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Studies on transverse spin effects at Jlab

Studies on transverse spin effects at Jlab. Harut Avakian. QCD Structure of the Nucleon June 12-16, 2006, Rome. Physics motivation k T -effects from unpolarized data TMD-studies from polarized target data Summary. Physics Motivation.

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Studies on transverse spin effects at Jlab

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  1. Studies on transverse spin effects at Jlab Harut Avakian QCD Structure of the Nucleon June 12-16, 2006, Rome Physics motivation kT-effects from unpolarized data TMD-studies from polarized target data Summary

  2. Physics Motivation • Describe the complex nucleon structure in terms of quark and gluon degrees of freedom using polarized SIDIS Cross section is a function of scale variables x,y,z n = E-E’ y = n /E x = Q2 /2Mn z = Eh /n • In 1D world (no orbital motion) • quarks polarized if nucleon is polarized, 3PDFs. • No azimuthal asymmetries in LO z Transverse spin effects are observable as correlations of transverse spin and transverse momentum of quarks.

  3. Transverse momentum of quarks Mulders & Tangerman • kT – lead to 3 dimensional description • kT – required to describe azimuthal distributions of hadrons and in particular SSAs. • kT - important for cross section description (also for exclusive production) • PT distributions of hadrons in DIS • exclusive photon production (DVCS) • - hard exclusive vector meson x-section Off diagonal PDFs related to interference between L=0 and L=1 light-cone wave functions. • Factorization of kT-dependent PDFs proven at low PT of hadrons (Ji et al) • Universality of kT-dependent distribution and fragmentation functions proven (Collins,Mets…) kT – crucial for orbital momentum and spin structure studies.

  4. SIDIS (g*p->pX) x-section at leading twist: TMD PDFs f1 h┴1 Unpolarized target Longitudinally pol. Transversely pol. g1 T-odd h┴1L f┴1T g1T h1 h┴1T kT-even kT-odd Studies of PDFs require three experiments The structure functions depend on Q2, xB, z, PhT

  5. y ST PT PT fC y fC sT sT PT fS sT fh fC fh fh fS’ fS’ x fS=fh x ┴ AUL∞h1L H1┴ Collins Effect: azimuthal modulation of the fragmentation function D(z,PT)=D1(z,PT)+H1┴(z,PT)sin(fh- fS’) unpolarized longitudinally polarized transversely polarized y fS Initial quark polarization AUT∞h1H1┴ Scattered quark polarization AUU∞h1┴H1┴ sin(fh- fS’)= h1┴H1┴ cos(2fh) fC Combination of Collins asymmetry measurements with 3 different targets will provide info to separate the chiral-odd distribution functions and measure the Collins function sin(2fh)

  6. kT-dependent SIDIS p┴= PT– zk┴+ O(k┴2/Q2) Anselmino et al data fit on Cahn effect→<m02>=0.25GeV2 EMC (1987) and Fermilab (1993) data

  7. Azimuthal Asymmetries in SIDIS • Intrinsic transverse momentum of partons (Cahn 1978) • Higher twists (Berger 1980, Brandenburg et al 1995) • Gluon bremsstrahlung (Georgi & Politzer, Mendez 1978) at z→1 All known contributions to <cosf> and <cos2f> are “flavor blind”

  8. Unpolarized target azimuthal asymmetries M.Osipenko CLAS 5.7 GeV preliminary • Significant cosf,cos2f observed at large PT at 5.7 GeV • New proposal in preparation for JLab PAC to study azimuthal moments at 11 GeV (large Q2 and PT)

  9. Tests of partonic picture CTEQ5M PDFs + Binnewies FF GRSV2000 X=0.3, Q2=2.5 GeV2, W=2.5 GeV E00-108 at JLAB JLab data consistent with partonic description (0.4<z<0.7, MX>1.5)

  10. Missing mass of pions in ep->e’pX p0 D++ p- p+ n D0 Large Delta(1232) contribution makes p- different (Mx>1.5 GeV applied)

  11. ALL : PT-dependence • Data is needed in small bins in x,Q2,PT,f to measure m0 andmDfor polarized and unpolarized targets andprobe their variations (Kotzinian et al) • m0uandm0d • mDfavandmDunfav • Expected flat in perturbative limit • Difference between neutral and charged pions

