Carl Gagliardi – WWND – Trans Spin at RHIC 1 Transverse Spin Physics in pp Collisions at RHIC Carl A. Gagliardi Texas A&M University Outline Introduction

Carl Gagliardi – WWND – Trans Spin at RHIC 1

Transverse Spin Physics in pp Collisions at RHIC

Carl A. GagliardiTexas A&M University

Outline

• Introduction• Forward rapidity measurements• Mid-rapidity measurements• Looking ahead


RHIC: the Relativistic Heavy Ion Collider

• Search for and study the Quark-Gluon Plasma• Explore the partonic structure of the proton• Determine the partonic structure of nuclei


From PDFs to polarized PDFs

• There are really three different sets of PDFs for the proton

q(x)g(x)

Δq(x)Δg(x)

δq(x)

Proton spin

+

Proton spin

Unpolarized PDF

“Polarized” distribution

“Transversity” distribution

Consider a proton moving toward the right


Current knowledge of the polarized distributions

• Quarks and antiquarks only carry ~30% of total proton spin• Proton “spin crisis”

gz

qz

pz LLGS

2

1

• Know very little about orbital motion

• Gluon and anti-quark polarizations have large uncertainties

RHIC transverse spin program

DSSV, PRL 101, 072001

Anselmino et al, arXiv:0807.0173

• Transversity also has large uncertainties


RHIC: the world’s first polarized hadron collider

• Spin varies from rf bucket to rf bucket (9.4 MHz)• Spin pattern changes from fill to fill• Spin rotators provide choice of spin orientation• “Billions” of spin reversals during a fill with little if any depolarization

BRAHMS

PHENIX

AGS

BOOSTER

Spin Rotators(longitudinal polarization)

Solenoid Partial Siberian Snake

Siberian Snakes

200 MeV PolarimeterAGS Internal Polarimeter

Rf Dipole

RHIC pC PolarimetersAbsolute Polarimeter (H jet)

AGS pC Polarimeters

Strong Helical AGS Snake

Helical Partial Siberian Snake

Spin Rotators(longitudinal polarization)

Spin flipper

Siberian Snakes

STAR

PHOBOS

Pol. H- SourceLINAC


Transverse single-spin asymmetries

• Definition:

• dσ↑(↓) – cross section for scattering to the left when incoming proton has spin up(down)

Two methods of measurements: • Single arm calorimeter:

R – relative luminosity

Pbeam – beam polarization

• Two arm (left-right) calorimeter:

Less sensitive to instrumental effects

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 to

left of the polarized beam


Transverse single-spin asymmetries at forward rapidity

• Large single-spin asymmetries at CM energy of 19.4 GeV

TqSN /pmαA

• Weren’t supposed to be there in naïve pQCD

FNAL E704


Forward π0 production at ISR energies

Bourrely and Soffer, EPJ C36, 371: NLO pQCD calculations underpredict the data at ISR energies Maybe the E704 results arise from soft physics?

√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


First AN measurement at RHIC

PRL 92, 171801 (2004)

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

Collins: spin and kT 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=19.4 GeV, 0.5 < pT < 2.0 GeV/c)

√s=200 GeV, <η> = 3.8

STARSTAR


Forward pp π0 + X cross sections at 200 GeV

NLO pQCD calculations by Vogelsang, et al.

PRL 97, 152302STARSTAR

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.


Sivers and Collins effects in pp collisions

Collins mechanism: final-state asymmetry in the forward jet fragmentation

Sivers mechanism: initial-state kT dependence in the parton distribution

SPkT,q

p

p

SP

p

p

Sq kT,πSensitive to proton spin – parton transverse motion correlations

Sensitive to transversity

Observed transverse single-spin asymmetries could arise from the Sivers effect or Collins

effect, or from a linear combination of the two


CollinsSivers

Sivers and Collins effects in deep-inelastic scattering

• Semi-inclusive DIS can distinguish the Sivers and Collins effects• HERMES finds both are non-zero


Recent π0 results at 200 GeV from STAR

• Sivers fit to HERMES SIDIS describes η = 3.3; overpredicts η = 3.7• Twist-3 fit to E704 plus preliminary STAR Runs 3 and 5 data

describes η = 3.7; underpredicts η = 3.3

STARSTAR PRL 101, 222001


Charged pion measurements at 200 GeV from BRAHMS

• Sign dependence of charged pion asymmetries seen in FNAL E704 persists to 200 GeV

BRAHMS

2.3 deg (~3.4)4 deg (~3)


3.0<<4.0

p+p0+X at s=62.4 GeV/c2 p+p0+X at s=62.4 GeV/c2

0 results at 62.4 GeV from PHENIX

• Asymmetries are comparable or larger at 62.4 GeV than they are at 200 GeV


Charged-hadron results at 62.4 GeV from BRAHMS

BRAHMS

Limitation of the BRAHMS measurements:Very strong correlation between xF and pT

from small acceptance

PRL 101, 042001

• Very large asymmetries!• K- (= su ) asymmetry is a surprise. Sea-quark Sivers

effect or disfavored fragmentation?


