ΔΔG/G Extraction From G/G Extraction From High-PHigh-Ptt Hadron Pairs at Hadron Pairs at

COMPASSCOMPASS

Ahmed El AlaouiNuclear Physics School, Erice, 16-24 September 2007

On Behalf Of COMPASS Collaboration

Outline

Introduction

COMPASS Experimental Setup

Data Analysis

Results

Summary and Conclusion

- To access the gluon contribution to the nucleon spin

- To understand the role of the Axial Anomaly in the explanation of the spin crisis

Nucleon Spin

Naive Quark Model

Pure valence description of constituent quarks:

∆u = + 4/3 ∆d = - 1/3 ∆Σ = 1

Relativistic Quark Model ∆Σ ≈ 0.75

QCD framework Hyperons β decay constants + SU(3) flavor symmetry

∆Σ = a0 ≈ 0.60 compatible with the Relativistic QM predictionHowever, the EMC measured ∆Σ = 0.12 ± 0.09 ± 0.14

SPIN CRISIS

a0 = ΔΣ – (3αs/2π)ΔG A measurement of ∆G is needed

How To Acess ΔG/G

Indirect Measurement:

QCD analysis: fit to the nucleon spin structure function g1(x)

Direct Measurement:

∆G/G can be accessed via Photon Gluon Fusion (PGF) process

Unfortunately, the limited range in Q2 does not allow for a precise determination of ∆G

PGF Process

Two approaches are used to tag PGF process

• q = c:

- Open Charm D0, D* decay- Clean signal- Combinatorial background- Low statistics

• q = u, d, s:- High-pt hadron Pairs

- Physical background- High statistics

APGF = aLL x (∆G/G) factorization theoremPGF

How To Acess ΔG/G

Direct Measurement:

∆G/G can be accessed via Photon Gluon Fusion (PGF) process

Three independent measurements were done at COMPASS

- Open Charm

- High pt hadron pairs production at Q2>1GeV2

- High pt hadron pairs production at Q2<1GeV2

Indirect Measurement:

QCD analysis: fit to the nucleon spin structure function g1(x)

Unfortunately, the limited range in Q2 does not allow a for precise determination of ∆G

COMPASS Collaboration

COMPASS

COmmon Muon and Proton Apparatus for

Structure and

Spectroscopy

250 Physicists

18 Institutes

12 Countries

LHC

SPS

luminosity: ~5 1032 cm-2 s-1

beam intensity: 2.108 µ+/spill (4.8s/16.2s)

beam momentum: 160 GeV/c

Experiment Layout

COMPASS Spectrometer

Tracking: SciFi, Silicon, MicroMegas, GEMs, MWPC, Straws

PID: RICH, Calorimeters, μ Filters

SM1

SM2

RICH

E/HCAL1

Muon Wall 1 Muon Wall 2E/HCAL2

Polarized Target

160 GeV μ beam

Polarization ~ 80%

50 m long

LAS

SASMicroMegas

DC

COMPASS Target

Two 60 cm long oppositely polarized cells

6LiD is used as a material

dilution factor ~ 0.4

Target Polarization ~ 50%

70 mrad acceptance (180 mrad for 2006 target) -100 -50 0 50 [cm]

Vertex distribution

High Pt Events Selection Primary vertex with at least μ, μ’ and 2 hadrons

minv(h1,h2) > 1.5 GeV

0.0 < z, xF < 1.0

Pt > 0.7 GeV

0.1 < y < 0.9 (Q2>1GeV2)

ECalo/P > 0.3

0.0 < z1+z2 < 0.95

ΣPt > 2.5 GeV22

0.35 < y < 0.9 (Q2<1GeV2)

High Pt Spin Asymmetry

The acceptance is not identical in both cells Asymmetry bias

μ

B

Aexp = (Nu - Nd)/(Nu + Nd)

