Diffractive ρ° production at COMPASS

Nicole d’Hose, CEA-SaclayOn behalf of the COMPASS collaboration

Results on spin dependence for exclusive ρ° production ρ° spin density matrix elements and R= σL/σT longitudinal double-spin asymmetry A1

ρ

DIS 2006, Tsukuba, Japan, April 20-24, 2006

Description of the diffractive ρ° production

At low energy W < 5 GeV: exchange of 2 quarks or of Reggeon ρ,ω (JP=1-), a2, f2 (JP=2+), a3, f3 (JP=3-), …

At higher energy : exchange of 2 gluons or of Pomeron

Regge theory,pQCD calculations at large Q2…

Experimental observations (NMC, E665, ZEUS, H1, HERMES): the helicity of γ* is approximatively retained by the ρ° meson SCHC the exchange object has natural parity P=(-1)J NPE

W

t

Q 2

*

N N’

μN μ’N’ρπ+π-

COMPASS: 3. 10-3< Q2 < 7 GeV2, <W>=10 GeV, t small

SCHNC

Spin properties of the production amplitudes

Angular Distribution of the production and decay of ρ π+π-

Spin density matrix elements bilinear combinations

of the helicity amplitudes A( γ*(λγ ) ρ(λρ ) ) Tλρ λγ

λγ= 1, 0 λρ= 1, 0

if NPE T- λ ρ -λγ = (-1) λρ-λγ

Tλρ λγ

9 helicity amplitudes reduce to five 5 independent amplitudes :

A(L L), A(TT) >> A(T L) > A(L T) > A(T -T)

i.e. T00 , T11 >> T01 > T10 > T-11

SCHC >> single helicity flip > double helicity flip

Physics from COMPASS:

F. Kunne

+ ΔG/G : K. Kurek A1

d and g1d : M. Stolarski

Transversity: H. Fischer

Λ Polarisation: B. GrubeProspects for GPDs: F-H Heinsius

COMPASS experiment

Veto

SM1 dipoleSM1 dipole

SM2 dipoleHCAL1

Muon-filter1,MW1

Micromegas,DC,SciFi

Gems,SciFi,DCs,straws

MWPC Gems Scifi

trigger-hodoscopes

Silicon

RICH_1 Gem_11

ECAL2,HCAL2

straws,MWPC,Gems,SciFi

straws Muon-

filter2,MW2

DW45

SciFi

Polarised Target 6LiD

~ 250 physicists from 28 institutes

μ 160 GeV from CERN SPS Beam Polarization ~ -76%

1 m

Tw o 6 0 cm lo n g ta rg e t ce lls w ith o p p o site p o la risa tio n

S u p erco n d u c tin g so len o id (2 .5 T )

The polarized 6LiD-Target

4 possible spin combinations:

3He – 4He dilutionrefrigerator (T~50mK)

μ

longitudinal transverse

Reversed once a week

Reversed every 8 hours

SuperconductingSolenoid (2.5T) + Dipole(0.5T)

Target Polarization ~ 50%

Dilution factor f ~ 0.36

Two 60cm long target cells with opposite polarization

Selection of Incoherent exclusive ρ0 production

Incoherent production0.15 < pt²< 0.5 GeV² scattering off a quasi-free nucleon

Assuming both hadrons are 0.5 < M< 1 GeV

M

Emiss

pt²

Exclusivity of the reaction Emiss=(M²X-M²N) /2MN

-2.5 < Emiss < 2.5 GeV

W

t

Q 2*

N N’

Kinematics: ν > 30 GeV Eμ’ > 20 GeV

quasi-free nucleons in 6LiD polarized target

Background ~12%

Kinematical domain of the final sample

<W> = 10.2 GeV

2002 : 800,000 evts 2003 : 1,600,000 evts

+ 2004 : not yet analyzed ~ same statistics as in 2003

Q²> 1GeV² ~60,000 evts

ρ° angular distributions W(cosθ, φ, Φ) depends on the Spin density matrix elements 23 (15) observables with polarized (unpolarized) beam

This analysis yet: - only 2002 data - only one-dimensional angular distribution

ρ° spin density matrix elements

φ

2

01

2

11

2

11TTL

T

2

00

2

010400

TTTN/σσR

δ)R)(ε(1N

Tδ)(εTr

Measurement of r0400

θ1)cos(3r)r(14

3) W(cos 204

000400

Distribution :

Spin density matrix elements:

Tλρ λγ are helicity amplitudesmeson photon

0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q² < 10 GeV2

TotσLσSCHC

2002

Determination of Rρ° =L/T

If SCHC holds :only T00≠0 T11≠0Then :

