Dan Watts (Edinburgh), Igor Strakovsky (GWU) and the CLAS

Collaboration

Daria Sokhan

New Measurement of Beam Asymmetry

from Pion Photoproduction on the Neutron using

CLAS

JLab Users’ Meeting – 9 June 2010

IPN Orsay, France

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Forecast…

Nucleon resonance spectrum

Pseudo-scalar Meson Photoproduction

Polarisation observables – Beam Asymmetry

The attraction of pions and importance of neutrons

CLAS and details of the g13 experiment

Highlights of analysis

Preliminary Beam Asymmetry in 1.6 < W < 2.3 GeV

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Nucleon excitation spectrum

Little known - predictions primarily from calculations on the Lattice and phenomenological models (eg: constituent quark-model, di-quark model, quark – flux tube model…)

Clear indication of resonances

Free proton photoabsorbtion cross-sections from various meson channels (PDG 2002)

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Observed resonance spectrum

PDG existence rating: **** Certain ** Evidence fair

Very likely to certain. Further confirmation desirable and / or quantum number, branching fractions, etc not well determined

***

* Evidence poor

PDG 2004

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Missing resonances

Not there in nature

Or simply not yet observed

• N* (I = 1/2)

• (I = 3/2)

Many more resonances are predicted by some models than observed

Missing resonances

Too many ambiguities as insufficient experimental observables measured!

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Real photons – well understood EM interaction, giving access to EM properties of resonances.

Meson Photoproduction

• 4 invariant complex reaction amplitudes • Experimentally, 16 single and double polarisation observables

Meson photoproduction – for pseudo-scalar mesons:

Polarised: Beam Target Recoil

12 double-polarisation observables&

Beam

Recoil

Target

Partial Wave Analysis (PWA) fits to observables are used to extract resonance parameters (eg: angular momentum, parity), eg: SAID, MAID

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Barker, A. Donnachie, J. Storrow, Nucl. Phys. B 95, 347 (1975)

Polarisation Observables

Direction of target polarisation

Component of recoil polarisation measurement

a) Notation: { ; ; }T RP P P

P

TP

RP

Beam polarisation

Linear polarisation at angle θ to scattering plane

( )L

C Circular polarisation

Complete measurement requires cross-

section, and four

double-polarisation observables!

W.-T. Chiang and F. Tabakin,Phys. Rev. C 55, 2054 (1997).

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Beam Asymmetry

Beam asymmetry, , from linearly polarised photons – crucial observable to constrain PWA.

0(1 cos2 )lin

dP

d

Many wide, overlapping resonances expected to couple to the pion channel.

Current experiment:

Advantages of pion photoproduction: sensitivity to many resonances, large cross-section, easy detection.

n p

JLab Users’ Meeting – 8 June 2010Daria Sokhan

EM interaction does not conserve isospin, so multipole amplitides contain isoscalar and isovector contributions of EM current:

The importance of the Neutron

0

12 2( )(0) (1) 31 1 2

3 3 3

II

p pA A A A

Proton data alone does not allow separation of the isoscalar, A(0), and isovector, A(1), components.

Need data on both proton and neutron!

12 2( )(0) (1) 31 1 1 2

3 3 32

II

p nA A A A

0

12 2( )(0) (1) 31 1 2

3 3 3

II

n nA A A A

12 2( )(0) (1) 3

1 1 1 2

3 3 32

II

n nA A A A

Proton Neutron

JLab Users’ Meeting – 8 June 2010Daria Sokhan

World Data: Σ off the Neutron

… has very few points from the neutron (most data is from proton).

… is in a limited polar-angle and energy range.

Alspector, PRL 28, 1403 (1972).• Abrahamian, SJNP 32, 69 (1980).• Adamyan, JPG 15, 1797 (1989).

