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
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Experiments with muon beam
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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
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Experiments with hadron beams
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Pion and Kaon polarizabilities
Diffractive and Central
production of exotic states
Search for glueballs
Light meson spectroscopy
Data taking
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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
August 21, 2006
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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
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Specifications
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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
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Polarization: >50%
Field reversal: 8h
Two 60 cm oppositely polarized cells
August 21, 2006
Largest polarized
target
in the
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This talk
Most recent COMPASS results on
 Quark contribution to the nucleon spin
 Gluon contribution to the nucleon spin
August 21, 2006
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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
GL
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
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QCD analysis – fit to g1(x) structure functions
F2 ( x )
g1 ( x )  A1 ( x )
2 x(1  R)
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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 !
August 21, 2006
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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  0.310.10
0.14 ( prg1)
G  0.260.04
0.06 ( prg1)
G  0.180.04
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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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)
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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)
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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+ppslow
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
August 21, 2006
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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
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Q2 > 1 (GeV/c)2
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Leading
Order
pQCD scale, good evaluation of
bgnd, use LEPTO MC
Lower statistics
QCD
Compton
Q2 < 1 (GeV/c)2
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Need to evaluate resolved photon
bgnd, use PYTHIA MC
More model dependent
Very large statistics (x10!)
August 21, 2006
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Resolved g
Q2 < 1 (GeV/c)2
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High-pt hadron pairs – Q2 and pt2 distributions
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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
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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
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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
August 21, 2006
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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 !
August 21, 2006
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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
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Measurement of the g1 deuteron structure function
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QCD NLO fit to most recent COMPASS/World data
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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
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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
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Outlook -- COMPASS Programme
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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
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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
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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
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Full description of the nucleon at LO - 3 nucleon PDFs
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Momentum distribution q(x)
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Helicity distribution q(x)
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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
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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 qh ( Z , M h )
 T q( x ).DqL ( z )
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Transversity in COMPASS
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Experimental method
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Transverse polarization
Field reversal by microwaves
 Once every 5-6 days
Single hadron distributions
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“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
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“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
August 21, 2006
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Transversity – Sivers asymmetries
Systematic errors are much smaller than statistical errors
Asymmetries compatible with 0
August 21, 2006
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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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Transversity – two hadrons
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Selection
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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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