Determination of the gluon polarisation at COMPASS & RHIC Sébastien Procureur (CEA - Saclay) Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur

Determination of the gluon polarisation at

COMPASS & RHICSébastien Procureur (CEA - Saclay)

Determination of G at COMPASS & RHIC PAVI06, Milos S.Procureur

STARSTAR

The nucleon spin structureThe nucleon spin structure

½= ½ΔΣ + ΔG + Lq + Lg

quark spin gluon spin orbital momenta

Prediction 2: (QCD & s=0)ΔΣ ~ 0.6

ΔΣ ~ 0.75Prediction 1: (Naive quark model)

Experiment: a0 = ΔΣ – (3S/2)ΔG ~ 0.2! (EMC, SMC, SLAC)

expect G(Q²) = dxg G(xg,Q²) ~ 1.5 @ Q² ~ 1GeV²

~ 2.5 @ Q² ~ 3GeV²Determination of G at COMPASS & RHIC PAVI06, Milos S.Procureur

The nucleon spin structureThe nucleon spin structureΔG contributes to scaling violation of g1

ΔG(Q²=1) = 0.53 +/- 1.93 (AAC 2000)

ΔG(Q²=1) = 0.50 +/- 1.27 (AAC 2003)

ΔG(Q²=1) = 0.47 +/- 1.08 (AAC 2006)

Small available Q² range for g1 large uncertainties on G

Direct measurement of G needed


hep-ph/0312112

Direct measurement of ΔGDirect measurement of ΔG


or

lepton beam

or

nucleon target

heavy flavor, high pT

or

or

proton beam

proton beam

0, (PHENIX),jets (STAR)

STARSTAR

polarized p-p collider (RHIC) polarized semi-inc. DIS

Hard probe

RHIC @ BNLRHIC @ BNL


AGSLINACBOOSTER

Polarized Source

Spin RotatorsPartial Snake

Siberian Snakes

200 MeV Polarimeter

AGS Polarimeter

Rf Dipole

RHIC pC Polarimeters Absolute Polarimeter (H jet)

PHENIX

PHOBOS BRAHMS & PP2PP

STAR

Siberian Snakes

Helical Partial Snake

Strong Snake

Spin Flipper

RHIC



LLA

P1

P2

RNN

RNN

PP 21

1

(R = L++/L-- : relative luminosity)

Run number Lintegrated P Figure of Merit (P4L )

Run 3 (2003) 350 nb-1 27% 1.86nb-1

Run 4 (2004) 120 nb-1 40% 3.1nb-1

Run 5 (2005) 3.8 pb-1 47% 185nb-1

G/G @ RHICG/G @ RHIC

22

~

q

q

q

q

G

G

G

G

Muon IDPanels

CentralArms

North MuonArm

South MuonArm

Ring ImagingCerenkov

EM Calorimeter

Muon TrackingChambers

Beam-BeamCounter

Multiplicity/VertexDetector

Time ExpansionChamber

Drift Chambers

Pad Chambers

Time of FlightPanels

Four spectrometer arms with excellent trigger and DAQ capabilities.

PHENIX detectorPHENIX detector


0 cross section0 cross section


Very good agreement with NLO

Xsection calculation!

ResultsResults


Theory model C.L.(%)

GRSV-std 17.1-21.7

GRSV-max (g=g) 0.0-0.0

GRSV g=0 16.7-18.4

GRSV g=−g 0.0-0.7

0 reconstruction (with background asym. subtraction)

17.2 pbLdt

Large acceptance TPC and EMC -1<η<2

STAR detectorSTAR detector


ResultsResults


STARSTAR Inclusive Jet Cross section

STAR Preliminary

14.0 pbLdt

Large G already disfavored

2003-2004 data:

+ results from 2005 soon…

RHIC projectionsRHIC projections


STARSTAR

Results of unprecedent precision in the next years:

pQCD analysis

+ PHENIX data on

COMPASS

SPS

LHC

Beam: 2 . 108 µ+/ spill (4.8s / 16.2s) Luminosity ~5 . 1032 cm-2 s-1 Beam polarization: -80%Beam momentum: 160 GeV/c

COMPASS @ CERNCOMPASS @ CERN


COMPASSCOMPASS


G/G @ COMPASSG/G @ COMPASS

LO Compton Photon gluon fusion (PGF)

Spin asymmetries ALL of the main diagrams in DIS:

Only PGF is sensitive to (G/G)(xg) (but S suppressed)

ALL = LLa )( gxG

G*ALL q

q


Open charm channel

q = c, then c D0 or D* K,

c

c

: no physical background

: low statistics

Selections of PGFSelections of PGF

scale: mc


ΔG/G with open charmΔG/G with open charm

)( gLL xG

Ga

BS

S

ALL =

D* D0+ S K+ + S


)(41.057.0)15.0~( statxG

Gg

D0 K+

Open charm channel

q = c, then c D or D* K,

c

c

: no physical background

: low statistics

High pT hadrons channel

(q,q) (h1,h2) with high transverse momentum wrt * direction

: physical background

: large statistics

Selections of PGFSelections of PGF

scale: mc scale: pT , Q²


ALL/D = Rpgf G/G aLLpgf

+ Rqcdc q/q aLL

qcdc

+ Rqq q/q aLLgq (G/G)

+ Rqg G/G aLLqg (q/q)

+ …

Need a Monte Carlo simulation

Theoretical inputs for polarisations of partons in virtual photon…

+ resolved photons!(low Q²)

G/G with high pT hadronsG/G with high pT hadrons


high Q²

pT

Monte Carlo simulationMonte Carlo simulationPYTHIA generator for low Q² + GEANT for spectro simul.

Good description of the data by the simulation!

RPGF 30%

RRes.Phot 50%


Data/MCData & MC

5.2)( gxG

16.0)( gxG

62.0)( gxG

ResultsResults


2002-2004 data

(2002-2003 data:PLB 633 (2006) 25-32)

Favours small values of G

Next stepsNext steps


Results from 2004 data for Q² > 1 GeV²

Improved high pT selection with Neural Network

New magnet larger acceptance larger xg

Improved RICH (particle identification)

Extraction of G/G at different xg

very soon!

New run in 2006

NLO analysis (theoretical inputs)

ConclusionConclusion• Efforts on G measurement start to pay!

• Agreement between RHIC and COMPASS data

• Very exciting results are expected in the next few years!

½= ½ΔΣ + ΔG + Lq + Lg

½0.2 + 0.4 + 0.0 + 0.0possible scenarios:½0.2 + 0.0 + 0.4

G not large ΔΣ small ( predictions)


… + … + …

SparesSpares


Resolved photon asymmetryResolved photon asymmetry

1st estimation of the resolved photon asymmetry!

• Problem: polarized PDFs of virtual photons not measured!

-q,npert(xp,0²) < q,npert(xp,0²) < q,npert(xp,0²) • Solution:

measured!

Allows us to obtain a range for (q/q) and (G/G) !

Adds a limited uncertainty to the estimation of (G/G)(xg)

q,pert(xp,²) calculable


Documents

Determination of the gluon polarisation at COMPASS & RHIC Sébastien Procureur (CEA - Saclay) Determination of G at COMPASS & RHICPAVI06, Milos S.Procureur