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Parity-Violating Electron Scattering
Jeff Martin
University of Winnipeg
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Parity-Violating Elastic Scattering of Electrons from Protons
• Two applications we will study tonight:– Strange quark structure of the nucleon.– Tests of standard electroweak theory.
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ElectromagneticElastic Electron Scattering
• Scattering cross-section depends on two “form factors” GE(Q2), GM(Q2).
• At small Q2, form factors are Fourier transforms of spatial distributions of charge and magnetization densities in the proton.
e p
k’
k
q = k – k’“4-momentum
transfer”
222 'kkqQ A useful variable:
radius) (chg. 6
mom.) (mag. )0(
charge) (total 1)0(
2
02 E
rdQ
dG
G
G
E
M
E
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• The charge and magnetization are carried by quarks
• We can do the same experiment for the neutron (udd)
Relationship to Quarks
psME
pdME
puME
q
pqMEq
pME GGGGeG ,
,,,
,,
,,, 3
1
3
1
3
2
nsME
ndME
nuME
q
nqMEq
nME GGGGeG ,
,,,
,,
,,, 3
1
3
1
3
2psME
puME
pdME GGG ,
,,,
,, 3
1
3
1
3
2 isospin
symmetry
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The Extra Handle:Z0 scattering
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
puME
pdME
nME
psME
pdME
puME
pME
GGGG
GGGG
GGGG
,,
2,,
2,,
2,,
,,
,,
,,
,,
,,
,,
,,
,,
sin3
41sin
3
41sin
3
81
3
1
3
1
3
23
1
3
1
3
2
0.0002 0.2312sin
angle mixing weak theis 2
W
W
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Parity Violating Asymmetry
2
e e pp
22
2
)()(24 pM
pE
AMEF
LR
LR
GG
AAAQGA
)()()sin41(
)()()(
)()()(
222
222
22
QGQGA
QGQGQA
QGQGA
MeAWA
MZMM
EZEE
eA
sM
sE
GGG
Q2
4M 2
1 2(1 )tan2 2 1
(1 2) (1 )
kinematical factors
forward ep
backward ep
backward ed
Note: Asymmetry is of order ppm
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24200Q
22
QQQ24
,QQQ24
2
2
QBG
FG
M
MA
pweak
F
ppweak
F
EM
NC
ZMEME GG ,, and contains
The Proton’s Weak Charge
measures Qp – proton’s electric charge measures Qpweak
– proton’s weak charge
MEMMNC
As Q2 0
W
pweakQ 2sin41
At tree level in the standard model:
A sensitive, low-energy extraction of the weak mixing angle.
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Physics: The Running of sin2W
present:“d-quark dominated” : Cesium APV (QA
W): SM running verified at ~ 4 level“pure lepton”: SLAC E158 (Qe
W ): SM running verified at ~ 6 level
future:“u-quark dominated” : Qweak (Q
pW): projected to test SM running at ~ 10 level
“pure lepton”:12 GeV e2ePV (QeW ): projected to test SM running at ~ 25 level
12 GeVQW (e)
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(published)±0.006
(proposed)
-
• Qweak measurement will provide a stringent stand alone constraint on Lepto-quark based extensions to the SM.
• Qpweak (semi-leptonic) and Moller (pure leptonic) together make a
powerful program to search for and identify new physics.
Qpweak & Qe
weak – Complementary Diagnostics for New Physics
JLab Qweak SLAC E158 (complete)
Erler, Kurylov, Ramsey-Musolf, PRD 68, 016006 (2003)
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DHB, 17 June 2005
Summary of PV Electron Scattering Experiments
K. Kumar
publishing, running
x2,
publishing, running
publishing, running
published x2, running
2008
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G0 Forward-Angle Measurements
• Elastic proton detection• toroidal focusing spectrometer• Time-of-flight distinguishes pions and protons
pions
inelastic protons
elastic protons
Det 8
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G0 beammonitoring girder
superconducting magnet (SMS)
detectors (Ferris wheel)
cryogenic supply
target service module
G0 Forward-Angle Configurationat Jefferson Lab
Beam
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Largest Systematic Effect: Backgrounds
Determined using fitting techniques Large asymmetry from hyperon production, decay, rescattering
detector 8
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GEs+GM
s, Q2 = 0.12-1.0 GeV2
2 test taking into account random and correlated errors:the non-vector-strangeness hypothesis is disfavored at 89%
G0 forward-angle experiment – final results
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Comparison to World Data
Q2=0.1 GeV2
Q2=0.48 GeV2
95.5% CL
31.062.0)1.0(
028.0013.0)1.0(
sM
sE
G
G
82.079.0
16.014.0
sM
sE
G
G
Q2=0.23 GeV2
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Compare GEs
with GEn, and
GMs with GM
p
Empirical Fit: GEs and GM
s Separately
-1/3s/p = -18%-1/3GE
s(0.2)/GEn(0.2)~40%
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Upcoming Data-Taking:The year of G0
In coming years, G0 will run at backward angles in order to truly separate the electric and magnetic form factors.
