Longitudinal Spin Program: Results and Plans

A.BazilevskyBrookhaven National Laboratory

For the PHENIX Collaboration

SPIN-Dubna-2011, Sep 20-24

Gluon helicity distribution G from ALL

(Anti-)quark helicity distribution from AL of W

+ STAR

For RHIC-Spin Collaboration

From DIS to pp: pp(semi)DIS

Probes G:

Q2 dependence of quark PDFs

Photon-gluon fusion

(Anti-)quark flavor separation:

Through fragmentation processes

Probes G:

Directly from gg and qg scattering

(Anti-)quark flavor separation:

Through and Wdu Wdu

Complimentary approaches2

RHIC as polarized proton collider

BRAHMS & now ANDY

STAR

PHENIX

AGS

LINACBOOSTER

Pol. Proton Source500 mA, 300 ms

GeVs

L

50050

onPolarizati%70

cms102 2132max

Spin Rotators

Partial Siberian Snake

Siberian Snakes

200 MeV Polarimeter AGS Internal PolarimeterRf Dipoles

RHIC pC PolarimetersAbsolute Polarimeter (H jet)

2 1011 Pol. Protons / Bunche = 20 p mm mrad

Year s [GeV]L [pb-1]

(recorded) Pol. [%]

2003 200 0.35 / 0.3 27

2004 200 0.12 / 0.3 40

2005 200 3.4 / 3.1 49

2006 200 7.5 / 8.5 57

2006 62.4 0.08 / - 48

2009 200 16 / 25 57

2009 500 14 / 12 39

2011 500 ~17 / ~12 ~52

Longitudinal Spin Running in PHENIX/STAR

3

Measuring Asymmetries in a Collider

L

LR

RNN

RNN

PPdd

ddALL ;

||

1

21

(N) Yield 0, , ±, h±, , e, etc.

(R) Relative Luminosity

(P) Polarization RHIC Polarimeters (at 12 o’clock) Local Polarimeters (in experiments)

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

Possibility for false asymmetries are greatly reduced4

PHENIX and STAR

STAR

STAR:Large acceptance with azimuthal symmetryGood tracking and PIDCentral and forward calorimetryUpgrades to higher rate capabilities, inner and forward tracking

PHENIX:High rate capabilityHigh granularityGood mass resolution and PIDLimited acceptanceUpgrades to wider acceptance, forward capabilities, inner tracking

5

Local PolarimetryZero Degree Calorimeter:

<2.5 mrad Utilizes spin dependence of very forward neutron production discovered in RHIC Run-2002 (PLB650, 325)

neutronchargedparticles

Before Run11, STAR also used BBCs (3.3<|h|< 5.0) utilizing spin dependence of hadron production at high xF

6

Local Polarimeter

Vertical f ~ ±p/2Radial f ~ 0Longitudinal no asymmetry

Measures transverse polarization PT , Separately PX and PY

Longitudinal component:P – from CNI polarimeters

22TL PPP

Vertical

Radial

Longitudinal

-/2 0 /2

Asymmetry vs

Longitudinal spin runs: 99.0PPL 7

Relative Luminosity (PHENIX) Two arrays of 64 elements, each a

quartz Cherenkov radiator with PMT Δη = ±(3.1 to 3.9), Δφ = 2π

144 cm

Beam-Beam Counters (BBC)

Cross checked with ZDC:

<2.5 mrad (>6)

Different physics signal, different kinematic region

ALL of BBC relative to ZDC is ~0

Results:

R ~ (25)10-4 ALL ~ (37)10-4 (for P~0.6)

8

GPolarized Gluon Distribution Measurements :

Use a variety of probes with variety of kinematicsAccess to different gluon momentum fraction xDifferent systematics

Use different beam energies Access to different gluon momentum fraction x

9

Unpol. Cross Section and pQCD in pp

PHENIX pp 0 XPRD76, 051106

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

PHENIX pp XPRL 98, 012002

s=200 GeVSTAR: ppjet XPRL 97, 252001

||<0.35||<0.35

10

Probing G in pol. pp collisionspp hX

hf

fXff

baba

hf

fXffLL

fXff

baba

LL Ddff

Dadff

dd

ddA

ba

baba

ˆ

ˆˆ

,

,

Double longitudinal spin asymmetry ALL is sensitive to G

11

ALL: jets

STAR Preliminary Run6 (s=200 GeV)10 20 pT(GeV)

