Experiments with Linearly Polarized Photons at CLASQuark Confinement and the Hadron Spectrum VII, 2006

1
Experiments with Linearly Polarized Photons at
CLASKen LivingstonUniversity of Glasgow and
The CLAS Collaboration

• Motivation
• Linear polarization and coherent bremsstrahlung
• The coherent bremsstrahlung facility at CLAS
• The g8 experimental program with preliminary
  results
• The future

2
Motivation linear polarization
Differential cross section
Photon beam asymmetry

• General advantages of linearly polarized photons
• Theoretical calculations can include photon
  polarization
• S can be more sensitive than s.
• S measurement experimentally easier than s.
• A step towards double polarization observables
  and a complete measurement

E2 / M1 ratio for N -gt ? transition.
p(?,p)p0 Daphne, Mainz. Beck et al, PRL 78,606
(1997)
3
Motivation vector meson production
Photon asymmetry for ?p -gt ?p measured at GRAAL
(Y. Assafiri, Proc. NSTAR 2000)

• Calculations from model of Zhao, Li and Bennhold.
  (Ajaka et al., PRL 96 (2006))
• Includes S11(1535), D13(1520), P11(1440),
  P11(1710), P13(1720), F15(1680), P13(1900),
  F15(2000)
• Solid line full calc. Other lines with
  labelled resonance removed.
• Asymmetry strongly sensitive to D13(1520),
  P13(1720), F15(1680)
• g8 results will extend this data

4
Motivation vector meson production
Extraction of spin density matrix elements
Calculations for ?0 decay (Roberts)

• Helicity reference frame
• Linear polarization gives access to 6 more matrix
  elements than unpolarized data.
• g8 aims to do this for ?0, F and ?.

26 resonances N3/2(1910) 26 N3/2(1910)
5
Motivation hyperon production
p(?,?)?0
Photon Asymmetry (SPring81)
Recoil Polarization (CLAS2)
Data compared with model of Janssen et al3. Core
set of resonances (black), (additional resonances
in (colors)

• Single polarization observables sensitive to
  different resonances
• g8 will extend S, improve recoil pol stats and
  measure double polarization observables Ox,Oz

1. Zegers et al., PRL 91 (2003), 2. McNabb
et al.,PRC 69 (2004), 3. Ireland et
al.,Nucl. Phys. A740 (2004)
6
The coherent bremsstrahlung facility at CLAS

• Requirements for coherent brem
• Low emittance, stable beam
• High quality thin crystal
• Collimation lt 0.5 characteristic angle
• Polarimetry

7
The coherent bremsstrahlung facility at CLAS
Goniometer, G.W. University
Rocking curve from SRS Daresbury
J.Kellie, Glasgow
Diamond
8
Producing polarized photons from a crystal

• Align the crystal with the electron beam
• Need to be aligned to within a few urad
• Install by eye to about 2 deg
• Scan small angular steps enhancement spectra
• Find relationship to beam and set required peak.

9
Polarization and polarimetry

• Measuring the degree of photon polarization
• Compare photon spectrum with bremsstrahlung
• calculation (eg. anb / mcb from Tuebingen)
• g8b reaches gt 90 (preliminary calculation)
• Many free parameters in calculation
• A more direct method would be useful

Peak gt 90 pol.
K.Livingston, Glasgow

• A pair (ee-) polarimeter (CUA, JLAB, NCCU, SC)
• Measure azimuthal distribution of pairs
• QED, well understood, analyzing power gt 20
• Tested at SPring8 and g8b, Jlab
• Should be implemented for next experiments
  (J.Santoro)

J.Santoro, CUA
10
The g8 experimental program

• Experiments comprising g8a / g8b
• Vector meson production (?,?,F)
• K?, KS production

• ?, ? production
• Others (p, ? production, )

• g8a (July 2001)
• The commissioning experiment for the polarized
  photon beam.
• Polarized photon energy range 1.9 2.1 GeV
• Events (single charged particle in CLAS) 1.3
  billion
• Status of analysis Preliminary results, (2 PhD,
  2 Masters theses)

• g8b (July 2005)
• High quality data set with high polarization.
• Polarized photon energy range 1.3 2.1 GeV
• Events (single charged particle in CLAS) 10
  billion
• Status of analysis Calibration almost complete

11
?0 photon asymmetry

• Reaction ? p ? ?0 p ? p p- p
• First measurement of photon asymmetry
• PhD thesis Chris Gordon, Glasgow
• Compared with prediction of diquark model of
  using ? paramaterisation. Zhao PRC 58 (1998).

Mean polarization 715
Event selection
210
270
360
?0
470
590
lt720
p p- inv. mass for different ?cm

• Clean ?0 at low ?cm (ie low t)

12
K ?0 photon asymmetry

• Reaction ? p ? K ?0 ? K p- p
• Extended range of photon asymmetry
• PhD thesis Joseph Melone, Glasgow
• Compared with calculations of Janssen et al.
• g8b data to be added

13
The future - linearly polarized photons

• g8b
• Analysis of many channels will begin soon.
• High statistics gt 10 billion events
• High photon polarization from 1.3 2.1GeV.
• Prelim analysis of ?p ? Np on a few runs with
  rough calibration. (M. Dugger ASU)
• P? estimated at 0.8
• Blue line is SAID prediction
• Data with statistical errors (no systematic)

preliminary

• Kaon production on deuterons
• A-rated at August 2006 PAC. (P. Turonski,
  E-06-103)
• Will use circularly and linearly polarized
  photons.
• Access to neutron channels.
• Repeat g8b kaon channels on proton in medium.
  Look for FSI effects.

14
The future polarized photons polarized targets

• Combined HD polarized target (LEGS).
• Target is polarized at LEGS and brought to Jlab
  on a truck. (Long relaxation time)
• Simultaneous measurement on polarized p and n
• Combine with circularly and linearly polarized
  photons
• A-Rated at Aug 2006 PAC (Sandorfi, E-06-101)
• and a complete determination of ? n ? K0 ?
  amplitude

15
The future a complete measurement
Observables in pseudoscalar meson production
(Barker, Donnachie Storrow Nucl Phys B95 (1975)
)

• 16 observables (red) 1 x unpol, 3 x single pol,
  3 x BT, 3 x TR, 3 x BR
• These are combinations of the 4 complex
  amplitudes which describe the reaction.
• Non-independent. ie we dont need to measure
  them all.
• Require gt 8 observables from 3 classes (classes
  single, BT, TR, BR)
• CLAS will have linearly circulary polarized
  photons, longitudinal transverse target
  polarization, recoil polarization (hyperons are
  self analysing)
• A complete measurement is in our sights.

16
Participants

• Institutes involved in coherent bremsstrahlung
  facility and related experiments
• (Alphabetic)
• Arizona State University
• Catholic University of America
• George Washington University
• Idaho State University
• North Carolina Central University
• Jefferson Lab
• Universidad de Los Andes, Columbia
• University of Glasgow, Scotland