dan watts (edinburgh), igor strakovsky (gwu) and the clas collaboration daria sokhan new measurement...
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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
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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
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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)
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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
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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!
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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
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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).
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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
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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
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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
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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.
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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.
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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
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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
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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
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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
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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.
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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.
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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
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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
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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),
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JLab Users’ Meeting – 8 June 2010Daria Sokhan
Preliminary Measurement I
from new SAID PWA fit: 2.6SAID 09 MAID 07
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JLab Users’ Meeting – 8 June 2010Daria Sokhan
Preliminary Measurement II
from new SAID PWA fit: 2.6SAID 09 MAID 07
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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%
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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
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JLab Users’ Meeting – 8 June 2010Daria Sokhan
THANK YOU!