Spin physics with STAR at RHIC Qinghua Xu（徐庆华) Shandong University(山东大学物理学院) • Introduction - spin structure of nucleon • STAR longitudinal spin program: results and status • STAR transverse spin program: results and status • Summary and Outlook
Spin of nucleon Spin, as a fundamental degree of freedom, was discovered in 1925 by Uhlenbeck and Goudsmit, and plays a paramount role in studies of fundamental interactions, particle property and hadron structure. Each particle has its spin, integer or half-integer. Important application- NMRI G.Uhlenbeck S.Goudsmit Nucleons (proton& neutron), the most abundant particles around us, is spin 1/2, and it has a internal structure of quarks and gluons, How is the proton spin 1/2 carried by its internal partons: quarks and gluons ? Proton Spin
Spin structure of nucleon In the naive Quark Model, the nucleon is made of three quarks - p(uud) The quark spins make up the nucleon spin, since the quarks are in the s-orbit: =1 1988 - European Muon Collaboration (polarized Deep Inelastic Scattering) “Spin Crisis”--- proton spin carried by quark spin is rather small: ~ 0.2
Spin structure of nucleon • Spin sum rule (longitudinal case): (X.D. Ji 1997) Quark spin, Best known (~30%) Gluon spin, Poorly known Orbital Angular Momenta Little known (DVCS) • Polarized parton densities:
Detailed knowledge on ∆q from global fit before RHIC x De Florian, Navarro, and Sassot, 2005
World efforts for spin physics RHIC@BNL pp@200GeV • Current running • Lepton-nucleon scattering HERMES, COMPASS, JLAB • Polarized proton-proton scattering, RHIC • Future facilities • eRHIC (BNL) • JPARC (Japan) • GSI-FAIR (Germany) HERMES@ DESY e+-p @27GeV SLAC E142-155 EMC@CERN COMPASS@CERN p@160GeV Jefferson Lab e-p@6,12GeV All these experiments have their unique coverage on q, g, Lq,g, and they are complementary as well
STAR Detector EndCap EMC Barrel EMC TPC BBC East BBC West = -ln[tan(/2)] Blue beam Yellow beam
The RHIC spin program • Helicity structure: determination of the parton distribution functions • Gluon polarization ∆g(x) in the nucleon -- results and status (inclusive jet, hadrons) -- future plan (di-jets, +jet, hadrons) • Flavor separation: sea quark polarization -- RHIC 500 GeV program (W prodction) • Transverse spin effects: • Single spin asymmetry AN (SSA) -- recent results on SSA of 0 from RHIC • QCD mechanisms (Sivers, Collins, high-twist) -- recent results of di-jets production on Sivers effects at STAR
Accessing ∆g(x) at a proton collider f1 f2 • Longitudinal spin asymmetry:
Results on jet X-section and spin asymmetry Experimental cross section agrees with NLO pQCD over 7 orders of magnitude PRL 100, 232003 (2008) 2005 PRL 97, 252002 (2006) 2006
RHIC constraints Impact of RHIC early results on g(x) de Florian, Sassot, Stratmann, Vogelsang, arXiv:0804.0422 Early RHIC data (2005, 2006) included in a global analysis along with DIS and SIDIS data. Evidence for a small gluon polarization over a limited region of momentum fraction (0.05<x<0.2)
Longitudinal asymmetry ALL with inclusive hadrons- complementary measurement for g
Future probes for g Upcoming Correlation Measurements :
Direct Photon - Jet Correlations • Direct +jet dominated by qg-Compton process: 90% from qg x2 W.Vogelsang x1 • Reconstruction of partonic kinematics • --> x-dependence of g !
Flavor separation of proton spin • Quark polarimetry with W-bosons: • Spin measurements:
Sensitivity of W measurements • Large asymmetries • dominated by quark • polarization: Consistency • check with DIS data. • With 300 pb-1 a strong • impact constraining the • unknown sea quark • polarizations.
