the strange form factors of the proton and the g 0 experiment jeff martin university of winnipeg...
Post on 19-Dec-2015
221 views
TRANSCRIPT
The Strange Form Factorsof the Proton
and the G0 Experiment
Jeff Martin
University of Winnipeg
Collaborating InstitutionsCaltech, Carnegie-Mellon, William&Mary, Grinnell College, Hampton,
IPN-Orsay, LPSC-Grenoble, JLab, Kentucky, LaTech, NMSU, TRIUMF, UConn,
UIUC, U Manitoba, U Maryland, UMass, UNBC, U Winnipeg, VPI, Yerevan
Z
N e
eA
ZM
ZE
Z GGGNJN ,, ||
neutral weak form factors,axial form factor
• Projected precision of NW form factors in 0.1 - 1 GeV2 Q2 range ~ 10% from the current generation of experiments
Nucleon form factors measured in elastic e-N scatteringNucleon form factors
•From scattering theory: form factor ~ Fourier transform of density•Well defined experimental observables• provide an important benchmark for testing non-perturbative QCD structure of the nucleon
N e
ME GGNJN , ||
electromagnetic form factors
• Measured precision of EM form factors in 0.1 - 1 GeV2 Q2 range ~ 2 - 4%
Flavour Decompositionof the Form Factors
e p
Species Charge Weak Charge
u
d
s
W2sin
3
81
W2sin
3
41
W2sin
3
41
3
2
3
1
3
1
psMEW
pdMEW
puMEW
pZME
psME
pdME
puME
pME
GGGG
GGGG
,,
2,,
2,,
2,,
,,
,,
,,
,,
sin3
41sin
3
41sin
3
81
3
1
3
1
3
2
0.0002 0.2312sin
angle mixing weak theis 2
W
W
exchange:
Z0 exchange:
Goal: Isolate GMs and GE
s by isolating the Z0 exchange graph
G0 Program
At a given Q2, full decomposition of GsE, Gs
M, GeA requires 3 new
parity-violation measurements:
Forward angle e + p (elastic) ← ← data-taking completedata-taking completeBackward angle e + p (elastic) ← ← data-taking 2005-2007data-taking 2005-2007
Backward angle e + d (quasi-elastic) ← ← data-taking 2005-2007data-taking 2005-2007
G0 will perform theseparation at threedifferent Q2 values -
0.3, 0.5, 0.8 (GeV/c)2
)()()()sin41(
)()()(
)()()(
222
222
22
QGQGA
QGQGQA
QGQGA
eAMWA
ZMMM
ZEEE
Parity-ViolatingElastic Scattering
2
e e pp
LR
LRA
unpol
AMEF AAAQG
224
2
where:
Scatter polarized electrons from unpolarized protons, and measure the asymmetry:
prevalent at forward angles
prevalent at backward angles
Note: A ~ 10-6 = 1 ppm
Experiment
General requirements:• High statistics (1013-1014) events
– high current, high polarization beam– large acceptance– high rate capability
• Systematics– control of helicity-correlated beam properties– background characterization and rejection
Forward-Angle Measurements
• Elastic proton detection• Toroidal focusing spectrometer• Time-of-flight distinguishes elastic protons from
pions, inelastic protons
G0 beammonitoring girder
superconducting magnet (SMS)
detectors (Ferris wheel)
cryogenic supply
target service module
G0 Forward-Angle Configurationat Jefferson Laboratory
Beam
Estimate of background yield under the elastic peak is performed by using
- data with full and empty (gas H2) targets, different pressures
- data with dummy entrance and exit windows (Al)
- Data with W radiator and dummy windows (electro/photo production)
Unfold backgrounds from target windows and inelastic LH2 processes
pions
elastic protons
inelastics
Fractional background in elastic cut
Dominant Systematic: Backgrounds
Yield
asymmetry
Detector 8
• For smaller detectors, yields and asymmetries are measured on each side of the elastic peak and smooth interpolation is reasonable.
• For larger detectors, background asymmetry is large and varies across the elastic peak.
Small increase of uncertainty in elastic asymmetry
due to background for small detectors.
Yield
asymmetry
Detector 13
Effect of backgrounds will likely dominate systematic uncertainty for larger detectors.
Background Correction
For preliminary result, used linear interpolation
Progress Background Correction
Constrain yield with Monte Carlo
motivated fit.
Constrain asymmetry by enforcing
constant elastic asymmetry.
ppm
Yield – Ring 8 Asymmetry – Ring 8
Forward angle asymmetries
- full statistics
- detectors 13 to 15 not shown.
- preliminary background corr.
- 25% blinding factor applied
Asy
mm
etr
y (
ppm
)
Increasing Q2
Detector number
Preliminary Results
Systematic uncertainties included for preliminary result:
- Deadtime corr 2%
- Beam parameters corr 0.01 ppm
- Leakage beam corr 0.10 ppm
- Beam Polarization 2%
- Background corr not included
- Q2 determination 1%
- Rad. Corr., EM FF’s small
BLINDING FACTOR APPLIED
G0 Daily NewsBox Has Been OpenedBlinding Factor RevealedAnalysis complete,
preparing for public release of results
Next Phase of G0: Backward-Angles
Electron detectionAdd Cryostat Exit Detectors (“CED’s”) to define electron trajectoryAdd aerogel Čerenkov counter to reject -
Measurements on H and D to separate GMs,
GAe
Ebeam (MeV) Q2 (GeV2)
424 0.3
585 0.5
799 0.8
beam
target
magnet
FPD #1
FPD #16 CED#9
CED#1
Čerenkov
inelastic e-
or photo -
elastic e-
Canadian Contributions to Backward-Angle Expt.
• Čerenkov counters built by:– U Manitoba, TRIUMF, UNBC– Grenoble (France)
• Čerenkov/CED support structure (TRIUMF) and CED construction.
• Electronics for Canadian Čerenkov counters:– U Winnipeg
• Acquisition and Analysis of Data– Pion asymmetries (U Winnipeg)
First run starts
Dec. 3, 2005
Summary
• Forward angle production run complete.• Analysis of forward angle data nearing
completion.• Preparations for back-angle run underway:
– additional scintillators, Čerenkov counters being tested.
– new electronics being tested.– new analysis software in preparation.
• Back angle running will begin in December 2005, will continue throughout 2006, 2007.