SSA in BRAHMS

Oct 3 Spin2006 J.H. Lee (BNL) 1/29

SSA in BRAHMS

J.H. Lee and F. Videbaek Physics Department

Brookhaven National Laboratory

for BRAHMS Collaboration

Preliminary Results on ,K,p Transverse Single Spin

Asymmetries at 200 GeV and 62 GeV at high-xF

Spin2006, Oct. 3, Kyoto Japan


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?


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 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


SSA measurements in pSSA measurements in p+p = +p = /K/p + X at 200/62 /K/p + X at 200/62 GeV GeV

BRAHMS measures identified hadrons (,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.


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).• 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%


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)

BRAHMS

Preliminary


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) at √s = 200 GeV/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


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

• 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.) )


Sivers Function description of FNAL/E704 (talk by U. Sivers Function description of FNAL/E704 (talk by U. D’Alesio)D’Alesio)

Collins function

Sivers function


Twist-3 calculation compared with FNAL/E704 (Talk by Twist-3 calculation compared with FNAL/E704 (Talk by Vogelsang)Vogelsang)


AANN(() at 2.3 deg. at √s = 200 GeV) at 2.3 deg. at √s = 200 GeV

• AN(): positive ~(<) AN(): negative: 4-6% in 0.15 <xF< 0.3


AANN(() at 2.3 deg. at √s = 200 GeV compared with ) at 2.3 deg. at √s = 200 GeV compared with Twist-3 Twist-3

• Solid lines: two-flavor (u, d) fit• Dashed lines: valence + sea, anti-quark• Calculations done only for <pT()> > 1 GeV/c

Curves: Twist-3 by F. Yuan


AANN(() at 2.3 deg. at √s = 200 GeV ) at 2.3 deg. at √s = 200 GeV compared with Sivers effect compared with Sivers effect

Curves: Sivers effect by U. D’Alesio


AANN(() at 4 deg. at √s = 200 GeV (high-p) at 4 deg. at √s = 200 GeV (high-pTT setting) setting)

• AN() decreases with pT


AANN(() at 4 deg. at √s = 200 GeV (high-p) at 4 deg. at √s = 200 GeV (high-pTT setting) setting) compared with Twist-3 calculationscompared with Twist-3 calculations



AANN(() at 4 deg. at √s = 200 GeV (high-p) at 4 deg. at √s = 200 GeV (high-pTT setting) + setting) + SiversSivers



• 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(+), AN(K-) ~0: disagreement with naïve expectations

AANN(K) at 2.3 deg at √s = 200 GeV(K) at 2.3 deg at √s = 200 GeV


AANN(K) at 2.3 deg at √s = 200 GeV compared with (K) at 2.3 deg at √s = 200 GeV compared with Twist-3Twist-3




• 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

proton at 2.3 deg. at √s = 200 GeVproton at 2.3 deg. at √s = 200 GeV


Kinematic coverage at √s = 62 GeV (FS at 2.3 and 3 Kinematic coverage at √s = 62 GeV (FS at 2.3 and 3 deg.)deg.)


AANN(() at √s = 62 GeV) at √s = 62 GeV

• Large AN(): 40% at xF~0.6 pT~1.3 GeV/c• Strong xF -pT dependence (“Alligator”)• |AN()/AN()| decreases with xF-pT


AANN(() at √s = 62 GeV compared with Twist-3) at √s = 62 GeV compared with Twist-3



AANN(() at √s = 62 GeV compared with Sivers) at √s = 62 GeV compared with Sivers



AANN(() vs –x) vs –xFF at √s = 62 GeV at √s = 62 GeV


AANN(K) at √s = 62 GeV(K) at √s = 62 GeV


AANN(K) at √s = 62 GeV compared with Twist-3(K) at √s = 62 GeV compared with Twist-3




AANN(K) vs –x(K) vs –xFF at √s = 62 GeV at √s = 62 GeV


• BRAHMS measures AN of identified hadrons at √s=62 GeV and 200 GeV , 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?

SummarySummary


“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!


BackupBackup

•


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


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


• 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)


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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SSA in BRAHMS

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