Proton spin structure from longitudinally polarized pp

collisions from PHENIXat RHIC

Alexander BazilevskyBNL

The 6th Circum-Pan-Pacific Symposium on High Energy Spin Physics

July 30 – August 2, 2007Vancouver BC, Canada

Nucleon Spin Structure

Naïve parton model:

1989 EMC (CERN):=0.120.090.14

Spin Crisis sdusdu

vv du 21

21

Determination of G and q-bar is the main goal of longitudinal spin program at RHIC

Gluons are polarized (G) Sea quarks are polarized:

Gqq 21

21

For complete descriptioninclude parton orbital angular momentum LZ:

ZLGqq 21

21

Parton Distribution Functions (PDF)• Quark Distribution

q(x,Q2)=

q(x,Q2)=

q(x,Q2)=

=unpolarised distribution

helicity distribution

g(x,Q2)=

g(x,Q2)=No Transverse Gluon Distribution in 1/2

• Gluon Distributionstransversity distribution

Polarized PDF from DISAsymmetry Analysis Collaboration

M. Hirai, S. Kumano and N. Saito, PRD (2004)

• Valence distributions well determined

• Sea Distribution poorly constrained

• Gluon can be either positive, 0, negative!

… To polarized pp colliderUtilizes strongly interacting probes

Probes gluon directly Higher s clean pQCD interpretation Elegant way to explore guark and anti-

quark polarizations through W production

Polarized Gluon Distribution Measurements (G(x)): Use a variety of probesAccess to different gluon momentum fraction xDifferent probes – different systematics

Use different energies s Access to different gluon momentum fraction x

RHIC as polarized proton collider

BRAHMS & PP2PP (p)

STAR (p)PHENIX (p)

AGS

LINAC BOOSTER

Pol. Proton Source500 A, 300 s

GeVs

L

50050

onPolarizati%70cms102 2132

max

Spin Rotators

Partial Siberian Snake

Siberian Snakes

200 MeV Polarimeter AGS Internal PolarimeterRf Dipoles

RHIC pC PolarimetersAbsolute Polarimeter (H jet)

2 1011 Pol. Protons / Bunch = 20 mm mrad

PHENIX for Spin

Electromagnetic Calorimeter

Drift ChamberRing Imaging Cherenkov Counter

JMuon Id/Muon Tracker

Relative LuminosityBeam Beam Counter (BBC) Zero Degree Calorimeter (ZDC)

Local Polarimetry - ZDC

Philosophy (initial design):

High rate capability & granularityHigh rate capability & granularity Good mass resolution & particle IDGood mass resolution & particle ID Sacrifice acceptanceSacrifice acceptance

PHENIX Long. Spin runs

Year s [GeV] Recorded L Pol [%]FOM (P4L)

2003 (Run 3) 200 .35 pb-1 32 3.7 nb-1

2004 (Run 4) 200 .12 pb-1 45 4.9 nb-1

2005 (Run 5) 200 3.4 pb-1 50 200 nb-1

2006 (Run 6) 200 7.5 pb-1 60 1000 nb-1

2006 (Run 6) 62.4 .08 pb-1 ** 48 4.2 nb-1 **

Unpol. Cross Section in pppp0 X : hep-ex-0704.3599 pp X: PRL 98, 012002

Good agreement between NLO pQCD calculations and data confirmation that pQCD can be used to extract spin dependent pdf’s from RHIC data.

