Recent measurement of ΔG/G at COMPASS Sébastien Procureur – CEA Saclay on behalf of the COMPASS...

Recent measurement of ΔG/G at COMPASS

Recent measurement of ΔG/G at COMPASS

Sébastien Procureur – CEA Saclay

on behalf of the COMPASS collaboration

EPS2005 23/07/2005

The Nucleon spin

½= ½ΔΣ + ΔG + Lq + Lg

Sébastien Procureur EPS2005

- Quark model: ΔΣ ≈ 0.75

- QCD + s = 0 (Ellis-Jaffe): ΔΣ ≈ 0.6

- Experimentaly: a0 (=ΔΣ?) ≈ 0.2

The Nucleon spin

½= ½ΔΣ + ΔG + Lq + Lg

quarks gluons orb. mom.

ΔΣ = a0 + 3(s/2)

ΔG


The Nucleon spin

½= ½ΔΣ + ΔG + Lq + Lg

quarks gluons orb. mom.

Necessary to measure ΔG

- Quark model: ΔΣ ≈ 0.75

- QCD + s = 0 (Ellis-Jaffe): ΔΣ ≈ 0.6


- Experimentaly: a0 (=ΔΣ?) ≈ 0.2

ΔΣ = a0 + 3(s/2)

ΔG

ΔG/G measurementmain DIS diagrams (up to 1st order):

LO PGF QCDC

only PGF is sensitive to ΔG/G


ΔG/G measurement

Apgf = ΔG/G * aLLpgf

if factorization holds (needa hard scale):

we need to select PGF events(S suppressed)… with a hard scale

hard part

soft part


PGF selections• Open charm channel J.Pretz’s talk

• 2 high pT hadrons channel:

(q,q) (h1,h2) with high pT

large stat, suppress LO,but still physical backgroundscale: pT², Q²

ALL/D = Rpgf G/G aLLpgf + (ALL/D)back

?

h1

h2


Physical background

ALL/D = Rpgf G/G aLLpgf

+ Rqcdc q/q aLLqcdc

all Q²


+ Rqq q/q aLLqq q/q

Physical background


+ Rqcdc q/q aLLqcdc

Q² < 1

all Q²


+ Rqg G/G aLLqg q/q

Physical background


+ Rqcdc q/q aLLqcdc

+ Rqq q/q aLLqq q/q

all Q²

Q² < 1


+ Rgq q/q aLLgq (G/G)

Physical background


+ Rqcdc q/q aLLqcdc

+ Rqq q/q aLLqq q/q

+ Rqg G/G aLLqg q/q

all Q²

Q² < 1


+ Rgg G/G aLLgg (G/G)

Physical background


+ Rqcdc q/q aLLqcdc

+ Rqq q/q aLLqq q/q

+ Rqg G/G aLLqg q/q


all Q²

Q² < 1


Inputs:

-generation of Monte Carlo evts-parametrization (world data)-pQCD calculations

… what about the rest (Q² < 1GeV²)??

Physical background


+ Rqcdc q/q aLLqcdc

+ Rqq q/q aLLqq q/q

+ Rqg G/G aLLqg q/q


+ Rgg G/G aLLgg (G/G)


Resolved photon processes

polarised parton distribution functions of (virtual…) photon areunknown!

BUT:

-q(x,²) < q(x,²) < q(x,²)

leads to 2 extreme (min&max) scenarios

additional (theoretical) uncertainty, but we can continue!

measured!


MC/data comparison

Q²

Generator PYTHIA is used for Q²<1GeV² (+ GEANT for spectrometer simulation)

.data

- MC

data/MC


MC/data comparison

pT

.data

- MC

.data

- MC

data/MC data/MC


Results from MC

The Monte Carlo describes our data quite well!


Rpgf ≈ 31%

Systematics due to MC Relevant parameters of the MC were varied to study the dependence

of our results with MC model


a typical one the critical one

Results for high pT low Q²• Asymmetry for 2002+2003 (pT > 0.7 GeV, pT² > 2.5 GeV²):

• G/G:

ALL/D = 0.002 ± 0.019(stat) ± 0.003 (syst)

ΔG/G(xg=0.1,²=3GeV²) = +0.024 ± 0.089(stat.) ± 0.014(exp.syst) ± 0.052(MC.syst) ± 0.018(photon)

0.057


Comparison with other resultsG = 2.48

G = 0.62

G = 0.16

(GRSV param.)


Summary• Most precise measurement of ΔG/G up to now:ΔG/G(xg=0.1,²=3GeV²) = +0.024 ± 0.089(stat.) ± 0.052(syst.)

ΔG is small or ΔG(xg) crosses 0 around xg = 0.1• For the 1st time, Res. Phot. asym. taken into account

• Next steps:– 2004 data analysis (ΔG/G)0.05 (also 2006 run)– NLO analysis

• Other channel: single high pT hadron


Spares


COMPASS acceptance


Results from RHIC-PHENIX


Asymmetry versus pT


Experimental systematics• Asymmetries calculated with low pT events

• Aexp = AAfalse according to microwave setting

A+Af

A-Af

1% of false asymmetries in 2003

data


Polarised 6LiD Target

PD: 50%

Dilution factor: 40%

Two 60 cm long target cellswith opposite polarisation

Superconducting Solenoid (2.5 T)

3He – 4He dilution refrigerator (T~50mK)

Dipole (0.5 T)

Reconstructed interaction vertices


Recent measurement of ΔG/G at COMPASS Sébastien Procureur – CEA Saclay on behalf of the COMPASS...

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