Measuring the gluon polarization F.Kunne – CEA Saclay 10 th International Symposium on Meson-Nucleon Physics and the Structure of the Nucleon Beijing, Aug 29- Sept

F.KunneMENU 04, Beijing, Sept Contents The nucleon spin problem; why measure  G ? Present knowledge of  G How to measure  G ? Ongoing experiments : RHIC spin, HERMES, COMPASS Summary and future prospects

F.KunneMENU 04, Beijing, Sept The Nucleon Spin quark contribution gluon contribution angular momentum Naive quark parton model + relativistic corr.  ~ 0.60 QCD ; Ellis- Jaffe assuming  s =0,  ~ 0.60  Quark contribution to spin expected to be large

F.KunneMENU 04, Beijing, Sept Measurement of  - quark contribution Need nucleon spin structure functions measurement in polarized inclusive Deep Inelastic Scattering  first moment of g 1 : SLAC, EMC, SMC & HERMES experiments Need knowledge of F & D meson octet decay constant  From these inputs, determine : singlet axial current matrix element a 0, related to 

F.KunneMENU 04, Beijing, Sept Polarized spin structure function g 1 Various pQCD global analysis of world data on g 1 ( p, d, n ) with different assumptions and parametrisations   ~ Quark contribution to spin  small, very different from QPM expectation 0.6 SLAC, EMC, SMC and HERMES data

F.KunneMENU 04, Beijing, Sept a 0 =  n f  s /2   G (Q 2 ) For a 0 = 0.3, need  G ~ 2.5 (and L z ~ - 2.3) to restore  ~ 0.6  Need independent measurement of  G Interpretation a 0 =   and  s cannot be accessed directly, they depend on  G - QCD (AB scheme) : - QPM :

F.KunneMENU 04, Beijing, Sept In all analyses :  u v (x) and  d v (x) well constrained  G(x) not: shape and sum unknown J.Blumlein & H.Bottcher (2002),Q 0 2 = 4 GeV 2 e.g : - SMC global QCD analysis at Q 0 2 = 1 GeV 2 : 1. +/ Blumlein-Bottcher at Q 0 2 =2 GeV 2 : 1. +/- 0.6 NLO QCD analysis of world data on g 1

F.KunneMENU 04, Beijing, Sept polarized – g 1 p (Q 2 ) data, just as g(x) has been extracted from unpolarized - F 2 p (Q 2 ), but   G from QCD evolution of g 1 Need more precise data on g 1 (x,Q 2 ) and better coverage in x and Q 2  s.  G constant at LO  G and  q NS mix in (x, Q 2 ) evolution In principle,  G can be extracted from evolution equations from

F.KunneMENU 04, Beijing, Sept HERMES e 27.5 GeV high p T hadrons COMPASS  160 GeV c c high p T hadrons Direct measurements of  G Lepton-Nucleon :  g q q RHIC ; PHENIX & STAR s 1/2 = GeV pp collisions pp  X pp jet X pp  X pp ccX

F.KunneMENU 04, Beijing, Sept Strong RHIC Spin collaboration : STAR, PHENIX, pp2pp, accelerator, RIKEN/BNL, … Polarized pp collider RHIC - Successful first runs and increasing performance : P beam : 0.2 in 2002, Now : 0.4 Goal : 0.7 Energy s 1/2 = 200 GeV now, 500 GeV later Luminosity : L ~ cm -2 s -1 ( Goal : L ~ cm -2 s -1 ) - First results from PHENIX on spin asymmetry

F.KunneMENU 04, Beijing, Sept  G at RHIC qg   q qg  gq gg  gg gg  qq C C A E Asymetries a LL for partonic processes pp  jet +X pp   +X pp  QQ Processes sensitive to g at LO All processes now calculated at NLO Various channels detected : a LL

F.KunneMENU 04, Beijing, Sept  G at RHIC; pp  jet (or hadron) +X gg  gg qg  gq qq  qq Ratios of partonic processes,  0 prod.

F.KunneMENU 04, Beijing, Sept First results from 2003 ! NLO calc B. Jaegger et al. PRD 67, (2003) Projected errors for 7 pb -1 First results on A LL pp   0 X PHENIX

F.KunneMENU 04, Beijing, Sept A LL prompt  PHENIX Projected error bars for 200 and 500 GeV

F.KunneMENU 04, Beijing, Sept Projected error bars for  G (G-S scenario A. B and C) STAR -  G from  +jet Xg reconstructed clean process

F.KunneMENU 04, Beijing, Sept  G at RHIC – summary and outlook Various channels, large coverage in x g - Jet/leading hadron production : First results of A LL pp   0 +X from PHENIX Very good perspective for  +- at PHENIX and jets at STAR - Direct photon production: hard scale 100 GeV 2, x g reconstructed in STA - Heavy flavours,... RHIC Spin program on track, major effort to reach performance Run s 1/2 (GeV) P beam L (pb -1 ) > > , Very powerful tool !

