1. Low Q tagging and spectroscopy with CLAS12
Raffaella De Vita
INFN – Genova
for the CLAS Collaboration

2. Photo-production at CLAS
Photo-production experiments represent a large fraction of the physics program at CLAS
study of the nucleon resonance spectrum
cross section measurements
polarization observables with circularly/linearly polarized photons on unpolarized/polarized nucleons
vector meson production at large momentum transfer
vector meson modification in the nuclear medium
hyperon production
cascade spectroscopy
meson spectroscopy and search for exotics
Aggiungere piu’ dettagli sul programma

3. The first CLAS result g p  p j  p K+ K-
E.Anciant et al. PRL (2000)
g p  p j  p K+ K-
Study of  photoproduction at large momentum transfer
POMERON  2- gluons
Nucleon wave function
Correlations between quarks
u-exchange around -tmax

4. Beam asymmetry in p- photoproduction on neutrons
g n  p-p
CLAS
World data
SAID-09 MAID-07 SAID-new PWA
D. Sokhan, Thesis Prize 2010

5. CEBAF Large Acceptance Spectrometer
Torus Magnet
6 Superconductive Coils
Electromagnetic Calorimeter
lead/plastic scintillator, 1296 PMTs
Target  start counter e mini-torus
Drift Chamber
35,000 cells
Cherenkov Counter e/ separation, 256 PMTs
Time of Flight
Plastic Scintillator, PMTs

6. Hall-B Photon Tagger Maximum photon energy of 5.7 GeV - Wmax  3.4 GeV
Photon beam produced from the primary electron beam via Bremsstrahlung
Gold and diamond radiator for In/Coherent Bremsstrahlung
Energy coverage: E0
Efficiency ~ 80%
Energy Resolution ~ 10-3
Timing Resolution ~100 ps
Maximum photon energy of 5.7 GeV
- Wmax  3.4 GeV
Beam intensity 107 g/s

7. Hall B at 12 GeV Options for Eg>6 GeV?
The existing PHOTON TAGGER will be available for energies up to Eg~6.1 GeV
Options for Eg>6 GeV?

8. Why Photoproduction at 12 GeV?
A tagged photon beam in CLAS12 will open new possibilities for high quality physics beyond the already approved program:
Meson spectroscopy
Spectroscopy on H target and search for exotics
Spectroscopy on 4He and other gas targets
Baryon spectroscopy
Heavy mass baryon resonances (Cascades and W-)
Hyperon production
φ radiative decays
Baryon/meson radiative decays
J/ψ production close to threshold and on nuclear targets
Time-like Compton scattering
Large -t physics

9. Hybrids and Exotics
Hybrids (qqg) are the ideal system to study qq interaction and the role of gluons
The existence is not prohibited by QCD but not yet firmly established. A possibility to identify unambiguously a meson as an hybrid state is to look for exotic quantum numbers
Normal mesons (qq) are classified according to their JPC where
P=(-1)L+1
C=(-1)L+S S1 S2 L
JPC=  (π,K,η,η’)
1--  (ρ,K*,ω,Φ)
1+-  (b1,K1,h1,h1’)
...
Some combinations of JPC are not allowed for conventional qq systems but can exist for “unconventional” states as hybrids
Normal meson: flux tube in ground state
m=0, PC=(-1)S+1 q
Hybrid meson: flux tube in excited state
m=1, PC=(-1)S q
Excitation of the flux tube leads to a new spectrum of hadrons that can have exotic quantum numbers JPC = 0+- , 1-+ ,
Lattice QCD calculations predict masses around 2 GeV, a range that can be explored at JLab

10. Search for Exotics in Photoproduction
Signals for exotic hybrids have been reported by various experiments but evidence is not yet fully convincing: more data are needed and experiments with different production modes (different probes) are crucial
Photoproduction: exotic JPC are more likely produced by S=1 probe
Pion Beam
Quark spins anti-aligned
JPC = 1-- , 1++
Quark spins already aligned
JPC = 0+- , 1-+ , 2+-
Photon Beam
A. Szczepaniak and M. Swat, Phys. Lett. B516 (2001) 72
regular Eg = 5GeV
X = a2
Exotic Eg = 8GeV
X = p1(1600)
Production rate for exotics is expected to be comparable to regular mesons
Few data (so far) but expected similar production rate as regular mesons

11. Search for the p1(1600) gpp+p+p-(n) CLAS-g6C PWA in CLAS is feasible!
Possible evidence of exotic meson p1(1600) in pp®(3p)-p (E852-Brookhaven)
Not confirmed in a re-analysis of a higher statistic sample
New evidence recently reported by Compass
CLAS-g6C
a2(1320)
Clear evidence of non-exotic 2++ state a2(1320)
No-evidence of exotic 1-+ state p1(1600)
PWA in CLAS
is feasible!

