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Parity violating electron backscattering
David Balaguer Rios
A4 CollaborationInstitut für Kern Physik, Mainz
February 9, 2009
David Balaguer Rios, February 9, 2009 1/37
Introduction
Experimental set up
Monte Carlo studies
Data analysis
Results
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Outline
Introduction
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The strange vector form factor
u
u
d u
u
dg
g
g
uu
d ss
d
◮ Nucleon: constituent quarks u,d, gluons, sea quarks qq̄ (q =u,d,s)
◮ s in nucleon: pure sea quark effect
◮ Contribution of s to the electromagnetic vector form factors ofthe nucleon?
◮ Measurement of the strange vector electric and magnetic formfactors: Gs
E and GsM
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Form factors flavor decomposition
Flavor decomposition
GpE,M =
2
3Gu
E,M −1
3Gd
E,M −1
3Gs
E,M
GnE,M =
2
3Gd
E,M −1
3Gu
E,M −1
3Gs
E,M
Weak form factors to access the strange vector form factor
G̃pE,M =
(
1
4−
2
3sin θW
)
GuE,M −
(
1
4−
1
3sin θW
)
GdE,M−
−(
1
4−
1
3sin θW
)
GsE,M
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Parity violating elastic electron scattering
◮ Parity violating elastic electron scattering to access G̃pE,M
◮ Parity violation: interference between electromagnetic and weak
amplitudes
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Parity violating asymmetry
◮ To measure: parity violating asymmetry in the cross section
APV =σR − σL
σR + σL
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Parity violating asymmetry
◮ To measure: parity violating asymmetry in the cross section
APV =σR − σL
σR + σL
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Parity violating asymmetry
◮ To measure: parity violating asymmetry in the cross section
APV =σR − σL
σR + σL
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Parity violating asymmetry
APV = A0 + As
◮ A0 calculated
◮ Input parameters: GpE, G
pM, Gn
E, GnM, G̃
p
A, Gµ, α, Q2, θ
Q2 = 0.23(
GeV/c)2
, A4 backwards kinematics
A0 = (−15.87± 1.22) · 10−6
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Parity violating asymmetry
As = −GFQ
2
4πα√2
{
−ǫGp
EGsE + τGp
MGsM
ǫ(GpE)
2 + τ(GpM)2
}
τ =Q2
4M2
ǫ =1
1 + 2(1 + τ) tan2 θ2
As = APV − A0
◮ Single measurement of As: linear combination of GsE and Gs
M.
◮ Two different measurements of As: separation of GsE and Gs
M.
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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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Target, calorimeter and luminosity monitors
◮ Cooling system:avoid boiling and ρtfluctuations
◮ Calorimeter energyresolution
3.9%/√E
◮ Calorimeter and
electronics: 20 ns
dead time
◮ LuMos:
measurement ofluminosity
fluctuations
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Detector at forward angles
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Rotated detector at backward angles
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PbF2 energy spectrum
◮ Elastic peak clearlyseparated
◮ Background of π0 → 2γenergetically separated.
◮ Elastic peak not separated
◮ Background of γs and elastic
peak: same energy range
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PbF2 energy spectrum
◮ Elastic peak clearlyseparated
◮ Background of π0 → 2γenergetically separated.
◮ Elastic peak not separated
◮ Background of γs and elastic
peak: same energy range
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Plastic scintillators
◮ Plastic scintillators detect charged particles. Neutral particlesnot detected.
◮ 72 plastic scintillators: two rings of 36 withoverlap.
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Plastic scintillators location
◮ Plastic scintillators located in front of calorimeter.
◮ One scintillator covers two frames: 14 modules.
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Generated histograms
◮ Pola-bit signal: two histograms: two polarization states
◮ Plastic scintillators add an Address-bit: two histograms: one of
neutral particles and one of charged particles
◮ Every 5 min four histograms are generated
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Neutral and charged particles spectra
◮ Separation of the elastic peak in the charged particles spectrum
◮ The scintillators do their job!
◮ Still some γ background mixed with elastic peak
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Neutral and charged particles spectra
◮ Separation of the elastic peak in the charged particles spectrum
◮ The scintillators do their job!
◮ Still some γ background mixed with elastic peak
David Balaguer Rios, February 9, 2009 20/37
Neutral and charged particles spectra
◮ Separation of the elastic peak in the charged particles spectrum
◮ The scintillators do their job!
◮ Still some γ background mixed with elastic peak
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Background in the charged particles spectrum
◮ π0 → 2γ and γ → e+ + e− in materials before calorimeter
Calorimeter
Plastic scintillators
Electron beamScattering chamber
◮ γ background in the spectrum of charged particles
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Outline
Monte Carlo studies
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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 PVA?
