1. Deeply Virtual Compton Scattering on the neutron at JLab with CLAS12
INFN Frascati,
INFN Genova,
IPN Orsay,
LPSC Grenoble
SPhN Saclay
University of Glasgow
Réunion GDR
Orsay, 11/02/10 e’ t
(Q2) e
gL*
x+ξ
x-ξ
H, H, E, E (x,ξ,t) ~ g p p’
CLAS12 Workshop, Genova, 2/27/08
S. Niccolai, IPN Orsay

2. Extracting GPDs from DVCS spin observables
leptonic plane
hadronic
plane p’ e’ e
A = Ds 2s
s+ - s-
s+ + s- =
x= xB/(2-xB)
k=-t/4M2
DsLU ~ sinf Im{F1H + x(F1+F2)H +kF2E}df ~
Polarized beam, unpolarized proton target:
Kinematically suppressed
Hp, Hp, Ep
Measured
or planned
at JLab
in Hall B
Unpolarized beam, longitudinal proton target:
DsUL ~ sinfIm{F1H+x(F1+F2)(H + … }df ~
Hp, Hp
Unpolarized beam, transverse proton target:
DsUT ~ sinfIm{k(F2H – F1E) + ….. }df
Hp, Ep
Hn, Hn, En ~
DsLU ~ sinf Im{F1H + x(F1+F2)H - kF2E}df
Polarized beam, unpolarized neutron target:
Suppressed because F1(t) is small
Suppressed because of cancellation
between PPD’s of u and d quarks
nDVCS gives access to E,
the least known and
least constrained GPD
that appears in Ji’s sum rule
Hp(ξ, ξ, t) = 4/9 Hu(ξ, ξ, t) + 1/9 Hd(ξ, ξ, t)
Hn(ξ, ξ, t) = 1/9 Hu(ξ, ξ, t) + 4/9 Hd(ξ, ξ, t)

3. Beam-spin asymmetry for DVCS: sensitivity to Ju,d
DVCS on the neutron
Ju=.3, Jd=.1
Ju=.8, Jd=.1
Ju=.5, Jd=.1
Ju=.3, Jd=.8
Ju=.3, Jd=-.5
The asymmetry for nDVCS is:
very sensitive to Ju, Jd
can be as big as for the proton
depending on the kinematics and on Ju, Jd
→ wide coverage needed
f= 60°
xB = 0.17
Q2 = 2 GeV2
t = -0.4 GeV2
Ee = 11 GeV
VGG Model
(calculations
by M. Guidal)

4. Beam-spin asymmetry for DVCS: sensitivity to Ju,d
DVCS on the proton
Ju=.3, Jd=.1
Ju=.8, Jd=.1
Ju=.5, Jd=.1
Ju=.3, Jd=.8
Ju=.3, Jd=-.5
f= 60°
xB = 0.2
Q2 = 2 GeV2
t = -0.2 GeV2
Ee = 11 GeV
VGG Model
(calculations
by M. G.)

5. Electromagnetic Calorimeter (PbF2) Active nucleon identified
First measurement of nDVCS: Hall A
Subtraction of quasi-elastic proton contribution deduced from H2 data convoluted with initial motion of the nucleon
Analysis done in the impulse approximation:
M. Mazouz et al., PRL 99 (2007)
Ee= 5.75 GeV/c Pe = 75 %
L = 4 ·1037 cm-2 · s-1/nucleon e’
HRS e
LH2 / LD2 target 
Electromagnetic Calorimeter (PbF2)
Active nucleon identified
via missing mass
Twist-2
Q2 = 1.9 GeV2
xB = 0.36
0.1 GeV2 < -t < 0.5 GeV2

6. nDVCS in Hall A: results
M. Mazouz et al., PRL 99 (2007)
Q2 = 1.9 GeV2 - xB = 0.36
F. Cano, B. Pire, Eur. Phys. J. A19 (2004) 423
S. Ahmad et al., PR D75 (2007)
VGG, PR D60 (1999)
Im(CIn) compatible with zero (→ too high xB?)
Strong correlation between Im[CId] and Im[CIn]
Big statistical and systematic uncertainties
(mostly coming from H2 and p0 subtraction)

7. Beam-spin asymmetry for DVCS: sensitivity to Ju,d
DVCS on the neutron
Ju=.3, Jd=.1
Ju=.8, Jd=.1
Ju=.5, Jd=.1
Ju=.3, Jd=.8
Ju=.3, Jd=-.5
f= 60°
xB = 0.17
Q2 = 2 GeV2
t = -0.4 GeV2
Ee = 11 GeV
VGG Model
(calculations
by M. Guidal)

8. nDVCS in Hall A: results
M. Mazouz et al., PRL 99 (2007)
Q2 = 1.9 GeV2 - xB = 0.36
F. Cano, B. Pire, Eur. Phys. J. A19 (2004) 423
Model dependent
extraction of Ju and Jd
S. Ahmad et al., PR D75 (2007)
VGG, PR D60 (1999)
Im(CIn) compatible with zero (→ too high xB?)
Strong correlation between Im[CId] and Im[CIn]
Big statistical and systematic uncertainties
(mostly coming from H2 and p0 subtraction)

