The neutron counter for the Central Detector of CLAS12

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The neutron counter for the Central Detector of
CLAS12
S. Niccolai, IPN Orsay
CLAS12 Workshop, Genova, 2/27/08
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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 RD SiPM, APDs, MCP-PMTs

S. Niccolai, IPN Orsay
CLAS12 Workshop, Genova, 2/27/08
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Deeply Virtual Compton Scattering and GPDs

• Q2 - (e-e)2
• xB Q2/2Mn nEe-Ee
• x?, x-? longitudinal momentum fractions
• t (p-p)2
• x ? xB/(2-xB)

e
conserve nucleon helicity
p
p
Vector H (x,?,t) Tensor E (x,?,t)
 Handbag  factorization valid in the Bjorken
regime high Q2 , ? (fixed xB), tltltQ2
flip nucleon helicity
Quark angular momentum (Jis sum rule)
3D quark/gluon image of the nucleon
X. Ji, Phy.Rev.Lett.78,610(1997)
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Extracting GPDs from DVCS spin observables
x xB/(2-xB) k-t/4M2
Polarized beam, unpolarized proton target


DsLU sinf ImF1H x(F1F2)H kF2Edf
Hp, Hp, Ep
Kinematically suppressed
Unpolarized beam, longitudinal proton target


Hp, Hp
DsUL sinfImF1Hx(F1F2)(H df
Unpolarized beam, transverse proton target
Hp, Ep
DsUT sinfImk(F2H F1E) .. df
Polarized beam, unpolarized neutron target


Hn, Hn, En
DsLU sinf ImF1H x(F1F2)H - kF2Edf
Suppressed because F1(t) is small
nDVCS gives access to E, the least known and
least constrained GPD that appears in Jis sum
rule
Suppressed because of cancellation between PPDs
of u and d quarks
Hp(?, ?, t) 4/9 Hu(?, ?, t) 1/9 Hd(?, ?,
t) Hn(?, ?, t) 1/9 Hu(?, ?, t) 4/9 Hd(?, ?, t)
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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)
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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

• 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

Ee 11 GeV
VGG Model (calculations by M. Guidal)
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First measurement of nDVCS Hall A
M. Mazouz et al., PRL 99 (2007) 242501
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 lt -t lt 0.5 GeV2
Twist-2
Subtraction of quasi-elastic proton contribution
deduced from H2 data convoluted with initial
motion of the nucleon
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nDVCS in Hall A results
M. Mazouz et al., PRL 99 (2007) 242501
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) 094003
VGG, PR D60 (1999) 094017
Im(CIn) compatible with zero (? too high
xB?) Strong correlation between ImCId and
ImCIn Big statistical and systematic
uncertainties (mostly coming from H2 and p0
subtraction)
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nDVCS with CLAS12 kinematics
Physics and CLAS12 acceptance cuts applied W gt
2 GeV2, Q2 gt1 GeV2, t lt 1.2 GeV2 5 lt qe lt
40, 5 lt qg lt 40
DVCS/Bethe-Heitler event generator with Fermi
motion, Ee 11 GeV (Grenoble)
ltpngt 0.4 GeV/c
More than 80 of the neutrons have qgt40 ?
Neutron detector in the CD is needed!
Detected in forward CLAS
Not detected
ed?eng(p)
Detected in FEC, IC
PID (n or g?) angles to identify the final
state
CD
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)
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CND constraints design

• 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
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Simulation of the CND

• Geometry
• Simulation done with Gemc (GEANT4)
• 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 gt threshold
• TOF (t1t2)/2, with
• t2(1) tofGEANT tsmear (l/2 z)/veff
• tsmear Gaussian with s s0/vEdep (MeV)
• s0 200 psMeV ½ (2 times worse than
• what obtained from KNUs TOF measurement)
• ß L/Tc, L vh2z2 , h distance between
• vertex and hit position, assuming it at mid-layer
• ? acos (z/L), z ½ veff (t1-t2)
• Birks effect not included (should be added in
  Gemc)
• Cut on TOFgt5ns to remove events produced in the
  magnet and rescattering back in the CND

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CND efficiency, PID, resolution
Efficiency 10-16 for a threshold of 5 MeV and
pn 0.2 - 1 GeV/c
Efficiency Nrec/Ngen Nrec events with
EdepgtEthr.
pn 0.1 - 1.0 GeV/c q 50-90, f 0
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
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nDVCS with CLAS12 CND expected count rates
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
  (CNDCTOFFD)

lttgt -0.4 GeV2 ltQ2gt 2GeV2 ltxgt 0.17
Dt 0.2 GeV2 DQ2 0.55 GeV2 DxB 0.05 Df 30
Count rates computed with nDVCSBH event
generator CLAS12 acceptance (LPSC Grenoble)
? DN 1- 5
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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-2s-1
• Maximum rate on a single paddle 22 MHz
• (1.5 MHz for Edepgt100KeV)
• This background can be reconstructed as a
  neutron
• with a 5 MeV energy threshold the rate is 3 KHz
• For these fake neutrons blt0.1-0.2 ? pn lt 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 Edepgt5 MeV
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Technical challenge TOF resolution B5T
SiPM

• SiPM - PROS
• Insensitive to magnetic field
• High gain (106)
• Good intrinsic timing resolution (30 ps/pixel)
• Good single photoelectron resolution

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)

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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
  (Stepans idea, used in IC hodoscope, x5 more
  ?s/mm2)
• Test of mchannel PMTs (collaboration with
  Glasgow)

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Preliminary results from Orsays test bench
s2test 1/2 (s2test,trig s2test,ref -
s2ref,trig - 4s2x/c2s) s2ref 1/2(s2test,ref
s2ref,trig - s2test,trig - 4s2x/c2s) s2trig
1/2(s2ref,trig s2test,trig - s2test,ref
2s2x/c2s)
Test
Ref
Trig
Double pe

• Next steps
• Complete measurement of 33 mm2 MPPC
• Try 55 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 1 SiPM Hamamatsu MPPC 3x3mm2
• rise time 5 ns (increased capacitance)
• more noise than 1x1 mm2, work in progress to get
  sTOF

• test PMT
• sTOF lt 90 ps
• nphe 1600

• 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)
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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 Jis 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 (qngt40), 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 plt1 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 theres 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 lt
  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

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SiPM signal from cosmic rays seen on
the oscilloscope
Work on electronics for DAQ is underway
Thanks to Thi Nguyen Trung and Bernard
Genolini (Orsay)