Introduction

1. Status of the ExperimentPolarizing antiprotonsE. Steffens – University of Erlangen-Nürnbergfor the PAX Collaborationhttp://www.fz-juelich.de/ikp/pax/ PAX Experiment

2. Introduction Wehaveproposeda methodtopolarizeantiprotonsby „spin-filtering“ PAX Experiment

3. Introduction New initiative, drivenbythe FAIR-projectat GSI High Energy Storage Ring (HESR) for a beam of antiprotons Isitpossible, and how, to providepolarizedantiprotonbeamsin HESR ? PAX Experiment

4. Introduction • Ourknowledgeabout pp interactionis limited: • Lack ofpolarizationdata! Restrictedtosingle-spin (A0n00, A000n) data.Nospin-correlationdata! • Attemptsat LEAR (1983-1996) toproducepolarizedstoredantiprotonswereunsuccessful! • Revival ofideasaboutstoredpolarizedantiprotons: → PAX experimentat FAIR (see PAX TP hep-ex/0505054) • Polarized antiprotons: A missing tool • Nucleon-antinucleon scattering at the parton level • Hadron spectroscopy • Nucleon-antinucleon scattering at low energy PAX Experiment

5. PAX→ Polarized Antiprotons Physics Case: Transversity distribution of the nucleon in Drell-Yan: • Last leading-twist missing piece of the QCD description of the partonic structure of the nucleon • Direct measurement of h1q(x,Q2) of the proton for valence quarks(ATT in Drell-Yan >0.2) • transversely polarized proton beam or target () • transversely polarized antiproton beam () • + many polarization observables in pp scattering PAX Experiment

6. Quark Transversity Distribution in Drell-Yan Double transversespinasymmetry: First directmeasurement: No competitive processes PAX Experiment

7. Plan of talk • Polarizingantiprotons … • Proposals, ideas, calculations, … • Presentstatusof pp interactionmodels • Experiments • FILTEX (TSR) • e p spinflip cross sectionmeasurement at COSY • Spin-filteringat AD/CERN • Spin-filteringat COSY • Conclusion • Task taken on bythe PAX Collaboration:~100 members • 16 institutions • Spokespersons: P. Lenisa (Ferrara), F. Rathmann (Jülich) PAX Experiment

8. TwoMethods: Spin-dependentlossversusspinflip For an ensembleofspin ½ particleswithprojections+() and– () PAX Experiment

9. Proposedmethods: Spin flip →Need for an experimental testofthisidea! PAX Experiment

10. epspinflipstudiesat COSY: Idea • Use protonbeam and co-moving electrons • Turn experiment around: p e →p e into p e → p e • i.e. observe depolarization of a polarized proton beam Velocity mismatch COSYelectron cooler(detuned) PAX Experiment

11. epspinflipstudies at COSY: Tools • Use (transversely) polarized proton beam circulating in COSY • Switch on (detuned) electron cooler to depolarize proton beam • Analyzeproton polarization with internal D2-cluster target of ANKE ANKE cluster target & STT e-cooler p Tp = 49.3 MeV PAX Experiment

12. epspinflipstudiesatCOSY: Feasibility After detuning, proton energy slowly follows electron energy: U = 245 V  ve = 1.5·10-3 c fR = 40 Hz in 5 s: vp/ ve~0.03 Detuning e-cooler for 5 s onlyensuresthatprotonmomentumstaysfixed. PAX Experiment

13. p D2 epspinflipstudiesatCOSY: Polarimetry pd elastic scattering:detection in two (L-R) symmetric Silicon Tracking Telescopes Deuteron identification d p PAX Experiment

14. epspinflip cross section at COSY: Result Nominal proton energy in electron rest frame (keV) 0 1 2 3 4107 D.Oellers et al., Physics Letters B 674 (2009) 269 2107 depol (barn) 0 -2107 -4107 0 110-3 210-3 310-3 |Relative velocity of electrons in proton rest frame| (c) No effect observed: measured cross sections at least 6 orders-of-magnitude smaller than the predicted 1013 b. Meanwhile, Mainz group discovered numerical problems in the calculation → two errata. Analytical calculation by Novosibirsk group (Strakhovenko et al): Spin flip negligible! PAX Experiment

15. Spin-dependentloss: Spin-filtering Polarizationbuild-upof an initiallyunpolarizedparticle beam byrepeatedpassagethrough a polarizedhydrogen target in a storage ring: PAX Experiment

16. P beam polarization Q target polarization k || beam direction σtot = σ0 + σ1·P·Q + σ2·(P·k)(Q·k) For initially equally populated spin states:  (m=+½) and  (m=-½) transverse case: longitudinal case: Unpolarized anti-p beam Polarized target Polarization Buildup PAX Experiment

