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!!!