Inclusive π0 AN(pT) in xF bins

• Combined data from three runs at <η>=3.3, 3.7 and 4.0

• Measured AN is not a smooth decreasing function of pT

as predicted by theoretical models

Kouvaris et al,PRD 74, 114013

STARSTAR PRL 101, 222001


A potential fly in the ointment?

• To date, the η meson has looked like a “high-mass, low-yield π0” in all measurements at RHIC

• AN for the η mass region is much larger at high xF

STAR 2006 PRELIMINARY

η ~ 3.66

STARSTAR


Mid-rapidity inclusive pion AN

• Mid-rapidity pion yields are gluon-dominated at these pT

• No Collins effect for gluons• May help to constrain the gluon Sivers function

PRL 95, 202001


Mid-rapidity inclusive jet AN

• Gluon-dominated at low pT

• Dominated by qg scattering at higher pT

• Within uncertainties, AN is consistent with zero

STAR Preliminary

STAR PreliminarySTAR Preliminary

STAR Preliminary

PT (GeV/c)

PT (GeV/c) PT (GeV/c)

PT (GeV/c)

-0.5< <0.0-0.9< <-0.5

0.0< <0.5 0.5< <0.9

STARSTAR


Sivers effect in di-jet production

180open

1

2

spin

di-jetbisector

kTx

>

180

for

k T

x > 0

Sivers effect:

• Left/right asymmetry in the kT of the partons in a polarized proton

• Spin dependent sideways boost to di-jets

• Measure the di-jet opening angle as a function of proton spin

• Requires parton orbital angular momentum


Mid-rapidity di-jet Sivers effect measurementPRL 99, 142003

• Observed asymmetries are an order of magnitude smaller than seen in semi-inclusive DIS by HERMES

• Detailed cancellations of initial vs. final state effects and u vs. d quark effects, coupled with very small gluon Sivers effect?

STARSTAR


Mid-rapidity di-hadron Sivers effect measurement

• PHENIX is performing a similar measurement• Back-to-back correlations between a trigger π0 and an away-side

charged hadron


Separating Sivers and Collins effects in pp collisions

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,πSensitive to proton spin – parton transverse motion correlations

Sensitive to transversity

• Need to go beyond inclusive hadrons to measurements of jets or direct γ


Large-acceptance forward detectors

• Both PHENIX and STAR have installed large-acceptance electromagnetic calorimeters in the forward direction

PHENIX Muon Piston Calorimeters3.1 < |η| < 3.7

STAR Forward Meson Spectrometer2.5 < η < 4.0


First look at jet-like events with the STAR FMS

• Comparisons of the jet profile and effective mass in data vs. PYTHIA + GEANT simulations


Transverse spin forward + mid-rapidity jet

Bacchetta et al., PRL 99, 212002

• Conventional calculations predict the asymmetry to have the same sign in SIDIS and +jet

• Calculations that account for the repulsive interactions between like color charges predict opposite sign

• Critical test of our basic theoretical understanding


In the further future: Drell-Yan

• In SIDIS, final-state interaction of outgoing quark with proton remnant involves opposite color charges – attractive

• In Drell-Yan, initial-state interaction of the incoming quark with the spectator components of the proton involves like color charges – repulsive

• Sign of AN should reverse


Conclusions

• RHIC has observed large transverse single-spin asymmetries for forward particle production

• These asymmetries may provide evidence for parton orbital angular momentum and/or quark transversity

• Measurements to identify the underlying cause(s) are underway

• Future measurements will provide a direct illustration of attractive vs. repulsive color-charge interactions

• RHIC, the world’s first polarized hadron collider, is generating a wealth of new data regarding the spin structure of the proton

• Stay tuned!



Low-pT forward transverse single-spin asymmetries

• Rotator tuning for longitudinal polarization requires local polarimetry

STAR BBCs

PHENIX ZDCs

Documents

Carl Gagliardi – WWND – Trans Spin at RHIC 1 Transverse Spin Physics in pp Collisions at RHIC Carl A. Gagliardi Texas A&M University Outline Introduction