1 April 21 DIS2006 J.H. Lee (BNL) SSA in BRAHMS J.H. Lee (BNL) for BRAHMS Collaboration DIS2006, Tsukuba, April 2006 Short Introduction Preliminary results on ,K,p SSA from RHIC/Run-5 Summary/Prospects

2 April 21 DIS2006 J.H. Lee (BNL) 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 (~  s m q /Q ) Non-zero SSA at partonic level requires -Spin Flip Amplitude, and -Relative phase SSA: Unravelling the spin-orbital motion of partons?

3 April 21 DIS2006 J.H. Lee (BNL) Beyond Naïve Parton Models to accommodate large SSA Spin and Transverse-momentum-dependent parton distributions -Final state in Fragmentation (Collins effect), -Initial state in PDF (Sivers effect) Twist-3 matrix effects -Hadron spin-flip through gluons and hence the quark mass is replaced by Λ QCD -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 x F,p T, -Flavor dependent SSA -Cross-section

4 April 21 DIS2006 J.H. Lee (BNL) SSA measurements in BRAHMS BRAHMS measures identified hadrons ( ,K,p,pbar) in kinematic ranges of 0<Y<3.5 and 0.2 <p T <4 p T dependent SSA in 0 < x < 0.35 (and < x < 0) utilizing blue (and yellow) beam (x F, p T, flavor dependent A N ) Cross-section of hadron production (Theoretical consistency) Data: Run-4: First SSA measurements in BRAHMS Run-5: pp at √s = 200 GeV 2.5 nb -1 recorded (this Talk) Run-6: pp at √s = 62 GeV planned (Energy dependence) This talk will focus on A N ( ,K,p,pbar) in 0.1 < x < 0.35 from Run-5

5 April 21 DIS2006 J.H. Lee (BNL) Braod RAnge Hadron Magnetic Spectrometers Designed to study nuclear reactions in broad kinematic range (y-p T ) 2 movable spectrometers with small solid angle measuring charged identified hardrons precisely Min-Bias Trigger Detector for pp (run05): ”CC” counter Local polarimeter, SMD in Run-5 pp (analysis in progress) 53 people from 12 institutions from 5 countries

6 April 21 DIS2006 J.H. Lee (BNL) Kinematic Variables and Acceptances The kinematic variables used for SSA: Feynman-x (x F ) and p T Shown is the BRAHMS acceptance for the data taken at  = 2.3° and the maximum field setting (7.2 Tm). Strong x F -p T correlation due to limited spectrometer solid angle acceptance The momentum resolution is  p/p~1% at 22 GeV/c

7 April 21 DIS2006 J.H. Lee (BNL) RICH Particle Identification Multiple settings PID for the analysis: Ring Image Cherenkov Counter ,K identification 17 GeV/c with efficiency ~ 97%

8 April 21 DIS2006 J.H. Lee (BNL) Covers ~70% of pp inelastic cross-section (41mb) 3.25 < |  |  < 5.25 range Vertex resolution  (z)~ 1.6cm Main relative luminosity monitor for SSA analysis ±80cm in z used in the analysis Min-Bias Trigger / Normalization Counter: “CC” (Cherenkov Radiators)

9 April 21 DIS2006 J.H. Lee (BNL) Determination of Single Spin Asymmetry: A N Asymmetries are defined A N          =  / P For non-uniform bunch intensities  = (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). The polarization P of the beam was ~50% in the RHIC Run-5 (Blue beam) Beam polarization P from on-line measurements: (systematic uncertainty ~15%)

10 April 21 DIS2006 J.H. Lee (BNL) Relative luminosity determination 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

11 April 21 DIS2006 J.H. Lee (BNL) A N of   and   FS (Front+Back) setting: 2.3 ° +2.3 ° and 4 ° +4 ° Statistical errors only Systematic error estimated ~ 25% A N (   ): positive ~(<) A N (   ): negative: 5-10% in 0.1 <x F < 0.3 p T range is limited by PID and statistics  

12 April 21 DIS2006 J.H. Lee (BNL) Acceptance and Statistics in p T vs x F FS(Front+Back) setting: 2.3 o +2.3 o 4 o +4 o FS(Front+Back) setting: 3 o +2.3 o ANAN Counts Statistically Challenging to do 2-d analysis Indication of p T dependence: A N decrease with p T at fixed x F Lower field settings to cover lower p T and extending PID is in progress

13 April 21 DIS2006 J.H. Lee (BNL) Twist-3 (initial state) calculations by Qiu and Sterman: Extrapolated to lower p T region Phys. Rev D (98) Data (  + ) Data vs Theory (Twist-3) for  +

14 April 21 DIS2006 J.H. Lee (BNL) A N for Kaons Anselmino and Murgia PLB442 (1998) BKK FF rescaledBKK FF K+K+ K-K- Strong strangeness FF dependence in prediction If main contribution to A N at large x F is from valence quarks: A N (K + )~A N (  + ), K - ~0 BKK (Binnewies, Kniehl, Kramer 1995)

