Qweak Transverse Asymmetry Measurements · 23 Summary Qweak have several interesting transverse...
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Qweak Transverse Asymmetry Measurements Buddhini Waidyawansa For the Qweak Collaboration Hall C Collaboration Meeting 02-21-2014
Transcript of Qweak Transverse Asymmetry Measurements · 23 Summary Qweak have several interesting transverse...
Qweak Transverse Asymmetry Measurements
Buddhini Waidyawansa For the Qweak Collaboration
Hall C Collaboration Meeting02-21-2014
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Outline
➔ Physics of transverse asymmetries
➔ Qweak transverse data set
➔ Analysis overview
➔ Analysis updates
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Transverse Asymmetry
● Generated by transversely polarized electrons scattering from unpolarized nucleons.
● Has an azimuthal dependence;
epk1
k2e
s s
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Transverse Asymmetry
● Generated by transversely polarized electrons scattering from unpolarized nucleons.
● Has an azimuthal dependence;
epk1
k2e
s s
Beam Normal Single Spin Asymmetry
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Beam Normal Single Spin Asymmetry
● An observable of the two photon exchange process.
● Parity even and time reversal odd.
● Arise from the interference between one and two photon exchange processes when the beam is transversely polarized
e.g. e+p scattering
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Beam Normal Single Spin Asymmetry
● An observable of the two photon exchange process.
● Parity even and time reversal odd.
● Arise from the interference between one and two photon exchange processes when the beam is transversely polarized
e.g. e+p scattering
Direct access to the imaginary part of two-photon exchange process
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Beam Normal Single Spin Asymmetry
● T1 - calculable using form factors of the nucleon.
● T2 - use Compton scattering from the nucleon to model two photon exchange.
µνµνγ πWl
qqE
kdeAbsT
k
⋅= ∫ 1
2)2( 22
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3
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2
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Leptonic tensor lμν
Hadronic Tensor Wμν : absorptive part of VVCS tensor
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Beam Normal Single Spin Asymmetry
● The hadronic tensor gets contributions from the ground state and the excited states of the nucleon
Ground state
Exactly calculable- on-shell electromagnetic
form factors
X
Excited states
X = p+pi, p+2pi, …
Dominant contributorNot exactly calculable. - large number of excited states- Rely on experimental inputs. e.g. GPDs, electroproduction amplitudes etc.
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BNSSA Calculations vs Measurements
G0
A4 [Source : PRL, 94, 082001, (2005)] G0 forward [Source :PRL 99, 092301 (2007]
D&M
P&V
P&V
MGA&M
Model Intermediate state Input
D&M – Diaconescui & Musolf [Phys. Rev.C. 70, 054003(3004)]
Nucleon Field theory calculation. No pions.
P&V – Pasquini & Vanderhaeghen [Phys.Rev. C70,045206 (2004)]
Nucleon+pion MAID electroproduction amplitude
M. Gorchtein [Phys.Rev. C73, 035312;055201(2006)]
Nucleon+Pion+Pion Photoproduction cross-section
A&M – Afanasev & Merenkov [Phys.Lett. B 599,48 (2004)]
Nucleon+Pion+Pion Photoproduction cross-section
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Physics Interest
● Nucleon structure information.
- GPDs, resonance form factors. Complementary to VCS, electroproduction results.
● Interpretation of radiative corrections.
- Ratio of the proton’s electric and magnetic form factors
- Higher order “box-graph” corrections to weak interaction observables.
● Potential false asymmetry in precision parity violating experiments.
Magnitude of Bn
Information on nucleon structure
Imaginary part of 2-photon exchange
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Bn as a False Asymmetry in PV Measurements
● PV experiments may need to correct for Bn
● Dedicated measurements with a transversely polarized beam are needed
..)sin()( detdet +−+= snTPVLmeasured BPAPA φφφ
Additional piece from BNSSA
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Residual transverse polarization in the beam
Broken azimuthal symmetry of the detectors
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Qweak Transverse Program Summary
Interaction Target Analysis Status
Elastic e+p at E = 1.165 GeV
Hydrogen Ready for publication
Aluminum Ongoing
Carbon Ongoing
Inelastic e+p with a Δ in the final state E=0.877 GeV and 1.165 GeV
Hydrogen, Al, C Ongoing
Elastic e+e at E=0.877 GeV Hydrogen Ongoing
Deep inelastic e+p at W=2.5GeV
Hydrogen Ongoing
Pion photoproduction at E=3.3GeV
Hydrogen Ongoing
Lots of interesting physics!
