Spin-physics and Polarized Antiprotons

Transcription

1 Spin-physics and Polarized Antiprotons dr. Paolo Lenisa Università di Ferrara and INFN - ITALY Polarized Antiprotons: How? Daresbury (UK)

2 Physics with polarized antiprotons Study of the proton spin 2

3 Where is the proton spin? the proton is an object we thought we knew so well, but which reveals another face when it spins. J. Ellis QPM: 1 1 = Δu v Δd v Δq s 2 2 gluons are important! ΔΣ = 1 sis i r C Σ=0.123 ±... EMC (1988) n i Sp SLAC, CERN, DESY: = ΔΣ ΔG 2 2 don t forget the orbital angular momentum! 1 1 = ΔΣ Lq ΔG L g 2 2 3

4 How to study the nucleon structure? deep-inelastic scattering (DIS) lab Q2 = q 2= 2EE' 1 cos θ q=1/λ (ph. wavel.) resolution lab 2 Q x= 2Mν P xp Proton momentum fraction carried by the struck quark inelastic deep high resolution: 2 M X M 2 2 Q >> M 2 proton has structure 4

5 Polarized deep inelastic scattering 1 1 = ΔΣ + L q + G + L g 2 2 =J q J g inclusive dis polarised beam and/or semi-inclusive DIS polarised target flavour separation exclusive dis inclusive hadrons 5

6 Experimental (e.g. HERMES) Polarized e+/e- beam Polarized internal target (Detector) 27.5 GeV (e+/e-) self-polarised electrons: e <Pb>~ 53±2.5 % <Pb>~ 53±2.5 % 6

7 Polarized semi-inclusive DIS flavour tagging π+ ud K- us π- ud distribution function fragmentation function h dσ z Δq f x dσ f f q h Df z, z=eh/ν 7

8 Quark/anti-quark helicity distributions u quarks large positive polarisation d quarks negative polarisation sea quarks (u, d, s) compatible with 0 (in measured x-range: ) 8

9 Where do we stand? the (spin) structure of the nucleon H1, ZEUS,++ HERMES COMPASS CERN RHIC SLAC HERMES, ++ 9

10 Where do we stand? the (spin) structure of the nucleon H1, ZEUS,++ HERMES COMPASS CERN RHIC SLAC Transversity h1, δq HERMES JLab HERMES, ++ 10

11 Quark structure of the nucleon 1 Tw2 1 Φ Corr x = { q x S L Δq x γ 5 δq x γ 5 γ S T } n 2 q= unpolarised quarks and nucleons Δq= longitudinally polarised quarks and nucleons δq= transversely polarised quarks and nucleons 11

12 Transversity q h 1= transversely polarised quarks and nucleons - Probes relativistic nature of quarks No gluon analog for spin-1/2 nucleon Different Q2evolution than Δq Sensitive to valence quark polarization h1 is chirally odd -> it needs a chirally odd partner Inclusive DIS Semi-inclusive DIS Drell-Yan HERMES,COMPASS,JLab 12

13 Transversity with SIDIS H1 Transversity Collins fragm. function Sivers moment h h h1 1 N h φ, φs N h φ, φs h A UT φ, φ S = = S T N h φ, φs N h φ, φs 13

14 Collins asymmetries ep π X q δq x H 1 z π+ first time: transversity & Collins FF are non-zero! π 14

15 First glimpse of transversity from Collins asymmetries sin φ φs A UT δq x q 1 H z e+ e- e p e' π X e d e' h X e e π jet 1 π jet2 X 15

16 Extraction Collins function (H1 (z)) xδd(x) xδu(x) Transversity (δ(x)) Anselmino et al. PRD75(2007) 16

17 Comparison with some models xδd(x) xδu(x) Transversity A. Bacchetta, Trento (2005) 17

18 Comparison with some models Tensor charges xδd(x) xδu(x) Transversity M. Wakamatsu arxiv: δu 0.39, δd

19 Situation First indirect extraction of transversity Combined analysis of HERMES, COMPASS, BELLE Disagreement between extracted h1 and theoretical models Disagreement with lattice QCD Questions: Are the theoretical models appropriate? Is extraction valid? (Universality, factorization ) More direct observables are needed! 19

