Few-Body 18 talk - IFT
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Transcript Few-Body 18 talk - IFT
Recent results from the
COMPASS experiment at CERN
DAPNIA
Stephane Platchkov
CERN, Geneva, Switzerland
and
CEA-Saclay, DAPNIA/SPhN
August 21, 2006
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COMPASS Collaboration: 230 physicists from
12 countries and 28 institutions
August 21, 2006
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Physics program of COMPASS
Experiments with muon beam
Gluon spin contribution
Quark spin contribution
Flavor decomposition of spin
distribution functions
Transverse spin distribution
functions
Transversity – two hadron
correlations
Vector meson production
Spin transfer in L-hyperon
production
Search for pentaquarks
Experiments with hadron beams
Pion and Kaon polarizabilities
Diffractive and Central
production of exotic states
Search for glueballs
Light meson spectroscopy
Data taking
2002, 2003, 2004
2005: SPS stop
2006: restart in July
COMPASS objectives: Study of QCD
First priority: spin structure of nucleon
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The COMPASS Set-up – Two spectrometers
Trigger-hodoscopes
μ
Filter
ECal & HCal
SciFi
SM2
RICH
MWPC
SM1
6LiD Target
Silicon
Gems
Straws
Drift chambers
Micromegas
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Polarized beam and target
~75%
≥50%
SAT, LAT, PID
0.003<x<0.5, 10-3<Q2<10 GeV2
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COMPASS Polarized target
Specifications
Superconducting solenoid : 2.5 T
16 trim coils
-4
Field homogeneity: 10
Dipole filed: 0.5 T
Temperature: 50 mK
Polarization by microwaves
DNP
Material (used): 6LiD
Dilution factor: 0.4
Acceptance: ±70 mrad…
Performances
Polarization: >50%
Field reversal: 8h
Two 60 cm oppositely polarized cells
August 21, 2006
Largest polarized
target
in the
S. Platchkov,
Few-Body
18 world
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This talk
Most recent COMPASS results on
Quark contribution to the nucleon spin
Gluon contribution to the nucleon spin
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Nucleon spin composition
1980’s
Naive parton model
3 valence quarks
1990’s
QCD: Contributions from
sea quarks and gluons
1 1
( u d )
2 2
But in 1990 EMC measured :
= 0.12 ± 0.16 !
August 21, 2006
2000’s
… and also orbital
momentum !
11 1
( u d
ss)) G
GL
22 2
What is the nucleon spin made of?
CERN, SLAC, DESY, JLAB, RHIC, …
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Quark contribution to nucleon spin – World results
d
A
Most recent data from COMPASS:
1 N N µd
d
d
; A D( A1 A2 )
2002, 2003, 2004
Pb PT f N N
Good agreement with previous experiments
Much improved statistics at low x
August 21, 2006
A1 compatible
with 0
at low
S. Platchkov,
Few-Body
18 x
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QCD analysis – fit to g1(x) structure functions
F2 ( x )
g1 ( x ) A1 ( x )
2 x(1 R)
Two different fit programs (Next to Leading Order)
World data from p, d, 3He (230 points, 43 from Compass)
Parton parametrisations at Q2=3 (Gev/c)2
Fit in MS scheme
Data are well described by two solutions: G < 0 or G > 0 !
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QCD analysis – fit to g1(x) structure functions
Leader, Sidorov, Stamenov,
Phys.Rev. D73(2006)034023.
Gluck et al.,
Phys.Rev. D63(2001)094005.
Blümlein Böttcher,
Nucl.Phys.B636(2002)225.
- Evolution shows little sensitivity to various fits
- Fits of LSS-05, GRSV, BB show similar trend at low x
- Sign of G is mainly determined by the low x behavior of g1(x)
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QCD analysis – parton distributions / fit results
G < 0
G > 0
0.32 0.01( stat )
0.28 0.01( stat )
G 0.310.10
0.14 ( prg1)
G 0.260.04
0.06 ( prg1)
G 0.180.04
0.03 ( prg 2)
G 0.19 0.01
0.10 ( prg 2)
QCD FIT RESULT: │G│ in the range 0.20 - 0.30
August 21, 2006
Sign
not yet
determined
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Few-Body
18
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QCD fit – Strange quark distribution
G > 0
G < 0
Fit result: ( s s )Q2 3(Gev / c )2 0.10 0.01( stat ) 0.01(evol )
In agreement with a determination from integral of g1(x)
Strange sea is polarized and negative
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Direct determination of G/G – two methods
Via Open Charm production (q=c)
Detect D° K-p+ and D*+D°p+
Clean channel (no uds quarks)
High combinatorial background
Limited S/B ratio
PhotonGluon
Fusion
Via High-pt hadron pairs (q=u,d,s)
Good S/B ratio
Physics background (QCDC, LODIS,
Resolved photons contributions)
Rely on MC estimates
2 cases: Q2>1 (GeV/c)2 -LEPTO MC
Q2<1 (GeV/c)2 -PYTHIA MC
August 21, 2006
N
PGF
Bgnd
ALL
aLL
G PGF ALL
tot
D
D G
D
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Determination of G/G – Open charm
Data 2002+2003+2004
Untaged D0 → K+p
Taged D* → D0pslow → K+ppslow
G / G 0.57 0.41 syst
xg=0.15, µ2= 13 GeV2
Systematic much smaller than statistics
D° and D* results within 1.7
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Determination of G/G – high-pt hadron pairs
Photon
Gluon
Fusion
pT > 0.7 GeV/c,
pT2 > 2.5 (Gev/c)2
PGF G
A|| RPGF aLL
A
Bkg
G
Q2 > 1 (GeV/c)2
Leading
Order
pQCD scale, good evaluation of
bgnd, use LEPTO MC
Lower statistics
QCD
Compton
Q2 < 1 (GeV/c)2
Need to evaluate resolved photon
bgnd, use PYTHIA MC
More model dependent
Very large statistics (x10!)
