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GPDs studies at COMPASS

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Etienne Burtin, CEA/Saclay, DAPNIA/SPhN

on behalf of the COMPASS collaboration

DVCS studies for a GPD experiment in 2010

Physics impact

Experimental issues

Recoil detector prototype

Exclusive r0 production in present COMPASS data

Principle

Preliminary 2002 results

SIR workshop 2005

May 19, 2005 – Newport News, VA

g*

g,p,r

Factorisation:

Q2 large, -t<1 GeV2

hard

x+x

x-x

soft

GPDs

P

P’

t

Generalized Parton Distributions

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

measured in DIS

for quarks :

4 functions H(x,x,t)

F(t)

measured inelastic scattering

Single Spin Asymmetry

Beam Charge Asymmetry

Polarised beam

+/- chargedbeam

Deep VCS

Bethe-Heitler

High energy beam

Lower energy=> use interference - holography

Cross section

COMPASS muon beam can do all !

Collimators

1 2 3 4

H V H V

scrapers

T6 primary

Be target

Compass

target

Be absorbers

Protons

400 GeV

Muon section 400m

Hadron decay section 600m

Polarized μ+andμ-beams

- Solution proposed by Lau Gatignon:
- To select Pπ=110GeV andPμ=100GeV
- to maximise the muon flux
- 2) To keep constant the collimator
- settings which define
- the π and μ momentum spreads
- Pol μ+ = -0.8andPol μ- = +0.8
- I(μ+) ~ 2. I(μ-)
- switch in ~10 mins (once per shift?)

Requirements for DVCS:

-same energy

-maximum intensity

-opposite polarisation to a few %

-change of beam charge frequent

2.108 muons/spill

1.3 1013protons/spill

dσ(μpμp) = dσBH + dσDVCSunpol + PμdσDVCSpol

+ eμ aBHReADVCS + eμ PμaBHImADVCS

μ’

*

θ

μ

p

φ

cos nφ sin nφ

Extraction of GPDs in the case of μ+ / μ-

Extraction of GPDs in the case of

t, ξ~xBj/2 fixed

Pμ+=-0.8 Pμ-=+0.8

E=190,

100GeV

Nx2

new LINAC 4 (SPS injection) in 2010

could COMPASS benefit from this ?

Collider :

H1 & ZEUS0.0001<x<0.01

Fixed target :

JLAB 6-11GeV SSA,BCA?

HERMES 27 GeV SSA,BCA

COMPASScould provide data on :

Cross section (190 GeV)

BCA (100 GeV)

Wide Q2 and xbj ranges

Limitation due to luminosity

model 1

model 2

BCA

Sensitivity of BCA to models

Model 1: H(x,ξ,t) ~ q(x) F(t)

Model 2: from Goeke, Polyakov

and Vanderhaeghen

H(x,0,t) = q(x) e t <b2>

= q(x) / xα’t

COMPASS

sensitivity to the different spatial

distribution of partons when xBj

Good sensitivity to models

in COMPASS xBj range

Beam Charge Asymmetry

L = 1.3 1032 cm-2 s-1

Ebeam = 100 GeV

6 month data taking

25 % global efficiency

6/18 (x,Q²) data samples

3 bins in xBj= 0.05, 0.1, 0.2

6 bins in Q2 from 2 to 7 GeV2

Model 1 : H(x,ξ,t) ~ q(x) F(t)

Model 2 :

H(x,0,t) = q(x) / xα’t

Good constrain for models

Additionnal equipment to the COMPASS setup

all COMPASS trackers:

SciFi, Si, μΩ, Gem, DC, Straw, MWPC

μ’

2.5 m Liquid H2 target

to be designed and built

ECAL 1 or 2

12°

COMPASS equipment

with additional calorimetry

at large angle

p’

μ

Recoil detector to insure exclusivity

to be designed and built

Source :Pythia 6.1generated DIS events

Apply DVCS-like cuts

one m’,g,p in DVCS range

no other charged & neutral in active volumes

detector requirements:

