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

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)

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

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

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

CLAS 12 Recent measurements at Orsay CEA – Orme des Merisiers Dec. 2-3, 2009 B. Genolini, T. Nguyen Trung, J. Pouthas http://ipnweb.in2p3.fr/~detect

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

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

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)

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

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)

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

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 ?