Target Single Spin Asymmetries in 3He(e,e’)

Nucleon and Nuclear structure studies using two photon exchange with a vertically polarized 3He target.

Program Goal: Measure the “vertical” target single spin asymmetry Ay in:

Todd Averett, College of William and MaryWilliamsburg, VA

On behalf of the Jefferson Lab Hall A and polarized 3He collaborations

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Graduate Students: Yawei Zhang, Joe Katich, Xin Qian, Ellie Long

• quasi-elastic 3He(e,e’) • deep-inelastic 3He(e,e’) • quasi-elastic 3He(e,e’n)

Born scattering and beyond

• Irritating correction to favorite diagram.

• Suppressed by αem relative to Born diagram

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• Jefferson Lab nuclear physicists’ favorite diagram (required for every talk):

Born scattering

Unpolarized elastic scattering

Born scattering and beyond

• How is it useful?• Loop integral contains entire

nucleon response.• How do we observe this?

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• Dominates unpolarized and most polarized N(e,e’) scattering.

Two Photon Physics

• Topic 1: Elastic N(e,e’) scattering with two photon exchange:

• h = electron helicity, λN (λ’N) = nucleon helicity, K=(k+k’)/2, P=(p+p’)/2

• The functions are complex and reduce to the usual (real) structure functions and form factors in 1γ exchange:

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A. Afanasev et al., Phys.Rev.D72:013008, 2005

Elastic form factor data

• Disagreement between recoil polarization and Rosenbluth separation measurements of proton form factors: two-photon exchange?

• Depends on the real part of the interference:

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A.J.R. Puckett, Phys.Rev. C85 (2012) 045203

A. Afanasev et al., Phys.Rev.D72:013008, 2005

Born TPEX TPEX

At large Q2, assume dominated by

interaction with a single quark

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At low Q2, entire nucleon is involved

2-photon exchange calculations

A. Afanasev et al., Phys.Rev.D72:013008, 2005

Elastic contribution to complex structure functions well known. Inelastic contribution estimated using e.g. form factors, resonance contributions, moments of GPD’s, ….

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pQCD Calculation of TPE corrections to elastic form-factor dataD. Borisyuk, A. Kobushkin, Phys.Rev. D79 (2009) 034001

Delta-resonance Contribution to Elastic TPE

Partonic Approach

S. Kondratyuk et al., Phys.Rev.Lett. 95 (2005) 172503

Hadronic Approach

Examples of models

Jefferson Lab GEp/GM

p vs. Q2

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Blunden et al.Phys. Rev. C72 (2005) 034612

A. Afanasev et al., Phys.Rev.D72:013008, 2005

GPD model Hadronic model

• Unpolarized e- beam incident on 3He target polarized normal to the electron scattering plane

• Note that unpolarized eN scattering, and double spin asymmetries (DSA) with beam and target polarization in-plane are dominated by 1-photon exchange. e.g. measurements of GE

n, GMn, F1, F2, g1, g2 <----(Born approximation)

• However, Ay=0 at Born level, sensitive to physics at order α3; two-photon exchange.

Target Single Spin Asymmetry (SSA)

3He

θ

e-

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Target Single Spin Asymmetry

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A. DeRujula et al., Nuc. Phys. B35 (1971) 365

Absorptive part Imaginary contribution

N. Christ-T.D.-Lee, Phys. Rev. 143 (1966) 1310

For inclusive scattering N(e,e’),

When we allow 2-photon exchange, the leading contribution is from 1γ + 2γ interference

• Measurement of Ay versus Q2 provides new constraints on nucleon models• Useful check on GPD-model correction to elastic form factor data.

Time reversal invariance, parity conservation, and the hermiticity of the electromagnetic current operator

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Existing Ay QE data

Expect Ay ~ -1.5% at Q2=1 GeV2

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TPEX in Target SSA

Target Single Spin Asymmetry (SSA) Unpolarized electrons scattering from nucleons or nuclei polarized perpendicular to the incident electron helicity. Target spin vector at angle ϕS relative to electron scattering plane

Imaginary part

The Target SSA (ϕS = π/2 ) for elastic scattering is:

-- Ay is zero for 1-photon exchange

-- Leading contribution is 1 x 2 photon exchange -- Direct access to details of TPEX formalism and models

-- New technique for studying nucleon substructure

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Trento Convention

A. Afanasev et al., Phys.Rev.D72:013008, 2005

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GPD-based model prediction

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• Elastic Contribution Known

• Inelastic Contribution calculated from GPD parameterizationo Neutron Dominated by GM

n

Predicted asymmetry at Q2=1 GeV2

Uses the same formalism, GPD moments, as those used for elastic form-factor corrections.

