Feasibility studies for DVCS and first results on exclusive r

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Feasibility studies for DVCS and first results on
exclusive r at COMPASS
Etienne Burtin, CEA/Saclay, DAPNIA/SPhN on
behalf of the COMPASS collaboration
DVCS studies Physics impact Experimental
issues Recoil detector prototype Exclusive r0
production Event Selection Mass and
angular distributions
4th Circum-Pan-Pacific Symposium On High Energy
Spin Physics August 4, 2003 - Seattle, WA
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Generalized Parton Distributions
g
g,p,r
Factorisation Q2 large, -tlt1 GeV2
hard
xx
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 in elastic
scattering
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DVCS observables
Deep VCS
Bethe-Heitler
High energy beam
Lower energy gt use interference - holography
Cross section
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Kinematical domain
Collider H1 ZEUS 0.0001ltxlt0.01
Fixed target JLAB 6-11GeV SSA HERMES
27GeV SSA,BCA
COMPASS could provide data on Cross section
(190 GeV) BCA (100 GeV) Wide Q2 and xbj
ranges
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Projected errors of a possible DVCS experiment
Beam Charge Asymmetry
Ebeam 100 GeV 6 month data taking 25 global
efficiency
6/18 (x,Q²) data samples
Model 1 simplest ansatz Model 2 from Goeke et
al.
Very promising Good constrain for models
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Experimental realisation
COMPASS equipment with additional calorimetry
New recoil detector to be designed and built
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Compass Set-up 2002-2003
magnets
muon filter
Calorimeters
200 detection planes Silicon, SciFi,
Micromegas, Drift chambers, GEM, Straw chambers,
MWPC
RICH
polarized target
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Recoil detector
Requirements - Identify and measure
protons - Identify neutrals Dgtnp,
p0gtgg - Be large and hermetic
COMPASS Set-up
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Physical Background
Source Pythia 6.1 generated DIS events
Apply DVCS-like cuts one m,g,p in DVCS range
no other charged neutral in active volumes
DVCS model Mossé, Guichon, Vanderhaegen
DVCS is dominant
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Proof of Principle
Tests performed at COMPASS (summer 2001)
Particle Identification Short Plastic
scintillators Energy Loss vs TOF
Target 60cm of CH2 3x(2.5m of Liquid H2)
Nominal beam conditions
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Recoil Detector Prototype
Goal full test of feasibility of a 45 degree
sector - scintillating material
studies (timing at 200 ps over 4 m)
- fast triggering and multi-hit ADC/TDC
system Candidate to funding by European
Community Bonn-Mainz-Saclay-Warsaw are
participating Milestones 2004 material
research, test bench and in-beam studies 2004/5
development of fast electronics 2005
construction gt2005 integration and operation
downstream of COMPASS
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Exclusive r0 production in COMPASS data
2002 Data E 160 GeV - 2 x 60 cm polarized
6LiD target - semi-inclusive trigger - 1/6 of
the data analysed Event selection - m, m
- 2 hadrons of opposite charge - no other
particles - ngt30 GeV, Emgt20 GeV - -2 lt DE
lt 2.5 GeV - t lt 0.5 GeV
Q²
Q²
Number of events (1/6 of 2002 data)
1300. 103 r0 all Q² 18. 103 r0 with
Q²gt1 GeV²
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r0 mass distribution
Söding parametrization
No acceptance corrections !
(subtracted)
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r0 angular distributions
f
Definitions in g-p CM frame
p
m
p
g
m
r
p
F
in r rest frame
q
p-
Assuming s-Channel Helicity Conservation
with
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r0 angular distributions

• 1/6 of 2002 data
• No acceptance
• corrections applied
• qualitative agreement
• with expected physics

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Conclusions Perspectives
Exciting physics case Theory
Experimental communities very active
Proposal for DVCS at COMPASS in preparation
COMPASS could provide important data
DVCS cross section Beam Charge
Asymmetry Exclusive meson production
High statistics Studies of SCHC
Double spin asymmetries Hard Exclusive
Meson Production
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Proton detection efficiency
Goal evaluate inefficiency due
to m interaction in the target

• GEANT Simulation
• - real attenuation length
• segmentation (24 sectors)
• vary target
• shielding thickness

Inefficiency 30 occupancy from Möller
electrons
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Existing Data
H1
CLAS at JLAB
HERMES
PRL 87 (2001) 182001
Phys.Lett.B517 (2001) 47
PRL 87 (2001) 182002