GPDs studies at COMPASS

DVCS studies for a GPD experiment in 2010 Physics impact Experimental issues Recoil detector prototype

Exclusive 0 production in present COMPASS data Principle Preliminary 2002 results

Etienne Burtin, CEA/Saclay, DAPNIA/SPhN on behalf of the COMPASS collaboration

SIR workshop 2005May 19, 2005 – Newport News, VA

Generalized Parton Distributions

H(x,0,0) = q(x) measured in DIS

F(t) measured in elastic scattering

GPDsx+ x-

P P’

hard

soft

* ,Factorisation:Q2 large, -t<1 GeV2

Generalized Parton Distributionsfor quarks : 4 functions H(x,,t)

t

),dxH(x, t

DVCS observables

1

1

),,(

xtxHdxDVCS P

Cross section

High energy beam

Single Spin Asymmetry

),,( txH

Polarised beam

Beam Charge Asymmetry

1

1

),,(

xtxHdxP

+/- charged beam

Deep VCS Bethe-Heitler

Lower energy => use interference - holography

2),,( tHi

COMPASS muon beam can do all !

Requirements for DVCS:-same energy-maximum intensity-opposite polarisation to a few %-change of beam charge frequent

Solution proposed by Lau Gatignon:1) To select Pπ=110GeV and Pμ=100GeV to maximise the muon flux2) To keep constant the collimator settings which define the π and μ momentum spreads

Pol μ+ = -0.8 and Pol μ- = +0.8 I(μ+) ~ 2. I(μ-) switch in ~10 mins (once per shift?)

Muon section 400mHadron decay section 600m

Compasstarget

scrapers

Be absorbersProtons400 GeV

T6 primary Be target

Collimators1 2 3 4H V H V

1.3 1013 protons/spill2.108 muons/spill

Polarized μ+ and μ- beams

Extraction of GPDs in the case of -μ μ-

t)ξ,ξ,H(xξxt)ξ,H(x,dx

iξxt)ξ,H(x,dx π1

111

A

i -P

DVCS)pp(

dσ(μpμp) = dσBH + dσDVCSunpol

+ Pμ dσDVCSpol

+ eμ aBH Re ADVCS + eμ Pμ aBH Im ADVCS cos nφ sin nφ

φθμ’μ *

p Pμ+=-0.8 Pμ-=+0.8

~σσ t)ξ,ξ,(x H μμ -

11

μμξxt)ξ,H(x,dx ~σσ P-

t, ξ~xBj/2 fixed

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

Kinematical domain

Collider :H1 & ZEUS 0.0001<x<0.01

Fixed target :JLAB 6-11GeV SSA,BCA?HERMES 27 GeV SSA,BCA

COMPASS could provide data on : Cross section (190 GeV) BCA (100 GeV) Wide Q2 and xbj ranges

Limitation due to luminosity

E=19

0, 1

00G

eV

Nx2

new LINAC 4 (SPS injection) in 2010could COMPASS benefit from this ?

BCA

model 1

model 2

COMPASS

Model 2: from Goeke, Polyakov and Vanderhaeghen

sensitivity to the different spatial distribution of partons when xBj

Model 1: H(x,ξ,t) ~ q(x) F(t)

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

= q(x) / xα’ t

Sensitivity of BCA to models

Good sensitivity to models in COMPASS xBj range

Projected errors of a possible DVCS experiment

Ebeam = 100 GeV6 month data taking25 % global efficiency

6/18 (x,Q²) data samples

Good constrain for models

Model 1 : H(x,ξ,t) ~ q(x) F(t)

Model 2 :

Beam Charge Asymmetry L = 1.3 1032 cm-2 s-1

H(x,0,t) = q(x) / xα’t

3 bins in xBj= 0.05, 0.1, 0.26 bins in Q2 from 2 to 7 GeV2

μ’

p’μ

Additionnal equipment to the COMPASS setup

COMPASS equipmentwith additional calorimetry at large angle

Recoil detector to insure exclusivity

to be designed and built

2.5 m Liquid H2 target to be designed and built

ECAL 1 or 2 12°

all COMPASS trackers:SciFi, Si, μΩ, Gem, DC, Straw, MWPC

Physical Background to DVCS

detector requirements: 24° coverage for neutral 50 MeV calorimeter threshold 40° for charged particles

Source : Pythia 6.1 generated DIS eventsApply DVCS-like cuts one ’,,p in DVCS range no other charged & neutral in active volumes

in this caseDVCS is dominant

12°

24°

4m

ECAL0

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

Vector meson production (ρ,ω,…) H & E

Pseudo-scalar production (π,η… ) H & E~ ~

Hρ0 = 1/2 (2/3 Hu + 1/3 Hd + 3/8 Hg)

Hω = 1/2 (2/3 Hu – 1/3 Hd + 1/8 Hg)

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

Single spin asymmetry ~ E/H

for a transverse polarized target

Cross section:

Hard Exclusive Meson Production

Can be investigated

with present COMPASS data

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

Compass Set-up 2002-2003

polarized target

magnets

RICH

Calorimeters

muon filter

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

at CERN 250 physicists 26 institutes

160 GeVpol. beam

Incoherent exclusive 0 sample selection

Exclusivity of the reactionEmiss=(M²X-M²N)/2MN-2.5 < Emiss < 2.5 GeV

Incoherent production:0.15 < pt²< 0.5 GeV² scattering off a quasi-free nucleon

Assuming both hadrons are 0.5 < M< 1 GeV

M

Emiss

pt²

Event topology:

Kinematics :• > 30 GeV• E’ > 20 GeV• Q² > 0.01 GeV²

’

N N’

*0

0 angular distributionsDefinitions :in *-p CM frame

’

pp’

in rest frame

The angular distribution W(cosdepends on the 0 spin density matrix elements 23 (15) measurable with polarised (unpol.) beamThis 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 r0400

θ1)cos(3r)r(143) W(cos 204

000400

201

211

211TTL

T

200

20104

00

TTTN/σσR

δ)R)(ε(1NTδ)(εT

r

Distribution :

Spin density matrix elements:

T are helicity amplitudesmeson photon

Measurement of r and Im r

2cos21 [21)( 04

11 rW

if SCHC holds:0r0r 3

1104

11 Im

041-1

31-1

weak violationbeam polarisation

]2sin12 2311 PrIm

R can be extracted

2002

Determination of R° =L/T

2003+

as SCHC holds …:only T00≠0 T11≠0Then :

0400

0400

T

L

r1r

δ)(ε1

σσR

High statistics from -production to hard regime

better coverage at high Q²more data to come …

Impact on GPD :L is dominant at high Q²

(factorisation only valid for L)

ConclusionsExclusive vector meson production in COMPASS data - preliminary results from 2002 data - allow test of SCHC - shows dominance of L at Q²>2GeV² - more data on tape with better coverage at high Q² to come - insight on GPDs : transverse asymmetry and cross section measurementsTowards 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-005http://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

Gatignoncompass

Task NameLINAC4

Design ref inment

Construction

Commissioning

Start operation with PSB

SPLDesign ref inment

Construction

Linac4 displacement + commissioning

Start operation as PS Injector

12/31

12/30

2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

DVCS+ Bethe Heitler

The high energy muon beam at COMPASSallows to play with the relative contributions DVCS-BHwhich depend on

1/y = 2 mp Eℓ xBj /Q2

Higher energy: DVCS>>BH DVCS Cross section

Smaller energy: DVCS~BHInterference term will provide the DVCS amplitude

φθμ’μ *

p

μp

μp BH calculable