Kazutaka Nakahara

G0 Forward Angle Measurement and the Strange Sea of the Proton

SLAC Seminar 1/19/06

• Strangeness (briefly)

• Parity violation

• G0 Experiment

• Physics results

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

SS

Hadronic Current

UqiFM

FU qqEMJ )

2( )

2(2)

2(1

uuejEM

EM coupling to pointlike fermion

Represents internal structure of the proton

FFGFFG

pppM

pppE

,2,1,

,2,1,

Sach’s FF ~ p, rp at q2=0

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

Nucleon Form FactorsWant to know and

GGGGps

MEw

pd

MEw

pu

MEw

pZ

ME

,

,

2,

,

2,

,

2,

, sinsinsin 3

41

3

41

3

81

GGGGps

ME

pd

ME

pu

ME

p

ME

,

,

,

,

,

,

,

, 3

1

3

1

3

2

= Proton EM form factor

charge symmetry

GGGGps

ME

pd

ME

pu

ME

p

ME

,

,

,

,

,

,

,

, 3

1

3

2

3

1

= Neutron EM form factor

Measure the neutral weak proton form factor

Access the neutral weak sector of elastic e-p scattering

Gps

E

,

Gps

M

,

Parity Violation

Parity Conserving Parity Violating

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

GpZ

ME

,

,

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

105

2

M

MM

Z

ZA

-3 to -40 ppm measurement!!

Parity Violation Proton Form Factors

GG

GGGGGGQGAp

Mp

E

eA

pMW

pZM

pM

pZE

pEF

,2

,2

,'2,,,,2sin41

24

GG

GGG

AAA

ps

M

ps

E

e

A

pZ

M

pZ

E

backwardD

backwardH

forward

,

,

,

,

,

,

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.

Theoretical Predictions

Qd

QGd

Q

r

G

sE

s

sMs

2

2

0

)(6

)0(

2

Most calculations attempt to calculate s.

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

• Loops• Poles• Lattice• Other

Previous PV Experiments

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

Experiment Mode Target Q2 (GeV/c)2

Asymmetry (ppm) Status

SAMPLE Backward H 0.1 -5.61 ± 0.67 ± 0.88 Completed

SAMPLE II Backward D 0.1 -7.28 ± 0.68 ± 0.75 Completed

SAMPLE III Backward D 0.04 -3.51 ± 0.57 ± 0.58 Completed

HAPPEX Forward H 0.477 -15.05 ± 0.98 ± 0.56 Completed

HAPPEX II Forward H 0.1 -1.14 ± 0.24 ± 0.06 Running

HAPPEX He Forward 4He 0.1 6.72 ± 0.84 ± 0.21 Running

PVA4 Forward H 0.1 -1.36 ± 0.29 ± 0.13 Completed

PVA4 Forward H 0.23 -6.30 ± 0.43 Completed

G0 Forward Forward H 0.12 to 1.0 -3 to –40 Completed

G0 Backward Backward H,D -- -- Scheduled/

proposed

HAPPEX III Forward H -- -- Proposed

Jefferson Laboratory

A B CInjector/Source

linacs

G0 Experiment Overview• 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)

Electron Beam

LH2 Target

SuperconductingCoils

Particle Detectors

Ebeam = 3.03 GeV, 0.33 - 0.93 GeV

Ibeam = 40 A, 80 A

Pbeam = 75%, 80%

= 52 – 760, 104 - 1160

= 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

Gs

E Gs

M

G0 Spectrometer (forward mode)

lead collimators

elastic protons

detectors

targetbeam

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

G0 in Hall C (JLab)

beammonitoring girder

superconducting magnet (SMS)

scintillation detectors

cryogenic supply

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.

Beam Structure• 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)

Helicity-Correlated Beam

Beam Parameter

Achieved “Specs”

Charge asymmetry

-0.14 ± 0.32

ppm

1 ppm

x position differences

3 ± 4 nm 20 nm

y position differences

4 ± 4 nm 20 nm

x angle differences

1 ± 1 nrad 2 nrad

y angle differences

1.5 ± 1 nrad 2 nrad

Energy differences

29 ± 4 eV 75 eV

Feedback shows convergence of HC beam properties

G0 Data Overview

Integrate yield over elastic region for + and – helicities

YY

YYA

...done? Not so fast

Various systematic effects must be corrected.

Final Data:

701 h at 40 A (101C)

19 x 106 quartets

76 x 106 MPS

Aphys

+ GEs GM

s

Blinding Factor

Analysis Overview

Raw Asymmetries, Ameas

“Beam” corrections:Leakage beam asymmetry

Helicity-correlated beam propertiesDeadtime

Beam polarization

Background correction

Q2

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.

iP

Y

ii

false pp

Y

YA

2

1

Beam Leakage Current• 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,

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

Aleak = -0.71 0.14 ppm (global uncertainty)

2ns

32ns

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

~ 1.6 pC G0 beam pulse

Background Subtraction2 step fitting procedure:

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

2. Fit asymmetry with,

)()())(1()( tAtfAtftA bemeas

Elastic Asymmetry Background asymmetry

Model:

Gaussian Yel, constant Ael

Pol’4 Ybkg, Pol’2 Abkg

Results

G0 Physics AsymmetriesNo “vector strange” asymmetry, ANVS, is A(GE

s,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

pE

pM

i G

GEQ

,2

NVSphyspE

pM

pE

F

sM

sE AA

G

GG

QGGG

22

2

24

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

G0 Forward Angle ResultsForward 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%.

Q2 = 0.1 GeV2

World data at Q2 = 0.1 including G0.

GEs and GM

s:

= -0.013 0.028

= +0.62 0.31

Gs

E

Gs

M

Q2 = 0.23 GeV2

• Negative ? Need more data• Backward angle measurement scheduled(G0 and PVA4)

Gs

E

Q2 = 0.477 GeV2

Summary

• 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 addResults:1. evolution of eta across Q22. Explanation of the “dip”3. Higher Q2 point band plots4. Add plot of “where we were”...consider moving the band plot with no

G0 to somewhere near here.

Apparatus:1. Explain “quartet”, pulse length (tof?), etc, somewhere...before beam

stuff or after?

Analysis1. Show chi2 of background fits.

Intro:1. 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

Strange Quarks in the Proton?

2.0 NdduuN

NssN

023.016.0095.0 s EMC

06.007.042.02 dus

Previous Experiments at Q2=0.1

1 contour

95.5% CI

World Data:

Each was at different angular kinematics.

GMs=0.550.28

GEs =-0.010.03

Polarized Electron Beam• 40 A of polarized electron beam

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

ii

false pp

Y

YA

2

1

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

Target

• 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

Magnet• 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

G0 Electronics

PMT Left

PMT Right

PMT Left

PMT Right

MeanTimer

MeanTimer

CoincTDC /LTD

Front

Back

Scalers:Histogramming

Particle identification through TOF separation.

Detect four-fold coincidence hits.

Fast counting electronics (~1MHz per detector).

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).

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!

Gs

EGs

M

Fit of G0 Data

Gs

E

Gs

M

)1( 33

221

1

bbb

aG

s

E

22

2

)1(

Q

aG

s

M

14/3.11/2

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

• : dipole form used