• Same comparison fails at lower energies

||<0.35

Probing G in pol. pp collisions pp hX

hf

fXff

baba

hf

fXffLL

fXff

baba

LL Ddff

Dadff

ddddA

ba

baba

ˆ

ˆˆ

,

,

Double longitudinal spin asymmetry ALL is sensitive to G

Measuring ALL

LLR

RNNRNN

PPddddALL ;

||1

21

(N) Yield (R) Relative Luminosity

BBC vs ZDC(P) Polarization

RHIC Polarimeter (at 12 o’clock)Local Polarimeters (SMD&ZDC)

Bunch spin configuration alternates every 106 ns Data for all bunch spin configurations are collected at the same time

Possibility for false asymmetries are greatly reduced

ALL: 0

pT(GeV)

Run3,4,5: PRL 93, 202002; PRD 73, 091102; hep-ex-0704.3599

5 10

GRSV model:“G = 0”: G(Q2=1GeV2)=0.1“G = std”: G(Q2=1GeV2)=0.4

Stat. uncertainties are on level to distinguish “std” and “0” scenarios? …

PHENIX Preliminary Run6 (s=200 GeV)

From soft to hard

exponential fit

Exponent (e-pT) describes our pion cross section data perfectly well at pT<1 GeV/c (dominated by soft physics):

=5.560.02 (GeV/c)-1

2/NDF=6.2/3

Assume that exponent describes soft physics contribution also at higher pTs soft physics contribution at pT>2 GeV/c is <10%

For G constrain use pi0 ALL data at pT>2 GeV/c

hep-ex-0704.3599

From pT to xgluon

Log10(xgluon)NLO pQCD: 0 pT=29 GeV/c xgluon=0.020.3

GRSV model: G(xgluon=0.020.3) ~ 0.6G(xgluon =01 ) Each pT bin corresponds to a wide range in xgluon, heavily overlapping with other pT bins

These data is not much sensitive to variation of G(xgluon) within our x range

Any quantitative analysis should assume some G(xgluon) shape

From ALL to G (with GRSV)Calc. by W.Vogelsang and M.Stratmann

“std” scenario, G(Q2=1GeV2)=0.4, is excluded by data on >3 sigma level: 2(std)2

min>9Only exp. stat. uncertainties are included

(the effect of syst. uncertainties is expected to be small in the final results)

Theoretical uncertainties are not included

Extending x range is crucial!Gehrmann-Stirling models

GSC: G(xgluon= 01) = 1 G(xgluon= 0.020.3) ~ 0

GRSV-0: G(xgluon= 01) = 0 G(xgluon= 0.020.3) ~ 0

GRSV-std: G(xgluon= 01) = 0.4 G(xgluon= 0.020.3) ~ 0.25

GSC: G(xgluon= 01) = 1

GRSV-0: G(xgluon= 01) = 0

GRSV-std: G(xgluon= 01) = 0.4

Current data is sensitive to G for xgluon= 0.020.3

G: what’s next

Improve exp. (stat.) uncertainties and move to higher pT More precise G constrain in probed x range Probe higher x and constrain G vs x

Different s Different x

Different channels Different systematics Different x gqg sensitive to G sign

GG

GG

gq g

GG

qq

QQgg

Improve exp. uncertaintiesNeed more FOM=P4 L (stat. uncertainty ~ FOM)

0: expectations from Run-8

Higher pT measurements probe higher x constrain G vs x

Different channelsNeed more FOM=P4 L (stat. uncertainty ~ FOM)

Different sensitivities of charged pions to u and d provide more sensitivity to sign of G through qg scattering

Predictions are sensitive to fragmentation functions

Xpp

Xpp

Different channelsNeed more FOM=P4 L (stat. uncertainty ~ FOM)

XJpp /Xpp

Complementary to 0 measurements Need fragmentation functions

Probe G with heavy quarksOpen charm will come soonNeed more theoretical input

pp + jet

Theoretically clean (no fragmentation at LO) Gluon Compton dominates sensitive to sign of G Requires substantial FOM=P4 L

PHENIX Projection

Different ss=62 GeV 0 cross section described by NLO pQCD within theoretical uncertainties

Sensitivity of Run6 s=62 GeV data collected in one week is comparable to Run5 s=200 GeV data collected in two months, for the same xT=2pT/s

s=500 GeV will give access to lower x; starts in 2009

Flavor decomposition

Measured through longitudinal single spin asymmetry AL in W production at s=500 GeV

First data expected in 2009-2010

Wdu

Wud

Wdu

Wud

Other measurements

Helicity correlated kT from PHENIX

May be sensitive to orbital angular momentum

PHENIX UpgradesSilicon Tracking

VTX (barrel) by 2009FVTX (forward) by 2011

Electromagnetic CalorimetryNCC by 2011MPC, already installed!

