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SSA in BRAHMS

SSA in BRAHMS. Preliminary Results on x F -dependent p ,K,p Transverse Single Spin Asymmetries at √s = 200 GeV and 62 GeV. 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 xF-dependent • p,K,p Transverse Single Spin Asymmetries • at √s = 200 GeV and 62 GeV 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 talk by U. D’Alesio J.H. Lee (BNL)

  10. Twist-3 calculation compared with FNAL/E704 talk by W. 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 especially at lower xF-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-section analysis at 62 GeV in progress) - 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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