Proton longitudinal spin structure- RHIC and COMPASS results

Transcription

1 Proton longitudinal spin structure- RHIC and COMPASS results Fabienne KUNNE CEA /IRFU Saclay, France Gluon helicity PHENIX & STAR: pp jets, pp p 0 COMPASS g 1 QCD fit + DG direct measurements Quark helicity Others Bjorken sum rule Quark Fragmentation Functions F. Kunne 1

2 Nucleon spin How is the nucleon spin distributed among its constituents? Nucleon Spin ½= ½ΔΣ + ΔG + L quark gluon orbital momentum ΔΣ : sum over u, d, s, u, d, s can take any value: superposition of several states Dq = q - q Parton spin parallel or anti parallel to nucleon spin Past: Theory: QPM estimations, with relativistic effects DS ~ 0.6 Experiment: Spin crisis in 1988, when EMC measured a 0 = DS = 0.12 ± 0.17 MS scheme Today: Precise world data on polarized DIS g 1 + SU f (3) a 0 = DS ~ 0.3 Confirmed by first results from Lattice QCD on DS u,d. (Results exist also on L u,d ) Large experimental effort on DG measurement also because a 0 = DS - n f (a s /2p) DG (AB scheme) F. Kunne 2

3 Three ways to study gluon spin contribution ΔG 1. Proton Proton collisions Gluon-Quark + Gluon-Gluon + 2. Lepton Nucleon Photon Gluon Fusion + + DG Dq G q DG DG G G + + A LL (p T ) RHIC : PHENIX & STAR DG/G(x) CERN: COMPASS 3. QCD Q 2 evolution of spin structure function g 1 (x,q 2 ): Indirect determination assuming a functional form DG(x). Global fits include polarized DIS, SIDIS and pp data F. Kunne 3

4 1. p p collisions at RHIC Lumi(pb -1 ) Longitudinal spin - hadron production for <DG> - W production for <Dq> F. Kunne 4

5 1. p p collisions at RHIC, channels for DG Various channels 3 processes contribute gg, qg, qq DG (x 1 ).DG (x 2 ) DG (x 1 ).Dq (x 2 ) Dq (x 1 ).Dq (x 2 ) More abundant channels p p p 0 X PHENIX p p jet X STAR F. Kunne 5

6 RHIC- PHENIX DG from p 0 production p p p 0 + X Measure double spin asymmetry A LL (p T ) Compare data to global fits with a given DG(x) parameterization earlier data 200 GeV & prelim data s = 510 GeV curves: DSSV14, which includes 2009 data Significant non-zero A LL (ΔG) observed x range extended to x~0.01 arxiv: F. Kunne 6

7 RHIC- STAR DG from jet production p p jet + X 2009 data at s = 200 GeV - A LL >0 for large p T, indicate DG>0 - agree with LSS10p (DG>0) PRL 115 (2015) Prelim data, s = 510 GeV - agree with 2009 data - LSS10p and DSSV14 which includes 2009 data arxiv: F. Kunne 7

8 Large impact of RHIC data on DG from QCD fits Significantly positive DG in measured range: Δg x dx Low-x vs high-x contributions to DG: low x part 0.001<x<0.05 Running DG from x min to 1 DSSV14 1 DSSV14 with 2009 data without Proj. with data 2015 DSSV14, PRL 113 (2014) DG trunc. high x part 0.05<x<1 arxiv Need data at small x x min F. Kunne 8

9 2.a DG/G from hadron prod. in DIS (Q 2 >1(GeV/c) 2 ) New COMPASS result all-p T - PGF m p h + h + X Photon Gluon Fusion Q 2 >1(GeV/c) 2 3 x-bins prelim. 2 solutions from COMPASS NLO QCD fit of g 1 world data (presented later) Dg/g (x=0.1) = 0.11 ± 0.04 ± 0.04 COMPASS DIS data indicate DG >0 at x ~0.1 caveat: extraction at LO only F. Kunne 9

10 2.b COMPASS DG from A LL (pt) low Q 2 data m p h + X New COMPASS result, Q 2 < 0.1(GeV/c) 2 quasi-real photoproduction of single hadrons, (A LL (p T ) à la RHIC ) calculation based on W.Vogelsang et al. (data-theory agreement for unpolarised case) deuteron min std max proton COMPASS low Q 2 A LL data prefer also DG>0 Caveat: NNL resummation missing for polar.case and large dependence on fragmentation functions arxiv: to be published in PLB F. Kunne 10