Good discriminative power between calculations with different assumption for G

12

0 ALL

PHENIX Run5+6: PRL103, 012003

0 pT wBG 2 GeV/c 20%5 8%10 5%

pT(GeV)5 10

The most abundant probe in PHENIX(triggering + identification capability)

13

From ALL(pT) to G

14

2(G)

binsp stat

theoryLL

dataLL

T

AA2

2

2

)3(2.0and)1(1.02.04:errorStat. 2.08.0

22]3.0,02.0[

GeVG xGRSV

1.0:error.expSyst.

Compare ALL(pT) data with calculated ALL(pT) for a variety of 1

0)( dxxgG

From pQCD:

pT = 212 GeV/c

xgluon = 0.02 0.3

Only weakly model dependent

PRL103, 012003 (2009)

G: Global FitDSSV:Daniel de FlorianRodolfo SassotMarco StratmannWerner Vogelsang

• Phys. Rev. Lett. 101, 072001(2008)• First truly global analysis of all available polarized data including

RHIC results

Uncertainty estimation:2=1 (optimistic)

2/2=2% (conservative)

… Truth is in between2/2=2%

A node?...

RHIC data

15

+ Run9 0 ALL : PHENIX Preliminary

Run5Run6Run9

Dataset <PB> (%) <PY> (%) Lanalyzed (pb-1) FOM (P4L)

Run5 50 49 2.5 0.15

Run6 56 57 6.5 0.66

Run9 57 57 14 1.5

Tendency to positive G? 16

Global Fit for +PHENIX Run9 0 ALL By S.Taneja et al (DIS2011)

ala DSSV with slightly different uncertainty evaluation approach

DSSV DSSV + PHENIX Run9 0 ALL

No node …Uncertainties decreased

A node at x~0.1 ? 17

+ Run9 0 ALL : STAR Preliminary

Run9:

3-4 smaller stat. uncertainties than in Run6: Trigger upgrade

(improved eff.)

DAQ upgrade (increased rate, lower ET threshold)

Run9 data will definitely have considerable impact on G global fit and its uncertainty

18

Other probes

±• Preferred fragmentation

u+ and d- ; u>0 and d<0 different

qg contributions for +, 0, -

access sign of G

• Analysis similar to 0

• Different flavor structure• Independent probe of G

PRD83,032001(2011)

PHENIX

19

Other probes

Heavy Flavor• Production dominated by gluon

gluon fusion• Measured via e+e-, +-, e, eX, X• Need more P4L

Direct Photon• Quark gluon scattering dominates• Direct sensitivity to size and sign

of G• Need more P4L

~80%

20

Extend x-range different s

2-2.5 GeV/c4-5 GeV/c9-12 GeV/c

2-2.5 GeV/c4-5 GeV/c9-12 GeV/c

0 at ||<0.35: xg distribution vs pT bin

s=500 GeV

s=62 GeV

s=200 GeV

21

0 at s=62 and 500 GeV:Unpolarized cross section

s=500 GeV: PHENIX Preliminary

May need inclusion of NLL to NLO

s=62 GeV: PHENIX, PRD79, 012003

Data below NLO at =pT by (3015)%22

s=62 GeV

Charged hadrons

Very limited data sample (0.04 pb-1, compared 2.5 pb-1 from Run2005 s=200 GeV)

Clear statistical improvement at larger x; extends the range to higher x (0.06<x< 0.4)

Overlap with 200 GeV ALL provides measurements at the same x but different scale (pT)

s=500 ALL results will be available soon (from Run2009 with L~10 pb-

1 and P~0.4)

0: PHENIX, PRD79, 012003

23

G: Path Forward

Improve precision of current measurements

Get more data

Extend xg-range

Move to forward rapidities

Constrain kinematics: map G vs xg

More exclusive channels: pp + jet and pp jet + jet

Limitations in current data: Limited x-range covered Weak sensitivity to the shape of G(x)

24

Get more data

STAR pp jet X : projections PHENIX pp 0 X : projections

25

Forward Calorimetry: PHENIX MPCMuon Piston Calorimeter (MPC): PbWO4

3.1 < || < 3.9

2 azimuth

Gives access to lower: x10-3

Fully available from 2008

26

MPC 0 500 GeV300 pb-1 P=0.55

STAR: 0 forward rapidityPRL 97, 152302

pQCD looks working even in forward rapidities

STAR: di-jet ALLConstrains kinematics shape of G(x)

2exp, 43

21

s

Mxx

Run9 data !

