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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 - PowerPoint PPT Presentation
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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