Polarized Antiprotons – the Quest for a Missing Tool

1. Polarized Antiprotons – theQuestfor a Missing Tool Erhard Steffens Physikalisches Institut, University of Erlangen-Nürnberg steffens@physik.uni-erlangen.de • Motivation • History • FAIR and PAX • Recent Workshops • - Workshop at Cockcroft Institute (August 2007) • - Heraeus Seminar at Physik Haus (June 2008) • New results on depolarizationstudy at COSY • Conclusions E. Steffens - Polarized Antiprotons

2. Motivation OurKnowledgeaboutinteractionislimited: lack of polarizationdata! Restricted to single-spin (A0n00, A000n) data. Attempts at LEAR (1983-1996) to producepolarizedstoredwereunsuccessful! FAIR project: FacilityforAntiproton and Ion Research. New chanceafterthe LEAR program to studyantiprotonphysics in a broadenergyrangefrom traps to Super-LEAR (= HESR). Revival of ideasaboutstoredpolarizedantiprotons: PAX experiment at FAIR. Outstandingphysicscase – see PAX proposal, and talks at thismeeting (e.g. Anselmino on Tue.). E. Steffens - Polarized Antiprotons

3. History of Polarized Anti-Protons* Early 1980‘s: CERN S Scollider and itspowerful-source. Enabled a low-energyprogram at LEAR (stretcher and storage ring). First ideasby Kilian and Möhl on polarizingbySpin Filtering. Spin filtering was proposedbyShapiro in 1966 to polarize thermal neutronsbyspin-dependentattenuation in a polarizedhydrogentarget. nowstandardmethod in neutronphysicsusingpolarized3He gas cells First successful test reported at the Madison conference (1970): intenseneutronbeamfrom a nuclearundergroundexplosionpolarizedbymeans of a DNP protontarget. *) seeProc. of the Workshop on Polarized Antiprotons Beams –How? – AIP Conf. Proc. 1008 (2008) p.1 E. Steffens - Polarized Antiprotons

4. Spin Filtering B.C. Diven: NuclearExplosions as a NuclearPhysics Tool. Ann. Rev. Nucl. Sci. 20 (1970) 79. Tower above the line of sight to a nuclear explosion, housing the different experiments Five neutron beams were extracted from the nuclear explosion and utilized. The tower on rails could be withdrawn within a few minutes after the explosion from the expected region of ground collapse. In such a tower, the first Spin Filtering experiment has been performed at the end of the 1960‘s. E. Steffens - Polarized Antiprotons

5. Workshop at Bodega Bay* List of topicsdiscussed at theworkshop: Polarized antiprotons from the decay in flight of antihyperons Spin filtering of antiprotons by a polarized hydrogen target in a storage ring Stochastic techniques à la ‘StochasticCooling’ DNP in flight using polarized electrons and microwave radiation Spontaneous Spin-Flip synchrotron radiation Spin-Flip synchrotron radiation induced by X-ray laser Polarization by scattering Repeated Stern-Gerlach deflection Polarized antiprotons via the formation of antihydrogen and application of the ABS method Polarizing during storage in a Penning trap Polarizing by Channeling Polarizing through interaction with polarized X-rays from a diamond crystal *) Proc. Workshop on Polarized Antiprotons, Bodega Bay, CA, 1985. A.D. Krisch, A.M.T. Lin, O. Chamberlain (Edts.), AIP Conf. Proc. 145 (1986) 207 E. Steffens - Polarized Antiprotons

6. FILTEX CERN proposal CERN/PSCC/85-80 (Nov. 5, 1985) E. St. et al, Proc. 3rd LEAR workshopTignes 1985, p. 245: „Proposalformeasurement of spindependence of theinteraction at lowmomenta“ • Spin Filtering test experimentpreparedforthe Heidelberg TSR bythe FILTEX collaboration (Heidelberg-Karlsruhe-Madison-Mainz-Rutgers-München). • Polarizedhydrogen Filter target of unprecedenteddensity (1014 atoms/cm3) to bedeveloped. • 1992 proof-of-principle: build-up of polarization of stored 23MeV protonsobserved! E. Steffens - Polarized Antiprotons

