1 Probing the Light Quark Sea Flavor Asymmetry and Measuring the Neutron Transversity in Semi-inclusive Charged Meson Electroproduction Xin Qian Duke University

2 Outline  Nucleon Structure and Electron Scattering  Flavor  Flavor structure: Probing light quark sea flavor asymmetry  Spin structure: Measuring neutron transversity  Summary

3 Nucleon Structure  Nucleon anomalous magnetic moment (Stern, Nobel Prize 1943)  Electromagnetic form factor from electron scattering (Hofstadter, Nobel Prize 1961)  Deep-in-elastic scattering, quark underlying structure of the nucleon (Freedman, Kendell, Feldman, Nobel Prize 1990) Understanding the underlying nucleon structure (Spin, flavor, charge, current distribution) from quantum chromodynamics (confinement region) is essential.

4 Electronuclear Scattering A powerful tool to study nuclear structure Inclusive: (the main tool) detecting electron only Semi-inclusive: (providing additional information) detecting electron and one of the hadrons coincidently Charge distribution: Spectrum: Energy

5 Cross Section Structure Functions: Transversity Distributions: Polarized and Unpolarized inclusive DIS γ*γ* Relations to Form Factor: Charge distribution: Magnetic moment distribution: Hadronic Part:

6 Semi-Inclusive DIS  A DIS reaction in which a hadron h, produced in the current fragmentation region is detected coincidently with scattered electron. SIDIS Parton distribution Function (PDF) Fragmentation function (FF) Semi-inclusive DXs~PDF  FF Current frag. Target frag.

7 Outline  Nucleon structure and electron scattering  Flavor structure: Probing light quark sea flavor asymmetry  Spin structure: Measuring neutron transversity  Summary

8 Flavor Asymmetry in the light nucleon sea  Gottfried sum rule:  A flavor-symmetric nucleon sea and isospin symmetry would lead  New Muon Collaboration result determined  The Drell-Yan measurement also supports the flavor asymmetry.

9 Semi-inclusive Pion production from proton and deuteron target  The Pion yield in unpolarized SIDIS can be expressed as:  The flavor asymmetry can be determined by four yields: will introduce systematic error.

10 Semi-inclusive Kaon production from proton and deuteron target  Fragmentation Function Ratio (ignored the strange quark contribution):  With PR

11 Outline  Nucleon structure and electron scattering  Flavor structure: Probing light quark sea flavor asymmetry  Spin structure: Measuring neutron transversity  Summary

12 Leading-Twist Quark Distributions No K ┴ dependence K ┴ - dependent, T-odd K ┴ - dependent, T-even ( Eight parton distributions functions) Transversity:

13 Eight fragmentation functions  T-odd, quark intrinsic momentum dependent H 1  (z, к T ’ ): related to Collins effect. Hadron momentum ~ к T ’ = -z к T ~ quark momentum --

14 The kinematics and coordinate  E’ is the energy of scattered electron  θ e is the scattering angle  ν=E-E’ is the energy transfer.  k  : quark transverse momentum DIS: Q 2 (1/λ) and ν is large, but x is finite.

15 Leading-Twist DXs in SIDIS Unpolarized Polarized target Polarized beam and target S L and S T : Target Polarizations; λ e : Beam Polarization Sivers Collins DXs ~ PDF  FF Transversity

16 Characteristics of Transversity  Some characteristics of transversity:  h 1T = g 1L for non-relativistic quarks  In non-relativistic case, boosts and rotations commute.  Λ QCD =200 MeV, m u and m d ~ 5 MeV, quark are relativistic.  Important inequalities: |h 1T q | ≤ f 1 q ; |h 1T q | ≤ (f 1 q + g 1L q )/2.  h 1T and gluons do not mix  Gluon can not be included in transversity for nucleon.  Q 2 -evolution for h 1T and g 1L are different N qq N Helicity state

17 Characteristics of Transversity  Chiral-odd → not accessible in inclusive DIS  In calculating the hadronic part in inclusive DIS, the gluon contribution cancel the quark mass term which contains the transversity distribution.  Decoupling mass term will turn off transversity distribution - +

18 Characteristics of Transversity  It takes two Chiral- odd objects to measure transversity  Drell-Yan (Doubly transversely polarized p-p collision)  Semi-inclusive DIS Chiral-odd distributions function (transversity) Chiral-odd fragmentation function (Collins function) Chiral-quark soliton model -

19 Asymmetry in Semi-Inclusive DIS with polarized target Unpolarized Polarized target Polarzied beam and target S L and S T : Target Polarizations; λe: Beam Polarization Sivers Transversity

20 Asymmetry in Semi-Inclusive DIS with polarized target Collins effect  Access to transversity  Artru model  Based on LUND fragmentation picture. Scattering plane

21 Asymmetry in Semi-Inclusive DIS with polarized target Sivers effect  Sivers effect  A new type of PDF, T-odd, depends on intrinsically quark transverse momentum quark orbital momentum Beam direction Into the page

22 Asymmetry in Semi-Inclusive DIS with polarized target Discussion  Can not separate two effects in the longitudinal case.  In longitudinal case, some higher twist distribution contributes.  Need transversely polarized target in order to separate. ~ 0.15 Hermes kinematics

