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Parity-Violating Electron Scattering at JLab. David S. Armstrong College of William & Mary. (replacing Juliette Mammei , who kindly provided most of the slides). MENU 2013 Rome, Italy Oct 2 2013. Parity-Violating Electron Scattering at JLab. Electroweak interaction (neutral current)

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Parity-Violating Electron Scattering at JLab

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Parity-Violating Electron Scattering at JLab

David S. Armstrong

College of William & Mary

(replacing Juliette Mammei, who kindly provided most of the slides)

MENU 2013

Rome, Italy

Oct 2 2013

Parity-Violating Electron Scattering at JLab

  • Electroweak interaction (neutral current)

  • - not one of the canonical probes for Hadron Physics

  • (i.e. strong or electromagnetic)

  • - nevertheless, much of the program should be

  • of interest to this community

  • Focus today:

  • - new results since MENU2010

  • - plans for 12 GeV era at Jefferson Lab


Intro to Parity-Violating Electron Scattering (PVES)

The Vector Strange Form Factors a ≈ completed program

Qweak: first results on the proton’s weak charge

Neutron radius in Heavy Nuclei

5) Standard Model Tests with PVES: plans at JLab-12 GeV

• Neutrino mass and their role in the early universe 0νββ decay, θ13 , β decay,…

• Matter-antimatter asymmetry in the present universe EDM, DM, LFV, 0νββ, θ13

• Unseen Forces of the Early Universe Weak decays, PVES, gμ-2,…

LHC new physics signals likely will need additional indirect evidence to pindown its nature

• Neutrons: Lifetime, P- & T-Violating Asymmetries (LANSCE, NIST, SNS...)

• Muons: Lifetime, Michel parameters, g-2, Mu2e (PSI, TRIUMF, FNAL, J-PARC...)

•PVES: Low-energy weak neutral current couplings, precision weak mixing angle

(SLAC, Jefferson Lab, Mainz)

new since MENU2010: first result from QWeak

2) Study nucleon and nuclear properties

  • Strange quark content of nucleon new since MENU2010: HAPPEX-III

  • Neutron radius of heavy nuclei new since MENU2010 – first PREx results

1) Search for physics Beyond the Standard Model

  • Low energy (Q2 <<M2) precision tests complementary to high energy measurements

MENU 2013

A brief history of parity violation

1930s – weak interaction needed to explain nuclear β decay

1950s – parity violation in weak interaction;

V-A theory to describe 60Co decay





1970s – neutral weak current events at


late 1970s – parity violation observed in electron scattering - SLAC E122

MENU 2013

Parity-violating electron scattering


Electroweak interference

MENU 2013

SLAC Experiments

  • SLAC E122 – crucial confirmation of WSG electroweak model

  • Electron-deuteron deep inelastic scattering

  • High luminosity: photoemission from NEA GaAs cathode

  • Rapid helicity-flip (sign of e- polarization)

  • Polarimetry to determine beam polarization

  • Magnetic spectrometer: backgrounds and kinematic separation

APV ~ 100 ± 10 ppm


  • SLAC E158 – 1999

  • electron-electron scattering - purely leptonic interaction

  • electron-electron weak attractive force had never been measured!

APV ~ -131 ± 14 ± 10 ppb


MENU 2013

Genesis of a Strange Idea

Puzzle: Initial DIS measurements of spin-structure of nucleon (EMC):

valence quarks contribute unexpectedly low fraction to total spin - “Spin Crisis”

Possible reconciliation: large fraction of spin from ?

eg. D. B. Kaplan and A. Manohar, Nucl. Phys. B310, 527 (1988).

Theoretical realization: not only did many available nucleon model calculations allow this, but they also allowed (and in some cases favored) large strange quark contributions to other properties of nucleon.

Consternation and excitement: at the time, data gave no constraint on strange contributions to charge distribution and magnetic moment!

Challenge: how to isolate strange vector form factors?

Answer: exploit the weak neutral current as a probe

strangequark contribution

Assume isospinsymmetry, and we have


and this

are well known

what about this?

(Assume neutral weak charges are known)


MENU 2013

Nucleon Form Factors

NC and EM probe same hadronic flavor structure, with different couplings:

Assume Charge Symmetry:

Strange Form Factors

MENU 2013

Qweak: Proton’s weak charge

  • Neutral current analog of electric charge:

The Standard Model makes a firm prediction of

“Accidental suppression”

sensitivity to new physics


Q-weak is particularly sensitive to the quark vector couplings ( and ) .

MENU 2013

Qweak: Proton’s weak charge

For electron-quark scattering:

Use four-fermion contact interaction to parameterize the effective PV electron-quark couplings (mass scale and coupling)

New physics:

Small θ

Large θ

Planned 4% measurement of proton’s weak charge - probes TeV scale new physics

new Z', leptoquarks, SUSY ...