7. FILTEX test experiment at TSR (Heidelberg) • Principle of Spin-Filtering • Polarizedhydrogentarget and detectors at the TSR • Target chamber and cooledstoragecell • Result on polarizationbuild-up(theonly experimental result so far) E. Steffens - Polarized Antiprotons

8. FILTEX test experiment at TSR (Heidelberg) • Result on polarizationbuild-up(theonly experimental result so far) • Build-up rate was ≈ 1/2 of the naive expectation (nuclearinteractiononly) • Quantitative explanationby H.O. Meyer and C.J. Horowitz [e.g. PRL 72 (1994) 3981]. Twomorecontributionsintroduced: frompolarizedelectrons, and byscatteringintoacceptance of thestorage ring • New Proposalby PAX collaboration [F. Rathmann et al, PRL 94 (2005) 014801], solelybased on spin-transferfrompolarized (bound) electrons: calculableinteraction! moreefficient! • New Proposalby Mainz group (Th. Walcher, H. Arenhövel, K. Aulenbacher et al) to utilizeSpin-transferfromfreepositrons to co-movingantiprotons at low relative velocity [e.g. Eur. Phys. J. A34 (2007) 447] – no losses! E. Steffens - Polarized Antiprotons

9. FAIR and PAX • FAIR: FacilityforAnti-proton and Ion Research – firstbeams in 2013(?) • Antiproton source: SIS100, productiontarget, collector (CR) and accumulator (RESR) rings • HESR: 15GeV/c antiprotonstorage ring (PANDA and possibly PAX) • APR: Antiproton Polarizer Ring forspinfiltering of antiprotons • PAX: Directmeasurement of transversity in double polarizedDrell-Yan (seetalkAnselmino) APR E. Steffens - Polarized Antiprotons

10. FILTEX test experiment at TSR (Heidelberg) • Result on polarizationbuild-up (theonly experimental result so far) • Build-up rate was ≈ 1/2 of the naive expectation (nuclearinteractiononly) • Quantitative explanationby H.O. Meyer and C.J. Horowitz [e.g. PRL 72 (1994) 3981]. Twomorecontributionsintroduced: frompolarizedelectrons, and byscatteringintoacceptance of thestorage ring • New Proposalby PAX collaboration [F. Rathmann et al, PRL 94 (2005) 014801], solelybased on spin-transferfrompolarized (bound) electrons: calculableinteraction! moreefficient! • New Proposalby Mainz group (Th. Walcher, H. Arenhövel, K. Aulenbacher et al) to utilizeSpin-transferfromfreepositrons to co-movingantiprotons at low relative velocity [e.g. Eur. Phys. J. A34 (2007) 447] – no losses! E. Steffens - Polarized Antiprotons

11. PolarizingMechanisms I/I0 0.8 0.6 0.4 0.2 0 2 6 4 t/tbeam • Trivial non-selectivelossesoccur at gas target: single-scatteringCoulomblosses. to beminimized! • Protons scattered off freeelectronsstaywithinacceptance! a lot of openquestions! spin-transferbyfreeelectrons: no losses! P E. Steffens - Polarized Antiprotons

12. Recent Workshops August 29-31, 2007 – Cockcoft Institute, Daresbury (UK) Polarized Antiproton Beams – How? S. Chattopadhyay, D.P. Barber, N. Buttimore, G. Court, E. St. (Organizers) AIP Conf. Proc. 1008 (2008) June 23-25, 2008 – Physik Haus, Bad Honnef (GER) 409. WE-Heraeus-Seminar: Polarized Antiprotons P. Lenisa & F. Rathmann; talks online at http://www.fe.infn.it/heraeus/program.html E. Steffens - Polarized Antiprotons

13. Polarized Antiproton Beams – How? • Key issues • How to understandthe FILTEX result? • Discussion of alternative methods. - HostedbytheCockcroftInsitute (Daresbury Campus, Warrington, UK) - S. Chattopadhyay (Liverpool and CI) - About30 participants and speakers S. Chattopadhyay, D.P. Barber, N. Buttimore, G. Court, E. St. (Edts.) AIP Conf. Proc. 1008 (2008) E. Steffens - Polarized Antiprotons

