Transverse SSA Measured at RHIC

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1 May 21-24, 2007 Jefferson Lab Transverse SSA Measured at RHIC Jan Balewski, IUCF Exclusive Reactions

moment of baryon octet d p BUT partons have an x distribution and

find quarks carry only ~1/3 of the proton s spin!

2 Where does the proton s spin come from? p is made of 2 u and 1d quark S = ½ = Σ S q u u Explains magnetic moment of baryon octet d p BUT partons have an x distribution and there are sea quarks and gluons Check via electron scattering and find quarks carry only ~1/3 of the proton s spin! ½ = ½ ΔΣ + ΔG + L q + L g Jets,pions,A LL Di-jets, Sivers A N 2 Transverse SSA from RHIC Jan Balewski (IUCF)

3 Transverse spin physics Basic, naïve QCD (leading-twist, zero quark masses) predicts: mq AN s = 0 PRL 92, (2004) STAR Experiments: E704(91), AGS(99), STAR(02) A N is large Study of transverse spin effects: Qui and Sterman (Initial-state twist-3), Koike (final-state twist-3) Quark & gluon field interference Sivers: k T in initial state correlates w/ proton spin, ISI and/or FSI is needed depends on orbital momentum Collins: kt in final state correlates w/ spin of quark depends on transversity Fundamental transverse spin sum rule: δ ( x, Q ) q i = 1 2 dxδq ( 2 ) + ( 2 x, Q L Q ) i i= q, q i= q, q, g S T i 3 Transverse SSA from RHIC Jan Balewski (IUCF)

4 RHIC - First Polarized pp Collider Different pol pattern of colliding beams AGS Helical Partial Snake 4 Transverse SSA from RHIC Jan Balewski (IUCF)

5 5 Transverse SSA from RHIC Jan Balewski (IUCF)

6 RHIC pp Absolute Cross Section Results Confirm Applicability of pqcd STAR pqcd works! Absolute cross sections for many channels -- p+p π 0 +X, jet + X, γ +X at s = 200 GeV are well reproduced in NLO pqcd calcs. down to p T ~ few GeV/c. BRAHMS Inclusive Production, s=200 GeV pp K + X PRL 97, (2006) BRAHMS Preliminary pp π + X pp px 6 Transverse SSA from RHIC Jan Balewski (IUCF)

7 Brahms Transvers beam pol Particle ID BRAHMS measured A N s=62.4 GeV and 200 GeV Large xf dependent SSAs seen for pions and kaons Collinear factorization and (NLO) pqcd describe unpolarized cross-section at RHIC in wide kinematic region 7 Transverse SSA from RHIC Jan Balewski (IUCF)

8 BRAHMS Transverse SSA TMD PDF (Sivers function) alone cannot describe polarized and unpolarized data Gluonic degree of freedom (Twist-3) is signicantly responsible for the large A N Sea quark contributions not well understood: A N (K ± ),A N (p) J.H.Lee, SPIN06 8 Transverse SSA from RHIC Jan Balewski (IUCF)

9 Phenix 9 Transverse SSA from RHIC Jan Balewski (IUCF)

10 Charged inclusive hadrons at mid-rapidity Phenix 10 Transverse SSA from RHIC Jan Balewski (IUCF)

11 Phenix 11 Transverse SSA from RHIC Jan Balewski (IUCF)

12 STAR Jan B. ca Transverse SSA from RHIC Jan Balewski (IUCF)

13 Motivation for pp Di-Jet Measurement HERMES transverse spin SIDIS asymmetries u and d quark Sivers functions of opposite sign, different magnitude. Sivers effect in pp spin-dependent sideways boost to di-jets, suggested by Boer & Vogelsang (PRD 69, (2004)) Both beams polarized, x +z x z can distinguish q vs. g Sivers effects. Do we observe q Sivers consistent with HERMES? Tests universality. First direct measurement of gluon Sivers effects. A r p r r ( k ) N S T x parton k T x proton spin 13 Transverse SSA from RHIC Jan Balewski (IUCF) y z Colliding beams

STAR Results vs. Di-Jet Pseudorapidity Sum Spin Pol quark Emphasizes Emphasizes (50%+ (50%+ ) ) quark quark Sivers Sivers +z +z Unpol qluon y Emphasizes (80%+) gluon Sivers x V&Y calcs.