  12. SSA: PT-dependence of sinf moment ssinfLU(UL) ~FLU(UL)~ 1/Q (Twist-3) ALUCLAS @4.3 GeV AUL(CLAS @5.7 GeV) AUTHERMES @27.5 GeV PRELIMINARY TMD pQCD Beam and target SSA for p+ are consistent with increase with PT In the perturbative limit is expected to behave as 1/PT

  13. Higher Twist SSAs Target sinf SSA (Bacchetta et al. 0405154) Discussed as main sources of SSA due to the Collins fragmentation In jet SIDIS only contributions ~ D1 survive The same unknown fragmentation function Beam sinf SSA With H1┴ (p0)≈0(or measured) Target and Beam SSA can be a valuable source of info on HT T-odd distribution functions

  14. Target SSA measurements at CLAS • Complete azimuthal coverage crucial for separation of sinf, sin2f moments ep→e’pX CLAS PRELIMINARY p1sinf+p2sin2f W2>4 GeV2 Q2>1.1 GeV2 y<0.85 0.4<z<0.7 MX>1.4 GeV p1= 0.059±0.010 p2=-0.041±0.010 p1=-0.042±0.015 p2=-0.052±0.016 p1=0.082±0.018 p2=0.012±0.019 PT<1 GeV 0.12<x<0.48 No indication of Collins effect for p0 (x20 more data expected)

  15. 60 days of CLAS+IC (L=1.5.1034cm-2s-1) Kotzinian-Mulders asymmetry: new CLAS proposal curves, cQSM from Efremov et al Hunf=-5Hfav Hunf=-1.2Hfav Hunf=0 • Provide measurement of SSA for all 3 pions, extract the Mulders TMD and study Collins fragmentation with longitudinally polarized target • Allows also measurements of 2-pion asymmetries • Prospects with polarized deuteron.

  16. First glimpse of Twist-2 TMD h1L┴ For Collins fragmentation use fit to HERMES (Efremov et al) Distribution functions from cQSM from Efremov et al PRELIMINARY CLAS-5.7GeV Systematic error only from unknown ratio of favored and unfavored Collins functions (R= H1d→p+/H1u→p+), band correspond to -2.5<R<0 • More data expected for p- & p0 • Exclusive 2 pion background may be important p- and p0 SSA will also give access to h1Ld

  17. Summary • Significant azimuthal moments in pion production in SIDIS were measured at CLAS providing information on transverse momentum distributions of quarks. • Measurement of Collins asymmetries at JLab with unpolarized and polarized targets will provide access to leading twist chirall-odd distribution functions (Boer,Mulders and transversity distributions) • SSA measurements in a wide range of Q2, would allow studies of higher twist effects and probe T-odd distributions • SSA measurements in a wide range of PT will allow to study the transition from non-perturbative to perturbative description.

  18. support slides…

  19. L Flavor decomposition of T-odd f┴ (g┴, f1T┴ ) In jet SIDIS with massless quarks contributions from H1┴ vanish gauge link contribution With SSA measurements for p++p-and p0 on neutron and proton (p=p++p-) assuming Hfav=Hu→p+≈ -Hu→p-=-Hunfav With H1┴ (p0)≈0(or measured) target and beam HT SSAs can be a valuable source of info on HT T-odd distribution functions

  20. PT-dependence

  21. PT-dependence

  22. CLAS12: kinematic distributions CLAS12 allow wide kinematical coverage of SIDIS

  23. SSA: PT-dependence of sinf moment ssinfLU(UL) ~FLU(UL)~ 1/Q (Twist-3) ALUCLAS @4.3 GeV AUL(CLAS @5.7 GeV) AUTHERMES @27.5 GeV PRELIMINARY TMD pQCD Beam and target SSA for p+ are consistent with increase with PT In the perturbative limit is expected to behave as 1/PT

  24. Azimuthal Asymmetries in semi-exclusive limit • Higher twists (Berger 1980, Brandenburg et al 1995) z→1 dominant contribution u+e- →e- p+ d Dominant contribution to meson wave function is the perturbative one gluon exchange and approach is valid at factor ~3 lower Q2 than in case of hard exclusive scattering (Afanasev & Carlson 1997)

  25. PT-dependence of beam SSA ssinfLU(UL) ~FLU(UL)~ 1/Q (Twist-3) In the perturbative limit 1/PT behavior expected 2.0 Perturbative region Non-perturbative TMD Asymmetries from kT-odd (g┴,h1┴) and kT-even (g1) distribution functions are expected to have a very different behavior.