Our experiment has added 1179 new data points to the previous

set of 67

JLab Users’ Meeting – 8 June 2010Daria Sokhan

CLAS

Multi-layer onion of detectors, optisimised for charged particle detection

Very large angular coverage:Near full coverage in azimuthal angle and from 8° to 140° in scattering angle.

JLab Users’ Meeting – 8 June 2010Daria Sokhan

The Photon Beam

Produced via bremsstrahlung of electron beam in a radiator

Energies up to 6 GeV

Photons “tagged” in the Tagger → culprit photon causing reaction can be identified and its energy measured.

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Coherent bremsstrahlung from unpolarised electrons passing through a highly ordered crystalline radiator, typically 20 – 50 µm diamond.

Crystal orientation chosen to produce a “coherent” peak of polarised photons at the required energy.

Polarisation up to > 90%.

Linearly polarised photons

JLab Users’ Meeting – 8 June 2010Daria Sokhan

The g13b Experiment

Experimental run: March – June 2007

Electron energies: 3.3 – 5.2 GeV

Linearly polarised photons produced via coherent bremsstrahlung

Six photon energy settings in range: 1.1 - 2.3 GeV, with two orthogonal polarisation orientations.

Target: liquid Deuterium

Single charged particle trigger. Total of 3∙1010 events

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Reaction Identification I Deuterium target – quasi-free reaction with spectator proton:

• Cut on events with two particles, momentum-dependent cut on proton and pion.

Identify the channel: channel:( , )n p ( )spectatord p p

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Reaction Identification II

• Cut on “missing mass” – for the spectator proton.

( )spectatord p p

• Fiducial cuts to remove areas of bad acceptance

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Cut on low “missing momentum” below 0.12 GeV where quasi-free contribution dominates.

deuterium p spectatorP P P P P

Cut on proton and pion back-to-back in CMS: coplanarity.

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Photon-spotting Energy of each photon measured by the tagger.

Identify exact photon from timing coincidence – beam in 2 ns bunches.

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Extracting the Asymmetry

Reaction axes:

pz p

p py

p p

in the Centre of Momentum System (CMS)

φ: angle of beam polarisation plane in CMS w.r.t. reaction plane.

Asymmetry from cos(2φ) fit to the φ-distribution of pions:

0(1 cos2 )lin

dP

d

JLab Users’ Meeting – 8 June 2010Daria Sokhan

To reduce systematics, beam polarisation plane rotated between two orthogonal directions during experiment.

extraction

(1 cos2 )

(1 cos2 )

cos2

N N P

N N P

N NP

N N

Fit with:

cos(2 )A B C

Where B = PΣ

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Check of FSI

< 30 MeV “missing momentum” 95 – 120 MeV “missing momentum”

• Quasi-free nucleon good approximation to a free nucleon: V. Vegna et al., Chin. Phys. C 33, 1249 (2009),

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Preliminary Measurement I

from new SAID PWA fit: 2.6SAID 09 MAID 07

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Preliminary Measurement II

from new SAID PWA fit: 2.6SAID 09 MAID 07

JLab Users’ Meeting – 8 June 2010Daria Sokhan

Preliminary

• Alspector, PRL 28, 1403 (1972).• Abrahamian,SJNP 32, 69 (1980).• Adamyan, JPG 15, 1797 (1989).

Existing data:

Only statistical error shown!

Systematics: ~ 10%

JLab Users’ Meeting – 8 June 2010Daria Sokhan

To conclude… Beam Asymmetry measured in range 1.6 < W < 2.3 GeV, for the channel

A sizeable asymmetry changing both with scattering angle and energy can be observed.

Data included in a new SAID PWA analysis – good 2, significant changes from previous SAID PWA observed

Greatly expanded the sparse world data set on the neutron with > 1000 additional points

Will aid in constraining amplitudes of PWAs, en route to a “complete measurement” of polarisation observables.

Shed new light on the nucleon excitation spectrum!

n p

JLab Users’ Meeting – 8 June 2010Daria Sokhan

THANK YOU!