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SSA in BRAHMS

SSA in BRAHMS. Preliminary Results on p ,K,p Transverse Single Spin Asymmetries at 200 GeV and 62 GeV at high-x F. J.H. Lee and F. Videbaek Physics Department Brookhaven National Laboratory for BRAHMS Collaboration. Spin2006, Oct. 3, Kyoto Japan.

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SSA in BRAHMS

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  1. SSA in BRAHMS Preliminary Results on • p,K,p Transverse Single Spin Asymmetries • at 200 GeV and 62 GeV • at high-xF J.H. Lee and F. Videbaek Physics Department Brookhaven National Laboratory for BRAHMS Collaboration Spin2006, Oct. 3, Kyoto Japan J.H. Lee (BNL)

  2. Single transverse Spin Asymmetry (SSA): Introduction • Large SSAs have been observed at forward rapidities in hadronic reactions: E704/FNAL and STAR/RHIC • SSA is suppressed in naïve parton models (~smq/Q ) • Non-zero SSA at partonic level requires • Spin Flip Amplitude, and • Relative phase • SSA: Unravelling the spin-orbital motion of partons? J.H. Lee (BNL)

  3. Spin and Transverse-Momentum-Dependent parton distributions -”Final state” in Fragmentation (Collins effect), -”Initial state” in PDF (Sivers effect) Twist-3 parton correlations -Hadron spin-flip through gluons and hence the quark mass is replaced by ΛQCD -Efremov, Teryaev (final state) -Qiu, Sterman (initial state) Or combination of above -Ji, Qiu, Vogelsang, Yuan… Challenge to have a consistent partonic description: -Energy dependent SSA vs xF, pT, -Flavor dependent SSA -Cross-section Beyond Naïve Parton Models to accommodate large SSA J.H. Lee (BNL)

  4. BRAHMS measures identified hadrons (p,K,p,pbar) in the kinematic ranges of - 0 < xF < 0.35 and 0.2 < pT < 3.5 GeV/c at √s=200 GeV - 0 < xF < 0.6 and 0.2 < pT < 1.5 GeV/c at √s=62 GeV for xF, pT, flavor, √s dependent SSA cross-section of unpolarized hadron production (constraint for theoretically consistent description) Data: Run-5: √s = 200 GeV 2.5 pb-1 recorded (polarization:45-50%) Run-6: √s = 62 GeV 0.21 pb-1 recorded (polarization:45-65%) Data from Forward Spectrometer at 2.3-4 deg. covering “high”-xF (0.15 < xF< 0.6) are presented. SSA measurements in p+p = p/K/p + X at 200/62 GeV J.H. Lee (BNL)

  5. Determination of Single Spin Asymmetry: AN • Asymmetries are defined as • AN= (s+ - s- )/(s+ + s-) = e /P • For non-uniform bunch intensities e = (N+ /L+ - N-/L-) / (N+ /L+ + N-/L-) = (N+ - L*N-) / (N+ + L*N-) where L = relative luminosity = L+ / L- • and the yield of in a given kinematic bin with the beam spin direction is N+ (up) and N- (down). • Most of the systematics in N+/N- cancel out • Uncertainties on relative luminosity L estimated to be< 0.3% • Beam polarization P from on-line measurements: • systematic uncertainty of ~18% • Overall systematic error on AN: ~ 25%-30% J.H. Lee (BNL)

  6. Charged Hadron production at Forward vs NLO pQCD BRAHMS Preliminary • NLO pQCD describes data at forward rapidity at 200 GeV • p- ,K+ are described best by mKKP (Kniehl-Kramer-Potter) than Kretzer FF • pbar is described best by AKK (Albino-Kniehl-Kramer) FF (light flavor separated) • (NLO pQCD Calculations done by W. Vogelsang. • mKKP: “modified” KKP for charge separations for p and K) J.H. Lee (BNL)

  7. BRAHMS FS Acceptance at 2.3 deg. and 4 deg./Full Field (7.2 Tm) at √s = 200 GeV FS @4deg. FS @2.3deg. • Strong xF-pT correlation due to limited spectrometer solid angle acceptance J.H. Lee (BNL)

  8. Twist-3 parton correlation calculation provide by F. Yuan - Kouvarius, Qiu, Vogelsang, Yuan - “Extended” with non-derivative terms (“moderate” effects at BRAHMS kinematics) - Two flavor (u,d) fit and valence+sea+antiquarks fit Sivers effect calculation provided by U. D’Alesio - Anselmino, Boglione, Leader, Melis, Murgia “Sivers effect with complete and consistent kT kinematics plus description of unpolarized cross-section” (Details: Talks by Vogelsang (Mon.) D’Alesio (Tues.) ) Calculations compared at the BRAHMS kinematic region J.H. Lee (BNL)

  9. Sivers Function description of FNAL/E704 (talk by U. D’Alesio) Collins function Sivers function J.H. Lee (BNL)

  10. Twist-3 calculation compared with FNAL/E704 (Talk by Vogelsang) J.H. Lee (BNL)

  11. AN(p) at 2.3 deg. at √s = 200 GeV • AN(p+): positive ~(<) AN(p-): negative: 4-6% in 0.15 <xF< 0.3 J.H. Lee (BNL)

  12. AN(p) at 2.3 deg. at √s = 200 GeV compared with Twist-3 Curves: Twist-3 by F. Yuan • Solid lines: two-flavor (u, d) fit • Dashed lines: valence + sea, anti-quark • Calculations done only for <pT(p)> > 1 GeV/c J.H. Lee (BNL)

