1. Overview of recent photon beam runs at CLAS
Ken Livingston, University of Glasgow
CLAS12 European Workshop, Feb , Genoa, Italy
Tagged photons at CLAS
Recent experiments and analysis
Polarization observables
g8, g13, g9
Summary, future experiments

2. Tagged photons at CLAS Tagged bremsstrahlung photons up to 6GeV.
Timing resolution < 1 beam bucket.
Circularly polarized
up to ~80% now standard
Linearly polarized
coherent bremsstrahlung up to >90%.

3. Experiments to measure polarization observables
CLAS photon runs
> 100 different analyses ! (
~20 publications – many more anticipated.
Most recent
“First Measurement of f0 Photoproduction on the proton”, M. Battaglieri et al. (The CLAS Collaboration),
submitted to PRL November 2008
Recent photon runs
Date
Run
Beam
Target
Primary physics goals
2005
g8b
γ lin
LH2
Missing resonances, meson photoproduction
2006
g13a
γ circ
LD2
Missing resonances , meson photoproduction (on neutron)
2007
g13b
2008
g9a
γ lin,circ,
C4H9OH,long
Missing resonances , meson photoproduction polarized proton
g12
Pentaquark states, new forms of matter
Date
Run
Beam
Target
Primary physics goals
2002
g1c
γ circ
LH2
N* and meson properties
2005
g8b
γ lin
Missing resonances, meson photoproduction
2006
g13a
LD2
Missing resonances , meson photoproduction (on neutron)
2007
g13b
2008
g9a
γ lin,circ,
C4H9OH,long
Missing resonances , meson photoproduction polarized proton
g12
Pentaquark states, new forms of matter
CLAS g12 update and search for new strangeonia in g12 - Mukesh Saini , in Hadron Spectroscopy session
Experiments to measure polarization observables

4. Missing resonances
Cross sections not enough to separate broad, overlapping resonances
Angular distributions and polarization observables.

5. Polarization observables
Linear Polarisation 
Circular polarisation 
Nucleon recoil
polarimiter x
+ N → N + m
→ Y + 
Hyperons are “self analysing” 
Transverse polarized nucleon targets ?
Longitudinally polarized nucleon targets 

6. Polarization observables in pseudoscalar meson production
4 Complex amplitudes - 16 real polarization observables.
A complete measurement from 8 carefully chosen observables.
Observable
g target recoil
I. S. Barker, A. Donnachie, J. K. Storrow, Nucl. Phys. B (1975).

7. Polarization observables - a simple example, S
Systematics of detector acceptance cancel out.
Only need to know Plin, the degree of linear polarization.

8. Polarized photon energy range: 1.3 – 2.1 GeV
g8b: July 2005
Polarized photon energy range: 1.3 – 2.1 GeV
Events (single charged particle in CLAS): 10 billion
preliminary results: p0p, p+n. Mike Dugger, ASU
CLAS data
SAID
High statistics. Good agreement with previous measurement.
Data set extends to photon energy up to 2.1GeV

9. g8b preliminary results: hp, h’p Mike Dugger, ASU
SAID
MAID
Nakayama and Haberzettl
effective lagrangian model
High stats for h.
Good agreement with previous measurements.
Data set extends to photon energy up to 2.1GeV
First measurement of S for h’
2 Eg bins one for coh peak at 1.9 and 2.1GeV

10. g8b preliminary results - K+L and K+S0 Craig Paterson, Glasgow
gp -> K+L -> K+pp gp -> K+S0 -> K+Lg -> K+pp-g
Single polarization observables S Photon asymmetry P Recoil polarization (induced pol. along y) T Target asymmetry Double polarization observables Ox Polarization transfer along x Oz Polarization transfer along z
L S0
figure by R.Schumacher, CMU

11. g8b preliminary results - K+L
Results compared with previous results from GRAAL
7, 50MeV Energy bins
1175 -> 1475MeV
Good agreement with previous results
PRELIMINARY

12. g8b preliminary results - K+L
Results compared with previous results from LEPS
6, 100MeV Energy bins
1550 -> 2050MeV
More bins for our data
Increase the angular coverage to backward angles
PRELIMINARY

13. g8b preliminary results - K+L
Ox/Oz extracted from fit to 2d asymmetry
T Target asymmetry from 2d asymmetry (not shown)
P Recoil pol. from acceptance corrected proton dist.
P Recoil pol.
g8b
GRAAL
CLAS J.W.C. McNabb, et al. (CLAS) Phys. Rev. C 69, (R) (2004).