F.KunneMENU 04, Beijing, Sept GeV e pure atomic H or D targets Spectrometer with complete hadron identification  G measurement at HERMES

F.KunneMENU 04, Beijing, Sept ΔG/G from high p T hadron pairs h1h1 h2h2 Leading process h1h1 h2h2 Gluon radiation (Compton) h1h1 h2h2 Photon Gluon Fusion A eN  hhN =  G/G a LL PGF R PGF + A 1 a LL LP R LP + A 1 a LL GR R GR a LL process = analysing power of process A 1 = g 1 /F 1 R process =  process /  TOT

F.KunneMENU 04, Beijing, Sept Statistics decreases with p T Background (also polarized) Extraction of result difficult. Need perfect description of background HERMES A LL (p T ) proton target

F.KunneMENU 04, Beijing, Sept HERMES result h+h+ h-h- Promising channel, but … theoretical uncertainty might be large : Contributions from leading process and Compton scattering difficult to estimate = 0.17 = 0.06 (GeV/c) 2 = 2.1 (GeV/c) GeV e on p Photon-gluon fusion, High pt hadrons A.Airapetian et al.

F.KunneMENU 04, Beijing, Sept HERMES A LL (p T ) deuterium target

F.KunneMENU 04, Beijing, Sept COMPASS at CERN SPS LHC

F.KunneMENU 04, Beijing, Sept  G at COMPASS Beam 160 GeV  P beam =0.80 Target 6 LiD  (f=0.5, P ~ 0.5) Large acceptance spectrometer, high flux and particle identification First run in 2002  G from photon-gluon fusion :  g  c c  D*  D 0  K   g  h h high p T hadrons

F.KunneMENU 04, Beijing, Sept COMPASS spectrometer SM1 Dipole SM2 Dipole RICH1 Polarised Target E/HCAL1 Muon Wall 1 Muon Wall 2, MWPC 160 GeV  beam Micromegas, SDC, Scifi Straws, Gems MWPC, Gems, Scifi,W45 E/HCAL2 Hodoscopes Scifi, Silicon

F.KunneMENU 04, Beijing, Sept COMPASS polarized target 3 He – 4 He dilution refrigerator (T~50mK) μ Reconstructed interaction vertices 6 LiD Dilution =0.4 Polarization ~ 0.5

F.KunneMENU 04, Beijing, Sept m 6 m 3 m photon detectors: CsI MWPC mirror wall vessel radiator: C 4 F 10 Particle identification with RICH Single event single photon:  = 1.2 mrad ring:  = 0.4 mrad photons/ringn ~14 3   /K sep.up to 40 GeV/c

F.KunneMENU 04, Beijing, Sept z D > 0.2 (background reduction) | cos(θ * ) | < 0.85 (background reduction) 10 < p K < 35 GeV/c(Rich πK sep.) define : M(D * ) – [M(D 0 )+M(π)] COMPASS  G from open charm D 0 K -  +  BR = 4% D *+ D   s + K -  +  s + c c

F.KunneMENU 04, Beijing, Sept < ΔM Kππ < 9.1 MeV 317 D 0 COMPASS - tagging 2002 prelim. analysis

F.KunneMENU 04, Beijing, Sept COMPASS - ΔG/G from high p T hadron pairs h h Leading process h h Gluon radiation (Compton) h h Photon Gluon Fusion (PGF) P T1 2 + P T2 2 > 2.5 GeV/c 2, P T1(2) > 0.7 GeV/c, no Q 2 cut preliminary (2002 data) events A  * d = ± (stat) ± (syst) ~ 0.1

F.KunneMENU 04, Beijing, Sept Uncertainty on ΔG/G from high p T hadron pairs Monte Carlo study is required to subtract contribution of background before anything can be said on Δ G/G. Work in progress.  ( ΔG/G) ~ 0.17 From preliminary analysis of 2002 data, expected stat. error assuming R PGF ~ ¼ (Phytia LO): COMPASS 2002 data C.Bernet PhD thesis

F.KunneMENU 04, Beijing, Sept COMPASS projections for  (  G/G) If 2004 as expected, projection for data:  (  G/G) = 0.05 high p T all Q 2  (  G/G) = 0.16 high p T Q 2 >1  (  G/G) = 0.24 charm to solve : contribution from resolved  NLO contribution

F.KunneMENU 04, Beijing, Sept COMPASS summary and outlook COMPASS is up and running First results of asymmetry from high-p T hadron pairs (π,K) First glance at open charm PGF 2004 run in progress 2005 CERN accelerator shutdown 2006 – 2010 –request in preparation; COMPASS will continue in 2006 more hardware to come : ECAL (π 0 ), RICH, DAQ, new target solenoid, 180 mrad acceptance

F.KunneMENU 04, Beijing, Sept STAR, PHENIX, HERMES, and COMPASS will cover different kinematical regions They will attack the problem from various channels Sources of systematic uncertainties, experimental and theoretical, will differ  G/G results and projections

F.KunneMENU 04, Beijing, Sept Conclusion Very active experimental field Difficult measurement : statistics systematics (small asymmetries, high background) theoretical interpretation (several processes mixed) Promising approach from HERMES, RHIC and COMPASS Various channels investigated Hope to get complementary measurements of  G (x,Q2)