High Pt Spin Asymmetry

Polarisation reversal each 8 hours

Aexp = (Nu - Nd)/(Nu + Nd)‘ ‘ ‘ ‘ ‘Aexp = (Nu - Nd)/(Nu + Nd)

To improve the statistical error, a weighted method is used in the asymmetry calculation:

w = fDPB (event-wise weight)

A||/D=(Aexp- Aexp)/2fPTPBD‘

μ

B

μ

B

f Dilution factorPT(B) Target(Beam) polarizationD Depolarization factor

∆G/G Extraction at Q2<1GeV2

Ri (fraction of the process i), aLL, ∆q, q, q and G are obtained from

- Monte Carlo Simulation based on PYTHIA generator and Geant.

- pQCD Calculation

- pdf in the nucleon from GRSV2000 and GRV98LO parametrization

- pdf in the photon from GRS parametrizationThe polarized pdfs in the photon ∆q and ∆G are not available. Therefore the positivity limit is used to constrain them which leads to 2 extreme scenarios.

∆G/G at Q2<1GeV2

A||/D = RPGF∆G/G aLL PGF

+ Rqq∆q/q aLL (∆q/q)

qq γ + Rqg∆G/G aLL (∆q/q) qg γ

+ Rgq∆q/q aLL (∆G/G)

gq γ + Rgg∆G/G aLL (∆G/G)

gg γ

γ

γ γ

Included as systematic error in the estimation of ∆G/G

+ RQCDC ∆q/q aLL QCDC

qq’ qq’

Resolved photon processes

γ

Monte Carlo vs. Data (Q2<1GeV2)

xBj

Process fractions (Q2<1GeV2)

Resolved photons processes

50%

32%12%

∆G/G Result at Q2<1GeV2

2002-2004 data

A||/D = 0.004 ± 0.013(stat.) ± 0.003(syst.)

∆G/G(xg,μ2) = 0.016 ± 0.058(stat.) ± 0.055(syst.)

xg = 0.085-

0.035

+0.070 μ2 = 3GeV2

Contribution to Syst. error comes from

- False asymmetry

- Monte Carlo tuning

- Resolved photon process

∆G/G Extraction at Q2>1GeV2

∆G/G at Q2>1GeV2

A||/D = RPGF∆G/G aLL + RQCDC ∆Q/Q aLL PGF QCDC

+ RLO ∆Q/Q aLLLO

At Q2>1GeV2 analysis, Lepto generator seems to describe the real data much better than PYTHIA. It was then used to estimate the fraction of each process

PGF QCDCLO

Contribution from resolved photon precesses is negligible in this case

Monte Carlo vs. Data at Q2>1GeV2

0

10

20

30

40

PGF QCDC LO

34%

∆G/G Result at Q2>1GeV2

• 2002 - 2003 Data

A||/D = -0.015 ± 0.080(stat.) ± 0.013(syst.)∆G/G(xg, μ2) = 0.06 ± 0.31(stat.) ± 0.06(syst.)

<xg> = 0.13

• 2004 Data

Analysis is in progress…

Contribution to Syst. error comes from

- False asymmetry

- Monte Carlo tuning

μ2 = 3GeV2

Results ΔG/G

Summary and Conclusion

- The new solenoid, installed in 2006, has an acceptance (180 mrad) three times larger than the previous one.

- High-Pt asymmetries at Q2>1GeV2 and Q2<1GeV2 were presented

- The measured ∆G/G is compatible with zero at xg = 0.1

- Analysis of 2004 data (at Q2>1GeV2) is almost finished. It will be released soon

Double the statistics obtained in 2004Access higher value of xg

Thank you

The most precise measurement up to now

Backup slides

Fit to g1(x) using DGLAP evolution equations provides 2 differents solutions : ∆G >0 and ∆G <0. Both solutions describe the data well. The first moment of ∆G obtained from the fit is

∆G(xg) dxg ∫ ≈ 0.2 - 0.3 ≡∆G

Muon Beam Line