20 Transversity q h 1= transversely polarised quarks and nucleons - Probes relativistic nature of quarks No gluon analog for spin-1/2 nucleon Different Q2evolution than Δq Sensitive to valence quark polarization h1 is chirally odd -> it needs a chirally odd partner Inclusive DIS Semi-inclusive DIS Drell-Yan HERMES,COMPASS,JLab 20

21 h1 from Drell-Yan q q γ l l rell-yan 2 2 d σ 4 πα 1 2 = e x 2 q x 1 q x 2 ] q [ q x1 q 2 22 x x dm dx F 9MM ss 1 2 q x F = x 1 x 2 2 x 1 x 2 = M /s τ dσ dσ ATT = =a TT dσ dσ x F =2Q L / s q=u, u,d, d,... M invariant Mass of lepton pair 2 e q q [ h1q x 1 h 1q x 2 h1 q x 1 h1 q x 2 ] 2 q e q [ q x 1 q x 2 q x 1 q x 2 ] 21

22 A possible facility (PAX proposal) s=200 GeV2 EXPERIMENT: Asymmetric collider: polarized protons in HESR (p=15 GeV/c) polarized antiprotons in CSR (p=3.5 GeV/c) 22

23 h1 from p-p Drell-Yan at PAX 2 e h x 1 h 1q x 2 h1 q x 1 h1 q x 2 ] dσ dσ q q [ 1q ATT = =a TT 2 dσ dσ e q q [ q x 1 q x 2 q x 1 q x 2 ] 1year run: 10 % precision on the h1u(x) in the valence region u-dominance h1u > h1d ATT a TT h1u x 1 h 1u x 2 Pp=30% u x 1 u x 2 PAX : Μ2/s=x1x2~ valence quarks (ATT large ~ ) Pp=10% Anselmino et al. PLB 594,97 (2004) Similar predictions by Efremov et al., Eur. Phys. J. C35, 207 (2004) 23

24 Perspectives for pbar-d, p-p p-p p-p, p-d M2/s = x1x2 ~ At x1=x2 ATT ~ h1u2 Direct meas. of h1u for 0.05<x<0.5 Extraction of h1d, h1q for x<0.2 p p, p p, p d : complete mapping of transversity Flavour separation of δq (equivalent to semi-inclusive DIS for q) 24

25 Spin-physics and Polarized Antiprotons Proton electromagnetic form-factors 25

26 Electromagnetic Form Factors - Describe int. structure of the nucleon -Information about proton ground state -Test for models of nucleon structure - Wavelength tunable with Q2: < 0.1 GeV2 integral quantities GeV2 internal structure > 20 GeV2 pqcd scaling TIMELINE: 1932 (Stern): anomalus magnetic moment of the proton 1950 (Hofstadter) first measurement of the proton s radius through elastic electron scattering (Nobel Prize in 1961) sis i r 2000 Polarization C transfer measurement in p d S 2n 26

27 Electromagnetic Form Factors p p q=k -k k J μ 2 μ =F 1 q γ κ 2 μν F 2 q iσ q ν 2M k One-photon-exchange approximation: Pauli-Dirac (F1 and F2) or Sachs (GE and GM) GM(q2) = F1(q2) + F2(q2) GE(q2) = F1(q2) + τ F2(q2) τ=q2/4m2 In the Breit reference system, Sachs FFs are the Fourier transform of the charge and magnetization distributions 27

28 Space-like and Time-like regions FFs are analytical functions. of t = q2 = -Q2. t=q2>0 (timelike) t=q2<0 (spacelike) real function complex function Annihilation _ e+ + e- => h + h Scattering e- + h => e- + h _ lim SL q 2 2 TL 2 F q =lim q2 F q 28

29 How to measure Space-Like FFs? (I) Rosenbluth separation (1950) - SL FFs scaling: GMp µp GEp charge and magnetization have the same distribution - FFs described ( 20%) by dipole formula Λ2 G D = 2 2 Λ +Q 2 Λ 0.8 GeV 29

