Nucleon Spin Structure: Overview

Transcription

1 Nucleon Spin Structure: Overview Jen-Chieh Peng University of Illinois at Urbana-Champaign Workshop on Spin Structure of Nucleons and Nuclei from Low to High Energy Scales EINN2015, Paphos, Cyprus, Nov. 1-7,

2 Spin structures of the nucleons Why is it interesting? Full of surprises and enigmas. Resolving them often led to important new insight on QCD. Close synergy between theory and experiment. The progress of lattice QCD allows direct comparison with the experiments and lattice. Novel parton distributions and their properties become accessible by experiments using lepton and hadron beams at existing and future facilities. 2

3 Intricate structure of the ivory ball Carved from a single piece of ivory A total of 21 nested concentric layers Each layer rotates freely Various windows allow one to view deeper and deeper inside 3

4 There was a time when nucleon structure was nice and simple u( x) d ( x) s( x) s( x) SU(3) symmetric sea From Frank Close s textbook (1980) Questions Is u ( x) 2 d ( x)? V Is u ( x) d ( x)? Is s ( x) u ( x)? Is s ( x) s( x)? Is u ( x) d ( x)? p Is g ( x) g ( x)? p n n V Actually, the nucleon is full of surprises!! 4

5 Is u d in the Proton? = Expect d u if sea quarks are produced in g qq The Gottfried Sum Rule 1 S [( F ( x) F ( x)) / x] dx G 0 p n ( u ( ) ( )) p x d p x 1 ( if u p d p) 3 dx New Muon Collaboration (NMC) obtains S G = ± ( Significantly lower than 1/3! ) d u? 5

6 d / u flavor asymmetry from Drell-Yan d dx dx 4 9sx x DY a e q ( x ) q ( x ) q ( x ) q ( x ) 2 a a 1 a 2 a 1 a 2 mass spectrum 800 GeV proton beam on hydrogen and deuterium pd pp 1 Dr ell-yan: / 2 (1 d ( x2) / u( x2) ) 2 6

7 Some Theoretical Models for d / u Asymmetry Meson Cloud Models Chiral-Quark Soliton Model Instantons nucleon = chiral soliton expand in 1/Nc Quark degrees of freedom in a pion mean-field Meson cloud has significant contributions to sea-quark distributions These models also have specific predictions on Asymmetry between s( x) and s ( x) Asymmetry between u( x) and d( x) 7

8 Is there a sign-reversal for d ( x) u( x) at large x? SeaQuest Experiment ( Unpolarized Drell-Yan using 120 GeV proton beam) Main goals: 1) Measure d / u flavor asymmet ry up to x ) Measure EMC effect of antiquarks up to x 0.45 Talk of Reimer 8

9 First lattice calculation of u( x) d ( x) Huey-Wen Lin 9

10 Where does the spin of the proton come from? EMC experiment in 1988/1989 q q "Spin Crisis" q 10

11 World-wide effort to measure g 1p, g 1 d Spin-dependent DIS CERN: EMC, SMC, COMPASS SLAC: E80, E142, E143, E154, E155 DESY: HERMES JLab: Hall A, B, and C (Talk of Armstrong) world data on g 1 p (hep-ex/ ) 11

12 NLO fit to world DIS g 1p, g 1 d data (arxiv: ) Q 3 GeV 2 2 integrals: G U D S

13 Photo-gluon fusion in di-hadron SIDIS PLB 718 (2013) 922 Photon-gluon fusion Di-hadron production data suggest positive Δg/g 13

14 Polarized p-p collider at RHIC (Talk of Eyser) 14

15 0 A jet from PHENIX from STAR LL Δg from RHIC-spin A LL PRD 90 (2014) PRL 115 (2015) Sensitive to Δg due to the q + g q + g underlying process 15

16 Δg from global fits dx g( x) 0 dx g( x) 0 Global fits favor positive Δg Extend to small-x in sphenix Extend to small-x in STAR Forward-upgrade 16

17 u ( x) and d ( x) from A of W production L STAR data u d consistent with some models and lattice 17

18 u ( x) and d ( x) from A of W production L arxiv: PHENIX data u d consistent with STAR result 18

