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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