Transcript Experiments with Linearly Polarized Photons at CLAS

Experiments with Linearly Polarized Photons at CLAS
Ken Livingston
University of Glasgow and The CLAS Collaboration
Motivation
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Linear polarization and coherent bremsstrahlung
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The coherent bremsstrahlung facility at CLAS
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The g8 experimental program with preliminary results
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The future
Experiments with Linearly Polarized Photons at CLAS
Quark Confinement and the Hadron Spectrum VII, 2006
Motivation – linear polarization
Differential cross section =
Photon beam asymmetry =
General advantages of linearly polarized photons
 Theoretical calculations can include photon polarization
 Σ can be more sensitive than σ.
 Σ measurement experimentally easier than σ.
 A step towards double polarization observables and a
complete measurement
E2 / M1 ratio for N -> Δ transition. p(γ,p)π0
Daphne, Mainz. Beck et al, PRL 78,606 (1997)
Experiments with Linearly Polarized Photons at CLAS
Quark Confinement and the Hadron Spectrum VII, 2006
Motivation – vector meson production
Photon asymmetry for γp -> ωp measured at GRAAL (Y. Assafiri, Proc. NSTAR 2000)
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θcm
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θcm
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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
Experiments with Linearly Polarized Photons at CLAS
Quark Confinement and the Hadron Spectrum VII, 2006
Motivation – vector meson production
Extraction of spin density matrix elements
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Helicity reference frame
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Linear polarization gives access to 6 more matrix elements
than unpolarized data.
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g8 aims to do this for ρ0, Φ and ω.
Calculations for ρ0 decay (Roberts)
26 resonances
N3/2+(1910)
26 – N3/2+(1910)
Experiments with Linearly Polarized Photons at CLAS
Quark Confinement and the Hadron Spectrum VII, 2006
Motivation – hyperon production
p(γ,Κ+)Λ0
Photon Asymmetry (SPring81)
Recoil Polarization (CLAS2)
Data compared with model of Janssen et al3. Core set of resonances (black), (additional resonances in (colors)
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Single polarization observables sensitive to different resonances
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g8 will extend Σ, improve recoil pol stats and measure double polarization observables Ox,Oz
1. Zegers et al., PRL 91 (2003),
Experiments with Linearly Polarized Photons at CLAS
2. McNabb et al.,PRC 69 (2004),
3. Ireland et al.,Nucl. Phys. A740 (2004)
Quark Confinement and the Hadron Spectrum VII, 2006
The coherent bremsstrahlung facility at CLAS
Requirements for coherent brem
 Low emittance, stable beam
 High quality thin crystal
 Collimation < 0.5 characteristic angle
 Polarimetry
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Photon
Tagger
Goniometer
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Diamond
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Pair
Spectrometer
LH2 target
23m
Experiments with Linearly Polarized Photons at CLAS
CLAS detector
Active
collimator
Polarimeter
Beam
Profiler
Quark Confinement and the Hadron Spectrum VII, 2006
The coherent bremsstrahlung facility at CLAS
Goniometer, G.W. University
Rocking curve
from SRS
Daresbury
J.Kellie, Glasgow
Diamond
Experiments with Linearly Polarized Photons at CLAS
Quark Confinement and the Hadron Spectrum VII, 2006
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.
Experiments with Linearly Polarized Photons at CLAS
Quark Confinement and the Hadron Spectrum VII, 2006
Polarization and polarimetry
Measuring the degree of photon polarization
 Compare photon spectrum with bremsstrahlung
calculation (eg. anb / mcb from Tuebingen)
 g8b reaches > 90% (preliminary calculation)
Peak > 90% pol.
 Many free parameters in calculation
 A more direct method would be useful
K.Livingston, Glasgow
A pair (e+e-) polarimeter (CUA, JLAB, NCCU, SC)
 Measure azimuthal distribution of pairs
 QED, well understood, analyzing power > 20%
 Tested at SPring8 and g8b, Jlab
 Should be implemented for next experiments (J.Santoro)
Experiments with Linearly Polarized Photons at CLAS
J.Santoro, CUA
Quark Confinement and the Hadron Spectrum VII, 2006
The g8 experimental program
Experiments comprising g8a / g8b
 Vector meson production (ρ,ω,Φ)
 η, η’ production
 KΛ, KΣ production
 Others (π, Δ 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
Experiments with Linearly Polarized Photons at CLAS
Quark Confinement and the Hadron Spectrum VII, 2006
ρ0 photon asymmetry
Reaction: γ p → ρ0 p → π+ π- p
 First measurement of photon asymmetry
 PhD thesis: Chris Gordon, Glasgow
Mean polarization = 71±5 %
 Compared with prediction of diquark model of
using ω paramaterisation. Zhao PRC 58 (1998).
Event selection
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ρ0
π+ π- inv. mass for different θcm
 Clean ρ0 at low θcm (ie low t)
Experiments with Linearly Polarized Photons at CLAS
Quark Confinement and the Hadron Spectrum VII, 2006
K+ Λ0 photon asymmetry
Reaction: γ p → K+ Λ0 → K+ π- p
 Extended range of photon asymmetry
 PhD thesis: Joseph Melone, Glasgow
 Compared with calculations of Janssen et al.
 g8b data to be added
Experiments with Linearly Polarized Photons at CLAS
Quark Confinement and the Hadron Spectrum VII, 2006
The future - linearly polarized photons
g8b
 Analysis of many channels will begin soon.
 High statistics > 10 billion events
 High photon polarization from 1.3 – 2.1GeV.
 Prelim analysis of γp → Nπ on a few runs with rough
calibration. (M. Dugger ASU)
• Pγ estimated at 0.8
• Blue line is SAID prediction
• Data with statistical errors (no systematic)
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.
Experiments with Linearly Polarized Photons at CLAS
Quark Confinement and the Hadron Spectrum VII, 2006
The future – polarized photons + polarized targets
FROST (Frozen Spin Polarized Target)
 Longitudinal and transverse target polarization
Transverse holding coil
(C,Keith, M. Seely)
 Combine with circularly and linearly polarized photons
 Scheduled for Nov 2006
 Double polarization observables
 Several proposals (http://clasweb.jlab.org/frost)
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 γ n → K0 Λ amplitude”
Experiments with Linearly Polarized Photons at CLAS
Quark Confinement and the Hadron Spectrum VII, 2006
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 don’t need to measure them all.
 Require >= 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.
Experiments with Linearly Polarized Photons at CLAS
Quark Confinement and the Hadron Spectrum VII, 2006
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
Experiments with Linearly Polarized Photons at CLAS
Quark Confinement and the Hadron Spectrum VII, 2006