12. New High Statistics Photon run: g12
Search for new forms of hadronic matter in photoproduction on the proton
Data taking completed in 2008
Photon Energy up to 5.5 GeV
More than 26 billion triggers (2-prong + 3-prong)
Total Luminosity: 68 pb-1 (~50 pb-1 for E> 4.4 GeV)
Data processing completed and physics analysis in progress
Several exclusive channels are being analyzed
gp  p+p+p-(n)
gp  pp+p-h
gp  pK+K- (h/p)
gp  K+K+ X*- (1530)
gp  p+K+K-(n)
gp  e+e-p
...
Search for exotic mesons
Study of strangeonium states
Cascade spectroscopy
...

13. a hint of g12... gpp+p+p-n gpp+p-hp gpK+K+X a2(1320) a2(1320)
C. Bookwalter (FSU)
D. Schott (FIU)
gpp+p+p-n
gpp+p-hp
a2(1320)
a2(1320)
a0(980)
X (1321)
Large statistics for many multiparticle final states
Clear peaks for known states are visible in the spectra
PWA in progress
gpK+K+X
X* (1530)
J. Goetz (UCLA)

14. EG6: Meson spectroscopy in coherent production on 4He
search for exotics in ph, ph’ final states
search for exotics in ph, ph’ final states
6 GeV electron beam on target on high pressure gas target
scattered electron at “0” degrees  quasi-real photo-production
recoiling nucleus detected in Radial TPC
hadronic final state detected in CLAS
data taking completed in fall 2009
analysis in progress
recoiling nucleus detected in Radial TPC
Strongest evidence of JPC=1-+ p1(1400) exotic meson p-p nhp0 in E852-Brookhaven
Search for a resonance in P-wave in p0h and p0h’
Known (non-exotic) resonances can be used as a benchmark (e.g. JPC=2++ a2(1232))
g 4He  4Hep0 h g4He  4Hep0 h’
Eliminate s-channel resonance background
Simpler PWA: S=I=0 target acts as spin and parity filter for final state mesons
Coherent Production on 4He
gp  pp+p-
Radial TPC with 7atm. He4 Target
Solenoid for forward-focusing of Moeller electrons and bending of recoil nucleus in the TPC
PbWO4 calorimeter for improved photon acceptance at forward angles
Ridurre ad una slide

15. CLAS12 Forward Detector: Central Detector: Proposed upgrades:
- TORUS magnet
- Forward SVT tracker
HT Cherenkov Counter
Drift chamber system
LT Cherenkov Counter
Forward ToF System
Preshower calorimeter
E.M. calorimeter (EC)
Central Detector:
SOLENOID magnet
Barrel Silicon Tracker
Central Time-of-Flight
Proposed upgrades:
Micromegas (CD)
Neutron detector (CD)
RICH detector (FD)
Forward Tagger (FD)

16. Photoproduction with CLAS12
The construction of a Low Q Tagging Facility or Forward Tagger has been proposed to continue the spectroscopy program with CLAS12 using quasi-real photoproduction
electron scattering at “0” degrees (LowQ, post-target tagging): low-Q2 virtual photon  real photon
photons are tagged by detecting the scattered electron
quasi-real photons are linearly polarized and the polarization can be determined event by event
high luminosity on thin (gas)- targets
can exploit CLAS12 PiD capabilities to study final states with strangeness
Complementary to GLUEX Coherent Bremsstrahlung photon beam

17. Quasi-Real Photoproduction
Studies at large W (~100 GeV) show a smooth transition between Q2=0 and Q20
Technique used in high energy experiments:
Q2 < W2
COMPASS: <1 GeV2
<Q2> ~ 10-1 GeV2
ZEUS: – 0.02 GeV2 <Q2> ~ GeV2
H1: <2 GeV2
Aggiungere un diagramma che spieghi la cinematica

18. Quasi-Real Photoproduction
Analysis of existing CLAS data shows clear peaks associated to known mesons in
ep(e’)pp0p0
ep(e’)pp0h
f0(980)
a0(980)
f2(1270)
Aggiungere un diagramma che spieghi la cinematica
The Technique works!!