23 JLab Hall-A E Experiment  High luminosity  15 μA electron beam on 10-atm 40-cm transversely polarized 3 He target  Measure neutron transversity  Sensitive to h 1 d, complementary to HERMES  Disentangle Collins/Sivers effects Measurement of Single Target-Spin Asymmetry in Semi-Inclusive Pion Electroproduction on a Transversely Polarized 3 He Target Argonne, CalState-LA, Duke, E. Kentucky, FIU, UIUC, JLab, Kentucky, Maryland, UMass, MIT, ODU, Rutgers, Temple, UVa, W&M, USTC-China, CIAE-China, Glasgow-UK, INFN-Italy, U. Ljubljana-Slovenia, St. Mary’s- Canada, Tel Aviv-Israel, St. Petersburg-Russia Spokespersons: J.-P. Chen (JLab), X. Jiang (Rutgers), J. C. Peng (UIUC)

24 Single Spin Asymmetry  With 100% polarization,  From azimuthal angular distribution, we can separate Collins effect and Sivers effect in this experiment. Comparison with HERMES projection

25 Experimental Configuration

26 Future plan  JLAB E will be my thesis experiment.  BigBite background simulation.  Work on target.  Doing the data analysis.  Plan to move to JLAB this summer.

27 Summary  Semi-inclusive DIS meson electroproduction can provide additional information to the inclusive DIS (transversity).  By measurement of SIDIS π + /π -, K + /K - yield ratio on hydrogen and deuterium target, we will independently check the light sea quark flavor asymmetry. The flavor dependent fragmentation function will be studied (flavor structure).  The Hall-A measurement on transversely polarized 3 He target should provide new information and powerful constraints on transversity of u-quark and d-quark, when combined with HERMES and COMPASS data (spin structure).

28 Thank you!

29 Supporting slides …..

30 Transversity (Chiral-odd)

31 Semi-inclusive Pion production from proton and deuteron target  The Pion yield in unpolarized DIS can be expressed as:  The flavor asymmetry can be determined as:  in which with and will introduce systematic error.

32 Current & target fragmentation

33 Quark-nucleon helicity amplitude  If use the quark-nucleon helicity amplitudes: Express three leading twist distribution function as amplitudes: h 1T (x) g 1L (x) f 1 (x)

34 Kinematics

35 Hermes data and detailed interpretations

36 Makins DNP04 talk π

37 Observation of Single-Spin Azimuthal Asymmetry Longitudinally polarized target ep → e’πxHERMES hep-ex/ ~ 0.15 Suggests transversity, δq(x), is sizeable Suggests Collins T-odd fragmentation function is sizeable Other effects (Sivers effect, higher twist) could also contribute

38

39 Why Collins π - asymmetries so large?  DIS on proton target dominates by u-quark scattering. …expect: positive. …expect: ~zero. Data indicate the disfavored fragmentation function is sizable and negative.

40 QCD Q 2 evolution

41 Nobel Prize this year! “Running” of Coupling Constants with energy scale is a key prediction 14

42 21

43 Probability of parton i going into parton j with momentum fraction z Calculable in pQCD as expansions in α S In Leading Order P ij (z) take simple forms P qq P qg P gq P gg Splitting Functions P ij (z)

44 b) Sum i) over q and q separately Fit to DGLAP equations c) Define: Valence quark density Singlet quark density I) Rewrite DGLAP equations a) Simplify notation N f … number of flavors i) ii) ia) ib) ← u,u,d

45 II) DGLAP equations govern evolution with Q 2 Do not predict x dependence: Parameterize x-dependence at a given Q 2 = Q 2 0 = 4 – 7 GeV 2 d) Rewrite DGLAP equations Valence quark density decouples from g(x,Q 2 ) Only evolves via gluon emission depending on P qq 55 parameters Low x behaviour High x behaviour: valence quarks

46 Proton Structure function F 2 (x,Q2)  Scaling violation explicitly seen …  Beyond the fixed target regime  H1 and ZEUS data in agreement. Further, pQCD predictions at NLO describe data impressively over many decades in x and Q2.  Studies have resulted in the determination of gluon distribution, precise determination of  S Rise in F 2 at low x

47 Polarized He 3 target

48 Why polarized 3 He is an effective neutron target? S-state about 90% D-state about 8% S ’ -state about 2%

49 Optical Pumping for Rubidium  37 Rb:  Rb vapor in a weak B field is optically pumped Buffer gas N 2 let the electrons decay without emitting photons 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 1

50 Polarized 3 He target description

51 NMR Polarimetry  The magnetic moment of a free particle of spin  When placed in an external B-field  Transform into a frame rotating  Effective field

52 NMR - Adiabatic Fast Passage (AFP)  Ramp the holding field from below the resonance to above it  Spin Flip (Twice)  Signal is the fitted amplitude

53 NMR-AFP Condition  The sweep rate is slow enough (Adiabatic)  The sweep rate is fast enough (Fast) T 1 and T 2 are the longitudinal and transverse relaxation times