Erler, Kurylov, and Ramsey-Musolf, PRD 68, 016006 2003

MENU 2013

Extracting the weak charge

Hadron structure enters here: electromagnetic and electroweak form factors…

Data rotated to

Reduced asymmetry more convenient:

One must extrapolate to .

We measure

at .




Hadronic term extracted from fit

The previous strange form factor program (experiments at MIT/Bates, JLab and MAMI) allow us to subtract our hadronic contribution

MENU 2013

The Qweak Apparatus

Main detectors

8-fold symmetry


Cleanup collimators


Shield wall

Acceptance defining collimator

Trigger scintillator

Lead collar

Vertical drift chambers (VDC)


e- beam

e- beam


Tungsten plug

Horizontal drift chambers (HDC)



MENU 2013

Reduced Asymmetry

4% of total data

in the forward-angle limit (θ=0)

Hadronic part

extracted through global fit of PVES data.

MENU 2013

MENU 2013

The C1q & the neutron’s weak charge

MENU 2013

The C1q & the neutron’s weak charge

Combining this result with the most precise atomic parity violation experiment

we can also extract, for the first time, the neutron’s weak charge:

MENU 2013

Qweak – first result

First result (4% of data set):


Lots of work to push down systematic errors, but no show-stoppers found….

Expect final result in 12-18 months time.

More details: Fundamental Symmetries 3, this afternoon

MENU 2013

A of Auxiliary Measurements

Qweak has data (under analysis) on a variety of observables

of potential interest for Hadron physics:

  • Beam normal single-spin asymmetry* for elastic scattering on proton

  • Beam normal single-spin asymmetry for elastic scattering on 27Al

  • PV asymmetry in the region.

  • Beam normal single-spin asymmetry in the region.

  • Beam normal single-spin asymmetry near W= 2.5 GeV

  • Beam normal single-spin asymmetry in pionphotoproduction

  • PV asymmetry in inelastic region near W=2.5 GeV (related to box diagrams)

  • PV asymmetry for elastic/quasielastic from 27Al

  • PV asymmetry in pionphotoproduction

  • *: aka vector analyzing power aka transverse asymmetry;

  • generated by imaginary part of two-photon exchange amplitude

  • (paceWim van Oers)

MENU 2013

Neutron skin – PREx

Exploit the large weak charge of neutron to extract radius of neutron distribution in heavy nucleus; theoretically clean probe.





First result of PRExexperiment at JLab:

PRL 108(2012)112502

APV = 0.656 ± 0.060 (stat) ± 0.014 (syst) ppm

MENU 2013


208Pb more closely approximates infinite nuclear matter

The 48Ca nucleus is smaller, so APV can be measured at a Q2 where the figure of merit is higher

and are expected to be correlated, but the correlation depends on the correctness of the models

The structure of 48Ca can be addressed in detailed microscopic models



Measure both and - test nuclear structure models over a large range of A

More info: yesterday’s talks by G. Urcioli and M. Thiel in Fundamental Symmetries 2

Theory from P. Ring et al. Nucl. Phys. A 624 (1997) 349

MENU 2013

Future: PVES at JLab in 12 GeV era

MOLLER - precision Standard Model test by measuring weak charge of electron in PV electron-electron scattering

(revisit SLAC E158)

SOLID - precision Standard Model test by measuring PV DIS on deuteron: access the quark weak axial couplings C2q

(also – a similar measurement was made at 6 GeV in Hall A at Jefferson Lab,

X. Zheng et al., being readied for publication, but I’m not authorized to

show the results; stay tuned…)

Large kinematic coverage: disentangle CSV and higher-twist effects

MENU 2013

MOLLER at 12 GeV

Coupling constants

Mass scale

2.3% MOLLER uncertainty



LEP2 (gLR and sum) mass scale sensitivity ~5.2 and 4.4 TeV



Doubly-charged scalar, heavy Z’, SUSY, dark Z…

Lepton compositeness – strong coupling – 47 TeV

MENU 2013

MOLLER and weak mixing angle

Reminder: at tree-level

QWe = (1 – 4 sin2θW)

Higgs discovery at LHC allows firm prediction of MOLLER asymmetry in Standard Model

MENU 2013

MOLLER and weak mixing angle

MENU 2013

The MOLLER Experiment

Detector Array

Hybrid Torus

Upstream Torus

Scattering Chamber



28 m

Incoming Beam

MENU 2013


SOLID – accessing the C2q’s


Z* γ*



Cahn and Gilman, PRD 17 1313 (1978) polarized electrons on deuterium

MENU 2013

Red ellipses are PDG fits

Blue bandsrepresent expecteddata: Qweak (left) and PVDIS-­6GeV (right)

Green bandsare proposed SOLID PVDIS

Small θ

Large θ

MENU 2013

SOLID – Large Acceptance Device

MENU 2013

SOLID – Parity-Conserving Physics

- SIDIS with Transversely Polarized 3He approved 90 days

- SIDIS with Longitudinally Polarized 3He approved 35 days

- SIDIS with Transversely Polarized Proton approved 120 days

- Near Threshold Electroproduction of J/Psi approved 60 days

PVDIS approved for 169 days (half of full request)

MENU 2013

PVES Experiment Summary

MENU 2013


  • Strange vector form factors of proton – small, consistent with zero.

