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

4

2

1

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

180

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

4

51

8

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