1. Physics with low energy recoil detector and options for instrumentation for the CLAS12
S. Stepanyan (JLAB)
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

2. Few physics topics with low energy recoil ( ) detector
neutron structure
nuclear structure (see also K. Joo’s talk)
meson spectroscopy
Options for a recoil detector
RTPC
LPMWPC&SSD
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

3. Neutron target and accessible physics
Free neutrons decay in 15 min
Can we explore the structure of the neutron if all we have are neutrons bound in nuclei? – nuclear model uncertainties
Low energy spectator tagging can be a solution for a neutron target:
PDF at large x
d/u at large x
quark-hadron duality on neutron
neutron elastic scattering
neutron DVCS
S. Bueltman et al., E
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy 3

4. d/u ratio at x  1 F2n/F2p = F2d/F2p -1 ??? CLAS12 European Workshop
February 25-28, 2009, Genova, Italy

5. Modifications to Simple Spectator Picture
Binding Effects
“BoNuS”
Modifications to Simple Spectator Picture
Final State Interactions
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

6. Spectator Tagging with BoNuS RTPC
E = GeV e p n
<Q2> = 1.19 (GeV/c)2
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy * 6

7. DVCS on nuclei Coherent DVCS
Incoherent DVCS
Coherent DVCS
Nuclear GPDs - partonic structure of nuclei,
momentum and spatial distribution of quarks in nuclei, generalized EMC effect
Incoherent DVCS
bound nucleon GPDs – nuclear medium effects, modification with respect to free nucleon ones
Both will require low energy recoil detector
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy 7

8. Nuclear DVCS Coherent and incoherent were not cleanly separated.
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy 8

9. CLAS experiment E08-024 (K. Hafidi et al.)
For nucleon DVCS:
4He nucleus
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy 9

10. Meson spectroscopy
Production on hydrogen - physics background from associated baryon resonance production
CLAS/g6: gp’np+p+p-
D(1232)
N(1520)
N(1680)
D, N*
a2(1320)
Strong kinematical cuts and a complex PWA are required
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

11. Spectroscopy in coherent production on nuclei
Method to eliminate the baryon background
All three charged modes can be studied using gas targets 3H, 3He, and 4He
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

12. Example: coherent production on 4He
Scattering off a spin and isospin zero target works as a spin and parity filter
with
JLAB experiment E07-009, Stepanyan et al.
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

13. Coherent production of p0h and p0h’ off 4He
Only C=-1 w-exchanges is allowed, Natural Parity Exchange (NPE)
The helicity of the produced state will be that of the incoming photon, l=lg, production of p0h (p0h’) in S- wave is forbidden
Test for above assumptions
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

14. Silicon Vertex Tracker Scintillator Counters, dt=50ps
CLAS12 central detector
5T SC Solenoid Magnet
Silicon Vertex Tracker
Scintillator Counters, dt=50ps
Low (200 MeV/c) energy protons in CLAS12 CD (M. Ungaro, GEMC)
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

15. g g p0 p0 qgg qgg CLAS12 Central detector
Protons
Deuterons
Specialized detector is needed for a broad physics program requiring low energy recoil detection
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy p0 g
qgg p0 g
qgg

16. BoNuS* RTPC** 7 atm D2 gas BoNuS Region 0.07 0.2 GeV/c
Helium/DME at 80/20 ratio
7 atm D2 gas
BoNuS
Region
VIPs
GeV/c
Drift Region
Gas Electron Multiplier
Readout pads and electronics
*BoNuS = Barely off-shell Nucleon Scattering
**RTPC = Radial Time Projection Chamber
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy 16

17. Performance and limitations of BoNuS RTPC
e- reconstructed in CLAS & RTPC
dE/dx from charge along the track   z
=8mm
=4º
=1.4º
Reasonable 3-vector reconstruction
Reasonable PID for a proton
Low rate of DAQ - ~500Hz
Poor PID for high mass fragments – gain uniformity of GEM pads
proof that the proton and electron had a common interaction point, this will be much cleaner when calibration is perfected (CLAS and BoNuS)
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

18. BoNuS-II (for EG6 run in October)
Lightweight (carbon-foam composite structure) rungs for stress-free, self support of GEMs – to avoid “wrinkles”
Ground and cathode foils, each 6mm thick
Target region - new cell – ID 4mm, 30mm thick wall
Open, 2p geometry – only 80% is accessible due to the GEM and readout pad sizes
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

19. Expected performance of BoNuS-II
Better gain uniformity – better PID:
new GEM foils
stress free support
calibration system (a-source and elastic scattering)
Lower momentum threshold (250 MeV for a’s)
thinner target walls
Increased DAQ rate kHz to 5 kHz
new RCUs, crate, backplane … work is done at INFN Genova (R. De Vita, M. Battaglieri, S. Minutoli)
Expected PID with fully calibrated RTPC
proof that the proton and electron had a common interaction point, this will be much cleaner when calibration is perfected (CLAS and BoNuS)
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

