Spin Physics at RHIC Ming Xiong Liu Los Alamos National Lab 8/3/091Ming Xiong Liu OCPA06

8/3/09Ming Xiong Liu OCPA06 2 Proton Spin Problem- an outstanding puzzle from polarized DIS experiments – Proton Spin Decomposition – Origin of Proton Spin: gluon, sea quarks, orbital angular momentum ? DIS can’t directly probe gluons and a new tool : the 1 st polarized proton collider – polarized PDF quark-gluon, quark-quark and gluon-gluon interactions directly explore gluon and sea quark distributions – study the role of spin in QCD strong interaction via A N Experimentally RHIC-Spin Program New Frontier of Nucleon Structure Research RHIC-Spin Program New Frontier of Nucleon Structure Research

3 RHIC: the world’s first polarized hadron collider Spin varies from rf bucket to rf bucket (9.4 MHz) Spin pattern changes from fill to fill Spin rotators provide choice of spin orientation “Billions” of spin reversals during a fill with little if any depolarization 8/3/09Ming Xiong Liu OCPA06

8/3/09Ming Xiong Liu OCPA064 Experimental Observables Asymmetries PHENIX and STAR: all BRAHMS: transverse beams only

8/3/09Ming Xiong Liu OCPA06 5 RHIC Spin Run History Pol L(pb^-1) Results % 0.15 first pol. pp collisions! %1.6 pi^0, photon cross section, A_LL(pi^0) % 3.0 absolute beam polarization with polarized H jet % 13 large gluon pol. ruled out (P^4 x L = 0.8) % 46 first long spin run (P^4 x L = 6) no spin running % (short) run GeV/200GeV just ended 06/2009

8/3/09Ming Xiong Liu OCPA066 The PHENIX detector 2 central spectrometers – Track charged particles and detect electromagnetic processes 2 forward muon spectrometers – Identify and track muons 2 forward calorimeters (as of 2007!) – Measure forward pions Relative Luminosity – Beam-Beam Counter (BBC) – Zero-Degree Calorimeter (ZDC) Philosophy: High rate capability to measure rare probes, limited acceptance.

8/3/09Ming Xiong Liu OCPA067 PHENIX BRAHMS &PP2PPPHOBOS STAR 1.2 km RHIC The STAR Detectors Time Projection Chamber |  |<1.6 Forward TPC 2.5<|  |<4.0 Silicon Vertex Tracker |  |<1 Barrel EMC |  |<1 Endcap EMC 1.0<  <2.0 Forward Pion Detector 3.3<|  <4.1

8/3/09Ming Xiong Liu OCPA06 8 Part I: The 20-year Old Proton Spin Puzzle Fairly well measured only ~30% of spin A future challenge The proton is viewed as being a “bag” of bound quarks and gluons interacting via QCD Spins + orbital angular momentum need to give the observed spin 1/2 of proton Beginning to be measured at RHIC

Physics Channels Sensitive to  g(x,Q 2 ) Pion or jet production; high rates; complex calculation required to extract physics; broad x g range needed to properly test models Photon production; low rates; simpler physics channels promise cleaner result Heavy-flavour production; separated vertex detection required for clean observation, independent channels 8/3/099Ming Xiong Liu OCPA06

8/3/09 10  f: polarized parton distribution functions For most RHIC kinematics, gg and qg dominate, making A LL for inclusive jets and hadrons sensitive to gluon polarization pT(GeV) Partonic fractions in jet production at 200 GeV 0 Longitudinally polarized pp collisions at RHIC Ming Xiong Liu OCPA06

RHIC Data 11 Ming Xiong Liu OCPA06 8/3/09 STAR GRSV curves with cone radius 0.7 and <  < jet data: PRL 100, (2008) 2005: PRD 76, : arXiv:  200 GeV

» First truly global analysis of polarized DIS, SIDIS and pp results » PHENIX & STAR  s = 200 and 62 GeV data used » RHIC data significantly constrain  G in range 0.05<x<0.3 Recent Global Fit: DSSV de Florian, Sassot, Stratmann, Vogelsang PRL 101, (2008)   g(x) small in current RHIC measured range  Best fit has a node at x~0.1  Low-x unconstrained RHIC range 8/3/0912Ming Xiong Liu OCPA06

Updated DSSV Global Fit [arXiv: ] In the context of this fit, RHIC Spin is straddling a node in  g(x), such that the contribution to our observable integral is nearly zero. We need to change our  s so that we can observe a different x-range. 8/3/0913Ming Xiong Liu OCPA06

8/3/09Ming Xiong Liu OCPA06 14 Future New  G Polarized hadron collisions – double longitudinal spin asymmetry – leading-order gluon interactions direct-photon production heavy-flavor production Other channels (di-jet etc.) proton beam  or  gluon photon proton beam  or  gluon heavy flavor

8/3/09Ming Xiong Liu OCPA06 15 Part II: Sea Quark Polarization & Asymmetry Nucleon is a complex system Sea quarks and gluons correlated Pion cloud model: Chiral soliton model:

Polarized Sea Quark Distributions 8/3/09Ming Xiong Liu OCPA0616 de Florian, Sassot, Stratmann, Vogelsang PRL 101, (2008) RHIC has an opportunity to provide strong constraints on the u and d sea distributions via the observation of the parity-violating single-spin asymmetry A L in W production. This requires running at 500 GeV with good polarization.