14. Double polarization ovservable Ox
g8b preliminary results - K+L
Double polarization ovservable Ox
Results compared with Regge-Plus-Resonance model from Gent group
- Large Polarizations!
Some evidence for an important role for missing D13(1900) state
Poor agreement at low energy
PRELIMINARY 14

15. Full lamba polarization ?
g8b preliminary results - K+L
Full lamba polarization ?
With circularly polarized photons – “fully polarized lambda”
R. Bradford et al. (The CLAS Collaboration), Phys. Rev. C 75, (2007).
- Full polarization at forward angles
- Not repeated over full kinematic range
- More relations can be tested
PRELIMINARY
(P2 + Ox2+Oz2)1/2 15

16. g13 meson photoproduction, polarized photons on LD2
gn (p) -> p+p (p)
Daria Sokhan, Edinburgh
g13a circularly polarized photons
~2GeV, ~2.6GeV electron beam
~2 months beamtime
g13b linearly polarized photons
1.3, 1.5, 1.7, 1.9, 2.1, 2.3 GeV coh peak
~ 2months beamtime
Single charge particle trigger
~7kHz event rate
PRELIMINARY
MAID
SAID
Previous data 16

17. g13 meson photoproduction, polarized photons on LD2
gp (n) -> K+L0 (n)
Russell Johnstone, Glasgow
See talk in Hadron Spectroscopy session
gn (p) -> K0s L0 (p)
Neil Hassall, Glasgow
PRELIMINARY
1st results
photon asymmetry S
Cosqcm ( )
Each plot is 50MeV photon energy bin MeV 17

18. g9 FROST – FROzen Spin Target
Meson photoproduction with linearly and circularly polarized photons on polarized target
E02-112: γp→KY (K+Λ, K+Σ0, K0Σ+)‏
E03-105/E04-102: γp→π0p, π+n
E05-012: γp→ηp
E06-013: γp→π+π-p
g9a running conditions
November 3, 2007– February 12, 2008
Longitudianally polarized target
Circularly and linearly polarized photon beam GeV
Trigger: at least one charged particle in CLAS
Target Pol > 80%, Relaxation time > 1600hrs – better that design goals

19. g9aFROST sample analysis of gpp+n Eugene Pasyuk, ASU
Select p+ applying cut on b vs. p
Vertex cuts
Select missing neutron
Raw asymmetry
Helicity asymmetry E
butanol
12C
CH2
butanol
12C
CH2
Preliminary
Based on ~2% of statistics

20. Recoil polarimetry possibilities ?
n(q,f) =no(q){1+A(q)[Pycos(f)–Pxsin(f)] How to do this for 4p detector
x and y (transverse) components
of nucleon polarisation q
Nucleon polarimeter for D. Watts, Edinburgh
Graphite
g8b data. g + p → p+ n with (n,p) scatter (detect p+p)
Mx(gp→p++X) q p+ n p g
Start counter
K.Livingston
Angle between Pncalc and Ppmeasured 20