19. A Forward Photon Tagger for CLAS12
Electron beam on target, electrons scattered between and 5 deg. are detected
Hadronic final state is detected in CLAS12
Effective photon flux for 1035 electron beam luminosity of 5x107 g/s
Compatible with the standard CLAS12 equipment
Photon detector for leading DVCS experiments
Extends the CLAS12 coverage for neutrals down to 2 deg.
Forward
Electromagnetic Calorimeter
High-Threshold Cerenkov Counter
Drift Chambers
Forward Tagger
Central Detector
Torus Magnet
Preshower Calorimeter
Forward Time of Flight

20. Forward Tagger Electron detection via Calorimeter+Tracker+Veto
Electron Detector E’
0.5-4 GeV n
GeV q
2-5 deg Q2
0.007 – 0.3 GeV2
Photon Flux
5 x 107 Le=1035
Rate
20 Le=1035
calorimeter to determine the electron energy with few % accuracy
tracker to determine precisely the electron scattering plane and the photon polarization
veto to distinguish photons from electrons
PbWO4 Calorimeter
424 Crystals
Different hardware options:
Calorimeter:
homogenous crystals (PbW04, LYSO, ..)
sampling device (W+Fibers, W+Si, ...
Tracker:
gems, micromegas, ...
Veto:
scintillator tiles, ...

21. Kinematics  Electron kinematics: E=0.5-4 GeV q=2-5 deg.
Q2= GeV2
Eg= GeV
Photon Polarization: 10-65% (determined on an event-by-event basis)

22. GEANT4 Simulations
Visualization of CLAS12 and FT with GEMC

23. Tracking
Electron track reconstructed by tracking detectors located between the CD and the FT:
Determine impact point on the calorimeter
Reduce background by matching information of different detector systems
Act as a veto for neutrals
Sensitivity to particle momentum due to the j bend in the solenoid field
First estimate based on GEANT4 simulations shows that % resolution on momentum and angles can be achieved with 2 tracking planes located in between the CD and FT calorimeter

24. Simulation Studies Acceptance and resolution studies
Determine acceptance for final states of interest
Determine resolution for exclusive channel selection and resonance mass reconstruction
Test of PWA in CLAS12
Generate events for final state of interest, according to a known a set of amplitudes
Project generated events onto CLAS12+FT acceptance and resolution
Extract PWA fitting the reconstructed events
Parallel tests in progress by JLab/CNU and Genova/Indiana/Edinburgh groups
g p  n R(M=1.6 GeV, G=150 MeV)
 n p+ p+ p-
g p  n p+ p+ p- p0
MX(gp p+p+p-X)
COSA è la LINEA BLU?????
M(p+p+p-)

25. Partial Wave Analysis
The development of robust PWA techniques is a crucial step for the successful completion of the meson spectroscopy program
Advancements in detectors, beams and experimental techniques in the last years are leading to high-precision, high-statistics data sets
Are the presently available PWA tools adequate for the new data that are and will be produced?
Workshop on Hadron Spectroscopy
INT - Seattle, November
Organizers: M. Battaglieri, C. Munoz Camacho, RDV, J. Miller, A.P. Szczepaniak
~ 40 participants from the theoretical and experimental community
address open issues in experimental techniques, partial wave analysis, and theoretical interpretation
interest from the theory community to work with experimentalists to develop more sophisticated analysis approaches, going beyond the isobar model
white paper being written
Workshop on Amplitude Analysis in Hadron Spectroscopy
ECT* - Trento, January
Organizers: C. Hanhart, M. Pennington, E. Santopinto, A.P. Szczepaniak (coordinator), U. Wiedner

26. Summary
Photo-production experiments at CLAS12 with 10-GeV photons can become a reality with a new Low-Q Tagging Facility
Broad physics program using (quasi-)real photon beam with post-target, forward tagging on nucleons and nuclei
Complementary technique to the Hall-D coherent Bremsstrahlung beam
Forward Tagger will be based on Calorimeter + Tracking + Veto and is being designed as an upgrade of the CLAS12 equipment
pTDR in preparation
LOI : “Hadron Spectroscopy with Low Q2 electro-scattering in CLAS”
LOI : “Production of the Strangest Baryon with CLAS12” approved by PAC35 and full proposal encouraged
Full proposal in preparation to be submitted to PAC in 2011
The new facility will add further strength to Jefferson Lab in the field of hadron spectroscopy
RIFARLO!!