17. P beam polarization Q target polarization k || beam direction Polarized anti-p beam σtot = σ0 + σ1·P·Q + σ2·(P·k)(Q·k) For initially equally populated spin states:  (m=+½) and  (m=-½) transverse case: longitudinal case: Unpolarized anti-p beam Polarization Buildup Polarizedtarget PAX Experiment

18. I/I0 0.8 Beam Polarization 0.6 0.4 0.2 0 2 6 4 t/τbeam Figure of Merit and optimum filtering time statistical error of a double polarization observable (ATT) Measuring time t to achieve a certain error δATT t ~ FOM = P2·I (N ~ I) Optimimum time for polarization buildup given by maximum of FOM(t) tfilter = 2·τbeam PAX Experiment

19. Spin-filteringat TSR: „FILTEX“ – proof-of-principle F. Rathmann et al., PRL 71, 1379 (1993) Polarization build-up process quantitatively understood! →Spin filteringworksforprotons PAX submitted new proposal to find out how well spin filtering works for antiprotons: Measurement ofthe Spin-Dependenceofthe pp Interaction attheAD Ring(CERN-SPSC-2009-012 / SPSC-P-337) • TSR spin filtering with protons:σexp=73 ± 6 mb • Averagetheoreticalvalue:σtheo=86 ± 2 mb • Goodagreement: ~2 σdiscrepancy • Brief summary in: D. Oellers et al., Phys. Lett. B 674, 269 (2009). PAX Experiment

20. pp interactionmodels PAX Experiment

21. Spin-dependence of the pbar-p interaction • Measurement of the polarization buildup equivalent to the determination of σ1andσ2 • Once a polarized antiproton beam is available, spin-correlation data can be measured at AD (50-500 MeV) Model A: T. Hippchenet al., Phys. Rev. C 44, 1323 (1991). Model OBEPF:J. Haidenbauer, K. Holinde, A.W. Thomas, Phys. Rev. C 45, 952 (1992). Model D: V. Mull, K. Holinde, Phys. Rev. C 51, 2360 (1995). PAX Experiment

22. PAX at the AD (the only place worldwide) Siberian snake Electron cooler PAX target section PAX Experiment

23. Experimental Setup Atomic Beam Source Target chamber: Detector system + storage cell Breit-Rabi Polarimeter Six additional quadrupoles PAX Experiment

24. Polarized Target Injected hyperfine states from ABS QxQyQz: Quantization axis is reoriented by a weak magnetic guide field of 10 G z y x BRP measures occupation numbers of different HFS + corrections → Target Polarization PAX Experiment

25. Atomic Beam Source and Breit-Rabi Polarimeter ABS Electric power Cooling water Target chamber COSY/AD beam axis Breit-Rabi polarimeter PAX Experiment

26. Atomic Beam Source and Breit-Rabi Polarimeter BRP vacuum HFT Coils power ABS/TC vacuum QMAs Bake-out PAX Experiment

27. Target chamber, cell and detector system Guide field coils (x, y, z) Atomic beam movable flow limiter Stored beam Storage cell: jet density 100 Silicon strip detectors PAX Experiment

28. Detector System 12 x 2 HERMES recoildetectors+ 12 x 1 PAX STT detectors 1 PAX Layer 1 PAX STT 2 HERMES Target cell + Detectorswithcoolingsystemandshielding Detectorssurroundingtheopenablestoragecell Fullsystemconsistingof36 Detectors PAX Experiment

29. Openable storage cell P. Belochitsky - CERN AD beam envelope at injection requires openable storage cell opened closed PAX Experiment

30. AD optics (P. Belochitsky - CERN) Beam is injected with low-βsectionmoderatelypowered on. Atexperimentenergy (T<450 MeV), β-functionsare „squeezed“ byfullypowering on low-βmagnets. Thenstoragecellisclosedand gas isinjected. “Squeezed” At injection PAX Experiment

31. Expectedpolarizations after filteringfortwolifetimes Limit Model A: Productionof longitudinal polarization θacc > 13 mrad→ P(2∙τb) changesonlymarginally PAX Experiment

32. Main phasesofinstallationat AD Installation of six magnets for the low-β insertion Phase 1 Installation of the target chamber: Machine acceptance studies. Stacking studies Phase 2 Spin-filtering measurements up to 70 MeV with transverse beam polarization Phase 3 • 2009 • 2010 • 2011 PAX Experiment

33. Time plan for AD PAX ready to set sequence into motion by installing low-β section into the AD end of this year! PAX Experiment

