Nicole dHose, CEA Saclay

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after the present COMPASS program ?g,
transversity, hadron spectroscopy
?
For a complete experiment
Nicole dHose, CEA Saclay
I3HP-Topical Workshop- St. Andrews - 31 August
2004
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3-dimensional picture of the partonic nucleon
structure
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Which information from the 3 dimensional picture
( Px,ry,z ) ? 1. Lattice calculation Negele
et al., NP Proc. Suppl. 128 (2004) ? fast
parton close to the N center ? small valence
core ? slow parton far from the N
center ? wider pion cloud 2. Dynamics
approach Strikman and Weiss, PRD69 (2004)
at large distance, gluon density generated by
the pion cloud it depends on xBj if
xBj lt mp/mp0.14 (COMPASS domain) 3. Chiral
quark-soliton model Goeke et al., Prog. Part in
NP47 (2001) Parametrization
based on Regge theory ltb2?gt a
ln 1/x H(x,0,t) q(x) e tltb?2gt
q(x)/xa t
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GPDs and relations to physical observables
?, p, ?, ?
factorization
x?
x-?
t
The observables are integral of GPDs over
Dynamics of partons in the Nucleon
Models Parametrisation
Fit of Parameters on the data
H, ,E, (x,?,t)
ordinary parton density
Elastic Form Factors
Jis sum rule
2Jq ? x(HE)(x,?,0)dx
x
x
H(x,0,0) q(x)
(x,0,0) ?q(x)
? H(x,?,t)dx F(t)
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Goal of an experiment control of the
factorization
Deeply Virtual Compton Scattering (DVCS)

?
?
?
Q2
?
Q2

hard
x ?
x - ?
x ?
x - ?
soft


GPDs
GPDs
Q2 large t small ?
p
p
p
p
t ?2
t ?2
Hard Exclusive Meson Production (HEMP)
meson
Q2
Q2
?
L
L
L
L
hard
x - ?
x ?
soft
GPDs
p
p
t ?2
Quark contribution
Gluon contribution
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Complementarity of the experiments in the world
At fixed xBj, study in Q2
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µ
µ
DVCS Bethe Heitler
p
p
The high energy muon beam at COMPASS allows to
play with the relative contributions
DVCS-BH which depend on 1/y 2 mp El xBj
/Q2
Higher energy DVCSgtgtBH ? DVCS Cross section

• Smaller energy DVCSBH
• Interference term will provide
• the DVCS amplitude

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Advantage of and
for Deeply virtual Compton scattering
(Bethe-Heitler )
t, ?xBj/2 fixed
Pµ-0.8 Pµ-0.8
Diehl
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Deeply Virtual Compton Scattering Beam Charge
Asymmetry (BCA) measured with the 100 GeV muon
beam at COMPASS
BCA
Q24?0.5 GeV2
x 0.05 0.02
BCA
Eµ 100 GeV
x 0.10 0.03
Q2 4 GeV2 x 0.10
efficiency25 6 months data taking
?
f
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Advantage of the kinematical domain of COMPASS
Model 1 H(x,?,t) q(x) F(t)
Model 2 considers that fast partons in the small
valence core slow partons at larger distance
(wider meson cloud)
H(x,0,t) q(x) e tltb?2gt q(x)/xa t
sensitivity ? when xBj ? range of COMPASS
Model 2 of Goeke, Polyakov, Vanderhaeghen
Implementation by Mossé, Vanderhaeghen
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Scaling predictions
hard
soft
Collins et al. (PRD56 1997) 1.factorization
applies only for ? 2. sT ltlt sL
L
?0 largest production present study ?0
?p p - with COMPASS
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Selection of ?
L
Pseudo-scalar meson (spin 0) as p ?
Rosenbluth separation
stotsT e sL
Vector meson (spin 1) as ?0 ? angular
distribution of ?0 ?p p


s-channel-helicity-conservation in p(?, ? )p
L
L
With COMPASS µ Complete angular distribution
? Full control of SCHC
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 Longitudinal  Meson production filter of
GPDs
Cross section
Vector meson production (?,?,?) ? H
E Pseudo-scalat production (p,? ) ? H
E


H?0 1/?2 (2/3 Hu 1/3 Hd 3/8 Hg) H? 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 (can be
realized at COMPASS during transversity
measurement)
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Quark and gluon contributions
NMC 94 E665 97 ZEUS 9395
Projected errors for COMPASS 2003
Gluon contribution
Quark contribution
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General requirements The highest
luminosity Nµ2.108 per SPS cycle (Radio
Protection limit) duration 5.2s
repetition each 16.8s With a new 2.5m long
liquid hydrogen target ? L1.3 1032
cm-2s-1 In 2010, benefit of the high intensity
H- source and of the new Linac-4, what could be
the limitation? The impact of an increased
intensity on the detectors will be studied
presently
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µ and µ- beams

• Solution proposed by Lau Gatignon
• To select Pp110GeV and Pµ100GeV
• to maximise the muon flux
• To keep constant the collimator
• settings which define
• the p and µ momentum spreads
• (fixed Collimators 1H and 3H and
• Scrapers 4V and 5V)
• ? Pol µ -0.8 and Pol µ- 0.8
• To fix Nµ- 2.108 per SPS cycle
• with the 500mm Be T6 target
• and Nµ 2.? Nµ-

