1. Speculative first look at neutron detection by (n,p) charge exchange in the central detector Dan Watts – University of Edinburgh

2. Neutron detector/polarimeter: CB at MAMI Scatter point (and therefore  n ) extrapolated from MWPC track. MAID SAID p(  0 )p E  =650 MeV MAID SAID p(  )p  CM =120  15 Photon energy (MeV ) CxCx CxCx D. Watts et. al., Chin. Phys. C 33:1183 (2009)

3. (n,p) in CLAS12 ?? Central detector - excellent proton detection capabilities (micromegas/SVT) Convert a fraction of neutron flux to protons - utilise existing detectors for neutron detection? Neutrons T=200 MeV 100k thrown Simple G4 simulation to take first look: Convertor material

4. 12 C conversion prob. ~2.3% with 2cm 56 Fe conversion prob. ~3% per 2cm PbWO 4 conversion prob. ~2.2% per 2cm 12 C conversion prob. ~4% with 4cm Proton energy (MeV) G4 simulation: 100k incident neutrons 56 Fe conversion prob. ~3% with 2cm PbWO4 conversion prob. ~2.2% with 2cm

5. Convertor placement “options” Would need convertor and tracker in limited space → Not favourable! Convertor placement options - outside

6. Micromegas : ~4cm thick 12 C before first MM cylinder (or replace 1 st cylinder?) SVT : additional convertor material between detector planes? Large acceptance neutron/proton polarimeter for free? Convertor placement options - inside

7. Summary Convertor could be a feasible fall back option for neutron detection Potential to add neutron (and proton) polarimetry to the suite of possibilities with CLAS12 Of course - many issues still to address..!

8. Detrimental side-effects of scatterer material  To hit polarimeter T N >100 MeV in (p,)N above the   Proton energy loss 100 MeV.  Multiple scattering 100 MeV  0.37 radiation lengths  conversion ~ 30% T p incident proton (MeV) T p exit proton (MeV) T p after graphite Energy loss Coulomb scattering Proton energy (MeV) FWHM scattering angle (deg)

9. In micromegas array - replace inner cylinder with ~4cm cylinder of graphite? Additional convertor material between Si detectors (~4cm gap)? Large acceptance neutron/proton polarimeter for free? Convertor placement “options” - inside

10. CND CTOF Central Tracker Convertor option for neutron detector/polarimeter

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

13. INFN Frascati, INFN Genova, IPN Orsay, LPSC Grenoble, SPhN Saclay, University of Glasgow 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 S. Niccolai, IPN Orsay The neutron counter for the Central Detector of CLAS12 CLAS12 Workshop, Genova, 2/27/08

14. SVT BST

15. JJ-Slice

16. BST Support / Cooling Fixture Downstream SideUpstream Side Internal Cooling Channel

17. Deeply Virtual Compton Scattering and GPDs e’ t (Q 2 ) e L*L* x+ξ x-ξ H, H, E, E (x,ξ,t) ~ ~  p p’ « Handbag » factorization valid in the Bjorken regime: high Q 2,  (fixed x B ), t<<Q 2 Q 2 = - (e-e’) 2 x B = Q 2 /2M  =E e -E e’ x+ξ, x-ξ longitudinal momentum fractions t = (p-p’) 2  x B /(2-x B )   0,x  ),(Ex q  2 1 Hxdx q  J G =  2 1 J q  1 1  )0,,( Quark angular momentum (Ji’s sum rule) X. Ji, Phy.Rev.Lett.78,610(1997) Vector: H (x,ξ,t) Tensor: E (x,ξ,t) Axial-Vector: H (x,ξ,t) Pseudoscalar: E (x,ξ,t) ~ ~ conserve nucleon helicity flip nucleon helicity «3D» quark/gluon image of the nucleon H(x,0,0) = q(x) H(x,0,0) = Δq(x) ~

