Nucleon spin and structure studies with COMPASS

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Nucleon spin and structure studies with COMPASS
Stephane Platchkov CERN and CEA-Saclay On
behalf of the COMPASS Collaboration
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The COMPASS collaboration

• Composed of
• 230 physicists
• 28 institutions
• 11 countries
• Data taking
• 2002, 2003, 2004
• 2005 SPS stopped

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The nucleon spin puzzle
Quark contribution to the nucleon spin
1990 - 2005 EMC, SMC, Hermes, SLAC DS ?
0.20 0.25 instead of 0.60 (QPM)
?The nucleon spin is more complicated than
anticipated
?
? Study Gluon contribution to the spin ? And also
many other spin observables
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Physics program of COMPASS

• Experiments with hadron beams
• Pion and Kaon polarizabilities
• Diffractive production of exotic states
• Search for glueballs
• Light meson spectroscopy
• Production of double charmed baryons

• 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
• Pentaquarks

Other Compass talks at Panic05 J.Nassalski
Delta-G A.Bressan Transversity-1h R.Joosten
Transversity-2h J.Friedrich Pentaquarks,
hadrons A.Sandacz r production, DVCS
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COMPASS first period (2002 2004)

• Large amount of data collected with muon beam
• Longitudinally polarized target
• About 78 of the total
• Transversely polarized target
• About 22 of the total
• First data with hadron beam
• Pilot hadron run in 2004
• This talk
• Quick overview of available COMPASS data
• Mainly 20022003
• Data from 2004 being analyzed

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The COMPASS Spectrometer
50 m
Two stage spectrometer
Polarized beam and target 75 50
SAT,
LAT,
PID
0.003ltxlt0.5, 10-3ltQ2lt10 GeV2
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Quark spin content inclusive DIS asymmetry
Data taken in 2002 and 2003
? Good agreement with previous experiments ?
Improved statistics at low x (Factor of 2)
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Quark spin content g1d(x) structure function

• Use

Data taken in 2002 and 2003
Phys.Lett. B612(2005)154.
Also new precise proton data from HERMES
New COMPASS QCD fit to the world data on g1(x)
(Q23 GeV2, NLO)
DS(MS) 0.22 0.03 without COMPASS data DS(MS)
0.25 0.02 with COMPASS data
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Semi-inclusive DIS

• Objectives
• Measure semi-inclusive spin asymmetries
• for p, p-, K K- Ks
• Determine individual flavor spin distributions
  (isoscalar target)

Preliminary data 2002 and 2003
? Kinematics range down to x 0.003, Q2gt1 ?
High accuracy data
In progress
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Transversity Collins and Sivers asymmetries
Full description of the nucleon
Nucleon PDFs Unpolarized Helicity
Transversity
Collins probes transverse spin quark
distribution Dtq(x) Sivers probes intrinsic
transverse momentum
Data taken in 2002 only
Collins
PRL 94 (2005) 202002
Sivers
See A.Bressantalk Session III.7
? Asymmetries are compatible with 0 ? Possible
explanation cancellation of DuT and DdT in
isoscalar targets?
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Collins asymmetries near future
COMPASS transverse spin polarimeters Collins
asymmetry Two-hadron correlation L-polarization
Covered by A. Bressan R. Joosten In progress
COMPASS Deuteron Combined 2002-2004 data
COMPASS Proton Expected 2006 data (30d)
? Accurate data to come
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Longitudinal polarization of L and?L hyperons

• Physics in the current fragmentation region
• Spin transfer from q to L
• Test of the strange quark asymmetry of the sea

Data from 2003 only
? With 2004 data much improved statistics
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Determination of DG/G

• Select Photon Gluon Fusion process
• Via Open Charm production
• Detect D ? K-p and D?Dp
• Clean channel (no c quarks)
• High combinatorial background
• Via High-pt hadron pairs
• Good S/B ratio
• Physics background (QCDC, LODIS, Resolved photons
  contributions
• Rely on MC estimates

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Determination of DG/G Two methods
Open Charm
High-pt pairs
Data 20022003
Calculation Glück et al.,Phys. Rev. D63 (2001)
094005
DG2.5
DG0.6
DG0.16
DG/G(xg0.1) 0.024 0.089(stat) 0.057(syst)
Most precise value of DG/G today
Present accuracy 0.73 With 2004 data ? 0.43 With
2006 data ? 0.24 (estimate)
See talk by J.Nassalski, PDF-II
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Search for F-- Pentaquarks
NA49 Peak at 1860 8 MeV

• Several evidence papers on pentaquarks
  published
• Motivated by NA49 findings
• Ali et al. PRL 92 (2004) 042003
• Search for the process

Check of COMPASS analysis Look for
COMPASS 1700 events NA49 150
events
? COMPASS does not confirm NA49
See talk by J.Friedrich, Pentaquarks session
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COMPASS upgrades for 2006

• New solenoid magnet
• Larger acceptance
• 70 mrad ? 180 mrad
• Expect 30 FOM
• RICH upgrade
• Central region maPMT system
• More photons
• Improved S/N
• Outer region New faster electronics
• Improved S/N
• Expect 60 FOM
• Other upgrades
• Large Drift Chamber
• Full ECAL coverage
• Etc

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COMPASS Programme

• The near future (2006 2010)
• Improved COMPASS Factor of Merit
• New OD magnet 1.3
• Upgraded RICH 1.5
• Full ECAL coverage 1.2
• Other 1.1
• Total FoM improvement 2.3
• Strategy
• Strategy muon beam in 2006,
• hadron beam in 2007,
• Precise data to come!
• Long term future (beyond 2010)
• Plans to use DVCS to measure GPD
• Benefit from the high muon energy (190 GeV)
• Try to access orbital momentum L
• Additional detectors being developed

Compass Factor of Merit
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Summary - Results

• New COMPASS results
• Improved measurement of the g1 deuteron structure
  function
• Most precise DG/G measurement up to now
• First COMPASS results on tranversity
• Accurate results on semi-inclusive DIS
• No evidence for pentaquarks
• And a lot of data being analyzed
• Better spectrometer in 2006
• Expected Factor Of Merit increase 2.3
• More precise results are coming.