Muon trigger upgradeBy 2009Momentum selectivity in the LVL-1 trigger

G from heavy flavor, photon-tagged jetsExpanded reach in x

Flavor separation of spin asymmetriesW physics at 500GeV

Transverse Spin Physics (see talk by M.Liu)

rapidity

See talk by I.Nakagawa

Summary RHIC is the world’s first and the only facility which provides collisions of

high energy polarized protons Allows to directly use strongly interacting probes (parton collisions) High s NLO pQCD is applicable

Inclusive 0 accumulated data for ALL has reached high statistical significance to constrain G in the limited x range (~0.020.3) G is consistent with zero Theoretical uncertainties might be significant

Extending x coverage is crucial Other channels from high luminosity and polarization Different s

PHENIX upgrades strengthen its capability in nucleon spin structure study Larger x-range and new channels (e.g. heavy flavor)W measurements for flavor decomposition

PolarimetryUtilizes small angle elastic scattering in the coulomb-nuclear interference (CNI) region

Pbeam N

ANpC

N NL NR

NL NR

Fast relative polarization measurements with proton-Carbon polarimeter

Single measurement for a few seconds

(Relatively) slow absolute polarization measurements with polarized atomic hydrogen jet target polarimeter

Used to normalize pC measurementstargetNtarget

beamNbeam

PAPA

targettarget

beambeam PP

Beam polarization in Run6: P ~ 60-65%

Polarization measurements in Run5: P/P~6% and (PBPY)/(PBPY)~9%

p p or C12

Backup: Rel. Lum. In PHENIX

Year [GeV] R ALL

2005 * 200 1.0e-4 2.3e-4

2006 * 200 3.9e-4 5.4e-4

2006 * 62.4 1.3e-3 2.8e-3

Backup: s=62 vs 200 GeV

Partonic Orbital Angular Momentum

Peripheral Collisions

Larger

Integrate over b, leftwith some residual kT

2Tk

Net pT kick

Central Collisions

Smaller 2TkJet 2

Jet 1

Jet 2 w/<kT>=0

Peripheral Collisions

Larger Jet 1

Jet 2 w/<kT>=0

Jet 2

Like helicities: Beam momenta

• Partonic orbital angular momentum leads to rotation of partons correlated with the proton spin vector

• This leads to different pT imbalances (pT-kicks) of jet pairs in semiclassical models

• Can be measured by measuring helicity dependence of <kT

2>

Partonic Orbital Angular Momentum

• Partonic orbital angular momentum leads to rotation of partons correlated with the proton spin vector

• This leads to different pT imbalances (pT-kicks) of jet pairs in semiclassical models

• Can be measured by measuring helicity dependence of <kT

2>

Integrate over b, leftwith different residual kT

Net pT kick

Central Collisions

Larger 2TkJet 2

Jet 1

Jet 2 w/<kT>=0

2Tk

Peripheral Collisions

Smaller Jet 1

Jet 2 w/<kT>=0

Jet 2

Unlike helicities: Beam momenta

Backup: W

W production» Produced in parity violating V-A process

— Chirality / helicity of quarks defined» Couples to weak charge

— Flavor almost fixed

W a b a bL

a b a b

u(x )d(x ) d(x )u(x )Au(x )d(x ) d(x )u(x )

xa>>xb: AL(W+) → u/u(x) - -xb>>xa: AL(W+) → d/d(x)

Backup: SIDIS for G

HERMES preliminary

Backup: GFrom M. Stratmann

Backup: from G to ALL

GRSV: G(Q2=1GeV2)= 1.76 +1.89

By Marco&Werner