1. The neutron counter for the Central Detector of CLAS12
CLAS12 Workshop, Genova, 2/27/08
S. Niccolai, IPN Orsay

2. The neutron counter for the Central Detector of CLAS12
GPDs and nDVCS
Neutron kinematics for nDVCS
Central Neutron Detector for CLAS12
Simulations: expected performances of CND
Ongoing and planned R&D: SiPM, APDs, MCP-PMTs
INFN Frascati,
INFN Genova,
IPN Orsay,
LPSC Grenoble,
SPhN Saclay,
University of Glasgow
CLAS12 Workshop, Genova, 2/27/08
S. Niccolai, IPN Orsay

3. ] [ ò Deeply Virtual Compton Scattering and GPDs x , + ) ( E 2 1 H xdx
Q2= - (e-e’)2
xB = Q2/2Mn n=Ee-Ee’
x+ξ, x-ξ longitudinal momentum fractions
t = (p-p’)2
x  xB/(2-xB) e’ t
(Q2) e
gL*
x+ξ
x-ξ
H, H, E, E (x,ξ,t) ~ g
conserve nucleon helicity p p’
Axial-Vector: H (x,ξ,t)
Pseudoscalar: E (x,ξ,t) ~
Vector: H (x,ξ,t)
Tensor: E (x,ξ,t)
« Handbag » factorization valid
in the Bjorken regime:
high Q2 ,  (fixed xB), t<<Q2
flip nucleon helicity
H(x,0,0) = q(x)
H(x,0,0) = Δq(x) ~
Quark angular momentum (Ji’s sum rule)
«3D» quark/gluon
image of
the nucleon x ] , + ) ( E q ò 2 1 H
xdx
- JG = = J [ -
X. Ji, Phy.Rev.Lett.78,610(1997)

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

5. 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. Guidal)

6. 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)

7. Electromagnetic Calorimeter (PbF2) Active nucleon identified
First measurement of nDVCS: Hall A
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)
Analysis done in the impulse approximation:
Active nucleon identified
via missing mass
Q2 = 1.9 GeV2
xB = 0.36
0.1 GeV2 < -t < 0.5 GeV2
Subtraction of quasi-elastic proton contribution deduced from H2 data convoluted with initial motion of the nucleon
Twist-2

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
Physics and CLAS12 acceptance cuts applied:
W > 2 GeV2, Q2 >1 GeV2, –t < 1.2 GeV2
5° < qe < 40°, 5° < qg < 40°
DVCS/Bethe-Heitler event generator
with Fermi motion, Ee = 11 GeV (Grenoble)
<pn>~ 0.4 GeV/c
More than 80% of the neutrons have q>40°
→ Neutron detector in the CD is needed!
Detected in
forward CLAS
Not detected
ed→e’ng(p)
Detected in
FEC, IC CD
PID (n or g?) + 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
compact readout needed
strong magnetic field
magnetic field insensitive photodetectors (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 underway 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
4 radial layers (each 2.4 cm thick)
30 azimuthal layers (to be optimized)
each bar is a trapezoid (matches CTOF)
inner r = 28.5 cm, outer R = 38.1 cm y x z
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 (~2 times worse than
what obtained from KNU’s TOF measurement)
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 (should 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°
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

13. Count rates computed with nDVCS+BH event generator + CLAS12 acceptance
nDVCS with CLAS12 + CND: expected count rates
<f (°)>
σ(nb GeV 4) N 16
5354 42
1873 74
824
104
520
134
410
165
379
195
377
225
417
256
513
286
835
317
1838
347
0.0182
5432
N = ∆t ∆Q2 ∆x ∆f L Time Racc Eeff
L = 1035cm-2s-1
Time = 80 days
Racc= bin-by-bin acceptance
Eeff = 15% neutron detector efficiency (CND+CTOF+FD)
<t> ≈ -0.4 GeV2
<Q2> ≈ 2GeV2
<x> ≈ 0.17
Dt = 0.2 GeV2 DQ2 =0.55 GeV2
DxB = 0.05 Df = 30°
Count rates computed with nDVCS+BH
event generator + CLAS12 acceptance
(LPSC Grenoble)
→ DN = 1%- 5%