8/3/09Ming Xiong Liu OCPA06 17 “New Probe”- W ± Production and A Bunce G. et al, hep-ph/ μ+μ+ μ-μ- ν ν

8/3/09Ming Xiong Liu OCPA0618 l+l+ PHENIX can make observations in forward, central, and backward rapidity.

Ming Xiong Liu OCPA06 19 Part III: Transverse Spin Physics Experiment: E704, Fermilab, 1991 Theory Expectation: Small asymmetries at high energies (Kane, Pumplin, Repko, PRL 41, 1689–1692 (1978) ) Right Left A N ~10 -1 measured A N ~10 -4 from theory π+π+ π-π- π0π0 E704 Spin structure of transversely polarized proton Define left-right asymetry 8/3/09

Ming Xiong Liu OCPA06 20 pQCD Proton Structure Collins Effect: Correlation between proton & quark spin + spin dependent fragmentation function Collins FF Quark transversity J. C. Collins, Nucl. Phys. B396, 161 (1993) Sivers distribution D. Sivers, Phys. Rev. D 41, 83 (1990) Possible origins of large A N Sivers Effect: Correlation between proton spin and quark transverse momentum Quark Fragmentation 8/3/09

Ming Xiong Liu OCPA06 21 Large asymmetries persist at high  s p  + p   ± + X,  s = 62 GeV Arsene et al. (BRAHMS), submitted to Phys. Rev. Lett. [arXiv:nucl-ex/ ] p T = GeV/c p  + p   + X,  s = 20 GeV  0 : E704, Phys.Lett. B261 (1991) 201.  +/- : E704, Phys.Lett. B264 (1991) 462. Fermilab, Fixed target, E704, 1991 RHIC, Brahms, 2007  s = 20 GeV  s = 62 GeV  s = 200 GeV RHIC, STAR, 2004 Examples: p  + p   0 + X,  s = 200 GeV Non-Perturbative cross section Perturbative cross section (STAR) Phys. Rev. Lett. 92 (2004) Gordon AGS/RHIC 2008

Ming Xiong Liu OCPA06 22 Sivers effect in heavy flavor production D meson production dominated by gluon-gluon fusion at RHIC energy Sensitive to gluon Sivers effect A N measured for muons from D decay (“prompt muons”) Smear by decay kinematics Anselmino et al, PRD 70, (2004) Gluon Sivers=0 Quark Sivers=Max Gluon Sivers=Max Quark Sivers=0 Calculations for D mesons Measurement for  - 8/3/09

Ming Xiong Liu OCPA06 23 Neutron production at very forward region neutron Large negative SSA discovered for x F >0 Diffractive physics Used for local polarimetry at PHENIX/STAR 8/3/09

Conclusions 24 Ming Xiong Liu OCPA068/3/09 What are the avenues for further important measurements and theoretical developments? we have just explored the tip of the iceberg you are here L q,g ss gg  u tot,  d tot  u,  d spin sum rule TMDs

Spin is fascinating Thank you for your attention N. BohrW. Pauli 8/3/0925Ming Xiong Liu OCPA06

Backup slides 8/3/0926Ming Xiong Liu OCPA06

8/3/09Ming Xiong Liu OCPA06 27 Extra dbar in proton g.s. Sea Quarks Carry Major Orbital Angular Momentum Component? Pion Cloud Model and the Orbital Angular Momentum?!

8/3/09Ming Xiong Liu OCPA06 28 Sea Asymmetry from Drell-Yan Processes Towell et al., Phys.Rev. D64 (2001) Evidence of Pion Cloud? FermiLab/E866

The Gluon Polarization 29 Ming Xiong Liu OCPA06 8/3/09 unpolarised cross sections nicely reproduced in NLO pQCD in NLO RHIC: many sub-processes with a dominant gluon contribution high-p T jet, pion, heavy quark, …

 0 A LL PHENIX Run6 (  s=200 GeV) arXiv: GRSV model: “  G = 0”:  G(Q 2 =1GeV 2 )=0.1 “  G = std”:  G(Q 2 =1GeV 2 )=0.4 Statistical uncertainties are on level to distinguish “std” and “0” scenarios 8/3/0930Ming Xiong Liu OCPA06

arXiv: Generate  g(x) curves for different Calculate A LL for each  G Compare A LL data to curves (produce  2 vs  G) Sensitivity of  0 A LL to  G (with GRSV) 8/3/0931Ming Xiong Liu OCPA06

Flavor asymmetry of the polarised light sea With Semi-Inclusive Hadron Asymmetries HERMES COMPASS Projection with 1 additional year of proton Meson cloud  QSM  Separation between extreme models  enters in NLO global fits From Nicole D'Hose (CEA/IRFU Saclay) at CERN “New Opportunities” Workshop They can do this in the next few years if approved! ??? Actual RHIC range