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SSA in BRAHMS

J.H. Lee (BNL)for BRAHMS Collaboration

Preliminary Results on ,K,p Transverse Single Spin

Asymmetries at 200 GeV and 62 GeV at high-xF

Sep. 29th 2006 RHIC-SPIN Collaboration Meeting

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Single transverse Spin Asymmetry (SSA): IntroductionSingle 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?

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Beyond Naïve Parton Models to accommodate large Beyond Naïve Parton Models to accommodate large SSASSA

• 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 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

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SSA measurements in BRAHMS at 200 GeVSSA measurements in BRAHMS at 200 GeV

BRAHMS measures identified hadrons (,K,p,pbar) in

kinematic ranges of 0<Y<3.5 and 0.2 <pT<4 GeV/c

• pT dependent SSA in xF 0 - ±0.35

(xF, pT, flavor-dependent AN)

• Cross-section of hadron production (Theoretical consistency)Data: • Run-5: pp at √s = 200 GeV 2.5 pb-1 recorded

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SSA measurements in BRAHMS at 62 GeVSSA measurements in BRAHMS at 62 GeV

• 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.

• pT dependent SSA in xF 0 – ±0.6

(xF, pT, flavor-dependent AN)

• Cross-section (not shown today)Data: • Run-6: pp at √s = 62 GeV 0.21 pb-1 recorded with polarization at 50-65% (see Vadim’s talk)

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Determination of Single Spin Asymmetry: ADetermination of Single Spin Asymmetry: ANN

• Asymmetries are defined as

AN = /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 ~18%)

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• Covers ~70% (~45%) of pp inelastic cross-section 41mb (36 mb) at 200 GeV (62 GeV)• 3.25 < ||< 5.25 range• Vertex resolution (z)~ 1.6cm• Main relative luminosity monitor for SSA analysis

Min-Bias Trigger / Normalization Counter:Min-Bias Trigger / Normalization Counter:“CC” (Cherenkov Radiators)“CC” (Cherenkov Radiators)

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Relative luminosity Relative luminosity L = L+ /L- determinationdetermination

• 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

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Particle Identification using RICHParticle Identification using RICH

Multiple settings

• PID for the analysis: Ring Image Cherenkov Counter• ,K identification < 30 GeV/c and proton,pbar > 17 GeV/c with efficiency ~ 97%

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AAN N ((±±,K,K±±,p,pbar) presented at DIS2006 (April 2006) ,p,pbar) presented at DIS2006 (April 2006)

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Charged Hadron production at Forward vs NLO pQCD Charged Hadron production at Forward vs NLO pQCD

• NLO pQCD describes data at forward rapidity at 200 GeV• - ,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 and K)

BRAHM

S

Preliminary

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Summary at DIS2006Summary at DIS2006

BRAHMS has obtained particle spectra and single transverse spin asymmetries for ±,K±, p, and pbar in √s =200 GeV pp

collisionsat RHIC in the xF range of 0.1 to 0.35

• Cross-section described by NLO pQCD (with updated/modified FFs)

• Proton production requires extra production mechanism in addition to fragmentation even at high-pT

• AN(): positive ~(<) AN(): negative: 5-10% in 0.1 < xF < 0.3

increasing with xF and decreasing with pT

AN() in agreement with Twist-3 calculations

• First SSA Results on K, p in 0.1 < xF < 0.3

- AN(K) ~ AN(K): positive

- AN(p) ~0, AN(pbar): positive

AN(K) in disagreement with naïve expectation of small sea

quark polarization. Lack of understanding of p, pbar polarization: Need more theoretical inputs.

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<p<pTT> vs x> vs xF F (200 GeV) at 2.3 and 4 deg with various (200 GeV) at 2.3 and 4 deg with various field settingsfield settings

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BRAHMS FS Acceptance at 2.3 deg. and 4 deg.BRAHMS FS Acceptance at 2.3 deg. and 4 deg./Full Field (7.2 Tm)/Full Field (7.2 Tm)

4deg.

2.3deg.

• Strong xF-pT correlation due to

limited spectrometer solid angle acceptance

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Calculations compared at the BRAHMS kinematic Calculations compared at the BRAHMS kinematic regionregion

• Twist-3 calculation provide by F. Yuan - Kouvarius, Qiu, Vogelsang, Yuan - “Extended” with non-derivative terms (“moderate” effects at BRAHMS kinematics) - Two flavor (u,d) and valence+sea+antiquarks Fits• 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 at this afternoon: D’Alesio, Vogelsang)

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AANN(() at 2.3 deg. at 200 GeV) at 2.3 deg. at 200 GeV

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AANN(() at 2.3 deg. at 200 GeV + Twist-3) at 2.3 deg. at 200 GeV + Twist-3

• Solid line: two-flavor (u, d) fit• Dashed line: valence + sea, anti-quark

• Calculations shown only for pT() > 1 GeV/c

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AANN(() at 2.3 deg. at 200 GeV + Sivers ) at 2.3 deg. at 200 GeV + Sivers

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AANN(() at 4 deg. at 200 GeV (high-pt setting)) at 4 deg. at 200 GeV (high-pt setting)

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AANN(() at 4 deg. at 200 GeV (high-pt setting) + Twist-3) at 4 deg. at 200 GeV (high-pt setting) + Twist-3

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AANN(() at 4 deg. at 200 GeV (high-pt setting) + Sivers) at 4 deg. at 200 GeV (high-pt setting) + Sivers

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AANN(K) at 2.3 deg at 200 GeV(K) at 2.3 deg at 200 GeV

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AANN(K) at 2.3 deg at 200 GeV + Twist-3(K) at 2.3 deg at 200 GeV + Twist-3

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proton 2.3 deg at 200 GeVproton 2.3 deg at 200 GeV

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62 GeV kinematic coverage at 2.3+3deg/half field 62 GeV kinematic coverage at 2.3+3deg/half field settingsetting

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AANN(() at 62 GeV) at 62 GeV

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AANN(() at 62 GeV + Twist-3) at 62 GeV + Twist-3

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AANN(() vs –x) vs –xFF at 62 GeV at 62 GeV

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AANN(K) at 62 GeV(K) at 62 GeV

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AANN(K) at 62 GeV + Twist-3(K) at 62 GeV + Twist-3

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AANN(K) vs –x(K) vs –xFF at 62 GeV at 62 GeV

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SummarySummary

• BRAHMS measures AN of identified hadrons at 62 GeV and 200 GeV • Large 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 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?

• We plan to finalize the SSA analysis (+ cross-section analysis) in the next few months

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“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!