Diffractive ?

1
Diffractive ? production at COMPASS
Nicole dHose, CEA-Saclay On behalf of the
COMPASS collaboration
Results on spin dependence for exclusive ?
production ? ? spin density matrix elements and
R sL/sT ? longitudinal double-spin asymmetry
A1?
DIS 2006, Tsukuba, Japan, April 20-24, 2006
2
Description of the diffractive ? production
Regge theory, pQCD calculations at large Q2
At low energy W lt 5 GeV exchange of 2 quarks
or of Reggeon ?,? (JP1-),
a2, f2 (JP2), a3, f3
(JP3-), At higher energy exchange of 2
gluons or of Pomeron
COMPASS 3. 10-3lt Q2 lt 7 GeV2,
ltWgt10 GeV, t small
Experimental observations (NMC, E665, ZEUS, H1,
HERMES) the helicity of ? is
approximatively retained by the ? meson ? SCHC
the exchange object has natural parity
P(-1)J ? NPE
3
Spin properties of the production amplitudes
Angular Distribution of the production and decay
of ? ? pp-
? Spin density matrix elements ? bilinear
combinations
of the helicity amplitudes A( ?(?? ) ??(?? ) )
? T?? ??

• ?? ? 1, 0 ?? ? 1, 0

• if NPE T- ? ? -?? (-1) ??-?? T?? ??

9 helicity amplitudes reduce to five 5
independent amplitudes

• A(L? L), A(T?T) gtgt A(T? L) gt A(L? T) gt
  A(T? -T)

i.e. T00 , T11 gtgt T01
gt T10 gt T-11
SCHC gtgt single helicity flip
gt double helicity flip

• SCHNC

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COMPASS experiment
trigger-hodoscopes
250 physicists from 28 institutes
DW45
straws
SM2 dipole
Muon-filter2,MW2
HCAL1
RICH_1
Gem_11
SM1 dipole
ECAL2,HCAL2
MWPC Gems Scifi
Polarised Target 6LiD
Muon-filter1,MW1

Physics from COMPASS F. Kunne
?G/G K. Kurek A1d and g1d M.
Stolarski Transversity H. Fischer

? Polarisation B.
Grube Prospects for GPDs F-H Heinsius
straws,MWPC,Gems,SciFi
Veto
Gems,SciFi,DCs,straws
Silicon
Micromegas,DC,SciFi
SciFi
160 GeV from CERN SPS Beam Polarization
-76
5
The polarized 6LiD-Target
3He 4He dilution refrigerator (T50mK)
Superconducting Solenoid (2.5T) Dipole(0.5T)
Target Polarization
50 Dilution factor f 0.36
µ
Two 60cm long target cells with opposite
polarization
4 possible spin combinations
longitudinal
transverse

• ? ? ?
• ? ? ?
• Reversed once a week

• ? ?
• ? ?
• Reversed every 8 hours

6
Selection of Incoherent exclusive ?0 production
Assuming both hadrons are p 0.5
lt Mpplt 1 GeV
Mpp

Q 2
W
N
N
Emiss
Exclusivity of the reaction
Emiss(M²X-M²N) /2MN -2.5 lt Emiss lt 2.5 GeV
t
quasi-free nucleons in 6LiD polarized target
Kinematics ? gt 30 GeV Eµ gt 20 GeV
Incoherent production 0.15 lt pt²lt 0.5 GeV²
scattering off a quasi-free nucleon
pt²
Background 12
7
Kinematical domain of the final sample
2002 800,000 evts 2003 1,600,000
evts 2004 not yet analyzed same
statistics as in 2003

Q²gt 1GeV² 60,000 evts
ltWgt 10.2 GeV
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? ? spin density matrix elements
? angular distributions W(cos?, f, F)
depends on the Spin density matrix elements ?
23 (15) observables with polarized (unpolarized)
beam
f
This analysis yet - only 2002 data - only
one-dimensional angular distribution
9
Measurement of r
04 00
0.01 lt Q² lt 0.05 lt Q² lt 0.3 lt Q² lt 0.6 lt
Q² lt 2.0 lt Q² lt 10 GeV2
Distribution
Spin density matrix elements
T?? ?? are helicity amplitudes
meson photon
10
Determination of R? sL/sT
If SCHC holds only T00?0 T11?0 Then