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Resolved g
Q2 < 1 (GeV/c)2
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High-pt hadron pairs – Q2 and pt2 distributions
August 21, 2006
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Compass simulation software
Good description of data by the Monte-Carlo simulation program
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Determination of G/G – high-pt hadron pairs
Q2 > 1 (GeV/c)2 from 2002+2003 data:
G/G = 0.06 ± 0.31 (stat) ± 0.06 (syst)
@ xg = 0.13 ± 0.08, µ2 ~ 3 (GeV/c)2
Q2 < 1 (GeV/c)2 from 2002+2003+2004 data:
G/G = 0.16 ± 0.06 (stat) ± 0.06 (syst)
@ xg = 0.085, µ2 ~ 3 (GeV/c)2
(Systematics:
exp=0.014, MC=0.052, Res.g=0.013)
Both values compatible with zero
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Results for G/G – summary
Calculation: Glück et al.,Phys. Rev. D63 (2001) 094005
G=2.5
G=0.6
G=0.2
Three independent results, all consistent with zero !
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PHENIX RESULTS: ALL asymmetry for p° production
p p p 0 X
from Horaguchi,
CIPANP, June 2006
Results exclude the GRSV-max. More statistics needed.
PHENIX results are also compatible with G = 0
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Vogelsang, MS (work in progress)
From Stratmann, DIS2006
ALL for several trial gluons:
1st moment
at input
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Summary - New COMPASS results
Measurement of the g1 deuteron structure function
QCD NLO fit to most recent COMPASS/World data
Largely improved error bars
Data show no negative values at low x
Strange quarks negatively polarized
Gluon polarization is small, or even negative
G/G measurement with 3 independent methods
G/G is consistent with 0 or even negative!
No contradiction from RHIC experiments
If G is small or even negative, the spin puzzle is still unsolved!
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SPARES
August 21, 2006
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August 21, 2006
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Outlook -- COMPASS Programme
Improved COMPASS Factor of Merit
New OD magnet
Upgraded RICH
Full ECAL coverage
Other…
Total FoM improvement:
≥2.3
1.3
1.6
1.2
1.1
Complete longitudinal data (G/G, etc…)
Take transversity data with a proton target
Near future
Compass Factor of Merit
Better spectrometer in 2006
2007: Hadron run: Diffractive & Central production of exotics
Several new equipments under preparation…
2008 – 2010: Complete muon and hadron approved programs
Long-term future
Plans for DVCS – Proposal to be prepared
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G / G 0.57 0.41 syst
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QCD fits at low Q2
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The nucleon spin puzzle
Quark contribution to
the nucleon spin
1
1
( u d s )
2
2
1990 - 2005 : EMC, SMC, Hermes, SLAC :
0.20 – 0.25 instead of ~0.60 (QPM)
The nucleon spin is more complicated than anticipated
?
1 1
ΔΣ ΔG Lq Lg
2 2
Study Gluon contribution to the spin
And also many other spin observables…
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QCD fits -- COMPASS
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G/G – Resolved photon contribution
• Problem: polarized PDFs of virtual photons are not
measured!
• Solution:
qgP(xp,²) calculable
-qgNP(xp,0²) < qgP(xp,0²) < qgNP(xp,0²)
measured!
Allows us to obtain a range
for (q/q)g and (G/G) g !
Adds a limited uncertainty to the estimation of (G/G)(xg)
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Transversity in COMPASS
Full description of the nucleon at LO - 3 nucleon PDFs
Momentum distribution q(x)
Helicity distribution q(x)
Transversity distribution Tq(x)
Suppressed in DIS – “chirality odd”
2
Does not mix with gluons – simple evolution with Q
Measurement through fragmentation
Three methods to access transversity
Azimuthal distribution of single hadron:
Azimuthal dependence of a hadron pair
Transverse polarization of Lambdas
August 21, 2006
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T q( x ).T Dqh ( z, pTh )
T q( x ).H qh ( Z , M h )
T q( x ).DqL ( z )
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Transversity in COMPASS
Experimental method
Transverse polarization
Field reversal by microwaves
Once every 5-6 days
Single hadron distributions
“Collins” – transverse q spin
AColl
e . q .D
A
D . f .P
e .q .D
2
a
sin c
UT
T
h
a
a
a
2
a
NN
a
h
a
a
“Sivers” – transverse q momentum
sin s
AUT
ASiv
f .P
August 21, 2006
2
a
h
e
.
f
.
D
a 1T a
a
2
h
e
.
q
.
D
a a a
a
Distinguishable: different combination
of angles
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Transversity – Collins Asymmetries
Systematic errors are much smaller than statistical errors
Asymmetries compatible with 0
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Transversity – Sivers asymmetries
Systematic errors are much smaller than statistical errors
Asymmetries compatible with 0
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Transversity – L production
L
T
P
e . q( x) .D ( z)
f .P .D
e .q ( x).D ( z )
2
a
T
a
L/a
a
T
2
a
a
L/a
a
Negative trend for Q2>1
August 21, 2006
Deviations from zeroS.not
yet Few-Body
significant
– wait for 2004 data
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18
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Transversity – two hadrons
Selection
August 21, 2006
h+ and h- hadrons
z1, z2 > 0.1
Xf1, xf2 > 0.1
Results still compatibleS. Platchkov,
with 0Few-Body
--- 182004 data in progress…
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August 21, 2006
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