Oct 3 Spin2006 J.H. Lee (BNL) 1/29 SSA in BRAHMS J.H. Lee and F. Videbaek Physics Department...

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 xF-dependent

,K,p Transverse Single Spin Asymmetries

at √s = 200 GeV and 62 GeV

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)

BRAHM

S

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

talk by U. D’Alesio


Twist-3 calculation compared with FNAL/E704 Twist-3 calculation compared with FNAL/E704

talk by W. 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 especially at lower xF-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-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?

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%

Oct 3 Spin2006 J.H. Lee (BNL) 1/29 SSA in BRAHMS J.H. Lee and F. Videbaek Physics Department...

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