COMPASS plans to measure GPDs

• The COMPASS experiment • GPDs at COMPASS• now: deep production• ≥2010: DVCS and HEMP

Jean-Marc Le GoffDAPNIA/CEA-Saclay

24 Feb 2006, Albuquerqueworkshop on Orbital Angular Momentum

The COMPASS experiment

The COMPASS Collaboration

(230 Physicists from 12 Countries)

Bielefeld, Bochum, Bonn (ISKP & PI), Erlangen, Freiburg, Heidelberg, Mainz, München (LMU & TU)

Helsinki

CEA-Saclay

Tel Aviv

Burdwan, Calcutta

Torino(University,INFN),

Trieste(University,INFN)

Warsaw (SINS), Warsaw (TU)

CERN

Nagoya

Prag

Dubna (LPP and LNP), Moscow (INR, LPI, State University), Protvino

Lisboa

COMPASS

SPS

LHC

COMPASS fixed target experiment at CERN muons hadrons π/K, p Beam: 2 . 108 µ+/ spill (4.8s / 16.2s) 2 . 108 h/spillBeam polarisation: 80%Beam momentum: 160 GeV/c 150-270 GeV/c

COMPASS COmmon Muon and Proton Apparatus for Structure and Spectroscopy

muon beamnucleon spin structure

Quark and Gluon Polarization in

(longitudinally) polarized nucleons

transverse spin distribution

function Tq(x)

Flavor dependent polarized

quark helicity densities q(x)

Lambda polarisation

Diffractive vector-meson

production

hadron beamsnucleon spectroscopy

Primakoff-Reactions- polarizability of and K

Exotics : glueballs and hybrids

charmed mesons and baryons- semi-leptonic decays- double-charmed baryons

Physics goals

SM1 dipoleSM1 dipole

SM2 dipole

Polarised Target

HCAL1

Muon-filter1,MW1

Micromegas,DC,SciFi

Gems,SciFi,DCs,straws

MWPC Gems Scifi

trigger-hodoscopes

Silicon

RICH_1

BMS

Gem_11

ECAL2,HCAL2

straws,MWPC,Gems,SciFi

straws Muon-

filter2,MW2

DW45

SciFi

Veto

Spectrometer 2002 2004

COMPASS and GPDs

meson

t =Δ2

hard

soft

p p’

γ

GPDs

γ*

x + ξ x - ξ

t =Δ2

Deeply Virtual Compton Scattering (DVCS):

Hard Exclusive Meson Production (HEMP):

Quark contribution

Q2

p p’GPDs

γ*

x + ξ x - ξhard

soft

LLQ2

γγ* Q2

p p’GPDs

x + ξ x - ξ

t =Δ2

meson

p p’GPDs

γ*

x + ξ x - ξ

t =Δ2

Gluon contribution

L

Q2

Q2 large

t << Q2

+ γ*L

Collins et al.

measurement of GPDs

JLabPRL87(2001)

Hermes PRL87(2001)

COMPASS plans H1 and ZEUSPLB517(2001) PLB573(2003)

E=

190,

100G

eV

At fixed xBj, study in Q2

Valence quarks Valence and sea quarksand Gluons

0.0001< xBj < 0.01 Gluons

Complementarity of experiments

if Nμ 2 Q2 < 11 GeV2

E=

190,

100G

eV

At fixed xBj, study in Q2

DVCS limited by luminosity

now Nμ= 2.108μ /SPS spill Q2 < 7.5 GeV2

Benefit of a higher muon intensity for GPDs study

if Nμ 5 Q2 < 17 GeV2

COMPASS

>>LINAC4

sharing CNGS/FT

new Linac 4 up to 10 times more p+improve line

flux at COMPASS in 2010 ?

From Lau Gatignon

What can we measure now ?

1/Q4

1/Q6

vector mesons pseudo-scalar

DVCS, HEMP ?• DVCS small, bckg

• HEMP factorization: L

• vector meson decay R=T/L

• ρ0 largest

• ρ0 π+ π- , charged particules

Vanderhagen et al.:

N

NXmiss M

MME

2

22

W

t

Q 2

*

N N’

μN μ’N’ρπ+π-

Diffractive 0 production

Exp (NMC, E665, Zeus, H1, Hermes):• ≈ S-channel helicity conservation SCHC• exchanged object has natural parity : P=(-1)J NPE

Spin properties of amplitudes

• angular dist. of ρ π+π- : spin density matrix elt

• they are bilinear combinations of helicity amplitudes :

• λγ= 1, 0 λρ= 1, 0 9 amplitudes

• if NPE 5 amplitudes

• T00, T11 >> T01, T10 >> T-11

SCHC 1 helicity flip 2 flips

)()(*

AT

TT )1(

r spin density matrix elt

TotσLσSCHC

Tot

200

20104

00

Tδ)(εTr

~

0400

θ1)cos(3r)r(14

3) W(cos 204

000400

Distribution :

in terms of amplitudes:

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

• If SCHC holds :

0400

0400

T

L

r1

r

δ)(ε

1

σ

σR

2002

2003 + 2004

Determination of R=L/T

• L is dominant at Q2>2

• 2002: high stat in large Q2 range

• 2003 and 2004 data : - much more stat - better high Q2 coverage

Conclusions on rho

• SCHC → R

• tot + R → L

• when Q2 > 2 → R > 1 : accurate L

• we have transv. target spin asym → E/H important for Ji sum rule (∫E+H)

