Probing the Light Quark Sea Flavor Asymmetry and Measuring the Neutron Transversity in Semiinclusive Charged Meson Electroproduction Xin Qian Duke University 1
Outline § Nucleon Structure and Electron Scattering § Flavor structure: Probing light quark sea flavor asymmetry § Spin structure: Measuring neutron transversity § Summary 2
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. 3
Electronuclear Scattering ------ A powerful tool to study nuclear structure Spectrum: Charge distribution: Energy Inclusive: (the main tool) detecting electron only Semi-inclusive: (providing additional information) detecting electron and one of the hadrons coincidently 4
Polarized and Unpolarized inclusive DIS Cross Section γ* Structure Functions: Transversity Distributions: Hadronic Part: Relations to Form Factor: Charge distribution: Magnetic moment distribution: 5
Semi-Inclusive DIS § A DIS reaction in which a hadron h, produced in the current fragmentation region is detected coincidently with scattered electron. Semi-inclusive SIDIS Fragmentation function (FF) Target frag. Current frag. DXs~PDF FF Parton distribution Function (PDF) 6
Outline § Nucleon structure and electron scattering § Flavor structure: Probing light quark sea flavor asymmetry § Spin structure: Measuring neutron transversity § Summary 7
Flavor Asymmetry in the light nucleon sea § Gottfried sum rule: Ø A flavor-symmetric nucleon sea and isospin symmetry would lead § New Muon Collaboration result determined § The Drell-Yan measurement also supports the flavor asymmetry. 8
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. 9
Semi-inclusive Kaon production from proton and deuteron target § Fragmentation Function Ratio (ignored the strange quark contribution): PR-04 -114 Ø With 10
Outline § Nucleon structure and electron scattering § Flavor structure: Probing light quark sea flavor asymmetry § Spin structure: Measuring neutron transversity § Summary 11
Leading-Twist Quark Distributions ( Eight parton distributions functions) No K┴ dependence Transversity: K┴ - dependent, T-odd K┴ - dependent, T-even 12
Eight fragmentation functions -- § T-odd, quark intrinsic momentum dependent H 1 (z, к. T’ ): related to Collins effect. Hadron momentum ~к. T’ = -zк. T ~ quark momentum 13
The kinematics and coordinate § E’ is the energy of scattered electron § θe 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. 14
Leading-Twist DXs in SIDIS DXs ~ PDF FF Unpolarized Transversity Collins Sivers Polarized target Polarized beam and target SL and ST: Target Polarizations; λe: Beam Polarization 15
Characteristics of Transversity § Some characteristics of transversity: Ø h 1 T = g 1 L for non-relativistic quarks ¡ In non-relativistic case, boosts and rotations commute. ¡ ΛQCD=200 Me. V, mu and md ~ 5 Me. V, quark are relativistic. Ø Important inequalities: Ø h 1 T |h 1 Tq| ≤ f 1 q ; |h 1 Tq| ≤ (f 1 q + g 1 Lq )/2. and gluons do not mix ¡ Gluon can not be included in transversity for q q nucleon. for h 1 T and g 1 L are different Helicity state Ø Q 2 -evolution N N 16
Characteristics of Transversity § Chiral-odd → not accessible in inclusive DIS - + Ø In calculating the hadronic part in inclusive DIS, the gluon contribution cancel the quark mass term which contains the transversity distribution. Ø Decoupling mass term will turn off transversity distribution 17
Characteristics of Transversity § It takes two Chiralodd objects to measure transversity Chiral-quark soliton model Ø Drell-Yan (Doubly transversely polarized p-p collision) Ø Semi-inclusive DIS Chiral-odd distributions function (transversity) Chiral-odd fragmentation function (Collins function) 18
Asymmetry in Semi-Inclusive DIS with polarized target Unpolarized Transversity Sivers Polarized target Polarzied beam and target SL and ST: Target Polarizations; λe: Beam Polarization 19
Asymmetry in Semi-Inclusive DIS with polarized target ----- Collins effect § Access to transversity Scattering plane § Artru model Ø Based on LUND fragmentation picture. 20
