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

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Kazutaka Nakahara

Strangeness (briefly)

Parity violation

G0 Experiment

Physics results

SLAC Seminar 1/19/06

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

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

2.

= Neutron EM form factor

3.

Measure the neutral weak proton form factor

Access the neutral weak sector of elastic e-p scattering

Want to know and

1.

= Proton EM form factor

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)

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

Most calculations attempt to calculate s.

Loops

Poles

Lattice

Other

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

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

linacs

Injector/Source

A

C

B

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

detectors

lead collimators

beam

target

- 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

(SMS)

cryogenic supply

beam

monitoring

girder

scintillation detectors

cryogenic target ‘service module’

electron beamline

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)

Feedback shows convergence of HC beam properties

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

Aphys

GE

GM

+ h

s

s

Blinding Factor

“Beam” corrections:

Leakage beam asymmetry

Helicity-correlated beam properties

Deadtime

Beam polarization

Background correction

Q2

EM form factors

- 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,

32 ns G0 beamOnly 1 out of 16 buckets should be filled!!

2ns

~ 1.6 pC G0

beam pulse

32ns

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

Aleak = -0.71 0.14 ppm (global uncertainty)

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

Model:

Gaussian Yel, constant Ael

Pol’4 Ybkg, Pol’2 Abkg

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

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

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

World data at Q2 = 0.1 including G0.

GEs and GMs:

- Negative ? Need
more data

- Backward angle
measurement scheduled

(G0 and PVA4)

- 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
Results:

- 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.
Apparatus:

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

1. Show chi2 of background fits.

Intro:

- Explain where form factors come from?
p6. Too cluttered. Try creating animation.

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

EMC

World Data:

Each was at different angular kinematics.

GMs=0.550.28

GEs =-0.010.03

1 contour

95.5% CI

- 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

Mean

Timer

Front

PMT Right

TDC

/

LTD

Scalers:

Histogramming

Coinc

PMT Left

Mean

Timer

Back

PMT Right

Particle identification through TOF separation.

Detect four-fold coincidence hits.

Fast counting electronics (~1MHz per detector).

- 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

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

Yields:

Linear interpolation, ±0.5 “detector” as uncertainty.

Asymmetries:

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!

- : parametrized in a fasion similar to “Kelly” form factors.
- : dipole form used