• exploratory measurement (no exclusivity, nuclear target)

Towards a dedicated experiment for DVCS and HEMP

μ’

p’μ

Exclusivity: at high energy M2 = (mp+mπ)2-mp

2] > 1 GeV2

need 0.25 GeV2 for M cut

hermetic detector

ECal 1 or 2

12°

2.5m liquid H2 targetto be designed and built

L = 1.3 1032 cm-2 s-1

Recoil detector to insure exclusivityto be designed and built

+ additional calorimeter at larger angle

Additions to COMPASS setup

12°

30°

4m

ECAL0

Funding by European Union (Bonn-Mainz-Warsaw-Saclay) 45° sector recoil detector - scintillating material studies (200ps ToF Resolution over 4m) - fast triggering and multi-hit ADC/TDC system

2004-2007:

A possible solution

DVCS: μp μp

with PYTHIA 6.1 simulate:• HEπ°P: μp μpπ° • Dissociation of the proton: μp μN*π° Nπ• DIS: μp μpX with 1, 1π°, 2π°,η…

Acceptance cuts:• max= 30°• Emin= 50 MeV• charged

max= 30°

DVCS backgroundnot included:•Beam halo with hadronic contamination•Beam pile-up•Secondary interactions•External Bremsstrahlung

GPD measurement

• xB/(2-xB)

and t are fixed

• loop over x

1

1

1

1 ( , , ) +

H( , , + i (

+ t), , )

DVCS

H

H x tT

x tP d

x

x

x

dx i

p p’

γ

GPDs

x + ξ x - ξ

t =Δ2

*

TDVCS:

Higher energy: DVCS>>BH DVCS Cross section

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

φ

θμ’μ

*

p

μ

p

μ

p

BH calculable

DVCS + Bethe-heitler

DVCS + BH with and .

-μ μ

a

t)ξ,ξ,H(xξx

t)ξ,H(x,dx

iξx

t)ξ,H(x,dx π

1

1

1

1 A

i -P

DVCS)pp(

dσ(μpμp) = dσBH + dσDVCSunpol

+ Pμ dσDVCSpol

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

φ

θμ’μ

*

p

Pμ+=-0.8 Pμ-=+0.8

~σσ t)ξ,ξ,(x H μμ a

1

1

μμ

ξx

t)ξ,H(x,dx ~σσ Pa

t, ξ~xBj/2 fixed

x = 0.05 ± 0.02

x = 0.10 ± 0.03

BCA

BCA

φ

Q2=40.5 GeV2

Model 2:

Model 1: H(x,0,t) ~ q(x) F(t)

H(x,0,t) = q(x) e t <b

2>

= q(x) / xα’t

1

1

μμ

ξxt)ξ,H(x,

dx ~σσ Pa

φφassuming:• L = 1.3 1032 cm-2 s-1

• 150 days• efficiency=25%

2 bins shown out of 18:•3 bins in xBj= 0.05, 0.1, 0.2•6 bins in Q2 from 2 to 7 GeV2

at 100 GeV μμ σσa

model 1

COMPASS

model 2

advantage of COMPASS kinematics

Model 1: H(x,0,t) ~ q(x) F(t)

Model 2: H(x,0,t) = q(x) e t

<b

2>

= q(x) / xα’t

sensitive to different spatial distributions at different x

1

1

μμ

ξxt)ξ,H(x,

dx ~σσ Pa

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

Transverse single spin asymmetry : ~ E/H

Cross section:

HEMP: filter of GPDs

HEMP in 2010

HEMP requires higher Q2 than DVCS

with liq H target and same flux as now:• Q2= 20 GeV2

• Q2= 7 GeV2

if more higher Q2

2004-2009: production: L and transverse spin asym

2005 : "Expression of Interest": SPSC-E0I-005

2004-2007: recoil detector prototype

2008 ? : proposal

2007-2009: construction of the recoil detector cryogenic target, ECal0 2010: dedicated GPD measurement

Roadmap for DVCS at COMPASS

Measure OAM ?

• Ji sum rule relates total quark spin to GPDs H and E :

1

1

)0,,()0,,(2

1

2

1 xExHxdxLJ qqqq

similar sum rule for gluonsTo do :1. Measure H and E2. Perform flavor decomposition 3. Extrapolate to t=04. Integrate over x at fixed

• Sivers asymmetries are also measured at COMPASS

SPARE

Careful study of and π0 production COMPASS program with hadron beam

ECAL2 from 0.4 to 2°: mainly lead glass GAMSECAL1 from 2 to 12° : good energy and position resolution for 2- separation in a high rate environment

ECAL0 from 12 to 30°: to be designed for background rejection

Key role of calorimetry

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

This analysis: only one-dimensional angular distribution

We will use also: ψ= φ - Φ

φ

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

φ

0r

0Tδ)(ε)T(T

r

311

Tot

10*11-1104

11

...

~

Im

Re

2

Measurement of r and Im r

φr [φ 2212

1 0411 cos)(

W

041-1

31-1

Distribution :

wea

k vi

olat

ion

beam polarisation

]sin φPr 212 2311 Im

If SCHC holds

Spin density matrices:

φ

HERMES

<x>=0.11 <Q2>=2.6

pepe

one single bin

COMPASS 6 angular distributions among 18: 3 bins in xBj=0.05, 0.1, 0.2 6 bins in Q2 from 2 to 7 GeV2

BCA in DVCS projections for 1 year