18. Extracting GPDs from DVCS spin observables  LU ~ sin  Im{F 1 H +  (F 1 +F 2 )H +kF 2 E}d  ~ Polarized beam, unpolarized proton target: Unpolarized beam, longitudinal proton target:  UL ~ sin  Im{F 1 H+  (F 1 +F 2 )(H + … }d  ~  = x B /(2-x B ) k=-t/4M 2 H n, H n, E n Kinematically suppressed H p, H p ~ A =           = ~   leptonic plane hadronic plane p’ e’ e  LU ~ sin  Im{F 1 H +  (F 1 +F 2 )H - kF 2 E}d  ~ Polarized beam, unpolarized neutron target: Suppressed because F 1 (t) is small Suppressed because of cancellation between PPD’s of u and d quarks H p, H p, E p ~ nDVCS gives access to E, the least known and least constrained GPD that appears in Ji’s sum rule H p (ξ, ξ, t) = 4/9 H u (ξ, ξ, t) + 1/9 H d (ξ, ξ, t) H n (ξ, ξ, t) = 1/9 H u (ξ, ξ, t) + 4/9 H d (ξ, ξ, t) Unpolarized beam, transverse proton target:  UT ~ sin  Im{k(F 2 H – F 1 E) + ….. }d  H p, E p

19. J u =.3, J d =.1 J u =.8, J d =.1 J u =.5, J d =.1  = 60° x B = 0.2 Q 2 = 2 GeV 2 t = -0.2 GeV 2 Beam-spin asymmetry for DVCS: sensitivity to J u,d VGG Model (calculations by M. Guidal) DVCS on the proton J u =.3, J d =.8 J u =.3, J d =-.5 E e = 11 GeV

20.  = 60° x B = 0.17 Q 2 = 2 GeV 2 t = -0.4 GeV 2 Beam-spin asymmetry for DVCS: sensitivity to J u,d The asymmetry for nDVCS is: very sensitive to J u, J d can be as big as for the proton depending on the kinematics and on J u, J d → wide coverage needed VGG Model (calculations by M. Guidal) DVCS on the neutron J u =.3, J d =.1 J u =.8, J d =.1 J u =.5, J d =.1 J u =.3, J d =.8 J u =.3, J d =-.5 E e = 11 GeV

21. First measurement of nDVCS: Hall A E e = 5.75 GeV/c P e = 75 % L = 4 · 10 37 cm -2 · s -1 /nucleon Q 2 = 1.9 GeV 2 x B = 0.36 0.1 GeV 2 < -t < 0.5 GeV 2 HRS Electromagnetic Calorimeter (PbF 2 ) LH 2 / LD 2 target  e’e’ e Subtraction of quasi-elastic proton contribution deduced from H 2 data convoluted with initial motion of the nucleon Analysis done in the impulse approximation: Active nucleon identified via missing mass Twist-2 M. Mazouz et al., PRL 99 (2007) 242501

22. nDVCS in Hall A: results S. Ahmad et al., PR D75 (2007) 094003 VGG, PR D60 (1999) 094017 M. Mazouz et al., PRL 99 (2007) 242501 Q 2 = 1.9 GeV 2 - x B = 0.36 Im(C I n ) compatible with zero (→ too high x B ?) Strong correlation between Im[C I d ] and Im[C I n ] Big statistical and systematic uncertainties (mostly coming from H 2 and  0 subtraction) Model dependent extraction of J u and J d F. Cano, B. Pire, Eur. Phys. J. A19 (2004) 423

23. nDVCS with CLAS12: kinematics More than 80% of the neutrons have  >40° → Neutron detector in the CD is needed! DVCS/Bethe-Heitler event generator with Fermi motion, E e = 11 GeV (Grenoble) Physics and CLAS12 acceptance cuts applied: W > 2 GeV 2, Q 2 >1 GeV 2, –t < 1.2 GeV 2 5° <  e < 40°, 5° <   < 40° ~ 0.4 GeV/c ed→e’n  (p) Detected in forward CLAS Detected in FEC, IC Not detected PID (n or  ?) + angles to identify the final state CD In the hypothesis of absence of FSI: p μ p = p μ p’ → kinematics are complete detecting e’, n (p, ,  ),  p μ e + p μ n + p μ p = p μ e′ + p μ n′ + p μ p′ + p μ  FSI effects can be estimated measuring en , ep , ed  on deuteron in CLAS12 (same experiment)

24. 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 CND CTOF Central Tracker CND: constraints & design Detector design under study: scintillator barrel MC simulations underway for:  efficiency  PID  angular resolutions  reconstruction algorithms  background studies

25. 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 Reconstruction:  Good hit: first with E dep > threshold  TOF = (t 1 +t 2 )/2, with t 2(1) = tof GEANT + t smear + (l/2 ± z)/v eff t smear = Gaussian with  =  0 /√Edep (MeV)  0 = 200 ps·MeV ½ (~2 times worse than what obtained from KNU’s TOF measurement)  β = L/T·c, L = √h 2 +z 2, h = distance between vertex and hit position, assuming it at mid-layer  θ = acos (z/L), z = ½ v eff (t 1 -t 2 )  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 z y x