11 3. QCD fits- World data on g 1 p and g 1 d proton deuteron DIS hep-ex/ to be published in PLB d g 1 -Dg( x, 2 d Log( Q ) Q 2 ) F. Kunne 11

12 COMPASS NLO QCD fit to world DIS data Need wide coverage in x and Q 2 Need to assume the functional form of the polarized PDFs DS, DG and Dq NS, at 2 a starting Q ref Fit to g 1 data, using DGLAP for Q 2 evolution Obtain parametrisations. 2 Explore various functional forms of polarized PDF, and wide range for Q ref 2 Example Q ref varied between 1 and 60 (GeV/c) 2 : DΣ DG All give similar good c2. Does not affect much DS, does change DG (~ equivalent to changing the functional form). F. Kunne 12

13 COMPASS NLO pqcd fit of g 1 world data 2 classes of solutions, DG>0 and DG<0 DS DG Du Dd Ds using different functional shapes 2 and Q ref 0.82 ΔU ΔD ΔS 0.08 hep-ex/ to be published in PLB Quark spin contribution : 0.26 < DS < 0.36 at Q 2 =3 (GeV/c) 2 Largest uncertainty comes from the bad knowledge of functional forms. Result in fair agreement with other global fits Gluon spin contribution: DG not well constrained, even the sign, using DIS only Solution with DG>0 agrees with result from DSSV++ using RHIC pp data F. Kunne 13

14 Results for Bjorken sum rule from g 1 COMPASS data Fundamental QCD sum rule, which relates proton and neutron spin structure functions g 1. Using COMPASS data alone: - Non-singlet fit: independent from DG - Reduce systematics 94% of the sum is from the measured range xg 1 NS COMPASS (g A /g V ) NLO = 1.22 ± 0.05 ± 0.10 x neutron b decay g A /g V = ± Bjorken sum rule verified to 8% Better statistics and extended systematics studies compared to past Note that experimental value increases from 1.22 to 1.25 when C 1 NS at NNLO F. Kunne 14

15 Quark helicities from W production in pp Dd Du Single spin asymmetry - STAR and PHENIX Parity violating No quark fragmentation function needed High energy scale F. Kunne 15

16 W results from STAR and global analysis Du DSSV++, incl.star data Dd PRL 113 (2014) run 11+12, η e < < x < 0.2 F. Kunne 16

17 PHENIX W +- production Also larger A L for W wrt to DSSV14 recent NNPDF1.1 includes RHIC W data Run 12+13, η < 0.35 arxiv: F. Kunne 17

18 W+- prod. from STAR and PHENIX and QCD fits - -NNPDF1.1 DSSV GRSV Ringer, Vogelsang: arxiv: F. Kunne 18

19 Quark helicities from semi-inclusive DIS l p l h +/ X Hadron tags quark flavor (quark fragmentation functions) Leading order extraction of quark helicities from spin asymmetries: COMPASS PLB693(2010)227, using DSS quark FFs u o HERMES PRD71(2005) DSSV at NLO u d s Q 2 =3 (GeV/c) 2 Full flavour separation x~0.004 Sea quark distributions ~ zero Good agreement with global fits - What about Ds? for which the integral is found negative from inclusive data (with SU3)? - Here from SIDIS data, x> ~ 0.005, Ds compatible with zero. Lower x? - NB: The extraction assumes quark Fragmentation Functions known (DSS here) F. Kunne 19

20 Quark Fragmentation Functions (FF) FFs : - Non perturbative object; needed to describe various reactions - Strange quark FF= largest uncertainty in Ds extraction from polarized SIDIS. Data exist from e + e - and pp reactions, but unsufficient and at too high Q 2 Measure p, K, p multiplicities in SIDIS μ + d μ + h ± X PDFs FFs at LO z = E h /(E m -E m ) PDFs depend on x, while FFs depend on z Data obtained in a fine binning in x, z, Q 2 Constitute an input to global NLO QCD analyses to extract quark FFs F. Kunne 20

21 p + COMPASS p and K multiplicities vs z in (x,y) bins preliminary K + p - K - ~400 data points for p and 400 for K Strong z dependance Mp + ~ Mp - and MK + > MK - F. Kunne 21

