (both transversely polarized) 2) Angular distribution of the asymmetry.

  42. 2 k events/day 22 GeV 15 GeV M (GeV/c2) • x1x2 = M2/s Drell-Yan cross section and event rate • M2 = s x1x2 • xF=2QL/√s = x1-x2 22 GeV 15 GeV M>4 GeV M>2 GeV • Mandatory use of the invariant mass region below the J/y (2 to 3 GeV). • 22 GeV preferable to 15 GeV

  43. unknown vector coupling, but same Lorentz and spinor structure as other two processes Unknown quantities cancel in the ratios for ATT, but helicity structure remains! Extension of the “safe” region Determination of h1q(x,Q2) not confined to the „safe“ region (M > 4 GeV) Anselmino et al. PLB 594,97 (2004) Efremov et al., Eur.Phys.J. C35,207 (2004) Cross section increases by two orders from M=4 to M=3 GeV →Drell-Yan continuum enhances sensitivity of PAX to ATT

  44. Dream Option: Collider (15 GeV) L > 1030cm-2s-1 to get comparable rates

  45. ATT asymmetry: angular distribution • Asymmetry is largest for angles =90° • Asymmetry varies like cos(2f). Needs a large acceptance detector (LAD)

  46. Theoretical prediction Magnitude of Asymmetry Angular modulation 0.3 0.25 T=15 GeV 0.2 T=22 GeV LAD 0.15 0 0.2 0.4 0.6 xF=x1-x2 FWD: qlab < 8° LAD: 8° < qlab < 50° P=Q=1

  47. LAD LAD Estimated signal • 120k event sample • 60 days at L=2.1× 1031 cm2 s-2, P = 0.3, Q = 0.85 ATT=(4.30.4)·10-2 • Events under J/y can double the statistics. • Good momentum resolution requested

  48. Detector concept • Drell-Yan process requires a large acceptance detector • Good hadron rejection needed • 102 at trigger level, 104 after data analysis for single track. • Magnetic field envisaged • Increased invariant mass resolution compared to calorimeter • Improved PID through Energy/momentum ratio • Separation of wrong charge combinatorial background • Toroidal Field: • Zero field on axis compatible with polarized target.

  49. Double Polarization Experiments  Azimuthal Symmetry Possible solution: Toroid (6 superconducting coils) • 800 x 600 mm coils • 3 x 50 mm section (1450 A/mm2) • average integrated field: 0.6 Tm • free acceptance > 80 % Superconducting target field coils do not affect azimuthal acceptance. (8 coil system under study)

  50. Outline WHY? Physics Case HOW? Polarized Antiprotons WHERE? FAIR Project at Darmstadt WHAT? Transversity Measurement WHEN?Time Schedule Conclusion

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