20. DAQ with new Readout Control Unit board
Rx - PASA - ALTRO
Max event rate 7 kHz
FEC
128 ch 13
Front-end bus
( 200 MB / s )
FEC
128 ch 2
RCU
FEC
128 ch 1
Data Proc.
and Memory Tx
Bus controller
( conf. & R/O )
DDL ( 200 MB/s )
DETECTOR
DAQ int.
(DDL-SIU)
DCS ( 1 MB/s )
FEC
128 ch 13
BOARD
Controller
DCS int.
(Ethernet)
Local Monitor
and Control
TTC optical Link
(Clock, L1 and L2 )
Trigger
and Timing
circuit
Trigger int.
(TTC-RX)
FEC
128 ch 2
Control Network
(I2C-serial link)
FEC
128 ch 1
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

21. LPAC&SSD* (NIKHEF NIM A409, 1998)
MeV H
3.2 2H
4.2 3H
4.9
3He 11
4He 13
Sci E
SiY DE or E, y H 2H 3H
3He
4He
8.8 12 14 31 35
MeV
SiX DE or E, x
7 mbar
AC DE, t(dt=100ps), y
Entrance foil – 1mm
* Low-pressure avalanche chamber and silicon strip detectors
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

22. LPMWPC (JLAB FF detector NIM A426, 1999)
Detector was developed by YerPhI group for detection of fission fragments for studies of the absolute photofission cross sections at MAX-lab (Lund, Sweden) and for hypernuclear experiments in Hall C at JLAB
Two windowless gas proportional counters with three electrodes, operating as a parallel plate (wire planes) avalanche counter
Silicon strip detectors to measure full energy
Options for running the chamber with helium, helium/heptane (99%/1%), or helium/hydrogen (86%/15%) are under studies (YerPhI/Idaho)
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

23. Test of LPMWPC in Hall B
Two symmetric arms placed on top and bottom of the beam centerline
The inner chambers active area x 21cm2
Distance of the anode plane from the beam line - 3 cm.
The outer chambers - 21 x 21 cm2
Target - 209Bi with a thickness of 2 mg/cm2.
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

24. RTPC and LPMWPC BoNuS RTPC LPMWPC&SSD Rate High Background
Insensitive to MIP
Momentum resolution
Dp/p~20%, full trajectory fitting
Two point position recopnstruction
Minimum energy
~6 MeV for a
Defined by target, can be ~1 MeV for a
Efficiency
~100%
Time resolution
None
~200ps
Use in the trigger No
Yes
Solid targets
No, high accidental rates, long drift time
Yes, good for heavier fragments
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

25. Summary
Broad physics program, ranging from nucleon and nuclear structure to meson spectroscopy, can be accessed with CLAS12 and a low energy recoil detector
Exploration of some of these physics topics is started with CLAS and BoNuS RTPC
2nd generation RTPC will have much improved characteristics for PID and data rate (3rd generation detector for CLAS12 can have ~2p azimuthal coverage)
Option with LPMWPC must be carefully studied, specially for the detection of a-s and the heavier fragments from solid targets
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

26. Backup slides CLAS12 European Workshop
February 25-28, 2009, Genova, Italy

27. CLAS12 27 Low Threshold Cherenkov Counters Pre-shower calorimeters
CLAS12 Solenoid
Electromagnetic calorimeters
Central Detector
Drift Chambers
High Threshold Cherenkov Counter (HTCC)
TOF Counters
CLAS12 Torus
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

28. EMC effect The asymmetry ratio for DVCS enhances the EMC effect.
4He/D
Gomez et al., PRD49, 1994
Medium Modification
S. Liuti & S. K. Taneja PRC72, 2005
The deviations from unity is mostly due to convolutions with k.
The asymmetry ratio for DVCS enhances the EMC effect.
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy 28

29. Electroproduction at “0” degrees [photoproduction]
Reaction ep’pggggX at 5.76 GeV
(e1-6 and e1-DVCS experiments)
Missing transverse momentum distribution for |MX|2<0.1 GeV
Invariant mass of 2 photon pairs in the reaction ep’pgggg(e’)
Data were taken with loosely defined electron trigger
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

30. Reaction ep’pp0p0(e’) and ep’pp0h(e’)
CLAS DVCS
CLAS e1-6
Note:
- data were taken with unfavorable trigger conditions
- transferred momentum squared, |t|>0.2 (GeV/c)2 (proton momentum >0.4 GeV/c)
- spectra contain the baryon background
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy

31. Event display of RTPC: BoNuS data on D2 and 4He
2xQD < QHe
Deuterium Target
Helium Target
CLAS12 European Workshop 
February 25-28, 2009, Genova, Italy