  • Qweak: First measurement of proton’s weak charge, consistent with Standard Model, 25x more data on tape

  • PREx: two-sigma evidence for neutron “skin” of 208Pb; will improve after

  • JLab comes online after 12 GeV upgrade, and will extend to 48Ca

  • MOLLER and SOLID: major programs after JLab upgrade

    • two complementary Standard Model tests.

Grazie to the MENU 2013 organizers for inviting Juliette Mammei to give this talk, and for accepting me as a poor substitute….

MENU 2013

Extra slides

MENU 2013

Qweak and weak mixing angle

MENU 2013

SLAC Experiment E122


• High luminosity from

photoemission from NEA


• Rapid helicity-flip (sign of

e- polarization)

Integrating detectors

Magnetic spectrometer

Background and kinematic separation

Huge achievement!

Highest P2Iever, by far. Developed forthisexperiment at SLAC and used eversince

MENU 2013

SLAC Experiment E122

Parity Non-Conservation in Inelastic Electron Scattering, C.Y. Prescott et. al, 1978

APV ~ 100 ± 10 ppm


GWS --‐ Nobel Prize 1979

Deep inelastic scattering:

Y dependence reflects quark axial/electron vector coupling strength

At high x

MENU 2013

Complementary to the LHC - Z΄

Assume LHC discovers a new spin 1 gauge boson with M =1.2 TeV

If the SM value is measured

MOLLER can distinguish between models

Erler and Rojas

Half-way betweenSM and E158 central value

MENU 2013

Isolating strange form factors

Backward angle

Forward angle

For a proton:

~ few parts per million

For 4He: GEs alone

For deuteron:

enhanced GAe sensitivity

MENU 2013

Electroweak Radiative Corrections

Electroweak Radiative Corrections

Transverse Asymmetry Measurement

Normal production running: 89% longitudinal beam polarization

Small amount of transverse polarization

  • Large parity conserving asymmetry (~5ppm)

  • Leaks into the experimental asymmetry through broken azimuthal symmetry

    Measurements of the Beam Normal Single Spin Asymmetry (BNSSA) provide:

  • Direct access to imaginary part of two-photon exchange

    We need to correct for this…

Horizontal and vertical components measured separately

Detector number

Transverse Asymmetry Measurement

Normal production running: 89% longitudinal beam polarization

Small amount of transverse polarization

  • Large parity conserving asymmetry (~5ppm)

  • Leaks into the experimental asymmetry through broken azimuthal symmetry

PhD thesis of BuddhiniWaidyawansa; being prepared for publication


This is the most precise measurement of Beam Normal Single Spin Asymmetry to date.

Global PVES Fit Details

□γZ RC

  • 5 free parametersalaYoung, et al. PRL 99, 122003 (2007):

    • Employs all PVES data up to Q2=0.63 (GeV/c)2

      • On p, d, & 4He targets, forward and back-angle data

        • SAMPLE, HAPPEX, G0, PVA4

    • Uses constraints on isoscalar axial FF

      • Zhu, et al., PRD 62, 033008 (2000)

    • All data corrected for E & Q2 dependence of

      • Hall et al., arXiv:1304.7877 (2013) & Gorchtein et al., PRC84, 015502 (2011)

    • Effects of varying Q2, θ, & λ studied, found to be small

MENU 2013

Qweak “Run 0” corrections and their corresponding sizes

Target aluminum windows are our largest (~30%) correction

Beam line background is currently our largest uncertainty


SLHC, 1 ab-1

F. Petriello et al, PRD 80 (2009)

MOLLER: if Z‘seen at LHC...

  • Virtually all GUT models predict new Z’s (E6, SO(10)…):

  • LHC reach ~ 5 TeV

  • With high luminosity at LHC, 1-2 TeV Z’ properties can be extracted

Suppose a 1 to 2 TeV heavy Z’ is discovered at the LHC…

LHC data can extract the

mass, width and AFB(s)

MOLLER: resolve signs on eR, eL






and Su, Phys. Rep. 456 (2008)

MOLLER: if SUSY seen at LHC...

MSSM sensitivity if light super-partners and large tan β

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