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

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

] [ 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)

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)

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)

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)

Electromagnetic Calorimeter (PbF2) Active nucleon identified 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 < -t < 0.5 GeV2 Subtraction of quasi-elastic proton contribution deduced from H2 data convoluted with initial motion of the nucleon Twist-2

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 Im[CId] and Im[CIn] Big statistical and systematic uncertainties (mostly coming from H2 and p0 subtraction)

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)

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

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

CND: efficiency, PID, resolution for a threshold of 5 MeV and pn = 0.2 - 1 GeV/c Efficiency: Nrec/Ngen Nrec= # events with Edep>Ethr. pn= 0.1 - 1.0 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

Count rates computed with nDVCS+BH event generator + CLAS12 acceptance nDVCS with CLAS12 + CND: expected count rates <f (°)> σ(nb GeV 4) N 16 0.01794 5354 42 0.00627 1873 74 0.00276 824 104 0.00174 520 134 0.00137 410 165 0.00127 379 195 0.00126 377 225 0.00140 417 256 0.00172 513 286 0.00279 835 317 0.00616 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%

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<0.1-0.2 → 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

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)

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)

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)

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