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GPDs studies at COMPASS. 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 r 0 production in present COMPASS data Principle

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GPDs studies at COMPASS

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Gpds studies at compass

GPDs studies at COMPASS

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


Generalized parton distributions

Generalized Parton Distributions

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


Dvcs observables

Single Spin Asymmetry

Beam Charge Asymmetry

Polarised beam

+/- chargedbeam

DVCS observables

Deep VCS

Bethe-Heitler

High energy beam

Lower energy=> use interference - holography

Cross section

COMPASS muon beam can do all !


Gpds studies at compass

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


Gpds studies at compass

dσ(μpμp) = 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


Kinematical domain

E=190,

100GeV

Nx2

new LINAC 4 (SPS injection) in 2010

could COMPASS benefit from this ?

Kinematical domain

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


Gpds studies at compass

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 <b2>

= q(x) / xα’t

COMPASS

sensitivity to the different spatial

distribution of partons  when xBj 

Good sensitivity to models

in COMPASS xBj range


Projected errors of a possible dvcs experiment

Projected errors of a possible DVCS experiment

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


Gpds studies at compass

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


Physical background to dvcs

Physical Background to DVCS

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


Gpds studies at compass

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


Gpds studies at compass

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


Compass set up 2002 2003

Compass Set-up 2002-2003

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


Incoherent exclusive r 0 sample selection

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

Incoherent exclusive r0 sample selection

Event topology:

  • Kinematics :

    • n > 30 GeV

    • Em’ > 20 GeV

    • Q² > 0.01 GeV²


R 0 angular distributions

r0 angular distributions

f

Definitions :

in g*-p CM frame

p’

m’

p

g*

m

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).


Measurement of r

Measurement of r

04

00

Distribution :

Spin density matrix elements:

Tlg are helicity amplitudes

meson photon


Measurement of r and im r

Measurement of r and Im r

04

1-1

3

1-1

beam polarisation

weak violation

if SCHC holds:

R can be extracted


Determination of r r s l s t

2002

2003+

Determination of Rr° =sL/sT

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 …


Conclusions

Conclusions

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 !


Proton luminosity upgrade at cern

Gatignon

compass

Proton luminosity upgrade at CERN


Gpds studies at 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


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