Requirements -same energy -maximum
intensity -opposite polarisation to a few
Collimators 1 2 3 4 H V H V
scrapers
T6 primary Be target
Compass target
Be absorbers
Protons 400 GeV
Muon cleaning section 400m
Hadron decay section 600m
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Necessity to complete at large angle the
high resolution COMPASS spectrometer
Many trackers SciFi, Si, µO, Gem,
Straw, MWPC
µ
Deeply Virtual Compton Scattering µp ? µp?
? ECal 1 2 ?? ? 10
p
µ
By a recoil detector to insure the exclusivity of
the reaction
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Resolution needed
At these energies (for µ, µ, and ?) the missing
mass technique is not adapted ?M2required
(mpmp)2-mp2 0.25 GeV2 ?M2observed
Si(?M2/?i ? ?i)2½ with iPµ,Pµ,P?,?µ,?µ,??,
ex ?M2(Pµ) 2 ? 4.10-3 ?100 0.8
GeV2 Q24
X0.1 ?M2(P?) 1.5 ? 3.10-2 ?20
0.9 GeV2 ? ?M2observed
gt 1 GeV2
Need of a recoil detector to insure the
exclusivity
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Competing reactions to DVCS DVCS µp ?
µp? HEpP µp ? µpp ?
?? Dissociation of the proton µp ?
µNp ? Np DIS µp ?µpX
with 1?, 1p, 2p,? Beam halo
with hadronic contamination Beam
pile-up Secondary interactions External
Bremsstrahlung
Selection DVCS/DIS with PYTHIA 6.1 Tune
parameters -maximum angle for photon detection
24 -threshold for photon detection
50MeV -maximum angle for charged particle
detection 40
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Additional equipment to the COMPASS setup
A possible solution (proposed in the Workshop
on the Future Physics at COMPASS 26 Sept 2002)
Goal of the JRA in the EU FP6 Realisation of
a prototype detector consisting of a 45
sector
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Requirements for a recoil detector _at_ COMPASS
Requirements ? be large and hermetic
? identify protons and measure their low
momentum
250 MeV/c ? P ? 750 MeV/c

?
? limited by
thickness of target ideal for proton/pion
separation tmin 0.06
GeV² tmax 0.68 GeV²

(better is 1 GeV2) ? momentum
resolution from 2 to 5 ?
polar and azimuthal angle resolution from 1 to 2
degree ? longitudinal vertex
reconstruction 1 to 2 cm
Economy ? Time of Flight of large volume, of
typical resolution of 200 ps
B L400cm t5cm
A L280cm t4mm
H2 target ? 3cm
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Relation between ToF resolution and dP/P and
dt/t

If ToF resolution 200ps
dt/t 2 dP/P ? 20 bins in t from tmin to 1
GeV2
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What has already been achieved ?
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Tests in the COMPASS environment in 2001
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challenges for the JRA
Tests in 2001 ? Goal of the
prototype LA40,LB70cm
LA280,LB400cm ??2p/48
??2p/24 ?(ToF)300ps
? 200ps ?TA(t4mm)290?30ps ?
140ps! ?TB(t5cm)180?30ps ?
140ps vA13cm/ns?dzA1.9cm ?
1.0cm vB19cm/ns?dzB1.7cm ? 1.3cm

Limitation obtained with CLAS 190-250ps L 3
to 4 m, t5cm
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? Challenges for High Resolution (200ps) Time of
Flight with long scintillators
scintillator t5cm
2 Solutions to study
B
or
scintillator t4mm
A
extra start
Limitations for intrinsic timing resolution
? number of photoelectrons detected (13 ?e for
a 1m fiber of 1mm
650 ?e for a 1m scint of 5cm
thickness) ? dispersion of path length from the
scintillation point to the photocathode ?
scintillation decay time ? transit time spread
of the photomultiplier
Methods

• ? use of fast and efficient scintillator
• with excellent surface reflectivity
  and long attenuation length
• ? comparison scintillating fiber and scintillator
• ? use of fast phototubes with small transit time
  spread,
• with good matching in geometry and
  optics
• ? simulation of light tracks and collection with
  e.g. code as LITRANI
• 
  Cf Michael Seimetz

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? Challenges for electronics and data acquisition
1. Development of analogue electronics and well
adapted light read-out Analyse of the pulse
shape with an Analogue Ring Sampler (ARS)
for high rates and pile-up
2. Processing of many pieces of information
And correlation in a short time (? 1 ?s) ? fast
trigger Use of look-up table based technology
and coincidence matrix system
? Challenges for mechanics
Design of a mechanical solution with a minimum of
distributing material Solution with carbon fiber
sheet and honeycom structure
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Conditions for the success of the recoil detector
prototype To give measurement of t in a
large range from 0 to 1 GeV2 With a very
good resolution numerous bins in t
To work in crowded environment
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Roadmap for GPDs at COMPASS
2004-2009 Present COMPASS experiment with a
polarized target Complete analysis of ?
production ? SCHC study in a large range in
Q2 0.02-30 GeV2 ? E/H measurement with the
transverse polarized target 2004-2006
Realization of the recoil detector prototype
within the JRA tests at MaMi, ELSA, COMPASS
JRA/FP6 Bonn, Mainz and Warsaw universities
and CEA Saclay 24-25 September 2004
Presentation of this future physics at
COMPASS during the SPSC meeting at
Villars
2007-2009 construction of the recoil
detector 2010-2012 GPDs data at COMPASS

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