34. Phase 1: • Layout of vacuum system, based on suggestions by the CERN group(→ NEG works) • Pumping crosses • Turbo pump 200 l/s • Ion getter pump 400 l/s • 2 x SAES GP500 NEG pump 1900 l/s • All AD magnets remain in place • Installation of six magnets for the low-β insertion. • Commissioning of the low-β section • Central quad taken out only after commissioning • Same performance of machine as before PAX Experiment

35. Spin-filtering studies at COSY Main purpose: Repeat spin-filtering with protons. No surprises expected Commissioning of the experimental setup for AD Proposal to COSY PAC submitted in July 2009 Low-βmagnet installation at COSY Target chamberwithstoragecellanddetectorsystem COSY-Quadupoles ABS BRP Low-βquadrupoles PAX Experiment

36. Time plan for COSY PAX Experiment

37. Conclusions • Polarized antiprotons: a missing tool for spin physics • First measurement of the spin-dependence of pbar-p interaction • AD unique machine! • Commissioning of equipment at COSY • Clear strategy and commitment by PAX Collaboration • Now and hereornever! PAX Experiment

38. Spare transparencies PAX Experiment

39. HadronPhysics „DreamMachine“ … an asymmetric (double-polarized) proton (15 GeV/c) – antiproton (3.5 GeV/c) colliderusing HESR, CSR and APR p p PAX Experiment

40. 1year run: 10 % precision on the h1u(x) in the valence region Pp=30% Pp=10% Anselmino et al. PLB 594,97 (2004) Similar predictions by Efremov et al., Eur. Phys. J. C35, 207 (2004) h1ufrom p-pDrell-Yan at PAX • u-dominance • |h1u|>|h1d| PAX : M2/s=x1x2~0.02-0.3 valence quarks (ATTlarge ~ 0.2-0.3) PAX Experiment

41. Proposedmethods: Somehistory … EPAC 1988 • Stern-Gerlach splitting never tried (huge effort) PAX Experiment

42. I/I0 0.8 Beam Polarization 0.6 0.4 0.2 0 2 6 4 t/τbeam Figure of Merit and optimum filtering time statistical error of a double polarization observable (ATT) Measuring time t to achieve a certain error δATT t ~ FOM = P2·I (N ~ I) Optimimum time for polarization buildup given by maximum of FOM(t) tfilter = 2·τbeam PAX Experiment

43. epspinflipstudiesatCOSY: Principle (1) • Use (transversely) polarized proton beam circulating in COSY • Switch on (detuned) electron cooler to depolarize proton beam • Analyzeproton polarization with internal D2-cluster target of ANKE ANKE cluster target & STT e-cooler p Tp = 49.3 MeV PAX Experiment

44. epspinflipstudiesatCOSY: Feasibility After detuning, proton energy slowly follows electron energy: U = 245 V  ve = 1.5·10-3 c fR = 40 Hz in 5 s: vp/ ve~0.03 Detuning e-cooler for 5 s onlyensuresthatprotonmomentumstaysfixed. PAX Experiment

45. tuned cycle detuned cycle Target off Target on 27095 4  108 Ecooler Voltage (V) Number of Beam Particles Telectron-off 49·5 s Tdetuned 49·5 s +245 2  108 26850 Tnominal 49·5 s Tnominal 49·5 s 1000 0 200 400 600 800 1000 0 200 400 600 800 time (s) Compare cycle-by-cycle: No electrons to detuned electrons epspinflipstudiesatCOSY: Cycle setup PAX Experiment

46. 0 epspinflipstudiesat COSY: Super Cycle Pdetuned Ptuned Beam current DeterminePtunedandPdetunedfromidenticalcycles, exceptfordetuned cooler PAX Experiment

47. Tuned and detuned beam polarizations 0.6 Beam Polarization 0.4 tuned cooler 0.2 detuned cooler 0 -4 -3 -2 -1 0 1 2 3 4 Proton kinetic energy in electron rest frame (keV) p D2 Depolarization Studies at COSY: Results (1) pd elastic scattering:detection in two (L-R) symmetric silicon tracking telescopes PAX Experiment

48. Measured ratio of polarizations vs Proton Kinetic energy in electron rest frame 1.2 Pdetuned/Ptuned 1 0.8 -4 -3 -2 -1 0 1 2 3 4 Nominal proton kinetic energy in electron rest frame (keV) p D2 epspinflipstudiesatCOSY: Results(1) pd elastic scattering:detection in two (L-R) symmetric silicon tracking telescopes PAX Experiment

49. epspinflipstudiesatCOSY: New calc´s ~ 1 mb→ No effect expected! PAX Experiment

50. Atomic Beam Source and Breit-Rabi Polarimeter BRP vacuum HFT Coils power ABS/TC vacuum QMAs Bake-out PAX Experiment