Projections for s=500 GeV

The same trend as from inclusive jets: Slightly positive G? …

27

PHENIX Silicon Vertex tracker (VTX & FVTX)

FVTX endcaps1.2<|h|<2.7 mini strips

VTX barrel |h|<1.2VTX: available from 2011FVTX: available from 2012

432

ees

px T

431

ees

px T

EMC and MPC: pT and photon VTX: jet

May be luminosity (and polarization) hungry

Rejects hadronic backgroundc/b separated measurements

g

g Q

QQ = c or b

q

g

g

q jet

28

(Anti)quark flavor separationDSSV: PRL 101, 072001 (2008)

Mainly from SIDIS:

Fragmentation functions to tag (anti)quark flavor

l+

p+p W (e/) +

Parity violating W production:

Fixes quark helicity and flavor

No fragmentation involved High Q2 (set by W mass)

)()()()(

)()()()(

baba

babaWL xuxdxdxu

xuxdxdxuA

29

W: AL vs l

STAR

Central (barrel) region (We , ||<1)

First data from 2009: PRL106, 062002 (2011)

Forward (endcup) region (We , 1<||<2) :

Forward tracker upgrade, first data in 2012

PHENIX

Central Arms (We , ||<0.35)

First data from 2009: PRL106, 062001 (2011)

Forward Arms (W , 1.2<||<2.4) :

Trigger upgraded, first data from 2011

u

uAW

L

u

uAW

L

d

dAW

L

d

dAW

L

u

u

d

dAW

L 2

1

d

d

u

uAW

L 2

1

30

Central region: W e from Run9 Triggered by energy in EMCal Momentum from energy in EMCal Charge from tracking in B field

e+

e+ e-

e-

PHENIX: |e|<0.35STAR: |e|<1

L=8.6 pb-1L=12 pb-1

31

Central region: W e from Run9

PHENIX: PRL106, 062001 (2011)STAR: PRL106, 062002 (2011)

P=0.39 L=8.6/12 pb-1 in PHENIX/STAR

Run11: larger sample with P~0.52

PHENIX AL

Cross section

STAR AL

32

e-

e+

e+ e-

PHENIX Forward Arm: W

First data collected in 2011: L~15 pb-1 P~0.52Data being analyzed

trigger rejection

trigger eff.

PHENIX 2kHz Bandwidth

Raw yields with different triggers and cuts

More challenging than We at ~0

Expected W yield

33

W l : ProjectionsPHENIX: We STAR: WePHENIX: W

34

Longer term upgradesp+p: s=500 GeV 650 GeV

Will increase W production cross section twice

p+3He: s=432 GeVWill allow full flavor separation for light quarks

Workshop “Opportunities for polarized He-3 in RHIC and EIC” at BNL, Sep 28-30, 2011

35

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

PHENIX&STAR continues producing results for different spin observables

Significant constraint on G for xg~0.02-0.03 from inclusive 0 and jet ALL at s=200 GeV

Other ALL measurements to be included in G global fit with different systematics and x-coverage

First AL results from We in central rapidity at s=500 GeV from 2009

First AL results from W in forward rapidity at s=500 GeV from 2011 coming soon

A major machine, PHENIX and STAR upgrades are under discussion Machine: Polarized source upgrade and electron lenses in RHIC to increase L and P;

s=500 GeV 650 GeV; polarized 3He

Experiments: considerably extended acceptance and improved capabilities in forward rapidity

36

Backup

37

Proton Spin

zLG 2

1

2

1Proton

Spin

1988 EMC (CERN): is small Proton Spin Crisis

From recent fits: ~1/4 (PRL101:072001,2008)

sdusdu

Determination of G is the main goal of longitudinal spin program at RHIC

(anti)quarkspin

Parton OrbitalMomentum

Gluonspin

Gluons carry ~1/2 of the proton momentum Natural candidate to carry proton spin

38

From DIS …

Semi-inclusive polarized DIS Probe gluon through photon-gluon fusion process. Record heavy mesons (fragmented from heavy quarks)