19 Spin Decomposition Xiang-Dong Ji 19

20 Spin = 25(12)% Glue = 28(08)% Orbital = 47(13)% DI part is important

21 Extraction of orbital angular momentum via Generalized Parton Distribution (GPD) 21

22 Gunar Schnell 22

23 DVCS at JLab Hall-B: DVCS cross-section on the proton in Hall-B (E01-113) arxiv: Solid curves: VGG model Dashed curves: bt Ae 23

24 Extraction of quark and gluon angular momenta from GPD and Ji s sum rule P. Kroll, arxiv:

25 DVCS kinematic coverage 25

26 Transverse Momentum Dependent (TMD) Quark Distributions Leading-Twist Quark Distributions ( A total of eight distributions) Three survive after K integration The other five are transverse momentum (K ) dependent (TMD) Transversity Sivers function Boer-Mulders function 26

27 Three parton distributions describing transverse momentum and/or transverse spin 1) Transversity Three transverse quantities: 1) Nucleon transverse spin 2) Quark transverse spin 3) Quark transverse S s momentum k N q q Three different correlations Correlation between 2) Sivers function Correlation between and 3) Boer-Mulders function S s q N and S k N q Correlation between s q and k q 27

28 Probing transversity via semi-inclusive DIS e p e' h X Transversely polarized target 1 Azimuthal modulation : sin( ) h H 1 h S UT h1 H1 : transversity : Collin Frag. Func. 28

29 Observation of sin( + S ) dependence in SIDIS Hermes (2005) "Collins" asymmetry : sin( S ) UT h H 1 1 Product of h ( x) H ( z) is non-zero A surprising flavor dependence : H / H 1 Extraction of Collins function H ( z) 1, unfavored, favored 1 1 h( x) requires an independent measurement of 29

30 sin( + S ) dependence from COMPASS 160 GeV/c muon on polarized 6 Li D target Consistent with zero! Compass (2005) Cancellation between proton and neutron in deuteron? Need an independent measurement on neutron 30

31 3 He (e,e π +/- )x at JLab Hall A HRS L 16 o g * BigBite 30 o π Polarized 3 He Target e e Obtain full coverage in Φ angle by rotating the target spin direction 31

32 Extraction of Transversity and Collins fragmentation function from SIDIS and Belle data Torino group, Anselmino et al., PRD 87, (2013) SIDIS on p SIDIS on d SIDIS on p Belle e e 32

33 Extraction of Transversity and Collins fragmentation function from SIDIS and Belle data Torino group, Anselmino et al., PRD 87, (2013) Transversity Collins pion FF Favored u quark d quark Unfavored 33

34 Extraction of nucleon tensor charge Torino group, Anselmino et al., PRD 87, (2013) 1 [ q ( ) q ( )] q h x h x dx u d 1 : Extractions from global fits (using two different Collins FF parameterizations) u d : Predictions from various models (including LQCD) Tensor charges are smaller than axial charge Discrepancy could be caused by neglecting sea transversity in the fit 34

35 Transversity at Jlab 12-GeV Hall-B Extensive SIDIS program with SoliD in Hall-A 35

36 Extraction of Sivers function from SIDIS data Torino group, Anselmino et al., Eur. Phys. J. A39 (2009) 89 (2013) uand d- quark Sivers functions have opposite signs Sea-quark Sivers functions are non-zero (from K data) 36

37 Outstanding questions on Sivers function Does Sivers function change sign between DIS and Drell-Yan? Sign and magnitude of the sea-quark Sivers functions? Q 2 -evolution of the Sivers function? 37

38 Sivers function on the lattice Musch, Haegler, Engelhardt, Negle & Schaeffer, PRD 85 (2012) SIDIS Drell-Yan As the vertical gauge link goes from (SIDIS) to - (Drell-Yan), the sign of Sivers function changes 38

39 Polarized Drell-Yan with 190 GeV/c pion beam Polarized target (Sivers asymmetry) Data-taking in 2015 and possibly in

40 Another proposal to measure sea-quark Sivers in polarized Drell-Yan at Fermilab 40

41 First A N result of W production from STAR 41

42 Summary and Outlook We have come a long way since the discovery of the spin crisis The spin contents of the nucleon are much better understood now, thanks to tremendous efforts in experiments and theories Many novel parton structures (Transversity, TMDs, GPDs) are being explored with electromagnetic and hadronic probes at various facilities. We anticipate new surprises leading to even deeper insights on the internal structure of the nucleons CERN Fermilab JLab RHIC EIC 42