Mar. 17, 2006 Imran Younus Probing Gluon Polarization with Longitudinally Polarized p+p Collisions...

Mar. 17, 2006

Imran Younus

Probing Gluon Polarization with Longitudinally Polarized p+p Collisions at s = 200 GeV

http://www.unm.edu/index.html

3/17/2006 Imran Younus, Univ. of New Mexico. 22nd Winter Workshop on Nuclear Dynamics 2

“Proton Spin Crisis”

)(019.0)(013.0123.0

9

1

9

1

9

4

2

1)(

1

0 1

syststat

sduxdxg p

EMC Experiment at CERNJ. Ashman et al., Nucl. Phus. B 328, 1 (1989)

z

Z

LG

Lxgxqqdx

2

1

)())(2

1 1

0(

2

1

Proton-spin sum rule:

= U + D + SG = gluon polarizationLZ = orbital angular momentum

Polarized DIS: contribution of quarks to proton spin is amazingly small 0.1- 0.3

Main candidate to carry proton spin – Gluons Orbital angular momentum?

G measurement – main RHIC-spin goal


QCD FactorizationThe cross-section is written as convolution of PDFs and partonic sub process cross-section, e.g., production:

)()(ˆ)()(

)(

212211

21

zDfXffxfxf

Xpp

fpp

)()(ˆ)()()( 21221121 zDfXffxfxfXpp fpp

Unpolarized case:

Polarized case:

)(),( xfxf pp Unpolarized, polarized parton distribution functions, determined from experimental data.

)(zD f Fragmentation function, determined from experiment.

)(ˆ 21 Xpp Paronic level hard-scattering cross section. Calculated by perturbative QCD. Process dependant.

f1p(x1)

f2p(x2)

p1

p2 X

Df

f2

f1 f


Parton Distribution Functions (PDF)

Quark distributions (q= u,d,s…)

Unpolarized dist.

Helicity dist.

Transversity

q(x,Q2) = =

q(x,Q2) =

q(x,Q2) =

Gluon distributions

Unpolarized dist.

Polarized dist.

g(x,Q2) =

g(x,Q2) =

• PDFs - probability of scattering off of a parton carrying a particular fraction of the proton’s momentum.• Polarized PDFs - the difference in probability between scattering off of a parton with one spin state vs. the other. Still as a function of the momentum fraction.

Universality! PDFs extracted from the data in one process, can be used to make predictions for other processes.


Precision Data From DIS

• Precision Data in Wide Kinematical Range - Q2 evolution agrees with pQCD

• Only Fixed Target Spin Experiments so far. Need a collider to extend kinematical coverage.


Polarized PDFs and Uncertainties

Valence Dist’s are determined well.

Sea Dist’ is poorly constrained.

Gluon Dist’ can be either >0, =0, <0.

Gluon polarization (G) remains poorly constrained

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


Measuring Gluon Polarization at RHIC

Spin asymmetries for cross-sections

ALL: double longitudinal spin asymmetry – useful in extracting g(x), q(x).

)()(

)()(

LLAVarious channels to extract g(x).

Inclusive o production

Prompt photon

Heavy quark production

Jets

),( XqqqgA oLL

)( XqgALL

),( XbbccggALL


RHIC as Polarized Proton Collider

Lmax= 21032s-1cm-2

70% Polarization

50 s 500 GeV

Run05: L = 4 pb-1

47% Polarization

s = 200 GeV

BRAHMS & PP2PP

STAR

AGS

LINACBOOSTER

Pol. H- Source

Spin Rotators(longitudinal polarization)

Solenoid Partial Siberian Snake

Siberian Snakes

200 MeV Polarimeter AGS Internal Polarimeter

Rf Dipole

RHIC pC PolarimetersAbsolute Polarimeter (H jet)

AGS pC PolarimetersStrong AGS Snake

Helical Partial Siberian Snake

PHOBOS

Spin Rotators(longitudinal polarization)

Spin flipper

Siberian Snakes

PHENIX


PHENIX Detector

•Central Arms:||<0.35, =2900

Charged particle ID and tracking; photon ID. EM Calorimetry.

Muon Arm:1.2<||<2.4Muon ID and tracking.

•Global DetectorsCollision triggerCollision vertex characterizationRelative luminosityLocal Polarimetry

•Philosophy:– High resolution at the cost of acceptance– High rate capable DAQ– Excellent trigger capability for rare events


, Cross sectionsMeasured un-polarized cross section at s=200 GeV are well described by the Next-to-Leading Order cross sections.Necessary Confirmation that pQCD can be used successfully at RHIC to extract PDFs.


Measuring ALL

L

LR

RNN

RNN

PPALL ;

||

1

21

P1,P2 – Polarization of the colliding beams.

N++(N+-) – experimental yields for same (opposite) helicity collisions.

R – Relative luminosity, determined at PEHNIX using BBC LL1 trigger counts.

