Two photon exchange and transverse spin asymmetries in the...
Transcript of Two photon exchange and transverse spin asymmetries in the...
Two photon exchange and transverse spinasymmetries in the A4 experiment.
David Balaguer Rios
PAVI09Bar Harbor
A4 CollaborationInstitut für Kern Physik, Mainz
June 25, 2009
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Introduction
Experimental set up
Data analysis
Results H2 data
Results D2 data
Summary and outlook
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Outline
Introduction
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Two photon exchange
◮ R = GpE/Gp
M discrepancy
◮ 2γ exchange amplitude A2γ
◮ Target and beam normal SSA sensitive to Im(A2γ).
◮ Dispersion relations between imaginary and real part.
◮ PVA experiments set up: transverse beam spin asymmetry A⊥
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Transverse spin asymmetry
Am⊥ =
σ↑ − σ↓
σ↑ + σ↓= A⊥(θ)~Pe ·
~S = A⊥ cos φ
~k
~k′
θe
φ-π2
~S⊥ =~k×~k′
|~k×~k′|
~P
φ
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Transverse spin asymmetry
Am⊥ =
σ↑ − σ↓
σ↑ + σ↓= A⊥(θ)~Pe ·
~S = A⊥ cos φ
~k
~k′
θe
φ-π2
~S⊥ =~k×~k′
|~k×~k′|
~P
φ
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Outline
Experimental set up
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MAMI floor plan
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Polarized electron beam source
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Calorimeter, target and luminosity monitors
◮ Detector covers 2πφ.
◮ Counts singleevents andmeasures energy.
◮ Target of liquid hy-drogen/deuterium.
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Plastic scintillators
◮ Plastic scintillators detect charged particles. Neutral particlesnot detected.
◮ 72 plastic scintillators: two rings of 36 withoverlap.
◮ One scintillator covers two frames: 14 modules.
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Medusa
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Measurements performed at backward angles
Target Energy (MeV) angle (deg) Q2 (GeV/c)2
H2 315.1 MeV 140 − 150 0.23D2 315.1 MeV 140 − 150 0.23
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Outline
Data analysis
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Energy spectra for H2 target
◮ Counts and energy: energy spectrum histograms every 5 min.
◮ Two histograms for each polarization state.
◮ Scintillators: a histogram for charged particles and one forneutral paticles.
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Energy spectra for H2 target
◮ Counts and energy: energy spectrum histograms every 5 min.
◮ Two histograms for each polarization state.
◮ Scintillators: a histogram for charged particles and one forneutral paticles.
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Energy spectra for H2 target
◮ Counts and energy: energy spectrum histograms every 5 min.
◮ Two histograms for each polarization state.
◮ Scintillators: a histogram for charged particles and one forneutral paticles.
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Energy spectra for H2 target
◮ Separation of the elastic peak in the charged particles spectrum
◮ Still neutral background from γ → e−e+
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Energy spectra for H2 target
◮ Separation of the elastic peak in the charged particles spectrum
◮ Still neutral background from γ → e−e+
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Understanding the energy spectrum
◮ Monte Carlo Geant4 simulation: e− processes and γs
◮ Background from Al walls: measurement with empty target
◮ Agreement above 125 MeV
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Background obtained from neutral spectrum
◮ γ background asymmetry?