Strange quark polarization • S~ -0.08 from inclusive DIS • under SU(3)_f symmetry D. de Florian et al, arXiv:0804.0422 • SDIS results at HERMES: arXiv:0803.2993 • Clear need to measure. • Can we do it with hyperons at RHIC? - hyperons contain at least one strange quark and their polarization can be determined via their weak decay.
DLL-Longitudinal spin transfer at RHIC • Expectations at LO show sensitivity of DLL for anti-Lambda to : GRSV00-M.Gluck et al, Phys.Rev.D63(2001)094005 Pol. frag. func. models Typ. range at RHIC Q. X, E. Sichtermann, Z. Liang, PRD 73(2006)077503 - Promising measurements---effects potentially large enough to be observed. - DLL of is less sensitive to s, due to larger u and d quark frag. contributions.
Transverse spin program • Basic QCD calculations (leading- • twist, zero quark mass) predict • AN~0 • ---AN~0.4 for + in pp at E704 (1991) • Understanding transverse spin effect: • Single transverse-spin asymmetry • Qiu and Sterman (initial-state) / Koike (final-state) twist-3 pQCD calculations • Sivers: spin and k correlation in initial state (related to orbital angular momentum) • Collins: spin and k correlation in fragmentation process (related to transversity) STAR, Phys. Rev. Lett. 92 (2004)171801
Recent results on SSA PRL97,152302(2006) hep-ex/0801.2990 • X-section reproduced with pQCD • AN increase with xF, in agreement • with pQCD model calculation.
Recent results on SSA STAR, PRL97,152302(2006) • X-section reproduced with pQCD • AN increase with xF, in agreement • with pQCD model calculation. • pQCD based models predicted • decreasing AN with pT , which • Is not consistent with data. hep-ex/0801.2990
AN pbeam (kT ST) jet Boer & Vogelsang, PRD 69 (2004) 094025 pbeam into page jet Idea: directly measure kT by observing momentum imbalance of a pair of jets produced in p+p collision and attempt to measure Sivers distribution if kT is correlated with incoming proton spin STAR AN results with di-jet production
VY 1, VY 2 are calculations by Vogelsang & Yuan, PRD 72 (2005) 054028 Emphasizes (50%+ ) quark Sivers STAR AN consistent with zero , different as the model predictions with Sivers fit from SDIS. Sivers distribution, k -dependent distribution is not universal, AN results with di-jet production STAR, PRL99,142003(2007) Both effects occur in pp di-jets production, may cancel each other.
Future plan for transverse spin physics • Extend measurements of transverse single spin asymmetries from hadron production to prompt photon+jet production, to verify the theoretical understanding of Sivers distribution. • Develop RHIC experiments for a future measurement of transverse single spin asymmetries for Drell-Yan production of dilepton pairs. A.Bacchetta et al., PRL99,2007
Summary STAR longitudinal spin program: • STAR ALL measurements via inclusive jets/hadron production: Important contribution to understanding of G! • Upcoming results on correlation measurements: Di-jets • Future measurements: ---Prompt photons and Flavor decomposition through W production • STAR transverse spin program: • Precise AN measurement of forward 0 production, pQCD model • calculation agree with the xF dependence, but not the pT dependence. • Di-jet AN measurement are found to be zero, in disagreement with • calculations based on quark Sivers functions from SDIS data.
Outlook • Projected integrated luminosity through 2013 at RHIC: • Goal of beam polarization: 70%. • ~60% @ 200 GeV has been achieved in 2006 • 500 GeV development achieved 45%
Measurements of longitudinal spin asymmetry • Ingredients: • Polarization P1,P2: measured by RHIC polarimeters • Relative Luminosity R measured with the STAR BBC & scaler system • Spin dependent yields N++,, N+- : number of detected jets/particles • for a given combination of beam polarization directions
from polarized inclusive DIS • How is determined in inclusive DIS? --together with neutron, hyperon decay data using SU(3)_f symmetry, U~ 0.84, D~ 0.43, S~ 0.08 = 0.33 0.030.010.03: (HERMES,Q2=5 GeV2) Where are the rest of proton spin?