15 April 21 DIS2006 J.H. Lee (BNL) A N of Kaon A N (K  ): positive ~ A N (K  ): positive ≠ 0 for 0.2 <x F <0.3 A N for lower and higher x F require including extended PID and lower field setting data: Work in progress but statistically challenging Consistent with un-scaled BKK FF: in disagreement with naïve expectations K+K+ K-K-

16 April 21 DIS2006 J.H. Lee (BNL) Forward proton and pbar production in pp at √s=200 GeV Very different production mechanism (baryon transport) for p and pbar at forward p/  + >> pbar/  - (~x10) BRAHMS Preliminary

17 April 21 DIS2006 J.H. Lee (BNL) A N of proton and pbar A N (pbar) ≠ 0 and positive A N (pbar) ~ A N (K - )? Accidental? A N (p) ~ 0: At this kinematic region, protons are mostly from polarized beam proton, but only ones showing A N ~0 Need theoretical inputs

18 April 21 DIS2006 J.H. Lee (BNL) Charged Hadron production at Forward vs NLO pQCD NLO pQCD describes data at forward rapidity at 200 GeV  -,K - are described best by KKP (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  and K) BRAHMS Preliminary

19 April 21 DIS2006 J.H. Lee (BNL)Summary BRAHMS has obtained preliminary results for single transverse spin asymmetries for  ±,K ±, p, and pbar in √s =200 GeV pp collisions at RHIC in the x F range of 0.1 to 0.35 A N (   ): positive ~(<) A N (   ): negative: 5-10% in 0.1 < x F < 0.3 A N (   ) for 0.2 <x in agreement with Twist-3 calculations First SSA Results on K, p in 0.1 < x F < A N (K  ) ~ A N (K  ): positive - A N (p) ~0, A N (pbar): positive A N (K) in disagreement with naïve expectation from valence quark fragmentation. Need more theoretical inputs. BRAHMS Also measured cross-sections for  ±,K ±, p, and pbar in the same kinematic ranges described by NLO pQCD (with updated/modified FFs)

20 April 21 DIS2006 J.H. Lee (BNL) BRAHMS SSA Measurement at 62 GeV BRAHMS will measure SSA at 62 GeV in RHIC/Run06 Acceptance will reach kinematic limit, but SSA measurement up to x F ~ 0.6 with statistical significance Measurements at 62 GeV offers an opportunity to address an intermediate energy (RHIC-FNAL) to clarify to what degree the SSA are describable by pQCD, or is a ‘soft’ physics effect.

21 April 21 DIS2006 J.H. Lee (BNL) Theoretical Challenges BRAHMS x F, p T, flavor, and energy-dependent SSA and cross- section measurements: ingredient for consistent (partonic) description at RHIC energy regime: Exciting theoretical challenges We will work on our part: -Minimizing experimental systematic uncertainties -Extending kinematic coverage

22 April 21 DIS2006 J.H. Lee (BNL) I.Arsene 7, I.G. Bearden 6, D. Beavis 1, S. Bekele 6, C. Besliu 9, B. Budick 5, H. Bøggild 6, C. Chasman 1, C. H. Christensen 6, P. Christiansen 6, R. Clarke 9, R.Debbe 1, J. J. Gaardhøje 6, K. Hagel 7, H. Ito 10, A. Jipa 9, J. I. Jordre 9, F. Jundt 2, E.B. Johnson 10, C.E.Jørgensen 6, R. Karabowicz 3, E. J. Kim 4, T.M.Larsen 11, J. H. Lee 1, Y. K. Lee 4, S.Lindal 11, G. Løvhøjden 2, Z. Majka 3, M. Murray 10, J. Natowitz 7, B.S.Nielsen 6, D. Ouerdane 6, R.Planeta 3, F. Rami 2, C. Ristea 6, O. Ristea 9, D. Röhrich 8, B. H. Samset 11, D. Sandberg 6, S. J. Sanders 10, R.A.Sheetz 1, P. Staszel 3, T.S. Tveter 11, F.Videbæk 1, R. Wada 7, H. Yang 6, Z. Yin 8, and I. S. Zgura 9 1 Brookhaven National Laboratory, USA, 2 IReS and Université Louis Pasteur, Strasbourg, France 3 Jagiellonian University, Cracow, Poland, 4 Johns Hopkins University, Baltimore, USA, 5 New York University, USA 6 Niels Bohr Institute, University of Copenhagen, Denmark 7 Texas A&M University, College Station. USA, 8 University of Bergen, Norway 9 University of Bucharest, Romania, 10 University of Kansas, Lawrence,USA 11 University of Oslo Norway The BRAHMS Collaboration The BRAHMS Collaboration

23 April 21 DIS2006 J.H. Lee (BNL) Backup Slides Backup Slides

24 April 21 DIS2006 J.H. Lee (BNL) MRS: h - in 0.5<p T <1 GeV/c and 1<p T <1.5 GeV/c