+ Theoretically clean (high energy scale established by quark heavy mass)

– Background, low statistics Record light mesons (fragmented from light quarks)

+ High statistics – Large background and low energy scale

(problematic theoretical interpretation)

Inclusive polarized DIS Only information about input and scattered

lepton (e, ) is recorded x and Q2 reconstructed from kinematics

Do not have direct access to gluon Probe it through scaling violation (Q2 dependence of quark PDFs) - with poor precision currently

39

… To polarized pp collider

Utilizes strongly interacting probes

Probes gluon directly Higher energies clean pQCD interpretation

Polarized Gluon Distribution Measurements (G): Use a variety of probes with variety of kinematics

Access to different gluon momentum fraction xDifferent systematics

Use different beam energies Access to different gluon momentum fraction x

g

40

PHENIX Detector in 2011

p0, , g hElectromagnetic Calorimeter: ||<0.35Muon Piston Calorimeter: 3.1<||<3.9

p±, e, J/ye+e- : ||<0.35Drift, Pad Chambers, VTX (||<1)Ring Imaging Cherenkov Counter, ToFElectromagnetic Calorimeter

, J/y+- : 1.2<||<2.4Muon Id/Muon Tracker

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

Local Polarimetry – ZDCSpin direction control

Philosophy (initial design): High rate capability & granularity Good resolutions & particle ID Sacrifice acceptance

41

42

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%

PRD76, 051106 (2007)

pT>2 GeV/c – hard scale?

43

From soft to hard

xT scaling:

)(1

3

3

Tn xGsdp

dE

Running (Q2)Evolution of PDF and FFHigher order effectsEtc.

n=n(xT,Ös)

Soft region: n(xT) increase with xT If ~exp(-pT)

Hard region: n(xT) decrease with xT

Stronger scale breaking at lower pT

2 GeV/c at Ös=62 GeV pT~2 GeV/c – transition from soft to hard scale?

2004.62log

log 4.62200 n

PRD76, 051106 (2007)

xT10-2 10-1

44

xT scalings=500/200 GeVs=200/62 GeV

45

RHIC Spin Measurements

gq pp

Jet

Other exp.

Check theory (pQCD) works

q yieldJet g

This exp. Theory

Extract polarized PDF from spin asymmetries using pQCD

This exp. Theory

Jet Yield Spin Asymmetry

q g

Other exp.

Extract! 46

From pT to xgluon

NLO pQCD: 0 pT=212 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

10-210-3 10-1 x

47

From ALL to G (with GRSV)Generate g(x) curves for different (with DIS refit)

Calculate ALL for each G

Compare ALL data to curves (produce 2 vs G)

1

0)( dxxgG

1.0:error.expSyst. )3(2.0and)1(1.02.04:errorStat. 2.0

8.022]3.0,02.0[

GeVG x

GRSV

48

G: theoretical uncertaintiesParameterization (g(x) shape) choice

• Vary g’(x) =g(x) for best fit, and generate many ALL

• Get 2 profile• At 2=9 (~3), consistent constraint:

-0.7 < G[0.02,0.3] < 0.5 Our data are primarily sensitive to the

size of G[0.02,0.3].

Theoretical Scale Dependence:

Vary theoretical scale : =2pT, pT, pT/2

0.1 shift for positive constraint Larger shift for negative constraint

49

MPC: 0 cross section

50

Forward Arm: W Momentum and charge: from tracking in B field

W

W is dominant(pT>20 [GeV/c])

Muon Tracking Chambers (Tr) 3 stations of Cathode strip chambers

Muon Identifier (ID) 5 layers of Iarocci tubes in x and y 80 cm of steel plate absorber (total) Provides trigger p >2 GeV (Rej ~100)

New Muon trigger (Rej ~100)Tr fast front end electronics

Provides momentum sensitive info for level1 trigger (p >7 GeV )

Two trigger RPC stations Another tracking point Precise timing to reject the

background

RPC3RPC1(to be in 2012)

ID

Tr

51

W cross section

52

sPHENIX

53