G 0 forward angle measurement and the strange sea of the proton
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G 0 Forward Angle Measurement and the Strange Sea of the Proton. Kazutaka Nakahara. Strangeness (briefly) Parity violation G0 Experiment Physics results. SLAC Seminar 1/19/06. Proton Structure. 3 valence quarks  not a bad approx. at high energies

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G 0 Forward Angle Measurement and the Strange Sea of the Proton

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G0 Forward Angle Measurement and the Strange Sea of the Proton

Kazutaka Nakahara

Strangeness (briefly)

Parity violation

G0 Experiment

Physics results

SLAC Seminar 1/19/06

Proton Structure

3 valence quarks  not a bad approx. at high energies

At low energy, things get messy. Sea includes gluons and “sea quarks”, and can contribute to proton structure

Strange quarks give

exclusive insight into the sea

and come in pairs  no net strangeness

Hadronic Current

 EM coupling to pointlike fermion

Represents internal structure of the proton

Sach’s FF ~ p, rp at q2=0

~ Fourier transform of the charge and magnetization distribution of the proton.

charge symmetry 


= Neutron EM form factor


Measure the neutral weak proton form factor

Access the neutral weak sector of elastic e-p scattering

Want to know and

Nucleon Form Factors


= Proton EM form factor

Parity Violation

can be determined through parity-violating elastic e-p scattering.

Parity Conserving Parity Violating

Measure asymmetry in elastic e-p cross section for + and – helicity incident electrons.

-3 to -40 ppm measurement!!

Measure at forward angles

(elastic e-p)

Measure at backward angles

(elastic e-p and quasi-elastic e-d)

Parity Violation  Proton Form Factors

Can QCD tell us these things? In principle, yes. But hard to calculate.

Most calculations attempt to calculate s.

Theoretical Predictions





Maybe negative, but not much consensus...try measuring.

Previous PV Experiments

  • Mostly low Q2 to probe the proton’s static (Q2 = 0) properties.

Jefferson Laboratory






G0 Experiment Overview

Ebeam = 3.03 GeV, 0.33 - 0.93 GeV

Ibeam = 40 mA, 80 mA

Pbeam = 75%, 80%

q = 52 – 760, 104 - 1160

DW = 0.9 sr, 0.5 sr

ltarget = 20 cm

L = 2.1, 4.2 x 1038 cm-2 s-1

A ~ -1 to -50 ppm, -12 to -70 ppm

  • Measure and at different Q2.

    - Gives different linear combination of u, d, and s contributions.

  • Measure asymmetries at forward and backward angles.

    - Forward angles  recoil protons

    - Backward angles  elastic and quasi-elastic electrons

     Separates electric and magnetic contributions.

  • Forward angle measurement complete (101 Coulombs)

elastic protons


lead collimators



G0 Spectrometer (forward mode)

  • 40 A polarized electron beam at 3 GeV

  • High power LH2 target

  • Toroidal superconducting magnet (5000A, 1.6 T-m)

  • High rate counting electronis~ 1 MHz / detector  deadtime well understood

  • 0.12 < Q2 < 1.0 (GeV/c)2

superconducting magnet


G0 in Hall C (JLab)

cryogenic supply




scintillation detectors

cryogenic target ‘service module’

electron beamline

JLab Accelerator

3 endstations. One laser for each hall shining a common GaAs cathode

1497 MHz SRF cavities

Each hall receives beam at 499 MHz  simultaneous 3 hall delivery possible.

~ 600 MeV / linac, 2 linacs per “pass” with up to 5 passes possible.

RF separator + Lambertson kicks beam into the correct hall.

  • 40 A at 3 GeV

  • Usual beam pulse at Jlab is 2 ns (499 MHz). G0 beam pulse = 32 ns (31 MHz)  high bunch charge

  • Helicity-flip every 1/30 sec (macropulse).

  • Macropulse arrange in quartet pattern  asymmetry from each quartet

  • Must control helicity-correlation in beam properties (I,X,Y,x,y,E)

Beam Structure

Helicity-Correlated Beam

Feedback shows convergence of HC beam properties

G0 Data Overview

Final Data:

701 h at 40 A (101C)

19 x 106 quartets

76 x 106 MPS

Integrate yield over elastic region for + and – helicities

...done? Not so fast

Various systematic effects must be corrected.

Raw Asymmetries, Ameas




+ h



Analysis Overview

Blinding Factor

“Beam” corrections:

Leakage beam asymmetry

Helicity-correlated beam properties


Beam polarization

Background correction


EM form factors

Helicity-Correlated Beam Parameters

  • Sensitivity of detector to beam fluctuations, , well understood.

  • Run-averaged HC beam parameters are small.

  • False asymmetry ~ 0.02 ppm.

    - 100x to 1000x smaller than the smallest physics asymmetry measured in G0 - Significantly smaller than the 5-10% total uncertainty expected for G0.

  • 499 MHz beam leaks into G0 beam. (~40-50 nA). Comes from “inperfect” diode (poor) and Ti-Sapphire (better) lasers not shutting off fast enough.