22 Extraction of quark FFs from COMPASS LO fits Assume isospin and charge symmetry: Pions : 2 independent FFs p p Assume also D s p + = D s p - = D unf p + Kaons : 3 independent FFs D fav K D unf K D str K Choose functional forms for FFs (z); Chose PDF set ; use DGLAP for Q 2 evolution Fit p + and p - multiplicity data and extract the independent FFs Idem for Kaons F. Kunne 22

23 Quark FFs from COMPASS LO fits Pions Kaons D fav p D fav K D unf K D unf p ratio DSS fit DSS fit As expected, D fav > D unf. COMPASS results ~agree with DSS and LSS NLO fits (not shown here) D fav > D unf. D fav and D unf larger than DSS and LSS NLO fits (which do not include these kaon data) D str K unstable (not shown; depends on choice of functional form) F. Kunne 23

24 Sum of z integrated multiplicities p + + p - & K + +K - For isoscalar target, simple dependence on FFs: M π+ + π = (1-2S/(5Q+2S)) D fav + D unf 5M K+ + K = D K K Q + S/Q D S where: high x data low x data ~ no x dependence expected p + + p - K + + K - COMPASS pion data: significantly below HERMES no x dependence (as in EMC h, but not shown here) COMPASS kaon data: significantly above HERMES one agree with MC simulation (LUND) Indicate smaller D SK,and larger D Q K F. Kunne 24

25 Summary DG: High impact data from RHIC STAR: pp jets and PHENIX pp p 0 : Δg x dx (undetermined at low x). COMPASS data at low Q 2 and at high Q 2 also favor DG>0. COMPASS NLO QCD fit of g 1 world data: 0.26< DS< 0.36 Uncertainty dominated by initial functional forms DG : Not constrained enough by DIS data alone Bjorken sum rule from COMPASS p and d data: Verified to 8% RHIC data on W production: constrain Du and Dd; & more to come Quark Fragmentation Functions from LO fit of multiplicities for h +,h-, p +, p -, K +,K - Large discrepancies between COMPASS and HERMES data in the sum of integrated multiplicities. Hint for smaller D str K from COMPASS? F. Kunne 25

26 Backup F. Kunne 26

27 RHIC Probed x g, for various p T of p 0 F. Kunne 27

28 A 1 p (Q 2 ) at various <x> 160 and 200 GeV data: no Q 2 dependence observed hep-ex/ to be published in PLB F. Kunne 28

29 Fit to proton, neutron and deuteron world data x-q 2 coverage of world data proton neutron deuteron NLO fit: Assume functional forms for DS, DG and Dq NS at a reference Q 0 2 =1(GeV/c) 2 Assume SU 3 Use DGLAP equations Fit world data e.g.: 495 points with W>10 GeV 138 are from COMPASS, 11 free parameters. F. Kunne 29

30 g 1 p and g 1 d COMPASS data and NLO QCD fit to world data xg 1 p g 1 p DG < 0 solution DG > 0 solution xg 1 d g 1 d g 1 p positive at low x Lower x data needed for sensitivity to DG F. Kunne 30

31 Ratio of z integrated multiplicities p + / p - Interesting because many systematic errors cancel in the ratio p + / p - h + / h - Pions: Good agreement COMPASS- HERMES. Jlab data higher, but at lower W Hadrons: Good agreement COMPASS - EMC. F. Kunne 31

32 Sum of z integrated multiplicities K + + K - 5M K+ + K = D Q K + S/Q D S K high x low x data where: COMPASS data significantly above HERMES one agree rather well with MC simulation LEPTO+JETSET (LUND) Hints on kaon fragmentation functions: from these data at high x: 0.70 from DSS analysis: 0.43 D QK COMPASS result >> DSS one, as seen in LO fit where D fav and D unfav both larger for COMPASS than DSS Low x data, agree well with MC/Lund Suggest much lower D str than DSS F. Kunne 32

33 Quark Fragmentation Functions (FF) - Non perturbative objects - Process independent - Needed to access strange quark polarization Ds from polarized SIDIS. strange quark FF = largest uncertainty in this extraction PLB 680 (2009) 217 F. Kunne 33

34 Lattice : quark spin and angular momentum MS at 4 GeV 2 Ph. Haegler, MENU 2010 Impressive results from lattice QCD Agreement with measurements for quark spin Predictions for angular momentum F. Kunne 34