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

Experiments with Linearly Polarized Photons at CLAS

Ken Livingston

University 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

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

Motivation – linear polarization

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

Differential cross section =

Photon beam asymmetry =

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

0 50 100 150 0 50 100 150θcm θcm

Photon asymmetry for γp -> ωp 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

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

Motivation – vector meson production

Extraction of spin density matrix elementsCalculations for ρ0 decay (Roberts)

26 resonances

N3/2+(1910)

26 – N3/2+(1910)

Helicity reference frame

Linear polarization gives access to 6 more matrix elements than unpolarized data.

g8 aims to do this for ρ0, Φ and ω.

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)

Single polarization observables sensitive to different resonances

g8 will extend Σ, improve recoil pol stats and measure double polarization observables Ox,Oz

Data compared with model of Janssen et al3. Core set of resonances (black), (additional resonances in (colors)

1. Zegers et al., PRL 91 (2003), 2. McNabb et al.,PRC 69 (2004), 3. Ireland et al.,Nucl. Phys. A740 (2004)

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

The coherent bremsstrahlung facility at CLAS

Goniometerand

Diamond

Photon Tagger

Active collimator

PairSpectrometer

Polarimeter

CLAS detector

BeamProfiler

23m LH2 target

Requirements for coherent brem

Low emittance, stable beam

High quality thin crystal

Collimation < 0.5 characteristic angle

Polarimetry022

044066

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

The coherent bremsstrahlung facility at CLAS

Goniometer, G.W. University

Diamond

Rocking curve from SRS Daresbury

J.Kellie, Glasgow

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

J.Santoro, CUA

Peak > 90% pol.

K.Livingston, Glasgow

Measuring the degree of photon polarization

Compare photon spectrum with bremsstrahlung

calculation (eg. anb / mcb from Tuebingen)

g8b reaches > 90% (preliminary calculation)

Many free parameters in calculation

A more direct method would be usefulA 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 Quark Confinement and the Hadron Spectrum VII, 2006

The g8 experimental program

Experiments comprising g8a / g8b

Vector meson production (ρ,ω,Φ)

KΛ, KΣ production

η, η’ 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

Mean polarization = 71±5 %

Reaction: γ p → ρ0 p → π+ π- p

First measurement of photon asymmetry

PhD thesis: Chris Gordon, Glasgow

Compared with prediction of diquark model of using ω paramaterisation. Zhao PRC 58 (1998).

ρ0

π+ π- inv. mass for different θcm

210 270 360

470 590 <720

Event selection

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.

preliminary

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

Combine with circularly and linearly polarized photons

Scheduled for Nov 2006

Double polarization observables

Several proposals (http://clasweb.jlab.org/frost)

Transverse holding coil

(C,Keith, M. Seely)

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