◮ From experimental spectrumof neutral particles
◮ Model to obtain γ background
◮ Parameters
◮ shift δ◮ scaling factor ǫ
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Background obtained from neutral spectrum
◮ Scaling shifting modelagrees with simulatedbackground:
◮ above 125 MeV
◮ energy range of ourinterest
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Extraction of the asymmetry from the spectra
◮ f =Nback
e
Nbacke + Nel
e
◮ Cuts applied f = 5, 9, 16%
◮ Ae =N+
e − N−
e
N+e + N−
e
◮ Aγ =N+
γ− N−
γ
N+γ
+ N−
γ
◮ Arawphys =
Ae − f Aγ
1 − f
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Extraction of the asymmetry from the spectra
◮ f =Nback
e
Nbacke + Nel
e
◮ Cuts applied f = 5, 9, 16%
◮ Ae =N+
e − N−
e
N+e + N−
e
◮ Aγ =N+
γ− N−
γ
N+γ
+ N−
γ
◮ Arawphys =
Ae − f Aγ
1 − f
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Extraction of the asymmetry from the spectra
◮ f =Nback
e
Nbacke + Nel
e
◮ Cuts applied f = 5, 9, 16%
◮ Ae =N+
e − N−
e
N+e + N−
e
◮ Aγ =N+
γ− N−
γ
N+γ
+ N−
γ
◮ Arawphys =
Ae − f Aγ
1 − f
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Extraction of the asymmetry from the spectra
◮ f =Nback
e
Nbacke + Nel
e
◮ Cuts applied f = 5, 9, 16%
◮ Ae =N+
e − N−
e
N+e + N−
e
◮ Aγ =N+
γ− N−
γ
N+γ
+ N−
γ
◮ Arawphys =
Ae − f Aγ
1 − f
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Extraction of the asymmetry from the spectra
◮ f =Nback
e
Nbacke + Nel
e
◮ Cuts applied f = 5, 9, 16%
◮ Ae =N+
e − N−
e
N+e + N−
e
◮ Aγ =N+
γ− N−
γ
N+γ
+ N−
γ
◮ Arawphys =
Ae − f Aγ
1 − f
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Extraction of the asymmetry from the spectra
◮ f =Nback
e
Nbacke + Nel
e
◮ Cuts applied f = 5, 9, 16%
◮ Ae =N+
e − N−
e
N+e + N−
e
◮ Aγ =N+
γ− N−
γ
N+γ
+ N−
γ
◮ Arawphys =
Ae − f Aγ
1 − f
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Combination of asymmetries
◮ Detector divided in sectors: azimuthal modulation of A⊥
◮ A⊥ averaged over the scattering angle θ
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Outline
Results H2 data
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Raw asymmetry dependence on φ / H2
Q2 = 0.23 (GeV/c)2
A4 backwards kinematics
f = 9%Aexp = A⊥ cos(φ + δ) + p
(deg)φ
0 50 100 150 200 250 300 350
pp
m
-60
-40
-20
0
20
40
60
80Chi2/ndf = 3.03/5
Prob = 0.70
A = (-66.41 +- 4.92) ppm
d = (5.09 +- 4.24) deg
p = (5.15 +- 3.39) ppm
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GVZ test of systematics
GVZ = OUT
(deg)φ
0 50 100 150 200 250 300 350
pp
m
-60
-40
-20
0
20
40
60
80
100
Chi2/ndf = 1.07/5
Prob = 0.96
A = (-71. 58 +/- 6.90) ppm
d = (6.93 +/- 5.52) deg
p = (11.74 +/- 4.76) ppm
GVZ = IN
(deg)φ
0 50 100 150 200 250 300 350
pp
m
-60
-40
-20
0
20
40
60
80 Chi2/ndf = 2.69/5
Prob = 0.75
A = (61.17 +/- 7.00) ppm
d = (2.90 +/- 6.56) deg
p = (1.63 +/- 4.83) ppm
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Preeliminary results for H2 data
◮ 5 inner rings: 730 crystals.
◮ 50 h of data taking.
◮ Altogether 1.8 · 1011 elastic events.
◮ Effective Pe = 70.0%, error ∆(Pe) = 4%
At A4 Backward kinematics and Q2 = 0.23 (GeV/c)2
A⊥ = (−86.65 ± 3.35 ± 3.46) ppm
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Outline
Results D2 data
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Neutral and charged particles spectra for D2
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Neutral and charged particles spectra for D2
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Raw asymmetry dependence on φ / D2
Q2 = 0.23 (GeV/c)2
A4 backwards kinematics
f = 16%Aexp = A⊥ cos(φ + δ) + p
(deg)φ
0 50 100 150 200 250 300 350
pp
m
-40
-20
0
20
40
Chi2/ndf = 4.23/5
Prob = 0.52
A = (-41.02 +/- 3.71) ppm
d = (-5.25 +/- 5.15) deg
p = (3.01 +/- 2.55) ppm
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GVZ test of systematics
GVZ = OUT
(deg)φ
0 50 100 150 200 250 300 350
pp
m
-40
-20
0
20
40
60
Chi2/ndf = 7.19/5
Prob = 0.21
A = (-39.79 +/- 5.37) ppm
d = (3.21 +/- 7.71) deg
p = (0.97 +/- 3.70) ppm
GVZ = IN
(deg)φ
0 50 100 150 200 250 300 350
pp
m
-60
-40
-20
0
20
40
60 Chi2/ndf = 0.9/5
Prob = 0.97
A = ( 44.96 +/- 8.95)
d = (-13.3 +/- 11.4) deg
p = (-10.2 +/- 6.2) ppm
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Preeliminary results for D2 data
◮ 5 inner rings: 730 crystals.
