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Polarization Observables in Hyperon Photoproduction with CLAS
Craig Paterson University of Glasgow Eurotag WorkshopGlasgow, 1st September 2008
Introduction
Search for Missing ResonancesUndetected baryon states predicted to couple strongly to andchannels
Strongly correlated diquarks?
Understand strangeness production
off the proton
Sensitive test of resonance structure
– PDG star rating for “missing resonances”
Polarization Observables
4 Complex amplitudes completely describe pseudoscalar meson photoproduction
This gives 16 real polarization observables
PossibleMeasurements
Require 8 observables for a “complete measurement”
Fully Polarized Lambda
Evidence from previous CLAS analysis for a fully polarized Lambda Hyperon
Creation of virtual strange quark – antiquark pair in a 3S1 state
Spin Couplings treated at the quark level
Analogous results for linearly polarized photons?!?
Reaction In CLAS
Use 4 CLAS detector to search for the reactions
p -> K+-> K+p-
p -> K+-> K+-K+p-
Photon Polarization
Photon polarization calculated to be > 90%Underestimate of polarization for certain energiesImproved measurements requiredCompare to previous data?Polarimeter would be nice!!!
Analysis: Event Selection
Sophisticated analysis algorithm to select and events
Initial selection based on mass
calculated from time-of-flight
Identify the correct photon
Fiducial cuts
Analysis: Event Selection
Correctly Identify Hyperon Events
Remove mis-identifed kaons/pions
Reconstruct hyperons from
Missing mas
Pions
LambdaSigma
Analysis: Extraction of Photon Asymmetry
Photon Asymmetry, , extracted from cos(2) fit to azimuthal kaon distribution
Fits shown for 1 energy bin
340 (20E, 17) kinematic bins
Almost full angular coverage
Photon Asymmetry Results –
Results compared with previous results from GRAAL7, 50MeV Energy bins
1175 -> 1475MeV
Good agreement with previous results
PRELIMINARY
Photon Asymmetry Results –
Results compared with previous results from LEPS6, 100MeV Energy bins
1550 -> 2050MeV
More bins for our data!!!
Increase the angular coverage to backward angles
PRELIMINA
RY
Photon Asymmetry Results –
Results compared with previous results from GRAAL7, 50MeV Energy bins
1175 -> 1475MeV
Good agreement with previous results
PRELIMINA
RY
Photon Asymmetry Results –
Results compared with previous results from LEPS6, 100MeV Energy bins
1550 -> 2050MeV
More bins for our data!!!
Increase the angular coverage to backward angles
PRELIMINA
RY
Analysis: Extraction of Ox/O
z /P and T
Ox/O
z extracted from fit to 2d asymmetry
Recoil Polarization extracted from acceptance corrected proton distribution
Target Asymmetry extracted from 2d Asymmetry
Only results for channel will be shown!!!
Results: Recoil Polarization results for
Results compared to previous CLAS and GRAAL resultsResults:
g8b
Prev CLAS
GRAAL
~Factor of 4 increase in statistics from best previous measurements
Results: Ox
Results compared with Regge-Plus-Resonance model from Gent group
- Large Polarizations!
- Some evidence for an important role for missing D
13(1900)
state
PRELIMINA
RY
Results: Ox
Results compared with Kaon-MAID calculations
- Calculations involving both “core” and missing resonances
- Failure at low energies
- Some evidence for an important role for missing D
13(1900)
statePRELIMINA
RY
Results: Oz
Results compared with Regge-Plus-Resonance model from Gent group
- Less polarization transfer along z-axis
PRELIMINA
RY
Results: Target Asymmetry
Results compared with Regge-Plus-Resonance model from Gent group
- Large polarizations
- Less discrimination between model calculations
PRELIMINA
RY
Results: Fully Polarized Lambda
Evidence for full lambda polarization with linearly polarized photons?
- Full polarization at forward angles
- Not repeated over full kinematic range
- Compare with previous results???
- More relations can be tested
PRELIMINA
RY
Summary
High Statistics, High Quality Data Set for and photoproduction
Large Improvement Over Previous Recoil and Photon Asymmetry Measurements
Large Improvement Over Previous Ox and O
z , and Target Asymmetry
Results
Measurements Display Good Discriminating Power Between Different Model Calculations – Sensitive to Resonant Contributions
Full Polarization Only Evident at Mid to Forward Angles
Further Theoretical Input Required
Better Calculation of Degree of Photon Polarization Required - Eurotag!!!