• March 15 – April 29, 2006: Q2 = 0.6 GeV2.• July 21-Sept. 1, 2006: Q2 = 0.23 GeV2.• Sept. 22-Dec. 22 2006: Q2 = 0.6 GeV2.• 2007: finish low Q2.
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Backward-Angle Measurements
Ebeam (MeV) Q2 (GeV2)
360 0.23
585 0.5
687 0.6
beam
target
magnet
FPD #1
FPD #16 CED#9
CED#1
Čerenkov
inelastic e-
or photo -
elastic e-
•Electron detection (Note: VERY different systematics)•Add Cryostat Exit Detectors (“CED’s”) to define electron trajectory•Add aerogel Čerenkov counter to reject -
•Measurements on H and D to separate GMs, GA
e
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Recent progress:- Target installed- Beamline/Shielding in progress- Upstream Girder in progress- Cosmics testing ongoing
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G0 contribution 2007-8
= 0.032GEs= 0.13GM
s = 0.22GAe
• Very soon – high precision data from Happex at 0.1 GeV2
theory: Lewis, Wilcox, Woloshyn
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35 cm Liquid Hydrogen Target
Polarized Electron Beam
Collimator With Eight Openings = 9 ± 2°
Toroidal Magnet
Eight Fused Silica (quartz)Cerenkov Detectors
5 inch PMT in Low GainIntegrating Mode on Each
End of Quartz Bar
Elastically Scattered Electrons
325 cm
580 cm
LuninosityMonitor
Region 3Drift Chambers
Region 2Drift Chambers
Region 1GEM Detectors
Polarized Electron Beam
35cm Liquid Hydrogen Target
Collimator with 8 openingsθ= 8° ± 2°
Region IGEM Detectors
Region IIDrift Chambers
Toroidal Magnet
Region III Drift Chambersand Quartz Scanner
Elastically Scattered Electron
Eight Fused Silica (quartz)Čerenkov Detectors
Luminosity Monitors
electronics
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beam
scattered e envelope
•8 toroidal coils, 4.5m long along beam•Resistive, similar to BLAST magnet • Pb shielding between coils• Coil holders & frame all Al
• Bdl ~ 0.7 T-m• bends elastic electrons ~ 10o
• current ~ 9500 A
QpWeak Toroidal Magnet - QTOR
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Quartz Scanner Detector
• Scans in 2D through scattered beam near the main Quartz detector for a variety of purposes:– Fiducialization and “light map” of main detector– backgrounds (inelastics)– confirm linearity of main detector response with beam current– Q2 determination
• Similar technique used in both E158 and HAPPEx• UWinnipeg RTI proposal to NSERC submitted Oct. 2005.
2” air-core light guide PMTquartz
Pb pre-rad
scattered beam
Č
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Qweak status
• Magnet assembly and verification beginning.• Main detectors under construction at JLab.• Tracking chamber development underway by US
university groups.• Target development underway.• Parasitic beam tests of some instruments
conducted simultaneously with G0
• First run 2008-2010: 8% → 4%• More running 2010-2012: 4% → 2.5%
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Summary
• PV electron scattering is a useful tool for:– strangeness form factor determination.
– extraction of sin2W for standard model test.
• G0 Forward angle results published.
• G0 Backward angle running 2006-7.
• Qweak beginning in 2008.
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Summary of Systematic Effects
Source Uncertainty
Electronics deadtime 0.05 ppm
Helicity-correlated differences in beam properties
0.01 ppm
499 MHz (2 ns) leakage beam 0.14 ppm
Beam polarization (Hall C Møller) 1 %
Transverse beam polarization 0.01 ppm
Inelastic background subtraction 0.2-9 ppm
Radiative corrections 0.3 %
Detector Q2 1 %
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Aphys /Aphys Qp
weak/Qpweak
Statistical (2200 hours production) 1.8% 2.9%Systematic:
Hadronic structure uncertainties -- 1.9% Beam polarimetry 1.0% 1.6% Absolute Q2 determination 0.5% 1.1% Backgrounds 0.5% 0.8% Helicity-correlated Beam Properties 0.5% 0.8%_________________________________________________________ Total 2.2% 4.1%
Aphys /Aphys Qpweak/Qp
weak
Statistical (2200 hours production) 1.8% 2.9%Systematic:
Hadronic structure uncertainties -- 1.9% Beam polarimetry 1.0% 1.6% