Feasibility studies for GPDs measurement at COMPASS

1- DVCS studies for the future at COMPASS Physics impact Experimental issues Recoil detector prototype

2- Exclusive 0 production in the present COMPASS data Event Selection Mass distribution Angular distributions

Jacques Marroncle, CEA/Saclay, DAPNIA/SPhN on behalf of the COMPASS collaboration

Spin-03 in Dubna - RussiaSeptember 16th – 20th, 2003

DVCS or Deep Exclusive ρ0 production GPDs

Generalized Parton Distributions

E ,H E, t),ξ, H(x,~~

H(x,0,0) = q(x)

F1(t)

GPDs

x-ξ

P P’

hard

soft

* , Kinematic conditions :

4 quark GPDs :

t

),tξ(x,H dxe q

qq

DIS Q2 and W large finite xBj

Factorization -t << Q2

DIS = forward limit (t→0)

Properties :

/2Bjxξ

x+ξ

High energy cross section

Low energy interference : SSA imaginary part of the amplitude BCA real part of the amplitude

DVCS observables

2 t)ξ,ξ,xπH i (

Deep VCS Bethe-Heitler

Q2=3 GeV2

xB=0.3

(pB/GeV/sr2)

B-H

DVCS

21

1-

t)ξ,ξ,H(x iξ-xt)H(x,ξ

dx ,

)(degθlab*γγ

DVCS

B-H

DVCS

B-Hγγlab

'lab

'lab

*

dΩdΩdk

dσ

P

COMPASS – 200 GeV HERMES – 27 GeV JLAB – 6 GeV

DVCS in the world

H1-ZEUS

Hermes

JLABCOMPASS

Ebeam 820+27 276 (11)

100

190

xBJ0.0001 - 0.01

0.1 –0.3

0.30.5

0.03 - 0.27

σ X XSSA X XBCA X X

COMPASS wide Q2 and xBj range

present future

Projected errors of a possible

DVCS experiment

Ebeam = 100 GeV6 month data taking25 % global efficiency

6/18 (x,Q²) data samples

Very promisingGood constraint for models

Model 1 : simplest ansatzModel 2 : from Goeke et al.(L. Mossé,M. Vanderhaegen)

Beam Charge Asymmetry

polarized target

magnets

RICH

Calorimeters

muon filter

~ 200 detection planesSilicon, SciFi, Micromegas,Drift chambers, GEM, Strawchambers, MWPC

Compass Setup (2002 – 2003)

p

’

100 GeV/c

2.5 m LH2

p ~ 80 GeV/c ~ 1 deg

p > 3 GeV/c < 10 deg

p : 250 → 750 MeV/c : 40 → 70 deg

µ’ and COMPASS equipmentwith additional calorimetry

Need of a new recoil detector to be designed and built

DVCS experimental conditions

{

{

{

(2 108 µ/spill)

Requirements : - Identify and measure protons - Identify neutrals →n+, 0→ - large detector - good hermeticity

+ COMPASS Setup

Recoil Detector Concept

DVCS modelMossé, Vanderhaegen

Source : Pythia 6.1 generated DIS events

Apply DVCS-like cuts final state ’++p in DVCS range no other charged & neutral in active volumes

DVCS is dominant

DVCS background

’

p

Tests performed at COMPASS (summer 2001)

Particle Identification Short Plastic scintillators Energy Loss vs TOF

Target: 10 cm of CH2

~ (2.5m of Liquid H2) Nominal beam conditions

Proof of principle

B

A

Goal : full test of feasibility of a 25 degree sector - scintillating material studies (timing at 200 ps over 4 m!) - fast triggering and multi-hit ADC/TDC system

Funding approved by European CommunityBonn-Mainz-Saclay-Warsaw are participating

Milestones : 2004 – material selection, test bench and in-beam studies 2004/5 – development of fast electronics 2005 – construction >2005 – integration and operation downstream of COMPASS

Recoil detector prototype

Exclusive 0 production with COMPASS data

2002 Data : E = 160 GeV - 2 x 60 cm polarized 6LiD target - semi-inclusive trigger - 1/6 of the data shown

Event selection: - , ’ - 2 hadrons of opposite charge - 0.5 <Mh+h-< 1 GeV - no other particles - >30 GeV, E’>20 GeV - -2 < E < 2.5 GeV - |t| < 0.5 GeV2

1300. 103 all Q² 18. 103 with Q²>1 GeV²

N

2N

2X

2MMM

ΔE

Q²

Number of events (1/6 of 2002 data):

Q²

0 mass distribution

fB)Γ(mimmm

)Γ(mmmA.

dmdN

2

ππρ2ρ

2ππ

ππρππ

ππ

Söding parametrization:

No acceptance corrections !

(subtracted)

0 angular distributions

θ1)cos(3r)r(143

θ) W(cos 20400

0400

cos2ψr2ε12π1

) W(Ψ 111

0400ris the fraction of longitudinal (helicity 0) ’s

Definitions :in *-p CM frame

’

pp’

Assuming s-Channel Helicity Conservation + parity

Φφψ

in rest frame

with

0 angular distributions

● 1/6 of 2002 data● No acceptance corrections applied● qualitative agreement with expected physics

Conclusions & Perspectives

Exciting physics case Theory & experimental communities very active

COMPASS could provide important data DVCS cross section Beam Charge Asymmetry

Exclusive meson production High statistics Studies of s-CHC Double spin asymmetries Hard Exclusive Meson Production

Proposal for DVCS at COMPASS in preparation

Proton detection efficiency

Goal : evaluate inefficiency due to interaction in the target

Inefficiency = 30 %occupancy from Möller electrons

GEANT Simulation: - real attenuation length - segmentation (24 sectors)- vary target shielding thickness

Existing Data

CLAS at JLAB HERMES H1PRL 87 (2001) 182002 PRL 87 (2001) 182001 Phys.Lett.B517 (2001) 47

1

1DVCS ξx

t)H(x,ξdxP σ

,

Cross section

High energy beam

Single Spin Asymmetry

t)ξ,ξ,H(xσσ

Polarised beam

Beam Charge Asymmetry

1

1 ξxt)H(x,ξ

dxPσσ,

+/- charged beam

Deep VCS Bethe-Heitler

Lower energy use interference - holography

2 t) πH(ξ,ξ i ,

DVCS observables

(SSA) (BCA)