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Polarized Antiprotons – the Quest for a Missing Tool

This article discusses the motivation and history behind the study of polarized antiprotons, as well as recent workshops and new results. It explores the potential of the FAIR project and the PAX experiment in advancing our understanding of antiproton physics. The text is in English.

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Polarized Antiprotons – the Quest for a Missing Tool

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  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

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