Asymmetry in Semi-Inclusive DIS with polarized target ----- Sivers effect § A new type of PDF, T-odd, depends on intrinsically quark transverse momentum quark orbital momentum Beam direction Into the page 21
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. <ST> ~ 0. 15 Hermes kinematics § Need transversely polarized target in order to separate. 22
JLab Hall-A E 03 -004 Experiment Measurement of Single Target-Spin Asymmetry in Semi-Inclusive Pion Electroproduction on a Transversely Polarized 3 He Target Argonne, Cal. State-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) § High luminosity Ø 15 μA electron beam on 10 -atm 40 -cm transversely polarized 3 He target § Measure neutron transversity Ø Sensitive to h 1 d, complementary to HERMES § Disentangle Collins/Sivers effects 23
Single Spin Asymmetry Comparison with HERMES projection § With 100% polarization, § From azimuthal angular distribution, we can separate Collins effect and Sivers effect in this experiment. 24
Experimental Configuration 25
Future plan § JLAB E 03 -004 will be my thesis experiment. Ø Big. Bite background simulation. Ø Work on target. Ø Doing the data analysis. § Plan to move to JLAB this summer. 26
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). 27
Thank you! 28
Supporting slides …. . 29
Transversity (Chiral-odd) 30
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. 31
Current & target fragmentation 32
Quark-nucleon helicity amplitude § If use the quark-nucleon helicity amplitudes: Express three leading twist distribution function as amplitudes: f 1(x ) g 1 L(x ) h 1 T(x) 33
Kinematics 34
Hermes data and detailed interpretations 35
π Makins DNP 04 talk 36
Observation of Single-Spin Azimuthal ep → e’πx Asymmetry HERMES Longitudinally polarized target <ST> ~ 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 hep-ex/0104005 37
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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. 39
QCD Q 2 evolution 40
Nobel Prize this year! 14 “Running” of Coupling Constants with energy scale is a key prediction 41
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Splitting Functions Pij(z) Probability of parton i going into parton j with momentum fraction z Pqq Pqg Pgq Pgg Calculable in p. QCD as expansions in αS In Leading Order Pij(z) take simple forms 43
Fit to DGLAP equations I) Rewrite DGLAP equations a) Simplify notation i) ii) b) Sum i) over q and q separately ia) ib) Nf … number of flavors c) Define: Valence quark density ← u, u, d Singlet quark density 44
d) Rewrite DGLAP equations Valence quark density decouples from g(x, Q 2) Only evolves via gluon emission depending on Pqq II) DGLAP equations govern evolution with Q 2 Do not predict x dependence: Parameterize x-dependence at a given Q 2 = Q 20 = 4 – 7 Ge. V 2 55 parameters Low x behaviour High x behaviour: valence quarks 45
Proton Structure function F 2(x, Q 2) Rise in F 2 at low x § Scaling violation explicitly seen… Beyond the fixed target regime § H 1 and ZEUS data in agreement. Further, p. QCD predictions at NLO describe data impressively over many decades in x and Q 2. § Studies have resulted in the 46
Polarized He 3 target 47
Why polarized 3 He is an effective neutron target? S-state about 90% D-state about 8% S’-state about 2% 48
Optical Pumping for Rubidium § 37 Rb: 1 s 22 p 63 s 23 p 6 4 s 23 d 104 p 65 s 1 § Rb vapor in a weak B field is optically pumped Buffer gas N 2 let the electrons decay without emitting photons 49
Polarized 3 He target description 50
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 51
NMR - Adiabatic Fast Passage (AFP) • Ramp the holding field from below the resonance to above it • Spin Flip (Twice) • Signal <M> is the fitted amplitude 52
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 53
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