14. How to explainthe FILTEX results? • Clarification of possiblemechanisms • N. Buttimore (Dublin): Analysis of Channelling, and of polarizationtransferbypolarizedleptons • D. O‘Brien (Dublin): Analysis of spinfiltering, evolutionequation • Analyticalcalculationsbased on NN and el.-magn. interaction • V. Strakhovenko (Novosibirsk) • Analysis of el.-magn. spin-flipamplitude • K. Nikolenko (FZ Jülich - talkpresentedby F. Rathmann) • → exactcancellation of spin-flip and ‚scatteringintoacceptance‘ whichwouldeleminateMeyer‘s 1994 re-analysis of the FILEX result E. Steffens - Polarized Antiprotons

15. Spin transferbyfreepolarizedelectron • New methodproposedby T. Walcher, H. Arenhoevelet al (Mainz): Eur. Phys. J. A34 (2007) 447. • Hugespintransfer cross section (H. Arenhoevel2007) in attractivesystemslikeorpredicted: 1013 barn at low relative velocities! • Doesnotcontradictslowpolarizationbuild-upwithpolarizedelectrons of thetarget at rest and 23MeV protons. • Wouldenable fast loss-freebuild-up of polarization! • → foundduringtheworkshop: different 3-spin quantity to becalculatedinstead of the 2-spin quantityconsidered E. Steffens - Polarized Antiprotons

16. Alternative methods Spin Splitter: separation of stored particles in opposite spin states by the repeated Stern-Gerlach effect (discussed already at Bodega Bay) D. Barber (DESY and CI): no experimental evidence yet. ‚Proofs‘ by means of simulations should be taken with great care due to the smallness of the underlying SG effect. DNP in Flight: Spin transferbetweenco-movingantiprotons and electronsinducedbyrf, analoguousto DNP. A. Krisch (Michigan): sketched a possible test set-upbased on theideasdiscussed at Bodega Bay (C. Jeffries). Not clearwhethertheconditionscouldbe met. E. Steffens - Polarized Antiprotons

17. Alternative methods PolarizingbyChannellingthroughbentcrystals: Ideaisthatdue to continuousscatteringto oneside, a polarization of theprojec-tileisaccumulated . M. Ukhanov (Protvino): presentednumericalexampleswhichwouldlead to high polarization – based on completelymodel-dependentassumptions. Experiments required! nothing in commonwitholdchannellingresultsdiscussed in thepast (M. Kaminsky 1969) M. Fiorini (Ferrara): Experimental studies of channelling, in particularbybentcrystalsusedforthedeflection of multi-TeVionbeams (haloclearing at the LHC?). → Channellingthroughbentcrystalsworks (polarization to bemeasured) E. Steffens - Polarized Antiprotons

18. Cockcroftworkshop: Conclusions H.-O. Meyer (Indiana) in his summarytalk: Scientistshaveproduced a large variety of polarizedbeams (p, n, d, 3He, 6Li, 7Li, 23Na and targets incl. 3He andHolmium! Even ifpolarizedantiprotonsoffer so muchresistance, weshouldnotgive up! Withoutempiricalback-upitisequallydifficult to dismiss a subjector to acceptit! Theonlyoptionis to improvethetheoreticalbasis, until a supportingorrefutingmeasurementcanbeidentified. At present, the best bet seem to bemethodsinvolving a storage ring and an internaltarget. E. Steffens - Polarized Antiprotons

19. 409. WE-Heraeus-Seminar Polarized Antiprotons June 23-25, 2008 Conveners: P. Lenisa (Ferrara) and F. Rathmann (FZ Jülich) • Presentunderstanding of Spin Filtering • - Rith, Strakovenko, Walcher, Haidenbauer • Spin filteringexperiments • - Nekipelov, Oellers, Nass, Prasuhn, Aulenbacher, Schleichert, Belochitsky • Physicswithpolarizedantiprotons • - Koop, Ratcliffe, Melis, Tomasi-Gustafson, Kroll • Spin physicsexperiments • - Meyer, Contalbrigo, Bradamante, Schäfer, Metz, Cisbani, Maas • Otherantiprotonphysicstopics • - Knie, Shevchenko, Oelert, Lehrach, St. & Stroeher (Summary) E. Steffens - Polarized Antiprotons