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Qweak Transverse Program Summary
Interaction Target Analysis Status
Elastic e+p at E = 1.165 GeV
Hydrogen Ready for publication
Aluminum Ongoing
Carbon Ongoing
Inelastic e+p with a Δ in the final state E=0.877 GeV and 1.165 GeV
Hydrogen, Al, C Ongoing
Elastic e+e at E=0.877 GeV Hydrogen Ongoing
Deep inelastic e+p at W=2.5GeV
Hydrogen Ongoing
Pion photoproduction at E=3.3GeV
Hydrogen Ongoing
Covered in this talk
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Measurement Setup
Kinematics Beam energy = 1.165GeVQ2= 0.026 (GeV/c)2
Scattering angle = 7.80 Beam Polarization ~ 89%
Transversely polarized (vertical/horizontal)Electron beam
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Analysis Overview
● Form individual bar asymmetries
● Remove helicity correlated changes in position, angle and energy using linear regression.
e.g. asymmetries from Hydrogen
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Analysis Overview
● Check for false asymmetry cancellation with the insertable half wave plate (IHWP)
e.g. regressed asymmetries from Hydrogen using vertical transverse polarization
(IN+OUT)/2 shows a good cancellation of helicity correlated false asymmetries.
Cerenkov Detector Array
2 3
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7 6
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Analysis Overview
● Extract the measured physics asymmetry by fitting the regressed detector asymmetries
e.g. transverse asymmetries from elastic e+p
Not corrected for backgrounds, polarization and other systematics
Fit →
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Analysis Overview
● Extract the measured physics asymmetry by fitting the regressed detector asymmetries
● Correct for beam polarization, backgrounds and other systematics to extract Bn.
Backgrounds (Bkg)● Aluminum target windows● Inelastics
Systematics ( R )● Radiative corrections● Acceptance averaging● Q2 variation
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BNSSA from elastic e+p (PRELIMINARY)
From ~ 50 hrs worth of data.
BNSSA from elastic e+p scattering :
Bn = -5.35 ± 0.07 (stat) ± 0.15 (sys) ppm
A 3% Measurement.Vertex kinematics : Q2 = 0.0250 ± 0.0006 (GeV/c)2
Energy = 1.155 ± 0.003 GeVScattering angle = 7.9 ± 0.3 degrees
Soon to be published!
Error source Preliminary
Polarization 2.2 %
Statistics 1.3 %
Q2 acceptance 1.2 %
Non-linearity 1.0 %
Regression 0.9 %
Backgrounds 0.3 %
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BNSSA from elastic e+p (PRELIMINARY)
Compare to world data on forward angle Bn measurements.
Experiment Beam (GeV)
Q2
(GeV/c)2
BNSSA (ppm)
Precision
A4 (Mainz) 0.569 0.106 -8.59 ± 0.89stat ± 0.79sys~ 14%
A4 (Mainz) 0.855 0.230 -8.52 ± 2.31stat ± 0.87sys~ 30%
Qweak (JLab)
1.160 0.026 -5.35 ± 0.07stat ± 0.15sys ~ 3%
HAPPEX (JLab)
3.000 0.099 -6.58 ± 1.47stat±0.24sys~ 23%
G0 forward (JLab)
3.031 0.150 -4.06 ± 0.99stat± 0.63sys~ 29%
0.250 -4.82 ± 1.87stat± 0.98sys ~ 44%
Most precise measurement of Bn by far.
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BNSSA from elastic e+p (PRELIMINARY)
Comparison to model calculations
Using single pion electro-production amplitudes (MAID).
multi-pion intermediate statesInput:photoproduction cross
sectionsBut:Different cross-section fitsDifferent Compton slopes
PRELIMINARY
Emphasizes the significant role played by multiple pion resonance intermediate states in two-photon exchange
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BNSSA from elastic e+Al and e+C
● Ongoing analysis.
● Preliminary estimates of the uncertainties of the measurements looks promising.
Original theory and data plot from PRL 109, 192501 (2012)
● a new Carbon data point
● Al27 data point will help to understand theory between A=12 and A=208.
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Summary
● Qweak have several interesting transverse asymmetry measurements.
– Some are first time measurements
– Good candidates to test model calculations
● A 3% measurement of BNSSA from elastic e+p scattering is ready for publication!
– This is the most precise measurement of BNSSA to-date.
– Emphasizes the role played by multi-pion resonance intermediate states in the two-photon exchange process
● Rest of the data analysis is on going and can be expected to be finalized within the next year.
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Thank you!
Thank you!
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Backup Slides
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Beam Normal Single Spin Asymmetry
● Model calculations – varies for different kinematics
Model Kinematics region Input
Diaconescui & Musolf [Phys. Rev.C. 70, 054003(3004)]
Threshold Field theory calculation.
Pasquini & Vanderhaeghen [Phys.Rev. C70,045206 (2004)]
Resonance electroproduction amplitude
M. Gorchtein [Phys.Rev. C73, 035312;055201(2006)]
High energy forward scattering
Photoproduction cross-section
Afanasev & Merenkov [Phys.Lett. B 599,48 (2004)]
Photoproduction cross-section
M. Gorchtein, P.A.M. Guichon, M. Vanderhaeghen [Nuc.Phys. A 741:234-248(2004)]
Hard scattering GPDs