Run5: R=10-4


Local Polarimetry at PHENIX

Longitudinal Pol.

Transverse Pol.

Left-Right Asymmetry of Neutrons in transversely polarized p+p was discovered at IP12. Use neutron asymmetry to estimate longitudinal and transverse components of polarization. ZDC/SMD make local polarimetry measurement at PHENIX

<PL/P> ~ 98 – 99 %

<PT/P> ~ 10 – 14%


ALL()f1

p(x1)

f2p(x2)

p1

p2 X

Df

f

2

f

1

f

ppX is sensitive to gggg and gqgq

(at low/moderate pT)

G

G

G

G

G

G

q

q

q

q

q

q

+ +

][][][ qqgqggA qqgqggLL

Fractional contribution to ppX at s=200 GeV at mid-rapidity

qqqq

gggg

gqgq


Calculating ALL()

Calculate ALL(+BG) and ALL(BG) separately.

Get background ratio (wBG) from fit of all data.

Subtract ALL(BG) from ALL( +BG):

ALL( +BG) = W · ALL() + WBG · ALL(BG)

WBG=1- W, and W= /(+BG).

+BG region :±25 MeV around peakBG region :Two 50 MeV regions

pT(GeV) stat. (WBG)

1-2 17484k (32%)

2-3 10156k (14%)

3-4 1971k (7%)

4-5 381k (5%)

5-6 93k (5%)

6-7 28k (5%)

7-8 11k (5%)

8-9 4.2k (5%)


Run5 ALL() Results

Run Int.Lum. Pol. F.O.M.(P4L)

Run 3 (2003) 220 nb-1 27% 1.17 nb-1

Run 4 (2004) 75 nb-1 40% 1.92 nb-1

Run 5 (2005) 1.8(2.7) pb-1 47% 87.8(132) nb-1

Consistent with GRSV-std and GRSV G=0.

Excludes GRSV-max.

Run5 results consistent with previous results.

GRSV: M. Gluck, E. Reya, M. Stratmann, and W. Vogelsang, Phys. Rev. D 53 (1996) 4775.


Run6 Expectations• We expect both polarization and luminosity increases• This will decrease our uncertainty by a factor of ~4

(~3 when all Run5 data is considered).

Run6 Expectation:

Int. Lum.: 11.4 pb-1

Polarization: 60%

F.O.M. (P4L): 1.48 pb-1


Heavy Quark Production

Provide direct access to gluon polarization

Gluon Fusion dominates at LO

PHYTHA estimate:

GeV Charm Beauty200 95:5 85:15500 97:3 92:8

J/

x

XJggLLLL a

xg

xg

xg

xgA /

2

2

1

1

)(

)(

)(

)(

)(:)( QQqqQQgg

aLL(ggJ/ + X) model dependant.


ALL(J/)

PHENIX, PRL 92, 051802 (2004)

Partonic asymmetry for ggJ/ X

CEM: aLL ~ +1 CSM: aLL ~ -1

COM: )/,/4( max22 ppxsma TTcLL

Theoretical predictions of J/ production at RHIC are in good agreement with the PHENIX data.COM process dominant–PRD 68 (2003) 034003 G. Nayak, M. Liu, F. Cooper–PRL 93 (2004) 171801 F. Cooper, M. Liu, G. Nayak


ALL(J/)J/ Yield measurement:

• J/+-, 1.2 | | 2.2

• Fill by Fill yields: N(+-) – {N(++) + N(--)}

Fixed mass range; Open charm and DY background. Purity of J/ sample: 92%

• Gaussian + Exponential Fit. Used for cross check and background determination.

Run05 data used for the analysis• 3.2 pb-1 dimuon trigger data.• N (J/) ~ 7300• 47% polarization.

M()


ALL()

iii

iii

LLLL

xqe

xqeQxA

qqgaxAxg

xgA

)(

)(),(

)()()(

)()(

22

22

21

211

1

Cleanest mode for extracting g(x)Gluon Compton Dominates– At LO no fragmentation function– Small (15%) contamination from annihilation

p2

p1

x2

x1

NLO pQCD describes cross-section well can be used to interpret ALL()


ALL() projection

Prompt photon cross-section measured with Run3 data.Run5 analysis is going on.

ALL() needs large luminosity. Run6 may give us first results.

2008-09 projection.

GRSV-std


Summery

RHIC as a polarized p+p Ccollider is a powerful tool to probe gluon structure!

Measurements at PHENIX will allow us to constrain the gluon contribution to the proton’s spin.

First significant results of ALL of o from RHIC Run05 conclusively exclude GRVS-max scenario.

Study of other channels, prompt photons, heavy quarks production, is going on.

Run6 will improve FOM by a factor of 10 over Run5, and will provide us first measurement of prompt photon asymmetry.

Mar. 17, 2006 Imran Younus Probing Gluon Polarization with Longitudinally Polarized p+p Collisions...

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