1. CLAS RTPC for 4 He Experiment & Light Nuclei Tagging S. Stepanyan (JLAB) For CLAS/eg6 group Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014

2. 2 Outline Experiments in CLAS/EG6 run – DVCS and meson spectroscopy in coherent scattering Second generation RTPC RTPC calibrations with elastic scattering off 4 He Preliminary results on coherent DVCS off 4 He New Low-Energy Recoil Detector for CLAS12 S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014

3. 3 CLAS/Eg6 Run Meson Spectroscopy and DVCS (coherent and incoherent) on 4 He Both experiments are making use of zero spin and isospin of the target Coherent DVCS on 4 He will allow to extract real and imaginary parts of Compton amplitude in model independent way from BSA measurment Production of  0  and  0  ’ off 4 He limits quantum numbers of exchange state and simplifies PWA analysis Both experiments required detection and identification of recoil  -particles  2 nd Generation Radial Time Projection Chamber with 20 cm long, 6 atm, 4 He gaseous target located inside of the Hall-B superconducting solenoid (used for RTPC momentum analysis and Møller reduction)  Production data with 6.6 GeV polarized electrons, 1 GeV run for RTPC calibration

4. 4 Target & RTPC assembly for eg6 New target cell, 6 mm ID & with 27  m wall thickness, target gas He @ 6 atm. (Bonus run with 50  m thick target cell) He @ 1 atm Working gas, NeDME @ 1 atm Open geometry (has not been utilized effectively) Improved backend readout and increase the trigger rate to 2.5 kHz Total material thickness on the way of particles at 90 o is ~39  m 2 aluminized mylar foils, 2 and 6  m thick S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014

5. 5 , N* Elastic scattering on 4 He at 1.2 GeV RTPC calibration S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014

6. 6 Selection of elastic events S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014 Inclusive electron W- distribution of all events  - vs. Z-RTPC distribution of elastic electron in CLAS and a track in RTPC Difference between measured and calculated from elastic kinematics polar angle of recoil 4 He

7. 7 Δ ϕ CLAS-RTPC (°) RTPC Efficiency: Detect Elastic 4 He With electron is selected, look for RTPC track with matching vertex and scattering angles  Each Δ-quantity below has a cut on the others’ peaks  Resolutions are roughly 8mm, 2°,3° on z,φ,θ Test cut sensitivity, particularly Δθ Result: ~400K Exclusive Elastic Events S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014 ΔZ CLAS-RTPC (cm) Δθ CLAS-RTPC (°) W (GeV) Pass Fail Elastic Cuts Inclusive

8. 8 RTPC Efficiency: Measure Yields The ration of the number of Exclusive and Inclusive Elastic events ( i.e. with and without 4 He detection) will be the RTPC efficiency Shown here is Gaussian signal and backgrounds  Physical models of background (quasi- elastic) and signal (radiated elastic peak) were also tried Efficiency is not highly sensitive to yield extraction method so long as it is consistent for exclusive and inclusive final states S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014 W (GeV) Inclusive Exclusive Example Fits

9. 9 RTPC Efficiency: Result What is the best variable(s)?  Ideally p,θ, ϕ,z simultaneously  Very limited p/θ range for elastics (50 MeV/c, 5°)  ϕ is tricky due to CLAS+RTPC acceptance Z-vertex is also important  Calibration is more difficult at ends of the detector  Sensitive to field effects  We find similar tracking efficiencies for LEFT/RIGHT, although we know noise and # dead channels is different S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014 Z-Vertex (cm) RTPC Efficiency Q 2 (GeV 2 ) Yield LEFT RIGHT

10. 10 RTPC gain calibration with elastic events S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014 PID in RTPC is based on a track energy loss in the drift volume of the detector – calibration of gains of individual pads are very important Large gain variation from pad-to-pad is due to non-uniformity of space between GEM plains

11. 11 DVCS Exclusivity Variables: Simulation vs. Data S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014 θ  X (°) Missing P T (GeV)Coplanarity θ  *He (°) M 2 X (e  4 He) (GeV 2 ) E X (e  4 He) (GeV) e 4 He  e  4 He Full Exclusivity Cuts |θ cop | < 2° |θ  X | < 2° P tX < 0.2 GeV |M 2 X | < 0.02 GeV 2