A. Afanasev et al., Phys.Rev.D72:013008, 2005

Experimental Design

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• Use two symmetric, independent spectrometers for singles electron detection. Jefferson Lab Hall A HRS spectrometers.

• 10-15 uA beam

• Luminosity ~ 1036 /cm2/s

• Energy 2.2 - 5.5 GeV

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Hall A Polarized 3He Target-- New vertically polarized 3He target was constructed

Bext

Top view, Jefferson Lab3-axis polarized target

Side view, Jefferson Lab vertical coils with

target cell

Polarization vs. time (arb.)

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Hall A polarized 3He target

• Effective polarized neutron target

• Spin Exchange Optical Pumping (SEOP) technology. Polarized alkali atoms exchange spin with 3He nucleus during collisions

• 5:1 ratio of K:Rb at 235 oC• Spectrally narrowed diode lasers

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Glass target cell

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Events Selection

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Cherenkov ADC spectrum

EM calorimeter ADCPre-shower vs. shower

Reconstructed interactionposition in glass target cell

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New Results for Ay QE

-- Q2 dependence in the quasi-elastic single spin asymmetry, Ay

-- Agrees with GPD model** at Q2 = 1.0 GeV2.

-- Asymmetry stays large at low Q2: WHY???

-- TPEX important at low Q2, -relevant for GE

p/GMp at low Q2 ??

3He neutron

GPD calculation

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Topic 2: What about Ay for n(e,e’) in DIS?

• The formalism remains the same:Ay=0 for 1-photon exchange

• For DIS, one assumes that the scattering is dominated by two photon exchange with a single quark.

• Scattering from a single quark in the naïve QPM gives Ay=0 at all orders in α

• Ay ≠ 0 arises from e.g. quark mass, gluon exchange (higher-twist effects)

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Ay in deep-inelastic scattering

Naïve QPM

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Taken From: A. Metz, INT Workshop on Orbital Angular Momentum in QCDFebruary 10, 2012

Predict: Ay ~αemmq/Q

~10-4

Photons Coupling to Different Quarks

Predict: Ay ~ 10-2

Kinematics

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Beam Polarimetry(Møller + Compton)

LuminosityMonitor

Measured 3He(e,e’) SSA using BigBite and Left HRS in singles mode.

HERMES proton data

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A. Airapetian et al,Phys. Lett. B682, 351 (2010)

New Results: Ayn in DIS

Measured average: Ay = 0.94 ± 0.32 x 10-2

NEW Ayn

Topic 3: SSA in quasi-elastic 3He(e,e’n)

• Detect recoil neutron during QE scattering.

• Christ-Lee theorem doesn’t apply for semi-inclusive scattering.

Ay not necessarily zero

• Sensitive to final state interactions

• PWIA predicts Ay=0

• Precise laboratory for studying details of 3He wavefunction

• Unpublished NIKHEF result showed Ay=50% at Q2=0.1 GeV2

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Semi-Inclusive Target SSA

Preliminary Results

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-- Detect recoil neutron using Hall A Neutron Detector (HAND)

-- Ay changes by 2 orders of magnitude between Q2 = 0.1 - 1.0 GeV2.

-- Agree with theory that includes FSI and MEC

-- Beautiful demonstration of transition from nucleus to nucleon degrees of freedom

Summary

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• First measurements of the inclusive target SSA using vertically polarized 3He in QE, DIS scattering.

• Large QE SSA observed at Q2=1 GeV2; Predicted by GPD moment model.

-- Remains large down to Q2=0.14 GeV2

• DIS SSA appears much larger than predicted by Afanasev et al., consistent with Metz et al.

• Measurements at high Q2 possible with Jefferson Lab at 12 GeV.

• Precision results for SSA in 3He(e,e’n). Strong Q2 dependence. Sensitive to few body physics in 3He

• Must pay attention to TPEX as precision of experiments improve

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S, S’, D, Δ-isobar contributions to 3He wavefunction

Phys. Rev. C65, 064317 (2002)

3He as a neutron

Check for False Asymmetries: Luminosity Asymmetry

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ppm

Backgrounds

• BigBite: Pair produced e+/e- pairs from πo decay.– Measure using positive polarity– >50% contamination in lowest momentum bin– 1% in largest momentum bin– Largest systematic uncertainty

• BigBite: π-/+ in e-/+ spectrum. No Cherenkov detector. EM pre-shower and shower calorimeter

• LHRS spectrometer, virtually background free.– Good PID– Highest momentum = negligible pair-electron contamination

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Contaminations

Good Agreement among all trigger types for positron contaminations with pion contamination corrected (central results only)

Pion contaminations dominated by systematic uncertainties.

Pion contamination are obtained by looking at the preshower energy deposition. Also compared with BigBite GEANT3 MC and checked with HRS Pion rejector for systematic uncertainties

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