Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 1 Transverse Spin Studies with STAR at...

Carl Gagliardi – STAR Transverse Spin Studies – DIS‘06 1

Transverse Spin Studies with STAR at RHIC

Carl A. GagliardiTexas A&M University

for the CollaborationSTARSTAR

Outline • Previous results• Separating the underlying effects• Current status• Future plans


STAR detector layout


Transverse spin runs at STAR• RHIC Run 2 (’01-’02)

– First polarized pp run– Used prototype Endcap EMC for forward π0 detection– ~0.15 pb-1 with Pbeam ~ 16%– AN measurement at <η> ~ 3.8

• Run 3 (’03)– Forward π0 detectors (FPD) installed– ~0.25 pb-1 with Pbeam ~ 27%– Measurements at <η> ~ ±3.7 and <η> ~ ±4.0

• Run 5 (’05)– 25 hours of transverse data during June, ’05– ~0.07 pb-1 and Pbeam ~ 46%

• Run 6 (’06)– Started transverse data taking on April 7th

– FPD++ and full Barrel+Endcap EMCs installed– Measurements underway at both forward and mid-rapidities


First AN Measurement at STAR

PRL 92, 171801 (2004)

Sivers: spin and k correlation in parton distribution functions (initial state)

Collins: spin and k correlation in fragmentation function (final state)

Qiu and Sterman (initial state) / Koike (final state): twist-3 pQCD calculations, multi-parton correlations

Can be described by several models:

Similar to result from E704 experiment (√s=20 GeV, 0.5 < pT < 2.0 GeV/c)

√s=200 GeV, <η> = 3.8

STARSTAR


Forward π0 production at ISR energies

Bourrely and Soffer, EPJ C36, 371: NLO pQCD calculations underpredict the data at low s from ISR Ratio appears to be a function of angle and √s, in addition to pT

√s=23.3GeV √s=52.8GeV

xF xF

Ed

3 d

p3 [b

/GeV

3 ]

Ed

3 d

p3 [b

/GeV

3 ]

NLO calculations with different

scales:

pT and pT/2

Data-pQCD differences

at pT=1.5GeV


pp π0 + X cross sections at 200 GeV

NLO pQCD calculations by Vogelsang, et al.

nucl-ex/0602011

STARSTAR

The error bars are statistical plus point-to-point systematic

Consistent with NLO pQCD calculations at 3.3 < η < 4.0

Data at low pT trend from KKP fragmentation functions toward Kretzer. PHENIX observed similar behavior at mid-rapidity.


Inclusive π0 AN from runs 3+5

• Sizable asymmetries for xF > 0.4

• Back angle data consistent with AN ~ 0

~ 6σ from 0 for xF>0.4


Combined AN(pT) for xF > 0.4 from runs 3+5Online calibration of CNI polarimeter

Data are consistent with the 1/pT expectation of pQCD


Separating Sivers and Collins effects

Collins mechanism: asymmetry in the forward jet fragmentation

Sivers mechanism: asymmetry in the forward jet or γ production

SPkT,q

p

p

SP

p

p

Sq kT,π

Need to go beyond π0 detection tojets and direct photons

Sensitive to proton spin – parton transverse motion correlations

Sensitive to transversity


Event yield with FPD when Nγ > 1

• ~16% arises from “jet-like” events with ≥3 observed photons

• Also expect γ/π0 > 1 for single photons with xF > 0.5

• Need to separate these contributions reliably


FPD FPD++ for run 6

West end of the STAR interaction region

Runs 3-5


Run 6

FPD FPD++ for run 6



FPD++ enables jet and direct photon detection

• Trigger on EM energy in small cells

• Large enough to integrate over a jet cone

• Large enough for direct photon isolation cuts


Projected sensitivity

• PYTHIA simulations• Trigger on small cells;

sum over entire side• Vector sum of photon

momenta reproduces most forward hard-scattered parton

• Expect 4~5σ effect for 5 pb-1 if observed π0 AN is due to Sivers effect

L.C. Bland, hep-ex/0602012


D.

Bo

er

an

d W

. V

og

els

an

g,

Ph

ys.

Re

v. D

69

, 0

94

02

5 (

20

04

)Back-to-back di-jets: access to gluon Sivers function

5 pb-1 measurement will be sensitive at level of model predictions

Measurements near mid-rapidity with STAR – search for spin-dependent deviation from back-to-back alignment

> 7 GeV trigger jet> 4 GeV away side jet


Level-2 trigger monitoring output after ~0.84 pb-1

• Clear back-to-back di-jet signature• 6-fold symmetry from Level-0 (single jet) trigger condition• Level-2 filter is insensitive to the granularity

Occasional high-order bit problem in 1 of 5520 EMC towers


Di-jet kTx>0Di-jet kTx<0

Related to di-jet Δφ

Level-2 trigger monitoring output after ~0.84 pb-1

• Clear back-to-back di-jet signature• 6-fold symmetry from Level-0 (single jet) trigger condition• Level-2 filter is insensitive to the granularity


Run 7 and beyond


Run 7 and beyond

Addition of FMS to STAR provides nearly continuous EMC from -1<η<+4

STAR Forward Meson Spectrometer


Conclusions

• STAR finds sizable AN for inclusive π0 at forward rapidity

• Separating the Sivers contribution from the Collins contribution requires going beyond π0 measurements

• Measurements are underway at both forward and mid-rapidity

• Stay tuned!


The STARSTAR CollaborationU.S. Labs:

Argonne, Lawrence Berkeley, and Brookhaven National Labs

U.S. Universities: UC Berkeley, UC Davis, UCLA, Caltech, Carnegie Mellon, Creighton, Indiana, Kent State, MIT, MSU, CCNY, Ohio State, Penn State, Purdue, Rice, Texas A&M, UT Austin, Washington, Wayne State, Valparaiso, Yale

Brazil: Universidade de Sao Paolo

China: IHEP - Beijing, IPP - Wuhan, USTC,Tsinghua, SINAP, IMP Lanzhou

Croatia: Zagreb University

Czech Republic: Nuclear Physics Institute

England:University of Birmingham

France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes

Germany: Max Planck Institute – Munich University of Frankfurt

India:Bhubaneswar, Jammu, IIT-Mumbai, Panjab, Rajasthan, VECC

Netherlands:NIKHEF/Utrecht

Poland:Warsaw University of Technology

Russia: MEPHI – Moscow, LPP/LHE JINR – Dubna, IHEP – Protvino

South Korea:Pusan National University

Switzerland:University of Bern

STARSTAR


Possible mechanisms

• Sivers effect [Phys Rev D 41, 83 (1990); 43, 261 (1991)]: Flavor dependent correlation between the proton spin (Sp), proton momentum

(Pp) and transverse momentum (kT) of the unpolarized partons inside. The unpolarized parton distribution function fq(x,kT) is modified to:

• Collins effect [Nucl Phys B396, 161 (1993)]:

Correlation between the quark spin (sq), quark momentum (pq) and transverse momentum (kT) of the pion. The fragmentation function of transversely

polarized quark q takes the form:


<z>

<xq>

<xg>

NLO pQCD

S. Kretzer

p p 0, 3.8, s 200GeV

Forward π0 production at a hadron collider

• Large rapidity π production (η~4) probes asymmetric partonic collisions

• Mostly high-x quark + low-x gluon

• 0.3 < xq< 0.7

• 0.001< xg < 0.1

• <z> nearly constant and high ~ 0.7-0.8

• A probe of low-x gluons

N

N

q

g

s 2EN

ln(tan(2))

xq xF / zEN

xqpxgp

xF 2E

s

zE

Eq

xg pTse g

EN

(collinear approx.)


xF and pT range of existing FPD data

STARSTAR


Separated xF and pT dependence

6

5

13

3

B

N

pxdp

dE B

TN

F

Similar to ISR analysisJ. Singh et al., Nucl. Phys. B140, 189 (1978).


Single Spin AsymmetryDefinitions

• Definition:

• dσ↑(↓) – differential cross section of when incoming proton has spin up(down)

Two measurement techniques• Single arm calorimeter:

R – relative luminosity (by BBC)

Pbeam – beam polarization

• Two arms (left-right) calorimeter:

No relative luminosity needed

dd

ddAN

L

LR

RNN

RNN

PA

BeamN

1

LRRL

LRRL

BeamN

NNNN

NNNN

PA

1

π0, xF<0

π0, xF>0

Left

Right

p p

positive AN: more 0 going

left to polarized beam

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