21. Summary, future experiments
g8b (lin. pol. on LH2) cooking complete, analysis well developed
Photon asymmetries for p0p,p+n, h, h’ KL, KS: prelim. results.
P, T Ox, Oz for KL, KS:
Also f analysis well developed (J. Salamanca, ISU).
2 p channel talk by C. Hanretty in Hadron spectrosopy session.
g13 (lin., circ. pol on LD2) cooking in progress, analysis beginning
Some very preliminary results for photon asymmetry.
g9 (lin, circ pol on L/T pol butanol) cooking about to begin
Circularly polarized photon beam on longitudinally polarized target
E P, Lx/Lz, Cx/Cz
Linearly polarized photon beam on longitudinally polarized target
, G, P, (Tx/Tz) Ox/Oz
Circularly polarized photon beam on transversely polarized target
T, F, P, Tx/Tz, Cx/Cz
Linearly polarized photon beam on transversely polarized target
, H, T, P, (Lx/Lz)‏
Upon completion of g9b for the first time we will have complete set of observables in Kaon photoproduction and nearly complete for all other mesons, which will allow model independent extraction of the amplitude for subsequent extraction of baryon resonances and their properties. (E. Pasyuk).
eg6 4He target, Bonus detector coherent meson production. (S.Stepanyan talk on Friday) scheduled 2009
HDice (A.Sandorfi, F.Klein) Polarized HD target –further extesion of polarization observables scheduled 2010
CLAS12
Tagger will only tag up to 6GeV primary beam
Development of Quasi real photon beam (Q2~0) S.Stepanya, R.DeVita, M. Battaglieri
Post target tagger – linearly polarized
High rates
scheduled 2010

22. CLAS coherent bremsstrahlung facility
Tagging spectrometer with high rate, good energy and timing resolution
High precision goniometer (GWU)
High quality, thin diamond (Glasgow)
Tight photon beam collimation (ISU)
Polarimetry
Photon energy
P > 90%
Peak > 90% pol.
“A device called a goniometer tilts the diamond, much like a lady turning her hand to admire the sparkle of a new ring.” - JLAB On Target Magazine 22

23. Measurements with photon beam profile detector D. Glazier, Glasgow
1st Measurement of 2D photon enhancement for coherent bremsstrahlung (MAMI,Mainz) paper in preparation
below peak
coherent peak
above peak
Coherent peak at 300Mev, MAMI electron beam energy 855MeV
Good agreement with coherent bremstrahlung calculations
Improvements in incoherent component, collimation + multiple scattering.
No evidence of high energy photons from quasi channeling.
Investigation of 2D strip detector for polarimetry

24. g8b preliminary results - K+L
K+L Photon Asymmetry, S, extracted from cos(2f) fit to azimuthal kaon distribution
Fits shown for 1 energy bin
340 (20E, 17q) kinematic bins
Almost full angular coverage

25. g8b preliminary results - K+S0
Results compared with previous results from GRAAL
7, 50MeV Energy bins
1175 -> 1475MeV
Good agreement with previous results
PRELIMINARY

26. g8b preliminary results - K+S0
Results compared with previous results from LEPS
6, 100MeV Energy bins
1550 -> 2050MeV
More bins for our data!!!
Increase the angular coverage to backward angles
PRELIMINARY

27. Polarimetry: from pair (e+,e-) production H.Schmieden, Bonn
Well described by QED, but experimentally difficult – small opening angle e+ e-
Pair production simulations by Kharkov group
Polarimeter built and tested at Sping8, recently tested at Jlab
PRELIMINARY
J.Santoro, CUA
Peak > 90% pol.
K.Livingston, Glasgow
Polarimeter to be based on Jlab design
Microstrip detectors, or pixel detectors (Atlas group)
Bonn student completed 10 months exchange in Glasgow now to be full time in Bonn.

28. Polarimetry: from hadronic reaction R. Beck, Mainz -> Bonn
Use reaction with a known photon asymmetry
Can be high statistics
Very good relative monitor of polarization
Combined beam, target polarization.
Non-indpendent – depends on specific expt
Need very good systematics or calibration
Awaiting MAMI polarized target and polarised photon beam in 2nd half of 2007
Recent preliminary results from JLab (g8b)‏
Proton target
Back to back charge particles in Start Counter
Atomic or hardonic ?
Asymmetry from ~20mins DAQ data
Constant with E from 1.3GeV – 1.9GeV 28