14 STAR Results vs. Di-Jet Pseudorapidity Sum Spin Pol quark Emphasizes Emphasizes (50%+ (50%+ ) ) quark quark Sivers Sivers +z +z Unpol qluon y Emphasizes (80%+) gluon Sivers x V&Y calcs. include: no hadronization 5 < p T parton < 10 GeV/c STAR η acceptance HERMES-fitted q Sivers no gluon Sivers fcns. A N consistent with zero both quark and gluon Sivers ~order of magnitude smaller in pp di-jets than in SIDIS quark Sivers asym. hep-ex/ Transverse SSA from RHIC Jan Balewski (IUCF)

15 STAR: Forward Pion Detector++ π 0 A N at s=200 GeV x F -dependence A N at positive x F grows with increasing x F A N at negative x F is consistent with zero Run 6 data at <η>=3.7 are consistent with the existing measurements Small errors of the data points allow quantitative comparison with theory predictions Theory expects the reverse dependence on η L.Nogach, SPIN06 hep-ex/ Transverse SSA from RHIC Jan Balewski (IUCF)

16 STAR: Forward Pions A N (p T ) at x F > 0.4 Run3+Run5 data (hep-ex/ ): Online calibration of CNI polarimeter Hint of A N decrease with increasing p T at p T ~1-2 GeV/c Run 2006 data: more precise measurements consistent with the previous runs in the overlapping p T region jet-like events can discriminate Collins and Sivers A N residual x F -dependence? A N (x F,p T ) mapping in planed L.Nogach, SPIN06 hep-ex/ Transverse SSA from RHIC Jan Balewski (IUCF)

17 Summary Transverse SSA Measured at RHIC Large A N at forward rapidity (Brahms, STAR) Consistent w/ zero A N at mid-rapidity (Phoenix, STAR) related to Sivers A N Implication for theory Measured in p+p quark Sivers A N differs from non-zero HERMES results for π + universality of Sivers fcn.? Recent theory development for p+p: u & d-quark Sivers fcn cancel Bomhof, Mulders, Vogelsang, Yuan, hep-ph/ factorization is violated for back-to-back hadrons Collins,Qiu, hep-ph/ Future RHIC plans: jet back-to-back Sivers fcn two hadrons correlations Sivers and Collins transversity 17 Transverse SSA from RHIC Jan Balewski (IUCF)

18 Backup 18 Transverse SSA from RHIC Jan Balewski (IUCF)

19 Separating Sivers and Collins effects Sivers mechanism: asymmetry in the forward jet or γ production Collins mechanism: asymmetry in the forward jet fragmentation S P k T,q S P p p p p Sensitive to proton spin parton transverse motion correlations Sensitive to transversity S q k T,π Need to go beyond π 0 detection to jets and direct photons 19 Transverse SSA from RHIC Jan Balewski (IUCF)

20 STAR L0 Jet Trigger ,, jet patch size:1x1 Δη x Δϕ 10 Luminosity 3*10 31 /cm 2 /s Di-Jet Rate to tape: 15 Hz 20 Transverse SSA from RHIC Jan Balewski (IUCF)

21 RHIC pp Absolute Cross Section Results at Mid-Rapidity Rapidity gg gg gq gq qq qq PHENIX pp π 0 X s=200 GeV STAR pp jet+x, s=200 GeV NLO pqcd s = 200 GeV Mid-rapidity NLO pqcd NLO pqcd Syst. Unc. dominated by PRL 91, (2003) jet E scale uncertainty PRL 98, (2006) 21 Transverse SSA from RHIC Jan Balewski (IUCF)

22 STAR: Forward Pion Detector ++ TPC: -1.0 < η < 1.0 FTPC: 2.8 < η < 3.8 BBC : 2.2 < η < 5.0 EEMC: 1 < η < 2 BEMC: -1 < η < 1 FPD++/FPD: η ~ 3.3/-3.7 FPD++: engineering test of the Forward Meson Spectrometer L.Nogach, SPIN06 22 Transverse SSA from RHIC Jan Balewski (IUCF)

23 STAR pp π 0 X, s=200 GeV High rapidity 23 Transverse SSA from RHIC Jan Balewski (IUCF)

Nucleon Spin Structure Longitudinal Spin of the Proton. HUGS Summer School Jefferson National Laboratory June 2, 2011 Lecture 5

Nucleon Spin Structure Longitudinal Spin of the Proton HUGS Summer School Jefferson National Laboratory June 2, 2011 Lecture 5 RHIC as a Polarized Proton Collider Absolute Polarimeter (H jet) pc Polarimeters