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
David Balaguer Rios, June 25, 2009 5/39
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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David Balaguer Rios, June 25, 2009 11/39
David Balaguer Rios, June 25, 2009 11/39
David Balaguer Rios, June 25, 2009 11/39
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.
David Balaguer Rios, June 25, 2009 16/39
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.
David Balaguer Rios, June 25, 2009 16/39
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
David Balaguer Rios, June 25, 2009 21/39
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
David Balaguer Rios, June 25, 2009 21/39
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
David Balaguer Rios, June 25, 2009 21/39
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
David Balaguer Rios, June 25, 2009 21/39
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
David Balaguer Rios, June 25, 2009 21/39
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
David Balaguer Rios, June 25, 2009 39/39
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