30 How to measure Space-Like FFs? (II) Polarization transfer (1958) Akhiezer et al., Sov. Phys. Jept. 6, 588 (1958) GE GM = P t E e E e ' Pl 2M tan θ 2 - Pt and Pl of the scattered proton measured simultaneously (using 12C) Experimental: Had to wait over 30 years for development of Polarized beam with high intensity (~100 µa) and high polarization (>70 %) (strained GaAs, high-power diode/ti-sapphire lasers) Polarized targets with a high polarization Beam polarimeters with 1-3 % absolute accuracy 30

31 Space-Like FFs: proton data Rosenbluth Polarization - Q2 dependence suggests different charge and magnetization spatial distributions inside the nucleon Is the proton round? 31

32 How to measure Time-Like FFs? dσ α 2 βc = G M 1 cos θ G E 2 sin2 θ d 4s τ { τ = s/ 4M2 GM dominates the cross section for s >> 4M2 GM = GE at the physical threshold s = 4M2 No independent extraction of both TL FFs performed (σ 1 nb) GE remains unmeasured (New data from BABAR coming ) 32 }

33 Time-Like FFs: proton data The dashed line is a fit to the PQCD prediction G M μp = C s 2 ln 2 s Λ2 The expected Q2 behaviour is reached quite early, however... 33

34 Time-Like FFs: proton data The dashed line is a fit to the pqcd prediction G M μp = C s 2 ln 2 s Λ2 The expected Q2 behaviour is reached quite early, however there is still a factor of 2 between timelike and spacelike. 34

35 Situation The new SL JLab results dramatically changed the picture of the Nucleon: - GEp/GMp decreases with Q2 - data suggest GEp crosses 0 at Q2 8 GeV2 Charge and magnetization distributions are different ( second proton spin crisis ) Time-like region much less studied Intrinsic interest in itself (possible resonance at threshold) No direct measurement of GE existing Need to reconsider some discrepancy found in the TL region and with respect to the SL Additional independent measurements are needed! 35

36 A possible facility (PAX-Phase I) EXPERIMENT: Fixed target experiment: polarized antiprotons protons in CSR (p>200 MeV/c) fixed polarized protons target P. Lenisa Physics with polarized antiprotons 36

37 Double polarized pbar-p annihilation E. Tomasi, F. Lacroix, C. Duterte, G.I. Gakh, EPJA 24, 419(2005) Most contain mduli GE, GM Independent GE-GM separation Test of Rosenbluth separation in the time-like region Access to GE-GM phase Very sensitive to different models (next transparencies) 37

38 Theoretical models Spacelike Magnetic Electric Magnetic neutron proton Electric Timelike QCD inspired Bosted PRC 51, 409 (1995) Extended VDM E.L.Lomon PRC 66, ) VDM : IJL F. Iachello..PLB 43, 191 (1973) Hohler NPB 114, 505 (1976) E. Tomasi, F. Lacroix, C. Duterte, G.I. Gakh, EPJA 24, 419(2005) 38

39 Polarization and Models in T.L. Region Ay Axx Ayy VDM : IJL Ext. VDM QCD inspired Axz R Azz E. Tomasi, F. Lacroix, C. Duterte, G.I. Gakh, EPJA 24, 419(2005) 39

40 (Single Spin Asymmetry) A. Z. Dubnickova et al. Nuovo Cimento A109, 241 (1996) S.J. Brodsky et al. PRD 69, (2004) Single-spin asymmetry in pp e+e Measurement of relative phases of magnetic and electric FF in the time-like region - Also sensitive to different models 40

41 A possible facility (PAX-Phase I) EXPERIMENT: Fixed target experiment (+ asymmetric collider): Npbar = 1x1011 f=lcsr/βc ppbar>200 MeV/c dt=1x1014cm-2 Q=0.8 (p pol) P=0.3 (pbar pol) s>3.76 GeV 2 ε=0.5 Running days to get O = 0.05 Beam (MeV/c) <s> (GeV2) σe+e(nbarn) L (cm-2s-1) Days DS Days SS x x x ΔO DS = QP N 1 1 ΔO SS = Q N SS 41

42 Spin-physics and Polarized Antiprotons Polarized pbar-p hard-scattering 42

43 Hard polarized scattering Unpolarized p-p elastic cross section Beam P Target dσ/dt Dividing dσ/dt at 90o c.m by 4 made all p-p elastic data fit on a single curve scaled p2 43

