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
CLAS12 Central Detector Meeting
Saclay, 12/02/09 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. 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)

3. CND: constraints & design
CTOF
Central
Tracker
limited space available (~10 cm thickness)
limited neutron detection efficiency
no space for light guides
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

4. Simulation of the CND
Geometry:
Simulation done with Gemc (GEANT4)
Includes the full CD
4 radial layers (or 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 → σ ~ 130 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

5. 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°
Layer 1
Layer 2
Layer 3
Layer 4
“Spectator” cut
Dp/p ~ 5%
Dq ~ 1.5°
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

6. CLAS 12
Recent measurements at Orsay CEA – Orme des Merisiers Dec. 2-3, 2009
B. Genolini, T. Nguyen Trung, J. Pouthas

7. Main issues w h l = 60 cm Requested time resolution < 200 ps RMS
Plastic scintillator (best ≈ 2.5 ns FWHM)
Large number of photoelectrons: > 100
High magnetic field (5T): no PMT
SiPM (MPPC, GAPD, etc.)
APD
MCP PMT
60-cm long scintillator:
Important light losses (wrapping, absorption)
Spread of the photon time distribution w
Plastic scintillator (BC408) h
l = 60 cm

8. The test bench at Orsay Scintillator: 60×3×3 cm^3, BC408
Reference readout PMT (XP20D0)
Coincidence scintillators
Mobile support
Trigger scintillator
Test readout:
PMT or
SiPM
Scintillator: 60×3×3 cm^3, BC408
Trigger: the time reference is taken from the thickest scintillator, validated by the coincidence of the two others
Mobile support to scan the scintillator
Test readout: PMT as the reference, or SiPM (in a box, for shielding)
Ref
Test
Trig

9. Results σ2test =1/2 (σ2test,trig + σ2test,ref − σ2ref,trig)
Thi Nguyen Trung
Bernard Genolini
S. Pisano
J. Pouthas
Ref
Test
σ2test =1/2 (σ2test,trig + σ2test,ref − σ2ref,trig)
Trig
Test = PMT
sTOF < 90 ps
nphe ~1600
Single pe
Test = 1 SiPM Hamamatsu
(MPPC 1x1 mm2)
sTOF ~ 1.8 ns
rise time ~ 1 ns
nphe ~1
Test = 1 SiPM Hamamatsu
(MPPC 3x3mm2)
rise time ~5 ns (> capacitance)
more noise than 1x1 mm2
Test = 1 APD Hamamatsu
(10x10 mm2 )
sTOF ~ 1.4 ns
high noise, high rise time
Test = 1 MCP-PMT Photonis/DEP
(two MCPs)
sTOF ~ 130 ps
tested in B field at Saclay
(end of November)

10. Extruded scint. + WLS fiber
Typical signals
Extruded scintillator made at Triumph
Wavelength shifting fiber (best results with multi cladding): > 10 pe
Measurement with a 1×1 m2 MPPC (Hamamatsu SiPM) and a PMT (Photonis XP20D0)
Time resolution: 1.4 ns RMS
100 ns
PMT averaged signal
20 ns

11. MCP-PMT Double-stage MCP (Photonis-DEP)
Time resolution without magnetic field = 130 ps
Test at CEA under magnetic field: not working at 5 T (amplitude ratio = 10-4)

12. Simulation of the light collection
2 layers
Scint. 1
Scint. 2
Simulations with Litrani
Pulse shapes
Relative light yields
Prototype (0 layer)
Scint. 1
Scint. 2
3 layers
Adjusted on the
Prototype measurements
Scint. 3
Time resolution along the scintillator length

13. Issues
1/Can we obtain ~150 ps time resolution give the existing constraints ? (B-field,
space, photodetector lifetime,…) ? (3m MCP-PMTs, APDs, SiPMs,…)
2/If not, can we afford to give up on the TOF measurement ?
TOF measurement has three purposes:
A/ n/g separation
B/ pn measurement
C/ q measurement
Energy deposition profile (1cm2 scintillator trapezoids) ?
Preshower ? Pulse shape analysis ?
Could measuring only (pe,qe,fe), (pg,qg,fg), (qn,fn)
be enough ?
Additional segmentation in q
3/Measuring the recoil proton instead ?