26. CND: efficiency, PID, resolution p n = 0.1 - 1.0 GeV/c  = 50°-90°,  = 0 ° Efficiency: N rec /N gen N rec = # events with E dep >E thr. Efficiency ~ 10-16% for a threshold of 5 MeV and p n = 0.2 - 1 GeV/c Layer 1 Layer 2 Layer 3 Layer 4  distributions (for each layer) for: neutrons with p n = 0.4 GeV/c neutrons with p n = 0.6 GeV/c neutrons with p n = 1 GeV/c photons with E = 1 GeV/c (assuming equal yields for n and  ) n/  misidentification for p n ≥ 1 GeV/c “Spectator” cut  p/p ~ 5%  ~ 1.5°

27. nDVCS with CLAS12 + CND: expected count rates σ(nb GeV 4 )N 160.017945354 420.006271873 740.00276824 1040.00174520 1340.00137410 1650.00127379 1950.00126377 2250.00140417 2560.00172513 2860.00279835 3170.006161838 3470.01825432  t = 0.2 GeV 2  Q 2 =0.55 GeV 2  x B = 0.05  = 30° L = 10 35 cm -2 s -1 Time = 80 days R acc = bin-by-bin acceptance E eff = 15% neutron detector efficiency (CND+CTOF+FD) N = ∆t ∆Q 2 ∆x ∆  L Time R acc E eff Count rates computed with nDVCS+BH event generator + CLAS12 acceptance (LPSC Grenoble) ≈ -0.4 GeV 2 ≈ 2GeV 2 ≈ 0.17 →  N = 1%- 5%

28. 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 10 35 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  <0.1-0.2 → p n < 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) , for Edep>5 MeV

29. Technical challenge: TOF resolution & B=5T SiPM - PROS: Insensitive to magnetic field High gain (10 6 ) Good intrinsic timing resolution (30 ps/pixel) Good single photoelectron resolution SiPM - CONS: Very small active surface (1-3 mm 2 ) → small amount of light collected (  TOF ~1/√N phel ) Noise SiPM 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) MCP-PMT

30. 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/mm 2 ) Test of  channel PMTs (collaboration with Glasgow) Tests on photodetectors with cosmic rays at Orsay “Trigger” PMTs (Photonis XP2020) Scintillator bar (BC408) 80cm x 4 cm x 3 cm “Trigger” scintillators (BC408) 1cm thick “Reference PMT” Photonis XP20D0

31. Preliminary results from Orsay’s test bench Single pe Double pe σ 2 test =1/2 (σ 2 test,trig + σ 2 test,ref − σ 2 ref,trig − 4σ 2 x /c 2 s ) σ 2 ref =1/2(σ 2 test,ref + σ 2 ref,trig − σ 2 test,trig − 4σ 2 x /c 2 s ) σ 2 trig =1/2(σ 2 ref,trig + σ 2 test,trig − σ 2 test,ref + 2σ 2 x /c 2 s ) Test Ref Trig test = PMT:  TOF < 90 ps nphe ~1600 test = 1 SiPM Hamamatsu MPPC 1x1 mm 2 :  TOF ~ 1.8 ns (~consistent with expectation) rise time ~ 1 ns nphe ~1 test = 1 SiPM Hamamatsu MPPC 3x3mm 2 : rise time ~5 ns (increased capacitance) more noise than 1x1 mm 2, work in progress to get  TOF … Thanks toT. Nguyen Trung, B. Genolini and J. Pouthas (IPN Orsay) test = 1 APD Hamamatsu 10x10 mm 2 + IC preamp:  TOF ~ 1.4 ns high noise, high rise time Next steps: Complete measurement of 3×3 mm 2 MPPC Try 5×5 mm 2 APDs Extruded scintillator + WLS fibers + SiPM Matrix of SiPM (cost?) Glasgow: in-field tests (5T) for MCP-PMT

32. Using scintillator as detector material, detection of nDVCS recoil neutrons with ~10-15% of efficiency and n/  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 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. 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 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 (  n >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!

33. SiPM (Hamamatsu) 3x3 mm “Reference” PMT Coincidence of “Trigger” PMTs SiPM signal from cosmic rays seen on the oscilloscope Thanks to Thi Nguyen Trung and Bernard Genolini (Orsay) Work on electronics for DAQ is underway