◮ 60 h of data taking.
◮ Altogether 2.5 · 1011 elastic events
◮ Effective Pe = 78.4%, error ∆(Pe) = 4%
At A4 Backward kinematics and Q2 = 0.23 (GeV/c)2
A⊥ = (−57.60 ± 2.54 ± 3.07) ppm
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Systematic errors
Systematic error contributionPolarization 2.30 ppm
False asymmetries 0.64 ppmModel parameter δ 1.3 ppmModel parameter ǫ 0.25 ppm
Target density 0.03 ppmSpin deviation 1.4 ppm
Aluminium —Pile-up —
Nonlinearity LuMo —Total 3.07 ppm
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Transverse spin asymmetry in neutralbackground
f = 12%
(deg)φ
0 50 100 150 200 250 300 350
pp
m
-40
-30
-20
-10
0
10
20
30
40
50Chi2/ndf = 4.2/5
Prob = 0.53
A = ( -36.6 +/- 3.0) ppm
d = (-0.36 +/- 4.68) deg
p = ( 4.22 +/- 2.06) ppm
f = 16%
(deg)φ
0 50 100 150 200 250 300 350
pp
m
-40
-20
0
20
40
60
Chi2/ndf = 1.45/5
Prob = 0.92
A = ( -48.65 +/- 2.65) ppm
d = (-1.68 +/- 2.87) deg
p = ( 2.01 +/- 1.68) ppm
f = 20%
(deg)φ
0 50 100 150 200 250 300 350
pp
m
-40
-20
0
20
40
60Chi2/ndf = 3.08/5
Prob = 0.69
A = ( -53.75 +/- 2.08) ppm
d = (-3.33 +/- 2.21) deg
p = ( -0.25 +/- 1.43) ppm
◮ Aback⊥ is large and depends on
the cut:
◮ A⊥ sensitive to backgroundsubtraction
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Summary of asymmetries
Target elastic backgroundH2 −86.65 ± 3.35 ppm −100.60 ± 2.78 ppmD2 −57.60 ± 2.54 ppm −62.37 ± 2.28 ppm
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Comparision of measured and calculated A⊥
Calculation of A⊥ for proton 1
-140
-120
-100
-80
-60
-40
-20
0
0 20 40 60 80 100 120 140 160 180
An
[ppm
]
θlab[deg]
1Resonance estimates for single spin asymmetries in elasticelectron-nucleon scattering. B. Pasquini et al. physical review c 70. 045206(2004)
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Outline
Summary and outlook
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Summary
◮ Transverse spin asymmetry with H2 and D2 at Q2 = 0.23(GeV/c)2 and backward angles.
◮ Detector of plastic scintillators to separate neutral background
◮ Understanding the energy spectrum and mixed γ background
◮ Modulation of the transverse spin asymmetry on cos φ
◮ Combination of whole data. Preliminary results
◮ Comparision of the measured transverse spin asymmetries withthe calculations.
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Outlook
◮ Analysis in progress: sensitivity of A⊥ to background subtraction.
◮ Investigation the transverse spin asymmetry in the neutralbackground.
◮ Analysis of data at backward angles with H2 and D2 andQ2 = 0.35 (GeV/c)2
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Previous A⊥ at A4 forward kinematics
570 MeV
-70
-60
-50
-40
-30
-20
-10
0
0 20 40 60 80 100 120 140 160 180
An
[ppm
]
θlab[deg]
855 MeV
-25
-20
-15
-10
-5
0
5
0 20 40 60 80 100 120 140 160 180
An
[ppm
]
θlab[deg]
Target l-H2
A⊥(Q2 = 0.11(GeV/c)2) = (−8.59 ± 0.89 ± 0.75) ppm
A⊥(Q2 = 0.23(GeV/c)2) = (−8.52 ± 2.31 ± 0.87) ppm
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