Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006

Experiments with real photons at CLAS

Ken Livingston

University of Glasgow

Some recent highlights * * * *

g8 and the coherent bremsstrahlung facility at CLAS* * * *

The future

Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006

Recent experiments

Run Pol Description Physics

g1 C Hyperon prod. on the protonPion production, eta, eta’

Hyperon polarisation, resonances (p)

g2 Hyperon prod. on DeuteriumSearch for Pentaquark

Hyperon polarisation, resonances (n)

g3 C Hyperon prod. on Nuclei

Compliment above + FSI, 2N currents

g10 Pentaquark on Dueterium

Find pentaquarks

g11 Pentaquark on the proton

Find pentaquarks

g8 L Vector meson on protonHyperon productioneta, eta’Pion prod.

Polarisation observables, resonances

http://www.jlab.org/Hall-B/physics/experiments/run_period.html

Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006

Some highlights: pentaquark searches

S. Stepanyan et al. PRL 91 (2003) B. McKinnon et al. PRL 96 (2006) M. Battaglieri et al. PRL 96 (2006)

going…… going…… gone !

g2c g10 g11

Where have all the pentaquarks gone?

Cross section upper limits measured for several reactions.

Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006

Some highlights: γp -> η’p

Missing mass γp -> pX

M. Dugger et al. PRL 96,062001 (2006)

Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006

Some highlights – hyperon production

p(γ,Κ+)Λ0

Photon Asymmetry (SPring81) Recoil Polarization (CLAS2)

Single polarization observables sensitive to different resonances

CLAS paper in preparation on 1st measurement of double polarization observables Cx, Cz

g8 will extend Σ, improve P 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 Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006

g8: linearly polarized photons at CLAS

General advantages of polatization observables

Theoretical calculations can include polarization

More sensitive than σ.

Measurement experimentally easier than σ.

Stepping towards double polarization observables and a complete measurement

Proposals

Vector meson production (ρ,ω,Φ) Σ, spin density matrix elements

“Hangers on”

KΛ, KΣ production Σ, P, Ox,Oz

η, η` Σ

π0p, π+n Σ

Common motivational thread: missing resonances, resonance spectroscopy

Polarization observables

Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 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 Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 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 Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 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 Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006

The coherent bremsstrahlung facility at CLAS

Goniometer, G.W. University

Diamond

Rocking curve from SRS Daresbury

J.Kellie, Glasgow

Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 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 Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006

g8 preliminary results

Peak > 90% pol.

K.Livingston, Glasgow

Some statistics

High statistics: > 10 billion events

Open trigger: ie many channels

5 peak positions: 1.3 – 2.1 GeV.

High polarization: eg. peak > 90% at 1.3GeV

g8a (July 2001)

The commissioning experiment for the polarized photon beam.

Status of analysis: Preliminary results, (2 PhD, 2 Masters theses)

g8b (July 2005)

High quality data set with high polarization.

Status of analysis: Calibration almost complete

Preliminary analysis: Some very early asymmetry measurements

Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006

g8a preliminary results: 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 Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006

g8a preliminary results: 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 Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006

Pγ estimated at 0.8

Blue line is SAID prediction

Data with statistical errors (no systematic)

preliminary

g8b preliminary results: γp → Nπ photon asymmetry

preliminary

Preliminary analysis by Mike Dugger, ASU

Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006

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.

KΛ, KΣ production Σ, P, Ox, Oz, Cx, Cz

Repeat g8b kaon channels on proton in medium. Look for FSI effects.

Beamtime begins mid October 2006: About 2 weeks from now.

The future: g13

Statisical uncertainties for 36 allocated days

γn --> Κ0s Λ

Blue: CSI model with D13(1900)

Red: CSI model without D13(1900)

Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 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 Feb 2007

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 Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 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 + circularly polarized photons, longitudinal + transverse target polarization, recoil polarization (hyperons are self analysing)

A complete measurement is in our sights.