  13. AN(p) at 2.3 deg. at √s = 200 GeV compared with Sivers effect Curves: Sivers effect by U. D’Alesio J.H. Lee (BNL)

  14. AN(p) at 4 deg. at √s = 200 GeV (high-pT setting) • AN(p) decreases with pT J.H. Lee (BNL)

  15. AN(p) at 4 deg. at √s = 200 GeV (high-pT setting) compared with Twist-3 calculations Curves: Twist-3 by F. Yuan J.H. Lee (BNL)

  16. AN(p) at 4 deg. at √s = 200 GeV (high-pT setting) + Sivers Curves: Sivers effect by U. D’Alesio J.H. Lee (BNL)

  17. AN(K) at 2.3 deg at √s = 200 GeV • AN(K+) ~ AN(K-): positive 2-5% for 0.15 <xF <0.3 • If main contribution to AN at large xF is from valence quarks: AN(K+)~AN(p+), AN(K-) ~0: disagreement with naïve expectations J.H. Lee (BNL)

  18. AN(K) at 2.3 deg at √s = 200 GeV compared with Twist-3 Curves: Twist-3 by F. Yuan • Solid lines: two-flavor (u, d) fit • Dashed lines: valence + sea, anti-quark • Calculations done only for <pT(p)> > 1 GeV/c J.H. Lee (BNL)

  19. proton at 2.3 deg. at √s = 200 GeV • AN(pbar), AN(K-) > 0: Accidental? Or contribution from sea-quarks • AN(p) ~ 0: At this kinematic region, significant fraction of proton are mostly from polarized beam proton, but only ones showing AN ~0 J.H. Lee (BNL)

  20. Kinematic coverage at √s = 62 GeV (FS at 2.3 and 3 deg.) J.H. Lee (BNL)

  21. AN(p) at √s = 62 GeV • Large AN(p): 40% at xF~0.6 pT~1.3 GeV/c • Strong xF -pT dependence (“Alligator”) • |AN(p+)/AN(p-)| decreases with xF-pT J.H. Lee (BNL)

  22. AN(p) at √s = 62 GeV compared with Twist-3 Curves: Twist-3 by F. Yuan J.H. Lee (BNL)

  23. AN(p) at √s = 62 GeV compared with Sivers Curves: Sivers effect by U. D’Alesio J.H. Lee (BNL)

  24. AN(p) vs –xF at √s = 62 GeV J.H. Lee (BNL)

  25. AN(K) at √s = 62 GeV J.H. Lee (BNL)

  26. AN(K) at √s = 62 GeV compared with Twist-3 Curves: Twist-3 by F. Yuan • Solid lines: two-flavor (u, d) fit • Dashed lines: valence + sea, anti-quark • Calculations done only for <pT(p)> > 1 GeV/c J.H. Lee (BNL)

  27. AN(K) vs –xF at √s = 62 GeV J.H. Lee (BNL)

  28. BRAHMS measures AN of identified hadrons at √s=62 GeV and 200 GeV p, K cross-sections at 200 GeV described by NLO pQCD Large xF dependent SSAs seen for pions and kaons Suggesting: - Sivers mechanism plays an important role. - described (qualitatively) by Twist-3 - main contributions are from leading (favored) quarks - power-suppression 1/pT set the scale Questioning: - where the large positive AN(K-) come from then? - Sea quark contributions not well understood: AN(K-) and AN(pbar) - how well pQCD applicable at √s=62 GeV (cross-sections at 62 GeV will be delivered) - what can (not) be learned from AN at pT < 1 GeV/c - AN(-xF) ~ 0 set limits on Sivers-gluon contribution? - can AN (p, pbar) be described in the consistent framework? - What are the theoretical uncertainties? Is pT ~ 1 GeV/c valid for pQCD description? Summary J.H. Lee (BNL)

  29. “Despite the conceptual simplicity of AN, the theoretical analysis of SSA of hadronic scattering is remarkably complex.” (hep-ph/0609242) Looks like theorists are having all the fun. Enjoy! J.H. Lee (BNL)

  30. Backup J.H. Lee (BNL)

  31. Braod RAnge Hadron Magnetic Spectrometers • Designed to study nuclear reactions in broad kinematic range (y-pT) • 2 movable spectrometers with small solid angle measuring charged identified hardrons precisely • Min-Bias Trigger Detector for pp: ”CC” counter • 53 people from 12 institutions from 5 countries J.H. Lee (BNL)

  32. Relative luminosity L = L+ /L- determination • Using CC in spin scaler ±80cm • Consistent with CC recorded in data stream • Relative luminosity calculated by Beam-Beam Counter and CC: < 0.3% • Systematic effect on bunch number dependent beam width: negligible J.H. Lee (BNL)

  33. Min-Bias Trigger / Normalization Counter:“CC” (Cherenkov Radiators) • Covers ~70% (~45%) of pp inelastic cross-section 41mb (36 mb) at 200 GeV (62 GeV) • 3.25 < |h|< 5.25 range • Vertex resolution • s(z)~ 1.6cm • Main relative luminosity monitor for SSA analysis J.H. Lee (BNL)

  34. Particle Identification using RICH Multiple settings • PID for the analysis: Ring Image Cherenkov Counter • p,K identification < 30 GeV/c and proton,pbar > 17 GeV/c with efficiency ~ 97% J.H. Lee (BNL)

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