44 Experimental Development of polarized proton beams ZGS ( ) T=12 GeV 19 dep. resonances PB>70 % AGS (1988 T=22 GeV 45 dep. resonances Polarized target 44

45 Hard p-p polarized scattering Spin e ant s i Cris m? a r e litt T=10.85 GeV The graeatest asymmetries in hadron physics ever seen by a human being (Brodsky) D.G. Crabb et al., PRL 41, 1257 (1978) The results challenge the prevailing theory that describes the proton s structure and forces (Krish, 1987) One of the unsolved mysteries of hadron physics (Brodsky, 2005) the thorn in the side of QCD (Glashow) It would be very interesting to performe this measurements with polarized antiprotons. 45

46 A possible facility (PAX-Phase I) EXPERIMENT: Fixed target experiment (+ asymmetric collider): Npbar = 1x1011 f=lcsr/βc ppbar>200 MeV/c dt=1x1014cm-2 Q=0.8 (p pol) P=0.3 (pbar pol) s>3.76 GeV2 ε=0.5 Cross section estimation for p=3.6 GeV/c, θ=120 dσ p p 4 mb 10 2 dt E 838 GeV GeV t E c 2 hours to get 2 BNL E ΔO DS = QP N DS 46

47 Spin-physics and Polarized Antiprotons Further perspectives Spectroscopy of hadrons Use of polarization degrees of freedom to decrease number of contributing amplitudes 47

48 J/ψ,ψ production NRQCD Able to reproduce the unpolarized xsec Fixed target exp Fails in predicting polarization vs pt at CDF F. Maltoni et al., hep-ph/

49 Spin-physics and Polarized Antiprotons Further perspectives Spectroscopy of hadrons Use of polarization degrees of freedom to decrease number of contributing amplitudes Low-t proton-antiproton scattering Investigation of spin and isospin dependence of nucleon-antinucleon interaction at low energy 49

50 Spin-physics and Polarized Antiprotons By-products of a facility for polarized antiprotons 50

51 Single-spin asymmetries Correlation functions q P pp PDFs Sivers effect = number of partons in polarized proton depends on P (p x k ) k q Pq Boer-Mulders effect = polarization of partons in unpolarized proton depends on Pq (p x k ) k pp π Pq pq FFs pq Collins effect = fragmentation of polarized quark depends on Pq (pq x k ) k PΛ Λ Polarizing FF = polarization of hadrons from unpolarized partons depends on PL (pq x k ) k These effects may generate SSA dσ d σ AN = dσ d σ 51

52 Sivers from SIDIS ep hx Sivers from hadron scattering pp πx Asiv φ φ S f 1T x D1 z BNL-AGS s = 6.6 GeV 0.6 < pt < 1.2 p p E704 s = 20 GeV 0.7 < pt < 2.0 p p 52

53 The Sivers function Test of Universality SIDIS :ep eπ X 2 2 f 1T x,p T SIDIS = f 1T x,pt DY PAX : p p e e X E 704: p p π X A.V. Efremov et al., M. Anselmino et al., Phys. Lett. B 612, 233 (2005) x 1/2= M 2 / s e ± y Phys. Rev. D72, (2005) 53

54 The Sivers function p p D X p p D X No Collins effect in s-channel q q c c p p γ X U.D Alesio and F. Murgia hep-ph/ No fragmentation process 54

55 Spin-physics and Polarized Antiprotons Polarized antiprotons will open the way to a new spin-physics era: * Proton-spin structure: Complete map of transversity Flavour separation * Electromagnetic Form Factors Independent extraction of moduli of GE-GM in Time-Like region Test of the Rosenbluth separation in TL Measurement of the phase * Hard p-pbar scattering Additional measurement in one of the most intriguing puzzles of HF * Hadron spectroscopy * *(Wealth of single-spin asymmetries) 55

56 Final Remarks Polarization data has often been the graveyard of fashionable theories. If theorists had their way, they might just ban such measurements altogether out of selfprotection. J.D. Bjorken St. Croix, 1987 Georg Christoph Lichtenberg ( ) Man muß etwas Neues machen, um etwas Neues zu sehen. You have to make something new, if you want to see something new 56

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