flemming videbaek physics department, bnl for the brahms collaboration

DIS, London, April 7-11, 2008 1

Flemming VidebaekPhysics Department, BNL

for the BRAHMS collaboration

Transverse Single Spin Asymmetries from BRAHMS in

pp collisions at 62 and 200 GeV

Partonic description of inclusive cross sectionSingle spin asymmetries for +-

Single spin asymmetries for kaonsComparisons to pQCD models


Partonic description of hadronic dynamics in p+p at RHIC

• Description of partonic dynamics without understanding of spin independent cross-section is incomplete

• Large SSA have been observed at forward rapidities in hadronic reactions in a wide energy range. √S ~ 20-200 GeV

• SSA: Probing internal degree of freedom. May reveal information on internal orbital angular momentum in the proton

• Present energy and flavor dependent SSA and cross section at large xF measured by BRAHMS at RHIC


Partonic description of hadronic dynamics in p+p at RHIC

• Description of partonic dynamics without understanding of spin independent cross-section is incomplete.

• Large SSA have been observed at forward rapidities in hadronic reactions in a wide energy range. √S ~ 20-200 GeV

• SSA: Probing internal degree of freedom. May reveal information on internal orbital angular momentum in the proton.

• Present energy and flavor dependent SSA and cross section at large xF measured by BRAHMS at RHIC

E704


Beyond Naïve Parton ModelsBeyond Naïve Parton Models

• 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 ( Koike (final state) -

Qiu, Sterman (initial state) , -Ji, Qiu, Vogelsang, Yuan…

Challenge to have a consistent partonic description:

- Cross-section pi K at large y.– Energy dependent SSA vs. xF, pT,

-Flavor dependent SSA


Un-polarized Cross-sections at Un-polarized Cross-sections at √√s=200 GeV .s=200 GeV .

Good description at 200 GeV over all rapidities down to pT of 1-2 GeV/c.


Charged separated fragmentation functionsCharged separated fragmentation functions

Recently deFlorian, Sassot and Stratman performed a global fit including new data from Brahms at high rapidity. PRD 75, 114010 (2007)

Brahms data: PRL 98, 252001 (2007)


Comparison of NLO pQCD calculations (Vogelsang) with BRAHMS - data at high rapidity. The calculations are for KKP (solid) and a scale factor of =pT, DSS with CTEQ5 and CTEQ6.5 are also shown.The agreement is surprisingly good, in apparent disagreement with earlier analysis of ISR o data at 53 GeV.


Kaon 62 GeV at y=3.2Kaon 62 GeV at y=3.2

The K- data are suppressed due order of magnitude (valence quark effect).NLO pQCD using the recent DSS fragmentation functions give approximately same yield (?) Related to fragmentation or PDFs?


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 systematic uncertainties for N+/N- cancels Uncertainties on relative luminosity L estimated to be < 0.3%

• Beam polarization P ~50% with systematic uncertainty dP/P of ~7.2% for blue beam


Calculations compared at the BRAHMS Calculations compared at the BRAHMS kinematic regionkinematic region

• Twist-3 parton correlationTwist-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) and valence+sea+antiquarks Fits• Sivers effect calculations provided by U. D’Alesio

- Anselmino, Boglione, Leader, Melis, Murgia

“Sivers effect with complete and consistent kT kinematics plus description of un-polarized cross-section”


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

• Large AN(): 0.3-0.4 at xF~0.6 pT~1.3 GeV

• Strong xF -pT dependence. Though |AN()| ~ !AN()| |AN()/AN()| decreases with xF-pT

Twist 3

sivers

Submitted to Physical Review LettersarXiv:0801.1078


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

• Behavior consistent with a slight decrease with increasing pT as evident in going from ~4 deg to 2.3 deg setting.

• Good agreement with twist-3 calculations which also has the 1/pt-dependence at higher pT.

4o 2.3o

AANN(() at √s = 200 ) at √s = 200 GeVGeV


AANN x xFF-p-pTT dependence at dependence at √√s = 62, 200 GeVs = 62, 200 GeV

62.4 GeV acceptance , 3 angle settings of spectrometer (2,3 and 6 deg)


xxFF dependence in p dependence in pTT slices slices

To gain more insight the data are separated in pT bin to study xF dependence 200 GeV + and -•At all pT increasing AN with xF.•Magnitude is approximately constant at pT>1.5 GeV/c


AANN x xFF-p-pTT dependence at dependence at √√s = 62 GeVs = 62 GeV

At low-pT AN(π) increases from low pT.(Constraint must be 0 at pT=0)


Viewing pViewing pTT dependence differently dependence differently

Ratio of 2.3 vs 3 deg AN vs. xF at 62.4 GeV.R < 1 : Increasing AN with pT

R > 1: Decreasing => AN flat or decreasing above 0.5 (~pT~1)


Unifying 62 and 200 GeVUnifying 62 and 200 GeV


Unifying 62 and 200 GeVUnifying 62 and 200 GeVBRAHMS + E704BRAHMS + E704

E704 data – all pt (small star) pt>0.7 red star.


Kaon SSA at 62 GeVKaon SSA at 62 GeV

• AN ~0 at negative xF

• AN() ~ AN(): positive ~20% at xF< 0.5-0.6


•If main contribution to AN at large xF is from valence quark: AN(K+) ~ AN(+) and AN(K-) ~0•Observation clearly different•Show different models comparisons (only pT>1)


Sensitivity to fragmentation functionsSensitivity to fragmentation functions


200 GeV Kaons200 GeV Kaons

Data shown at DIS 2007.


SummarySummary

• BRAHMS measured AN of identified hadrons at 62 GeV and 200 GeV. , K cross-section at large rapidities at 200 GeV are described by NLO pQCD .

• At 62 GeV results shows that pQCD may actually still be valid at large y, and moderate pT. This make plausible the use pQCD approach to SSA at this energy.

• Large SSAs seen for pions and kaons – Increases with pT to ~ 1.0 GeV/c

Suggesting: - described (qualitatively) by Twist-3 - main contributions are from leading (favored) quarks Open Questions

- where the large positive AN(K-) come from?

- What are the theoretical uncertainties; pT ~ 1 GeV valid for QCD description?– What is pT dependence at larger values above 1 GeV/c?


• Backup Slides.


NLO pQCD comparisons to BRAHMS data

Calculations done by W. Vogelsang. Only one scale =pT and the same fragmentation functions as used for the PHENIX/STAR comparisons.

KKP FF does a better job compared to Kretzer, Pi and Kaon production still dominated by gg and gq at these rapidities apart from the highest pT

KKP has only 0 frag. Modifications were needed to produce charged pions


AANN for Kaons for Kaons

Anselmino and Murgia PLB442 (1998) 470-478

BKK FF rescaledBKK FF

K+

K-

• Strong strangeness FF dependence in prediction

• If main contribution to AN at large xF is from valence quarks: AN(K+)~AN(+), K- ~0

• BKK (Binnewies, Kniehl, Kramer 1995)


Owen et al ISROwen et al ISR

flemming videbaek physics department, bnl for the brahms collaboration

Documents