◮ From experimental spectrum
of 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 simulated
background:
◮ above 125 MeV
◮ energy range of ourinterest
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Outline
Data analysis
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Extraction of the asymmetry from the spectra
◮ f =Nbacke
Nbacke + Nel
e
(1 − 10%)
◮ Cuts applied
◮ Ae =N+e − N−
e
N+e + N−
e
◮ Aγ =N+
γ− N−
γ
N+γ + N−
γ
◮ Arawphys =
Ae − fAγ
1− f
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Extraction of the asymmetry from the spectra
◮ f =Nbacke
Nbacke + Nel
e
(1 − 10%)
◮ Cuts applied
◮ Ae =N+e − N−
e
N+e + N−
e
◮ Aγ =N+
γ− N−
γ
N+γ + N−
γ
◮ Arawphys =
Ae − fAγ
1− f
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Extraction of the asymmetry from the spectra
◮ f =Nbacke
Nbacke + Nel
e
(1 − 10%)
◮ Cuts applied
◮ Ae =N+e − N−
e
N+e + N−
e
◮ Aγ =N+
γ− N−
γ
N+γ + N−
γ
◮ Arawphys =
Ae − fAγ
1− f
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Extraction of the asymmetry from the spectra
◮ f =Nbacke
Nbacke + Nel
e
(1 − 10%)
◮ Cuts applied
◮ Ae =N+e − N−
e
N+e + N−
e
◮ Aγ =N+
γ− N−
γ
N+γ + N−
γ
◮ Arawphys =
Ae − fAγ
1− f
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Extraction of the asymmetry from the spectra
◮ f =Nbacke
Nbacke + Nel
e
(1 − 10%)
◮ Cuts applied
◮ Ae =N+e − N−
e
N+e + N−
e
◮ Aγ =N+
γ− N−
γ
N+γ + N−
γ
◮ Arawphys =
Ae − fAγ
1− f
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Extraction of the asymmetry from the spectra
◮ f =Nbacke
Nbacke + Nel
e
(1 − 10%)
◮ Cuts applied
◮ Ae =N+e − N−
e
N+e + N−
e
◮ Aγ =N+
γ− N−
γ
N+γ + N−
γ
◮ Arawphys =
Ae − fAγ
1− f
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Data analysis program
◮ Asymmetry is extracted for every module and every run.
◮ Statistical treatment of the complete set of asymmetries.
◮ Correction of systematics and evaluation of systematic errors.
◮ Normalization to the electron beam polarization Aphys =Aexp
Pe
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Statistical error
◮ 5 inner rings of the calorimeter: 730 crystals
◮ 1100 hours of data taking
◮ Altogether 3 · 1012 elastic events
◮ Statistical error: σ(A) =1
Pe
√Nel
⇒ σ(A) = 0.82 ppm
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Systematic errors
Correction(ppm) Error(ppm)
Polarization −5.58 0.69Helicity corr. beam diff. 0.14 0.39Dilution of γ backgr. −1.49 0.28Al windows 0.29 0.04Random coinc. events −0.19 0.02Sum syst. errors 0.89
◮ Effective Pe = 68.3%, error ∆(Pe) = 4%
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GVZ checking
◮ Half of measurement with GVZ in. The rest with GVZ out
◮ GVZ in: physics asymmetry should flip sign. Same magnitude.
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Outline
Results
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Physics asymmetry
◮ Physics asymmetry at Q2 = 0.23 (GeV/c)2 and A4 backwards
kinematics
APV = (−17.23± 0.82stat ± 0.89syst) · 10−6
◮ Expectation A0
A0 = (−15.87± 1.2) · 10−6
◮ Origin of ∆A0
G̃p
A = −0.78± 0.28
GpE = +0.5746± 0.0095 (1.7%) G
pM = +1.621± 0.030 (1.9%)
GnE = +0.0525± 0.0057 (11%) Gn
M = −1.092± 0.024 (2.2%)
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Measurements and predictions of GsE,M
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Measurements and predictions of GsE,M
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Measurements and predictions of GsE,M
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Measurements and predictions of GsE,M
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Measurements and predictions of GsE,M
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Separation of GsE and Gs
M
◮ Disentangling the linear combinations:
GsE = 0.050± 0.038± 0.019
GsM = −0.14± 0.11± 0.11
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Summary
◮ The A4 experimental objective: measurement of GsE and Gs
M atQ2 = 0.23 (GeV/c)2 with different kinematics
◮ Demanding experimental set up to measure APV ∼ 10−5
◮ Rotation of calorimeter to measure at backwards angles
◮ Detector of plastic scintillators to separate neutral background
◮ Understading the energy spectrum and mixed γ background
◮ Successful extraction of PVA from single spectra
◮ Combination of whole data. Systematic errors. Pe correction.
◮ Separation of GsE and Gs
M from different measurements
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Outlook
◮ A measurement of parity violating asymmetry with deuterium as
target is done.
◮ Analysis in progress.
◮ This measurement allows the separation of the GsM and the G̃
pA.
◮ Detector rotated back to forward angles to measure the parity
violating asymmetry at 1.5 GeV, Q2 = 0.62 (GeV/c)2
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