0400

0400

T

L

r1

r

δ)(ε

1

σ

σR

Impact on GPDs study:- easy determination ofL

dominant at Q2>2 GeV2

-factorisation well esta- blished for L

2003 + 2004

- High statistics from γ-production to hard regime- Better coverage at high Q2 with 2003 and 2004 data

Measurement of r and Im r

φr [φ 2212

1 0411 cos)(

W

041-1

31-1

Distribution :

wea

k vi

olat

ion ?

furt

her s

tudy

with

2003 d

ata

beam polarisation

]sin φPr 212 2311 Im

If SCHC holds

Spin density matrices:

φ

0r

0δ)R)(ε(1N

Tδ)(ε)T(Tr

311

T

10*11-1104

11

...Im

Re 2

2002

2002

A measurable asymmetry can arise from:

- an interference between a tiny Unnatural Parity Exchange contribution (π, a1(1260) Reggeon) with a dominant NPE contribution (ρ, ω, a2(1320), f2 Reggeon or a Pomeron)

- in Generalized Vector Meson Dominance Model: Fraas, NPB113 (1976) if xBj<0.2 + some assumptions

- in pQCD with gluon GPDs at small xBj < 0.01 and large Q2 > 1 GeV2 Goloskokov,Kroll, EPJC42 (2005) and Martin,Ryskin,Teubner, PRD62 (2000) Ryskin, Phys.Atom.Nucl.62 (1999) parton-hadron duality A1

ρ related to generalized spin-dependent gluon distributions

longitudinal double-spin asymmetry A1ρ

2321

2321

//

//

ρ1A

transverse photons and longitudinally polarized nucleons

N

N

ss

ss

23

21

/

/

211211

211211ρ1

1/21/21/21/2

1/21/21/21/2

TT

TTA

1111

1111DIS1

1/21/21/21/2

1/21/21/21/2

TT

TTA

mm

mmif SCHC

DISDISDISρ1 1

211 212 AAAA ))(/(

meas

bt

0LL LL.

.Pf.P1

σσ

σσ)ρN'' N ( AA

cell'upcell'down

cell'upcell'down

celldowncellup

celldowncellup

measLL

N N

N N

NN

NNA

2

1

)ηD( ρ2

ρ1LL AAA

3/21/2

3/21/2ρ1 σσ

σσ

A3/21/2

LTρ2 σσ

σ 2

A

Experimental method (2002+2003 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:η ~10-4 10-2 in COMPASS kinematics η (effects in syst. err.)

Rρ2 A

ρ2A

Depolarization factor D for incoherent exclusive ρ° production

/4)yyR)(12(1))(1/Q2m(1y

/Qm2yy)y/2)(2(1y)QD(y, 22222

μ2

22μ

222

222 /Q R: E665 parametrization

Dilution factor f for incoherent exclusive ρ° production

A DAAD

D

)σ/σ(nn

nf ~~

nD, nA: numbers of nucleons in deuteron and nucleus A in the target

DA σ σ ~~: cross sections per nucleon for exclusive incoherent ρ° production on deuteron and nucleus A

1- )(QPA

2

A σ σ ~

A1ρ (deuteron) from 2002+2003 COMPASS data

Vertical bars: statistical errors

Blue bands: total systematic errors including false asymmetries, uncertainties on Pt, Pb, parametrization of D, f effect of η

Background corrections presently studied and indicating tot. syst. err < stat err.

ρ2A

COMPASS results <W>=10 GeV

A1ρ (deuteron) consistent with 0

extention of the kinematical range by almost 2 decades both in Q2 and xBj

HERMES EPJC29 (2003) <W>= 4 GeV

A1ρ (proton) exhibits

a positive tendency

A1ρ (deuteron) consistent with 0

COMPASS preliminary and HERMES results for A1ρ(d)

at small xBj : small UNPE

Conclusions and perspectives

High statistics data for SDME and R for incoherent exclusive ρ° 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

ρ (deuteron) consistent with zero over a wide Q2 and xBj range

Spin asymmetries for transversally polarized target (also for GPD study)

Exclusive production of and J/ also observed and are studied

0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q²

Statistical error only, limited by MC

Preliminary :

- Corrected for acceptance, smearing and efficiency(MC:DIPSI gen)

- Background not subtracted

0.01 < Q² < 0.05 < Q² < 0.3 < Q² < 0.6 < Q² < 2.0 < Q² < 10 GeV2

Angular distributions for 2002 data

φ

0 mass distribution

PSfBmimmm

mmmA

dmdN

2

22 )(

)(

Söding parametrization:

m

m

q

qm

3

0

)(

No acceptance corrections yet !(subtracted)