12. 12 BSA in coherent DVCS - S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014 0 1-d t-bins 83.7% beam polarization is taken into account 1-σ MINOS uncertainties

13. 13 –Significant trends in t and x B, similar to Guzey’s calculation Model from EG6 proposal Kinematics are close but not matched to data Compton Form Factor H He, Re and Im parts S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014 EG6 Range 1 < Q 2 < 2.3 GeV 2 0.05 < -t < 0.2 GeV 2 0.1 < x B < 0.25 GS: Guzey & Strikman, Phys. Rev. C 68 (2003) 015204

14. 14 Results: BSA and Generalized EMC Ratio Beam Spin Asymmetry @ 90  Significantly non-zero and relatively flat ~25%  Consistent with HERMES ((e  X, no 4 He detection) A. Airapetian et al, Phys. Rev. C 81 (2010) 035202 S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014 GS: Guzey & Strikman, Phys. Rev. C 68 (2003) 015204 LT: Liuti & Taneja, Phys. Rev. C 72 (2005) 032201 Generalized EMC Ratio –Binning chosen to match published e1dvcs kinematics for the denominator –We only cover eg1dvcs’s lowest t-bin –A hint of the predicted behavior

15. 15 New LERD for CLAS12 S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014 N. Baltzel (ANL) Low-pressure wire chamber (LPWC), surrounded Si-strip layers, and scintillator fibers Insensitive to MIP, LPWC provides fast signal for triggering and PID All inside low-pressure vessel, vessel inside the solenoid (for simulation CLAS12 solenoid at 60% field has been used Momentum from tracking, PID from energy loss and time information Lessons learned from GEM RTPCs: o big gain variations from pad-to-pad (two mechanically different detector designs, in both the main problem is uniformity of diisdance between GEM planes) - limits PID based on dE/dX o no triggering capability, limits trigger rates, target windows have big effect in the trigger

16. 16 Low energy recoil detector and CLAS12 CD Silicon Tracker Scintillator Counters,  t=60ps 5T SC Solenoid Magnet Replace CLAS12 barrel tracker with LERD Use CTOF to extend momentum range of LERD S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014

17. 17 Particle ID S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014

18. 18 S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014 Summary  CLAS experiment for coherent photo- and electro-production on a spin and isospin zero target, 4 He, run in 2009 (with shorter than PAC approve beam time)  Second generation RTPC à la Bonus detector has been deployed for detection and identification of low energy recoil particles The new RTPC had few improvements – open geometry for a drift volume, lower material budget on a way particles, a much faster trigger rate …  Although Bonus and eg6 RTPCs have very different mechanical design, the non-uniformity of distances between GEM planes remind a main issue for pad gain uniformity and hence was the limiting factor in PID reach  For reliable detection and identification of wide verity of low energy particles - p, d, 3 H, 3 He, and 4 He, a fast timing information from the detector is desirable  Fast timing information can be used in the trigger as well – trigger rate was a limiting factor for high luminosity running in eg6, since with only CLAS trigger the 60% of events did not have RTPC track, and 40% come from target windows  A new detector concept has been developed for CLAS12 based on LPMWC, Si- strips and scintillator fibers, simulations and conceptual design is in progress

19. 19 S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014 Backup slides

20. 20 ForwardCentral Detector Angular range Tracks 5 0 – 40 0 35 0 – 125 0 Photons 2 0 – 40 0 --- Resolution  p/p (%) < 1 @ 5 GeV/c < 5 @ 1.5 GeV/c  (mr)< 1 < 10 - 20  (mr)< 3 < 5 Photon detection Energy (MeV)>150---  (mr)4 @ 1 GeV--- Neutron detection N eff < 0.7 (EC+PCAL)n.a. Particle ID e/  Full range---  p< 5 GeV/c< 1.25 GeV/c  /K< 2.6 GeV/c< 0.65 GeV/c K/p  GeV/c< 1.0 GeV/c    Full range--- CLAS12 – Design Parameters Forward Detector Central Detector S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014

21. 21 Helium bag downstream window (15  m Al) Target downstream window (15  m Al) Target upstream window (  4mm, 15  m thick Al) 4 He at 1 atm Target gas Passage of 100 nA 6 GeV through 30 cm long “straw” target RTPC and high pressure helium target S. Stepanyan, Exploring Hadron Structure with Tagged Structure Functions, Jefferson Lab, January 16-18, 2014