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Polarization Observables in Hyperon Photoproduction with CLAS
Craig Paterson University of Glasgow Eurotag WorkshopGlasgow, 1st September 2008
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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”
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Polarization Observables
4 Complex amplitudes completely describe pseudoscalar meson photoproduction
This gives 16 real polarization observables
PossibleMeasurements
Require 8 observables for a “complete measurement”
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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?!?
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Reaction In CLAS
Use 4 CLAS detector to search for the reactions
p -> K+-> K+p-
p -> K+-> K+-K+p-
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Photon Polarization
Photon polarization calculated to be > 90%Underestimate of polarization for certain energiesImproved measurements requiredCompare to previous data?Polarimeter would be nice!!!
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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
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Analysis: Event Selection
Correctly Identify Hyperon Events
Remove mis-identifed kaons/pions
Reconstruct hyperons from
Missing mas
Pions
LambdaSigma
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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
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Photon Asymmetry Results –
Results compared with previous results from GRAAL7, 50MeV Energy bins
1175 -> 1475MeV
Good agreement with previous results
PRELIMINARY
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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
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Photon Asymmetry Results –
Results compared with previous results from GRAAL7, 50MeV Energy bins
1175 -> 1475MeV
Good agreement with previous results
PRELIMINA
RY
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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
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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!!!
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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
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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
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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
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Results: Oz
Results compared with Regge-Plus-Resonance model from Gent group
- Less polarization transfer along z-axis
PRELIMINA
RY
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Results: Target Asymmetry
Results compared with Regge-Plus-Resonance model from Gent group
- Large polarizations
- Less discrimination between model calculations
PRELIMINA
RY
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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
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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!!!