Beam Asymmetry measurement in Pion ... - MENU 2013menu2013.roma2.infn.it/talks/tuesday_baryons_4/4... · Daria Sokhan LPSC, Grenoble – 21 April 2011 Daria Sokhan University of Glasgow,

LPSC, Grenoble – 21 April 2011 Daria Sokhan

Daria Sokhan University of Glasgow, UK

on behalf of the CLAS Collaboration

MENU2013, Rome, Italy 1st October 2013

Beam Asymmetry measurement in

Pion Photoproduction on the

neutron using CLAS


Meson Photoproduction

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

Meson photoproduction – for pseudo-scalar mesons:

Σ T R

Polarised: Beam Target Recoil

12 double-polarisation observables &

Beam

Recoil

Target

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

MENU2013, Rome, Italy – 1st Oct 2013


From observables to the resonance spectrum

New measurement of observable

Partial Wave Analyses

Multipoles

World data set, all measured observables

Partial Wave Analyses

Resonance parameters



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



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, Σ, T, R

and four double-polarisation observables!

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



What makes up a cross-section



Experiment: linearly polarised beam, unpolarised target

Focus on: Beam Asymmetry Σ



Measuring Beam Asymmetry

Beam asymmetry, Σ, 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. Large cross-section: precision data.

Experiment using CLAS @ JLab – exclusive measurement of Σ in 1.6 < W < 2.3 GeV on a quasi-free neutron :

( )spectatord p p



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

Why neutrons?

0

12 2( )(0) (1) 3

1 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) 3

1 1 1 2

3 3 32

II

p nA A A A

0

12 2( )(0) (1) 3

1 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



CEBAF: Continuous Electron Beam Accelerator Facility:

CLAS @ Jefferson Lab (Virginia, USA)

Duty cycle: ~ 100%

Energy up to ~6 GeV

Electron polarisation up to ~85%

CLAS in Hall B:

Drift chambers

Toroidal magnetic field

Cerenkov Counters

Scintillator Time of Flight

Electromagnetic

Calorimeters

Very large angular coverage – near full azimuthal and 8° to 140°

scattering angle


The Photon Beam

Goniometer for tilting radiator

e-

Linearly polarised (up to > 90%) photon beam: bremsstrahlung of unpolarised electrons in 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.



The Photon Beam


“Tagger” – momentum analyses deflected electrons

e- e-

γ



Culprit photon causing reaction can be identified and its energy calculated



The Photon Beam


“Tagger” – momentum analyses deflected electrons

Target within CLAS

e- e-

γ γ



Culprit photon causing reaction can be identified and its energy calculated



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



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). •Mandaglio, PRC 82, 045209 (2010).



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.

Our experiment has added ~1180 new data points to the previous

set of 166

Alspector, PRL 28, 1403 (1972). • Abrahamian, SJNP 32, 69 (1980). • Adamyan, JPG 15, 1797 (1989). •Mandaglio, PRC 82, 045209 (2010).



Reaction Identification

Deuterium target – quasi-free reaction with spectator proton:

Identify the channel: ( , )n p

( )spectatord p p

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

Identify p, π- final state



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.

𝑝𝑥 CM angle



Photon-spotting

Energy of each photon measured by the tagger.

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



Photon-spotting

Energy of each photon measured by the tagger.

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



Measuring the Beam Asymmetry Σ

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

Reaction plane



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

Σ from fits

At its simplest, fit with:

cos(2 )A B C

Where B = PΣ

𝑁∥ = 𝑁𝜃(1 − 𝑃Σ𝑐𝑜𝑠2𝜙)

𝑁⊥ = 𝑁𝜃(1 + 𝑃Σ𝑐𝑜𝑠2𝜙)

𝑃Σ𝑐𝑜𝑠2𝜙 = 𝑁∥ − 𝑁⊥

𝑁∥ + 𝑁⊥

Actual fit function more complicated, to account for imperfect normalisation, etc…



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

Σ from fits

At its simplest, fit with:

cos(2 )A B C

Where B = PΣ

𝑁∥ = 𝑁𝜃(1 − 𝑃Σ𝑐𝑜𝑠2𝜙)

𝑁⊥ = 𝑁𝜃(1 + 𝑃Σ𝑐𝑜𝑠2𝜙)

𝑃Σ𝑐𝑜𝑠2𝜙 = 𝑁∥ − 𝑁⊥

𝑁∥ + 𝑁⊥

Actual fit function more complicated, to account for imperfect normalisation, etc…



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),


PRELIMINARY !!!


• Graal: Madaglio et al, PRC 82 045209 (2010) MAID 07 SAID 11 BoGa 11

Preliminary Σ Measurement: θ > 90º ~ 1/4 of the g13 dataset shown below



Preliminary Σ Measurement: θ < 90º

MAID 07 SAID 11 BoGa 11

~ 1/4 of the g13 dataset shown below

• Graal: Madaglio et al, PRC 82 045209 (2010)



A closer took at discrepancy with GRAAL

Discrepancies too large to be due entirely to systematics!

Different treatment of Fermi momentum?

CLAS data: Fermi-momentum calculated from quasi-exclusive reconstruction of the reaction: 𝛾 + 𝑑 → 𝜋− + 𝑝 + 𝑋. GRAAL data: Fermi momentum reconstructed through an iterative match to a Monte Carlo simulation.

• Graal: Madaglio et al, PRC 82 045209 (2010)



Incompatible bins?

The beam asymmetry varies dramatically with the angle – slight differences in the composition of the bin may yield significant differences in Σ

Reasons for CLAS – GRAAL discrepancy under investigation!

c2 from SAID-09 PWA

fit including GRAAL data: 89

c2 from SAID-09 PWA

fit including g13 data, but not GRAAL: 2.6

A closer took at discrepancy with GRAAL


PRELIMINARY !!!


In Conclusion…

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

Comparison with the most recent PWA predictions shows significant differences in certain W regions. Inclusion of preliminary data into SAID09 PWA gave a good c2

Discrepancy with Graal measurement under investigation. 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.

Beam Asymmetry Σ measured in range 1.6 < W < 2.3 GeV, for the channel ( )spectatord p p



THANK YOU !


Documents

Beam Asymmetry measurement in Pion ... - MENU 2013menu2013.roma2.infn.it/talks/tuesday_baryons_4/4... · Daria Sokhan LPSC, Grenoble – 21 April 2011 Daria Sokhan University of Glasgow,