Stephane Platchkov

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• Stephane Platchkov
• DAPNIA-laboratory, CEA-Saclay, France
• For the COMPASS collaboration
• MENU-2007, Forschungszentrum Jülich, September
  10-14, 2007

RECENT RESULTS FROM COMPASS
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OVERVIEW PHYSICS OF COMPASS

• Hadron beams
• Pion and Kaon polarizabilities
• Diffractive and Central production of exotic
  states
• Search for glueballs
• Light meson spectroscopy
• Data taking
• 2002, 2003, 2004, 2006,2007
• 2005 CERN SPS stop

• 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

29,
COMPASS objectives Study of QCD First priority
until now spin structure of the nucleon
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This talk results on nucleon spin structure

• Two COMPASS results
• How large is the gluon contribution to the
  nucleon spin?
• QCD analysis of the inclusive g1(x, Q2) data
  (Q2-evolution)
• Possible now, thanks to the high accuracy of the
  data
• What is the size of the polarized valence quark
  distribution?
• Determined from Semi-Inclusive Deep Inelastic
  Scattering
• Infer also the light quark sea polarization
• Future prospects
• COMPASS program in the next years

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The COMPASS Spectrometer
50 m
Polarized beam and target 75 50
SAT,
LAT,
PID
0.003ltxlt0.5, 10-3ltQ2lt10 GeV2
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• How large is the gluon contribution to the
  nucleon spin?
• (using a QCD fit to world data of g1(x))

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DIS asymmetries World results on A1d
Most recent data from COMPASS 2002 2003
2004 - PLB 647(2000)8

• ? Good agreement with previous experiments
• Much improved statistics at low x
• A1 compatible with 0 at low x

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A1 was also measured for Q2lt1
COMPASS acceptance
Asymmetry compatible with 0 down to x0.0005
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Polarized g1(x) structure function
R
F2
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QCD analysis of world g1(x) data

• From DGLAP evolution equations
• A common fit to the world data allows a
  separation of all flavors a3, a8, DS, Dg !
• Two different fit programs (Next to Leading
  Order)
• World data (except HERMES 2006) from p, d, 3He
  (230 points)
• Parton parameterizations at Q23 (GeV/c)2
• Fit in scheme

• All Pij and Ck are known to NLO
• Mertig, van Neerven, 1996
• Vogelsang, 1996

Polarized g1(x) structure functions
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QCD FIT RESULTS
The fit provides two solutions one with DG gt 0
and one with DG lt 0, All other parton densities
are nearly identical
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QCD fit results
DG lt 0
DG gt 0
Quark contribution to the nucleon spin 0.30
0.02 Total gluon polarization 0.2-0.3 DG lt 0
or DG gt 0 !
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Strange quark distribution
DG lt 0
DG gt 0
Fit result In agreement with a determination
from integral of g1(x)
Strange sea is polarized and negative In
agreement with Hermes (talk of P. Kravchenko)
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The two QCD fit results for DG
We also have 3 direct measurements of DG
(Photon-Gluon Fusion) - High-pt hadron pairs
with Q2gt0 - High pt hadron pairs with Q2lt0 -
Open charm events (D -- Kp)
? Talk by Roland Kuhn, Session 18
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Gluon polarization measurements
The direct measurements of DG(x) in agreement
with the QCD fit
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A provocative digression

• What about the proton spin?
• (in a very naïve model)
• Everything started from here
• Assume DG positive, 0.25
• Assume DG negative, -0.25

W. Pauli
N. Bohr
Pauli and Bohr puzzled by the spin physics in the
30s
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• What is the size/shape of the polarized valence
  quark distribution?
• Is the light quarks sea flavor symmetric?

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Access to the valence spin distribution

• Use hadron asymmetries in semi-inclusive DI
  scattering
• Combine h and h- in difference asymmetry

Q2 range
1 70 GeV

• Advantages (LO)
• Fragmentation functions cancel
• No Particle Identification
• Better statistical errors

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Valence quark spin distribution
De Florian, Navarro, Sassot, 2005
Unpolarized distributions from MRST04 (Martin,
Stirling and Thorne, 2004 At large x, use
inclusive asymmetry (sea very small)
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Comparison with previous experiments
Reasonable agreement, improved statistical errors
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Is the light quark sea symmetric?

• Incluisve DIS measures valencesea contribution
• Semi-inclusive DIS measures valence contribution
• Take the difference through the first moments

Two expectations, depending on the flavor
symmetry of the sea
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First moment (integral) of DuvDvv

• Unmeasured contribution
• (0.7 lt x lt 1.0) 0.004 (est).

HERMES PR D71(2005)012003
Gv 0.410.070.05 Our result favors a
flavor-asymmetric sea (within 2.5s)
This analysis of the HERMES data therefore
favors a symmetric polarized light flavor sea
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Flavor separation with 2006 and 2007 data
Improved statistics, much lower x values
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Outlook and future prospects

• OUTLOOK
• Gluon contribution to the nucleon spin was
  measured
• likely to be small or zero
• Valence spin distribution was determined
• Light quarks flavor-asymmetric polarized sea is
  favored
• (Near-) future prospects
• 2006 Data on the deuteron being analyzed more
  statistics to come
• 2007 Polarized data on the proton (L T) is
  being collected better insight into DIS, SIDIS,
  transversity
• 2008 Program with hadron beams pion
  polarizability (talk by M.-L. Collantioni,
  Thursday, session 28), search for exotics,
  light-meson spectroscopy

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Spare slides
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Shape of parton distributions
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g1(x) for Q2lt1
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Fit results and low x behavior
Leader, Sidorov, Stamenov, Phys.Rev.
D73(2006)034023. Gluck et al., Phys.Rev.
D63(2001)094005. Blümlein Böttcher, Nucl.Phys.B63
6(2002)225.
- Evolution shows little sensitivity to various
fits - Fits of LSS-05, GRSV, BB show similar
trend at low x - Sign of DG is mainly determined
by the low x behavior of g1(x)
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Fit results on p, d, n