14. Electromagnetic background
Electromagnetic background rates and spectra for the
CND have been studied with Gemc (R. De Vita):
The background on the CND produced by the beam
through electromagnetic interaction in the target
consists of neutrals (most likely photons)
Total rate ~2 GHz at luminosity of 1035 cm-2·s-1
Maximum rate on a single paddle ~ 22 MHz
(1.5 MHz for Edep>100KeV)
This background can be reconstructed as a neutron:
with a 5 MeV energy threshold the rate is ~ 3 KHz
For these “fake” neutrons b< → pn < 0.2 GeV/c
The actual contamination will depend on the
hadronic rate in the forward part of CLAS12
(at 1 KHz, the rate of fake events is 0.4 Hz)
b, for Edep>5 MeV

15. Technical challenge: TOF resolution & B=5T
SiPM - PROS:
Insensitive to magnetic field
High gain (106)
Good intrinsic timing resolution (30 ps/pixel)
Good single photoelectron resolution
SiPM
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 (10K euros/PMT)

16. 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
Plan:
Measure TOF resolution with 2 standard PMTs
Substitute PMT at one end with one SiPM, one APD
Try with a matrix of SiPMs
Redo the same measurements with extruded scintillator (FNAL) + WLS fiber (Kuraray) + SiPM (Stepan’s idea, used in IC hodoscope, ~ x5 more γ’s/mm2)
Test of mchannel PMTs (collaboration with Glasgow)

17. Preliminary results from Orsay’s test bench
σ2test =1/2 (σ2test,trig + σ2test,ref − σ2ref,trig − 4σ2x/c2s)
σ2ref =1/2(σ2test,ref + σ2ref,trig − σ2test,trig − 4σ2x/c2s)
σ2trig =1/2(σ2ref,trig + σ2test,trig − σ2test,ref + 2σ2x/c2s)
Ref
Test
Trig
Single pe
Double pe
Next steps:
Complete measurement of 3×3 mm2 MPPC
Try 5×5 mm2 APDs
Extruded scintillator + WLS fibers + SiPM
Matrix of SiPM (cost?)
Glasgow: in-field tests (5T) for MCP-PMT
test = 1 SiPM Hamamatsu MPPC 1x1 mm2:
sTOF ~ 1.8 ns (~consistent with expectation)
rise time ~ 1 ns
nphe ~1
test = PMT:
sTOF < 90 ps
nphe ~1600
test = 1 SiPM Hamamatsu MPPC 3x3mm2:
rise time ~5 ns (increased capacitance)
more noise than 1x1 mm2, work in progress to get sTOF…
test = 1 APD Hamamatsu 10x10 mm2 + IC preamp:
sTOF ~ 1.4 ns
high noise, high rise time
Thanks toT. Nguyen Trung, B. Genolini
and J. Pouthas (IPN Orsay)

18. Conclusions and outlook
nDVCS is a key reaction for the GPD experimental program: measuring its beam-spin asymmetry can give access to E and therefore to the quark orbital angular momentum
(via the Ji’s sum rule)
A large kinematical coverage is necessary to sample the phase-space, as the BSA is expected to vary strongly
The detection of the recoil neutron is very important to ensure exclusivity, reduce
background and keep systematic uncertainties under control
The nDVCS recoil neutrons are mostly going at large angles (qn>40°), therefore a
neutron detector should be added to the Central Detector, using the (little) available space
LoI submitted to PAC34, encouraged to submit full proposal
Are you interested in detecting neutrons at large angles and p<1 GeV/c?
Are you interested in the photodetectors studies (useful for CTOF too)?
→ You are more than welcome to join in!
CTOF and neutron detector could coexist in one detector, whose first layer can be used
as TOF for charged particles when there’s a track in the central tracker, while the full
system can be used as neutron detector when there are no tracks in the tracker.
Using scintillator as detector material, detection of nDVCS recoil neutrons with
~10-15% of efficiency and n/g separation for p < 1 GeV/c seems possible from simulations,
provided to have ~120 ps of TOF resolution,
The strong magnetic field of the CD and the limited space available call for magnetic-field
insensitive and compact photodetectors: SiPM are a good candidate, but their timing
performances need to be tested
Tests on timing with SiPM and APDs in cosmic rays are underway at Orsay
Ongoing tests for MCP-PMTs in magnetic field at Glasgow University