24° coverage for neutral

50 MeV calorimeter threshold

40° for charged particles

in this case

DVCS is dominant

24°

ECAL0

12°

4m

Possible solution for the recoil detector proposed in the Workshop on the Future Physics at COMPASS 26 Sept2002

Received funding by EU FP6 (Bonn-Mainz-Saclay-Warsaw )

Goal : full test of a prototype of a 45 degree sector

- scintillating material studies (timing at 200 ps over 4 m)

- fast triggering and multi-hit ADC/TDC system

Also studying a scintillating fibers solution (Monte-Carlo)

with good timing provided by incoming or scattered muons

Hard Exclusive Meson Production

It comes for free with the recoil detector and existing COMPASS trackers

Cross section:

Vector meson production (ρ,ω,…) H & E

Pseudo-scalar production (π,η…) H & E

~

~

Hρ0 = 1/2 (2/3Hu + 1/3Hd + 3/8Hg)

Hω= 1/2 (2/3Hu – 1/3Hd + 1/8Hg)

H = -1/3Hs - 1/8Hg

Can be investigated

with present COMPASS data

Single spin asymmetry ~ E/H

for a transverse polarized target

at CERN

250 physicists 26 institutes

magnets

muon filter

Calorimeters

160 GeV

pol. m beam

~ 200 detection planes

Silicon, SciFi, Micromegas,

Drift chambers, GEM, Straw

chambers, MWPC

RICH

polarized

target

m’

p-

m

Mpp

Assuming both hadrons

are p:

0.5 < Mpp< 1 GeV

r0

g*

p+

N

N’

Emiss

Exclusivity of the reaction

Emiss=(M²X-M²N)/2MN

-2.5 < Emiss < 2.5 GeV

Incoherent production:

0.15 < pt²< 0.5 GeV²

scattering off a

quasi-free nucleon

pt²

Event topology:

- Kinematics :
- n > 30 GeV
- Em’ > 20 GeV
- Q² > 0.01 GeV²

f

Definitions :

in g*-p CM frame

p’

m’

p

g*

m

r°

p+

F

inr° rest frame

q

p-

The angular distribution

W(cosq,f,F)

depends on the r0 spin density matrix elements

23 (15) measurable with polarised (unpol.) beam

This analysis :

one-dimension distributions

test of s-Channel Helicity Conservation SCHC

Distributions are corrected for acceptance.

Background not subtracted.

Statistical error limited by MC sample (except last Q² bin).

04

00

Distribution :

Spin density matrix elements:

Tlg are helicity amplitudes

meson photon

04

1-1

3

1-1

beam polarisation

weak violation

if SCHC holds:

R can be extracted

2002

2003+

as SCHC holds …:

only T00≠0

T11≠0

Then :

Impact on GPD :

sL is dominant at high Q²

(factorisation only valid for sL)

High statistics from

g-production to hard regime

better coverage at high Q²

more data to come …

Exclusive vector meson production in COMPASS data

- preliminary results from 2002 data

- allow test of SCHC

- shows dominance of sL at Q²>2GeV²

- more data on tape with better coverage at high Q² to come

- insight on GPDs :

transverse asymmetry and cross section measurements

Towards a GPD experiment using COMPASS…

- COMPASS is complementary to other experiments

- has good sensitivity to GPD models through BCA

- has good Q² range for 0.03<xbj<0.2

- will be able to measure Hard Exclusive Meson Production

- seeking collaborators…

This initiative is now an “Express of Interest” : SPSC-EOI-005

http://doc.cern.ch//archive/electronic/cern/preprints/spsc/public/spsc-2005-007.pdf

COMPASS should also be a good place to study GPD in 2010 !

Gatignon

compass

μ’

*

θ

μ

p

φ

μ

μ

DVCS+ Bethe Heitler

p

p

BH calculable

The high energy muon beam at COMPASS

allows to play with the

relative contributions DVCS-BH

which depend on

1/y = 2 mp Eℓ xBj/Q2

Higher energy: DVCS>>BH

DVCS Cross section

- Smaller energy: DVCS~BH
- Interference term will provide
the DVCS amplitude