9. nDVCS with CLAS12: kinematics
More than 80% of the neutrons have q>40°
→ Neutron detector in the CD is needed!
DVCS/Bethe-Heitler event generator
with Fermi motion, Ee = 11 GeV (Grenoble)
Physics and CLAS12 acceptance cuts applied:
W > 2 GeV2, Q2 >1 GeV2, –t < 1.2 GeV2
5° < qe < 40°, 5° < qg < 40°
<pn>~ 0.4 GeV/c CD
Detected in
forward CLAS
Not detected
ed→e’ng(p)
Detected in
FEC, IC
PID (n or g?), p, angles
to identify the final state
pμe + pμn + pμp = pμe′ + pμn′ + pμp′ + pμg
In the hypothesis of absence of FSI:
pμp = pμp’ → kinematics are complete
detecting e’, n (p,q,f), g
FSI effects can be estimated measuring
eng, epg, edg on deuteron in CLAS12
(same experiment)

10. CND: constraints & design
CTOF
Central
Tracker
limited space available (~10 cm thickness)
limited neutron detection efficiency
no space for light guides (in front)
compact readout needed
strong magnetic field (~5 T)
magnetic field insensitive photodetectors (APDs, SiPMs or Micro-channel plate PMTs)
CTOF can also be used for neutron detection
Central Tracker can work as a veto for charged particles
MC simulations done for:
efficiency
PID
angular resolutions
reconstruction algorithms
background studies
Detector design under study:
scintillator barrel

11. Simulation of the CND Geometry: Simulation done with Gemc (GEANT4) y
Includes the full CD
3/4 radial layers (3 if MCP-PMTs are used)
30 azimuthal layers (can still be optimized)
each bar is a trapezoid (matches CTOF)
inner r = 28.5 cm, outer R = 38.1 cm z y x
Reconstruction:
Good hit: first with Edep > threshold
TOF = (t1+t2)/2, with
t2(1) = tofGEANT+ tsmear+ (l/2 ± z)/veff
tsmear = Gaussian with s= s0/√Edep (MeV)
s0 = 200 ps·MeV ½ → σPMT ~ 120 ps for MIPs
β = L/T·c, L = √h2+z2 , h = distance between
vertex and hit position, assuming it at mid-layer
θ = acos (z/L), z = ½ veff (t1-t2)
Birks effect not included (will be added in Gemc)
Cut on TOF>5ns to remove events produced in the
magnet and rescattering back in the CND

12. CND: efficiency, PID, resolution
for a threshold of 5 MeV
and pn = GeV/c
Efficiency: Nrec/Ngen
Nrec= # events with Edep>Ethr.
pn= GeV/c
q = 50°-90°, f = 0°
b distributions (for each layer) for:
neutrons with pn = 0.4 GeV/c
neutrons with pn = 0.6 GeV/c
neutrons with pn = 1 GeV/c
photons with E = 1 GeV/c
(assuming equal yields for n and g)
n/g misidentification
for pn ≥ 1 GeV/c
“Spectator” cut
Dp/p ~ 5%
Dq ~ 1.5°

13. Technical challenge: TOF resolution & B=5T
SiPM
SiPM - PROS:
Insensitive to magnetic field
High gain (106)
Good intrinsic timing resolution (30 ps/pixel)
Good single photoelectron resolution
MCP-PMT
SiPM - CONS:
Very small active surface (1-3 mm2)
→ small amount of light collected (sTOF~1/√Nphel)
Noise
MCP-PMT
APD – PROS:
insensitive to magnetic field
bigger surface than SiPM → more light collected
APD – CONS:
low gain at room temperature
timing resolution?
MCP-PMT – PROS:
resistant to magnetic field ~1T
big surface
timing resolution ~ordinary PMT
MCP-PMT – CONS:
behavior at 5T not yet studied
high cost (2K euros/PMT)
lifetime?

14. Tests on photodetectors with cosmic rays at Orsay
“Trigger” PMTs (Photonis XP2020)
“Reference PMT”
Photonis XP20D0
Scintillator bar (BC408)
80cm x 4 cm x 3 cm
“Trigger” scintillators
(BC408) 1cm thick
Measure TOF resolution with 2 standard PMTs
Substitute PMT at one end with one SiPM, one APD
Test of MCP-PMTs
Redo the same measurements with extruded scintillator (FNAL) + WLS fiber (Kuraray) + SiPM (used in IC ~ x5 more γ’s/mm2)
Test of MCP-PMTs in magnetic field (Saclay, mid November)

15. Results from Orsay’s test bench
T. Nguyen Trung
B. Genolini
S. Pisano
J. Pouthas
Ref
Test
σ2test =1/2 (σ2test,trig + σ2test,ref − σ2ref,trig)
Trig
Test = 1 SiPM Hamamatsu
(MPPC 3x3mm2)
rise time ~5 ns
(> capacitance)
more noise than 1x1 mm2
Single pe
Test = 1 SiPM Hamamatsu
(MPPC 1x1 mm2)
sTOF ~ 1.8 ns
rise time ~ 1 ns
nphe ~1
Test = PMT
sTOF < 90 ps
nphe ~1600
Test = 1 APD Hamamatsu
(10x10 mm2 )
sTOF ~ 1.4 ns
high noise, high rise time
Test = 1 MPPC 1x1mm2
Extruded scint. + WLS fiber
sTOF ~ 1.4 ns
WLS -> Width ~ 15 ns
Test = 1 MCP-PMT
Photonis/DEP (two MCPs)
sTOF ~ 130 ps
will be tested in B field
at Saclay (end of November) 15

16. x~4
Results of tests with
Saclay 17T solenoid
x~3
Drop of a factor of ~3600
(~300x12) from 0 to 5.5 Tesla !

17. Design: J. Bettane
(IPN Orsay)

18. Design: J. Bettane
(IPN Orsay)