     2ns “background” spectra under the G0 spectra.

  • Large charge asymmetry associated with leakage current. (A ~ 600ppm)

  • BCMs integrate charge  insensitive to micro-structure,

Beam Leakage Current

32 ns G0 beamOnly 1 out of 16 buckets should be filled!!


~ 1.6 pC G0

beam pulse


Use cut0 region to determine the leakage asymmetry. Agrees with leakage-only runs.

Aleak = -0.71  0.14 ppm (global uncertainty)

Background Subtraction

2 step fitting procedure:

  • Fit the yield spectra, and determine the background fraction, f(t), (e.g. background rate / total rate) bin by bin

  • Fit asymmetry with,

Background asymmetry

Elastic Asymmetry


Gaussian Yel, constant Ael

Pol’4 Ybkg, Pol’2 Abkg


G0 Physics Asymmetries

No “vector strange” asymmetry, ANVS, is A(GEs,GMs=0)

Inner error band is stat., outer band is stat. + pt-pt. Global error band dominated by leakage and background corrections.

Forward angle results: http://www.npl.uiuc.edu/exp/G0/Forward

Strange Quark Contribution

“Kelly form factors”: Kelly PRC 70 (2004) 068202

G0 Forward Angle Results

Forward angle results over 0.12 < Q2 < 1.0 GeV2. Model uncertainty from EW radiative corrections.

3 types of nucleon form factors  shift in baseline

No-vector-strange hypothesis disfavored at 89%.

= -0.013  0.028

= +0.62  0.31

Q2 = 0.1 GeV2

World data at Q2 = 0.1 including G0.

GEs and GMs:

Q2 = 0.23 GeV2

  • Negative ? Need

    more data

  • Backward angle

    measurement scheduled

    (G0 and PVA4)

Q2 = 0.477 GeV2


  • Forward angle measurement shows consistency with previous experiments

  • Strange quarks do appear to contribute to the static properties of the proton.

  • Interesting Q2 dependence

  • Backward angle measurements scheduled for 2006/2007.

  • Things to add


  • evolution of eta across Q2

  • Explanation of the “dip”

  • Higher Q2 point band plots

  • Add plot of “where we were”...consider moving the band plot with no G0 to somewhere near here.


  • Explain “quartet”, pulse length (tof?), etc, somewhere...before beam stuff or after?


    1. Show chi2 of background fits.


  • Explain where form factors come from?

    p6. Too cluttered. Try creating animation.

Strange Quarks in the Proton?

Proton is the only known stable hadron. Extensively studied, but the details of its composition is not well known.

  • Spin contributions

  • Mass contributions  N- term

  • Momentum Distribution

     NuTeV

  • Charge and magnetization distribution.

     Measure through Parity-Violation


Previous Experiments at Q2=0.1

World Data:

Each was at different angular kinematics.


GEs =-0.010.03

1 contour

95.5% CI

Polarized Electron Beam

  • 40 A of polarized electron beam

  • Helicity-correlated beam properties (I,X,Y,x,y,E) must be minimized!!

Minimize through active IA (charge) and PZT (position) feedback.


  • 20 cm LH2, aluminum target cell

  • longitudinal flow, v ~ 8 m/s, P > 1000 W!

  • negligible density change < 1.5%

  • measured small boiling contribution

    • 260 ppm/1200 ppm statistical width


  • 8 coil superconducting torus

  • single cryostat

  • 5000 A, 144 turns/coil; 5.8 MA-turns, total

  • stored energy ~ 5.5 MJ

  • field integral ~ 1.5 T·m

  • bend angle 35 – 87o

  • lead collimators for , acceptances

    •  acceptance 44% of 2

  • line-of-sight shielding for neutrons

PMT Left




PMT Right







PMT Left




PMT Right

Particle identification through TOF separation.

Detect four-fold coincidence hits.

Fast counting electronics (~1MHz per detector).

G0 Electronics

Electronics Deadtime Measurements

  • Deadtime at three stages

    • CFD, mean-timer, coincidence

    • scale CFD, mean-timer rates

    • measure coincidence deadtime directly with “buddy” system

      • e.g. oct. 5. det. 4 with oct. 1, det. 4

    • careful treatment of combined effects

  • Comparison

    • from measured components above

    • from slope of yield asymmetry vs. charge asymmetry

      • from large induced charge asymmetry runs (~1000 ppm)

    • consistent with measurements of yield as function of beam current

Background Subtraction (Det15)

Interpolate asymetries and yields over detectors 12 through 16 (for each timebin).


Linear interpolation, ±0.5 “detector” as uncertainty.


Smooth interpolation from lower detectors, ±1 “detector” and ± 0.5 ns time shift as uncertainty

Result shows good agreement with sideband asymmetries.

Fit suggests a positive and a negative bump in . Is this model realistic or too simplistic? There are some preliminary results that may support this claim. Otherwise, stay tuned for more results!

Fit of G0Data

  • : parametrized in a fasion similar to “Kelly” form factors.

  • : dipole form used

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