20. Heraeus Seminar: News New calculation of the relevant spintransfer cross sectionbyArenhoevel. Same order of magnitudefound: 1013barn = 10-11 cm2 ! • Isthenumericsstable at theselow relative velocities? Note thatexperiencewithelectroncoolingtellsusthat at vrel = 0 depolarizationisverylow! • Howcan a processbased on dipole-dipoleinteractionreach out as far as rint ≈ 18nm? E. Steffens - Polarized Antiprotons

21. Heraeus Seminar: News D. Oellers (Jülich): Measurement of the depolarizing p-e cross section using co-moving electrons (seetalk F. Rathmann Tue.) • Idea: Electronsat theappropriatevrelshoulddepolarizecirculatingprotons at a comparable rate! • Use Cooler electronsdetuned to the right vrelto check theArenhoevelprediction. E. Steffens - Polarized Antiprotons

22. taken from talk F. Rathmann Depolarization Studies at COSY: Idea • Use proton beam and co-moving electrons • Turn experiment around: p e  p into p e  p • i.e. observe depolarization of a polarized proton beam Velocity mismatch COSYelectron cooler(detuned) E. Steffens - Polarized Antiprotons

23. taken from talk F. Rathmann Depolarization Studies at COSY: Results Nominal proton energy in electron rest frame (keV) 0 1 2 3 4107 Preliminary 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 section at least 6 orders-of-magnitude smaller than predicted 1013 b

24. Heraeus Seminar: News D. Oellers (Jülich): Measurement of the depolarizing p-e cross section using co-moving electrons (seetalk F. Rathmann Tue.) • Electrons at theappropriatevrelshoulddepolarizecirculatingprotons at a comparable rate! • Use Cooler electronsdetuned to the right vrelto check theArenhoevelprediction. • Presentresultsindicatethat Spin Transfer frompolarizedfreeelectrons to co-movingprotons has a cross section of at most 107barn. • Note thatanalyticalcalculationsbythe Novosibirsk groupexist: effectshouldbenegligible(seeProc. Cockcroftworkshop) • → Loss-freepolarizationbuild-up of protons (antiprotons) byfreepolarizedelectrons (positrons) isnot an option!  E. Steffens - Polarized Antiprotons

25. Presentpicture • Spin Filtering of protonsissolelybased on NN interaction! • Thebuild-up of ‚high‘ polarization in the order of ≥ 0.2 requires a low-betasection (b ≤ 0.3m) in order to minimizetheCoulomblosses → Experimental demonstrationwithprotons at COSY. E. Steffens - Polarized Antiprotons

26. taken from talk F. Rathmann Spin-filtering at COSY: Optimize, understand FILTEX p Need a „low-ß section“ in COSY pp = 0.3… GeV/c ABS + storage cell, Si-tracking det´s (HERMES) Superconducting quadrupole (new)

27. Presentpicture • Spin Filtering of protonsissolelybased on NN interaction! • Thebuild-up of ‚high‘ polarization in the order of ≥ 0.2 requires a low-betasection (b ≤ 0.3m) in order to minimizetheCoulomblosses → Experimental demonstrationwithprotons at COSY. • The layout of an antiprotonpolarizer ring (APR) requires to studyspinfiltering of antiprotons. • The Antiproton Deccelerator ring at CERN istheonlyoptionforthenearfuture – ongoingcollaboration of PAX with AD machinephysicists to work out a realistic plan. E. Steffens - Polarized Antiprotons

28. Outlook Spin filteringstudies at COSY (FZ Jülich) withprotons: • determine & optimizebeamlifetime: requiredforlongfilteringtimes (nextbeam time in Nov. 2008); • set up HERMES-ABS and recoildetectors (ready in 2009); • implementset-up in ring (2010-11), performmeasurements. • Spin filteringstudies at AD (CERN) withantiprotons: • fullproposalworked out and submitted to CERN, • moveequipment, repeatexperimentswithantiprotons (≈ 2011-12):  By 2012/13 weknowhow best to do spin-filteringwith ! • Design the APR: readyfor PAX at FAIR! Letusworktogether in order to add a missing, verypowerfulinstrument to ourtool box! E. Steffens - Polarized Antiprotons