• Impact on GPDs study
• - easy determination of sL
• dominant at Q2gt2 GeV2
• factorisation well esta-
• blished for sL

- High statistics from ?-production to
hard regime
- Better coverage at high Q2 with 2003
and 2004 data
11
Measurement of r and Im r
04 1-1
3 1-1
f
Distribution
2002
beam polarisation
Spin density matrices
2002
If SCHC holds
12
? longitudinal double-spin asymmetry A1?
transverse photons and longitudinally
polarized nucleons

if SCHC
A measurable asymmetry can arise from - an
interference between a tiny Unnatural
Parity Exchange contribution (p, a1(1260)
Reggeon) with a dominant NPE contribution
(?, ?, a2(1320), f2 Reggeon or a Pomeron) - in
Generalized Vector Meson Dominance Model Fraas,
NPB113 (1976) if xBjlt0.2 some
assumptions - in pQCD with gluon GPDs at
small xBj lt 0.01 and large Q2 gt 1 GeV2
Goloskokov,Kroll, EPJC42 (2005) and
Martin,Ryskin,Teubner, PRD62 (2000)
Ryskin,
Phys.Atom.Nucl.62 (1999)
parton-hadron duality
A1? related to generalized spin-dependent gluon
distributions
13
Experimental method
(20022003 data)
Polarized muon beam and target (with 2 cells with
opposite spin configuration)
Relation between muon-N cross asymmetry and
virtual photon-N asymmetry
D depolarization factor
Positivity limit ? 10-4 ?10-2 in COMPASS
kinematics ? ? (effects in syst.
err.)
14
Depolarization factor D for incoherent exclusive
? production
R E665 parametrization
15
Dilution factor f for incoherent exclusive ?
production
nD, nA numbers of nucleons in deuteron and
nucleus A in the target
cross sections per nucleon for exclusive
incoherent ? production on deuteron and
nucleus A
16
A1? (deuteron) from 20022003 COMPASS data
Vertical bars statistical errors Blue
bands total systematic errors including
false asymmetries, uncertainties on Pt, Pb,
parametrization of D, f effect of ?
Background corrections presently studied
and indicating tot. syst. err lt stat err.
17
COMPASS preliminary and HERMES results for A1?(d)
HERMES EPJC29 (2003) ltWgt 4 GeV A1? (proton)
exhibits a positive tendency
A1? (deuteron) consistent with 0
COMPASS results ltWgt10 GeV A1? (deuteron)
consistent with 0 extention of the kinematical
range by almost 2 decades both in Q2 and xBj
at small xBj small UNPE
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Conclusions and perspectives
High statistics data for SDME and R for
incoherent exclusive ? prod. in a wide Q2
range from photo-production to 7 GeV2 weak
violation of SCHC observed Double spin asymmetry
for longitudinally polarized target A1?
(deuteron) consistent with zero over a wide Q2
and xBj range Spin asymmetries for transversally
polarized target (also for GPD study) Exclusive
production of f and J/Y also observed and are
studied
19
(No Transcript)
20
longitudinal double-spin asymmetry A1? in
exclusive ? production at COMPASS
transverse photons and longitudinally
polarized nucleons
A measurable asymmetry can arise from - an
interference between a tiny Unnatural
Parity Exchange contribution (p, a1(1260)
Reggeon) with a dominant NPE contribution
(?, ?, a2(1320), f2 Reggeon or a Pomeron) - in
Generalized Vector Meson Dominance Model Fraas,
NPB113 (1976) if xBjlt0.2 some
assumptions - in pQCD with gluon GPDs at
small xBj lt 0.01 and large Q2 gt 1 GeV2
Goloskokov,Kroll, EPJC42 (2005) and Ryskin,
Phys.Atom.Nucl.62 (1999) A1? related to
spin-dependent gluon generalized parton
distributions
21
Angular distributions for 2002 data
0.01 lt Q² lt 0.05 lt Q² lt 0.3 lt Q² lt 0.6 lt Q² lt
2.0 lt Q² lt 10 GeV2

f
Preliminary - Corrected for acceptance,
smearing and efficiency (MCDIPSI gen) -
Background not subtracted

Statistical error only, limited by MC
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r0 mass distribution
Söding parametrization
No acceptance corrections yet !
(subtracted)