1. nDVCS measurement with BoNuS RTPC M. Osipenko December 2, 2009, CLAS12 Central Detector Collaboration meeting

2. 2 z y x Two Alternatives Neutron detectorBoNuS detector Neutron is detected in range: polar angles θ from 35 to 145º, full azimuthal angle φ coverage, 3-momentum p n =0.3-1 GeV/c, resolution Δθ n =1.5º resolution Δφ n =12º resolution momentum Δp n /p n =5% Proton is detected in range: polar angles θ from 35 to 145º, full azimuthal angle φ coverage, 3-momentum p p =70-200 MeV/c, resolution Δθ p =3º resolution Δφ p =1.5º resolution momentum Δp p /p p =few %

3. 3 Kinematics Impulse Approximation: - affected, frame dependent -affected In OPE approximation: - unaffected Beyond Impulse Approximation: - neutron detector and- BoNuS detector - unaffected, but resolution is worse than   leptonic plane photon plane p’ e’ e

4. 4 ISI & FSI Main effect: obtained DVCS cross section is on the off- shell neutron, region of large-x is critical. Main effect: mixing of different physical kinematics in each measured point, region of low-t is critical.

5. 5 Hardware & FSI BoNuS nDVCS Ciofi degli Atti and Kopeliovich, Eur. Phys. J. A17(2003)133 Inclusive BoNuS FSI FSI is small when Both setups allow to suppress FSI via kinematic cuts, provided that neutron is fully reconstructed (momentum and angles).

6. 6 Resolutions in IA - neutron detector - BoNuS detector Neglecting both nucleon momenta with respect to masses and assuming struck neutron going forward one obtains: BoNuS gives better resolution on missing mass. The calculation likely overestimates the ratio of resolutions, but the conclusion sounds sensible.

7. 7 MC simulations Naïve geometrical simulations were performed (no efficiency or CLAS acceptance). The physics model is approximated as a simple function factorized in 4 independent dependencies: 1) 1/y in the range y=0.1-1 2) 1/Q 4 – in the range Q 2 =1-4 GeV 2 3) e bt with b=5 GeV -2 in range from t min to 4 GeV 2 4) Flat φ distibution from 0 to 2  5) Fermi motion with k F =120 MeV Fermi gas model

8. 8 MC

9. 9 Cuts BoNuS proton nDVCS neutron 20 cm long 10 cm R in 66 cm long R=33 cm θ min =45º ? long target 12% effect 8% effect

10. 10 Results We are interested in ratio of yields: - standard CLAS12 luminosity Assume 160 nA beam current, target thickness is 12.6 mg/cm 2 (20 cm x 7 bar pressure D gas) DAQ rate limiting BoNuS RTPC was not estimated here (2 kHz for above conditions is mentioned in NIM A592 for 6 GeV beam energy). EG6 run on ~20 mg/cm 2 target at 130nA with DAQ rate 2.5 kHz.

11. 11 Summary 1.Detection of neutron or spectator proton are equivalent as far Impulse Approximation is concerned, 2.Detection of spectator proton allows to suppress possible FSI effects by the angular cut (with relative loss of statistics), 3.The expected yields of good events for these two scenarios are similar in the ORDER OF MAGNITUDE ESTIMATES. Desirable Improvements 1.Physical cross section in the Monte Carlo model, better momentum distribution in deuteron, 2.Realistic CLAS12 acceptance for e- and , 3.Z-vertex distribution for long target in BoNuS case, 4.Final setup of nDVCS option, 5.Realistic resolutions for both detectors, 6.Physical background to estimate losses in channel identification cuts.

12. 12 The Tile Neutron Detector Neutron incoming direction Light is collected at the back with a large R optic fibre The geometry has been implemented in Geant4

13. 13 The Geant4 Simulations Scintillator without reflective wrapping Scintillator with reflective wrapping

14. 14 The Geant4 Simulations With 0.9 reflectivity 1.5% photons reach the optic fibre Optic fibre transmission not yet implemented Timing has not yet studied Considering 5 MeV threshold, one may expect 50000/2x0.015x0.3x0.2 =20 photoelectrons (assuming 30% of photons arriving to the fiber entrance window at any angle are transmitted to PMT) Timing resolution ~ 1ns/Sqrt(20)~250ps For P n =550 MeV/c  c equal to velocity of light in plastic, and therefore indetermination in the interaction point cancels out.