COMPASS plans to measure GPDs
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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
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The COMPASS experiment
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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
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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
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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
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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
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COMPASS and GPDs
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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
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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
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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
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COMPASS
>>LINAC4
sharing CNGS/FT
new Linac 4 up to 10 times more p+improve line
flux at COMPASS in 2010 ?
From Lau Gatignon
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What can we measure now ?
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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
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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
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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(
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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
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• 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
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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)
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Towards a dedicated experiment for DVCS and HEMP
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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
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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
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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
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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:
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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
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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
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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
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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
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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
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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
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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
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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
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SPARE
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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
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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
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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
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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
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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