Download and the CLAS Collaboration Daria Sokhan New Measurement of

New Measurement of Beam
Asymmetry from Pion
Photoproduction on the
Neutron using CLAS
Daria Sokhan
IPN Orsay, France
Dan Watts (Edinburgh), Igor Strakovsky (GWU)
and the CLAS Collaboration
JLab Users’ Meeting – 9 June 2010
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
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
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
Daria Sokhan
Free proton photoabsorbtion cross-sections from
various meson channels (PDG 2002)
JLab Users’ Meeting – 8 June 2010
Observed resonance spectrum
PDG 2004
PDG existence rating: **** Certain
** Evidence fair
* Evidence poor
*** Very likely to certain. Further confirmation desirable and / or quantum
number, branching fractions, etc not well determined
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
Missing resonances
• N* (I = 1/2)
•D
(I = 3/2)
 Many more resonances are predicted
by some models than observed
Missing resonances
Not there in nature
Or simply not yet observed
 Too many ambiguities as insufficient
experimental observables measured!
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
Meson Photoproduction
 Real photons – well understood EM interaction, giving access to EM
properties of resonances.
 Meson photoproduction – for pseudo-scalar mesons:
• 4 invariant complex reaction amplitudes
• Experimentally, 16 single and double polarisation observables
Polarised:

Beam
Target

Beam
Target
Recoil
S
T
R
&
12 double-polarisation
observables
Recoil
 Partial Wave Analysis (PWA) fits to observables are used to extract
resonance parameters (eg: angular momentum, parity), eg: SAID, MAID
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
Polarisation Observables
Complete
measurement
requires crosssection, S, T, R and
four doublepolarisation
observables!
W.-T. Chiang and F. Tabakin,
Phys. Rev. C 55, 2054 (1997).
a) Notation: {P ; PT ; PR }
P
Beam polarisation
L( ) Linear polarisation at angle
θ to scattering plane
C Circular polarisation
Barker, A. Donnachie, J. Storrow, Nucl. Phys. B 95, 347 (1975)
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
PT
Direction of target
polarisation
PR
Component of recoil
polarisation
measurement
Beam Asymmetry
 Beam asymmetry, S, from linearly polarised photons –
crucial observable to constrain PWA.
d
  0 (1  PlinS cos 2 )
d
 Many wide, overlapping resonances expected to couple to
the pion channel.
 Current experiment:
  n    p
 Advantages of pion photoproduction: sensitivity to many
resonances, large cross-section, easy detection.
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
The importance of the Neutron
 EM interaction does not conserve isospin, so multipole
amplitides contain isoscalar and isovector contributions of
EM current:
Proton
A p 0 p
A p  n
1
1

   A(0)  A(1) 
3
3

I1
1  1 (0) 1 (1) 

A  A 
3
2  3

Neutron
2
I1
2 ( I 2 )
 A 3
3
2
2 ( I 23)

A
3
A n 0n
A n  n
1
1

  A(0)  A(1) 
3
3

I1
1  1 (0) 1 (1) 

A  A 
3
2  3

2
2 ( I 2 )
 A 3
3
I1
2
2 ( I 23 )

A
3
 Proton data alone does not allow separation of the isoscalar,
A(0), and isovector, A(1), components.
 Need data on both proton and neutron!
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
World Data: Σ off the Neutron
 Alspector, PRL 28,
1403 (1972).
• Abrahamian, SJNP 32,
69 (1980).
• Adamyan, JPG 15,
1797 (1989).
 … 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 1179 new data
points to the previous
set of 67
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
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.
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
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.
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
Linearly polarised photons
 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%.
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
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
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
Reaction Identification I
 Deuterium target – quasi-free reaction with spectator proton:

spectator
  d   p  ( p
 Identify the
n( ,  ) p
)
channel:
• Cut on events with two particles, momentum-dependent b
cut on proton and pion.
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
Reaction Identification II
  d     p  ( pspectator )
• Fiducial cuts to
remove areas of bad
acceptance
• Cut on “missing
mass” – for the
spectator proton.
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
 Cut on low “missing momentum” below 0.12 GeV where
quasi-free contribution dominates.
P  P deuterium  P   P p  P spectator
 Cut on proton and pion back-to-back in CMS: coplanarity.
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
Photon-spotting
 Energy of each photon measured by the tagger.
 Identify exact photon from timing coincidence – beam in
2 ns bunches.
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
Extracting the Asymmetry
 Reaction
axes:
z
y
p
in the Centre of
Momentum System
(CMS)
p
p   p 
p   p 
 φ: angle of beam polarisation
plane in CMS w.r.t. reaction plane.
 Asymmetry from cos(2φ) fit to the φ-distribution of pions:
d
  0 (1  PlinS cos 2 )
d
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
S extraction
 To reduce systematics, beam polarisation plane rotated
between two orthogonal directions during experiment.
N  N (1  PS cos 2 )
N   N (1  PS cos 2 )
S P cos 2 
N  N
N  N
Fit with:
A  B cos(2  C )
Where B = PΣ
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
Check of FSI
• Quasi-free nucleon good approximation to a free nucleon:
V. Vegna et al., Chin. Phys. C 33, 1249 (2009),
< 30 MeV “missing momentum”
95 – 120 MeV “missing momentum”
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
Preliminary S Measurement I
SAID 09
Daria Sokhan
MAID 07
c2 from new SAID PWA fit: 2.6
JLab Users’ Meeting – 8 June 2010
Preliminary S Measurement II
SAID 09
Daria Sokhan
MAID 07
c2 from new SAID PWA fit: 2.6
JLab Users’ Meeting – 8 June 2010
Preliminary S
Only statistical
error shown!
Systematics:
~ 10%
Existing data:
• Alspector, PRL 28,
1403 (1972).
• Abrahamian,SJNP
32, 69 (1980).
• Adamyan, JPG 15,
1797 (1989).
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
To conclude…
 Beam Asymmetry
S measured in range
1.6 < W < 2.3 GeV, for the channel
  n    p
 A sizeable asymmetry changing both with scattering angle and
energy can be observed.
 Data included in a new SAID PWA analysis – good c2, 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!
Daria Sokhan
JLab Users’ Meeting – 8 June 2010
THANK YOU!
Daria Sokhan
JLab Users’ Meeting – 8 June 2010