  26. xF>0 (current fragmentation) Single pion production in hard scattering h xF<0 (target fragmentation) xF- momentum in the CM frame Target fragmentation Current fragmentation h h h h M PDF PDF GPD 1 -1 0 xF Fracture Functions kT-dependent PDFs Generalized PDFs Wide kinematic coverage of large acceptance detectors allows studies of hadronization both in the target and current fragmentation regions

  27. SIDIS (g*p→pX) cross section: Unpolarized target e Unpolarized target • cosf (Boer-Mulders function h1┴) and sinf (g┴) azimuthal moments of the x- section as a function of x, Q2, PT, z • cosf, cos2f azimuthal moments and Cahn and Berger effects • Transition from non-perturbative to • perturbative description at large PT • Target fragmentation (Lambda, azimuthal moments) at leading twist Study the transverse polarization of quarks in the unpolarized nucleon.

  28. e p Longitudinally pol. target SIDIS (g*p→pX) cross section: polarized target at leading twist • sinf (Mulders function h1L┴) and sinf (fL┴) azimuthal moments of the x-section as a function of x, Q2, PT, z • A1 and flavor decomposition (g1), • PT-dependence of A1 • Target fragmentation (Lambda, azimuthal moments) Study the transverse polarization of quarks in the longitudinally polarized nucleon.

  29. Transversely pol. target e p SIDIS (g*p→pX) cross section: polarized target • sinf-fS (Sivers, f1T┴), sinf+fS(transversity, h1) and cosfS (g1T) azimuthal moments of the x-section as a function of x,Q2,PT,z • 2 pion SSA (h1) • Target fragmentation (Lambda, azimuthal moments) at leading twist Study the transverse polarization of quarks in the Transversely polarized nucleon.

  30. cos2f: predictions V.Barone • Significant asymmetry predicted for HERMES • Asymmetry is LT! (not decreasing with 1/Q) The only mechanism with sign change from p+ to p- Projections for CLAS12 in progress..

  31. Unpolarized target e p Longitudinally pol. target Transversely pol. target e p SIDIS (g*p→pX) cross section at leading twist (Ji et al.) e Boer-Mulders 1998 Kotzinian-Mulders 1996 Collins-1993 structure functions = pdf × fragm × hard × soft (all universal) Off diagonal PDFs related to interference between L=0 and L=1 light-cone wave functions. To observe the transverse polarization of quarks in SIDIS spin dependent fragmentation is required!

  32. y ST PT fC sT fS fh fS fS’ = p-fS fS’ sin(fh+fS) spin of quark flips wrt y-axis x FUU∞h1 ┴H1┴ ┴ sT(p×kT)↔ h1┴ (sTkT)(pSL)↔ h1L sinfC=sin(fh- fS’) y PT sT fC fh fS=fh ┴ cos(2fh) FUL∞h1L H1┴ Collins Effect: azimuthal modulation of the fragmentation function FUT∞h1H1┴ sT(q×PT)↔H1┴ y fC PT fC sT D(z,PT)=D1(z,PT)+H1┴(z,PT)sin(fh- fS’) fh fS’ x fC fS= p/2+fh x sin(2fh) fS’ = p-fS= p-fh fS’ = p-fS= p/2-fh sin(2fh)

  33. Higher Twist SSAs Target sinf SSA (Bacchetta et al. 0405154) Discussed as main sources of SSA due to the Collins fragmentation In jet SIDIS only contributions ~ D1 survive The same unknown fragmentation function Beam sinf SSA With H1┴ (p0)≈0(or measured) Target and Beam SSA can be a valuable source of info on HT T-odd distribution functions

  34. CLAS12: kinematic distributions Large Q2 accessible with CLAS12 are important for cos2f studies

  35. Acceptance generated reconstructed Extract acceptance moments from MC

  36. sUL ~ KM Collins Effect and Kotzinian-Mulders Asymmetry longitudinally polarized target Study the Collins fragmentation with longitudinally polarized target. Measure the twist-2 Mulders TMD (real part of interference of L=0 and L=1 wave functions) Non-zero asymmetry measured at 5.7 GeV, new proposal will improve erors by a factor ~3 Measurement at 11 GeV will allow extend the Q2 and x range and perform a flavor decomposition of u/d contributions.

  37. sUL ~ KM CLAS12 : Mulders TMD projections Simultaneous measurement of, exclusive r,r+,w with a longitudinally polarized target important to control the background.

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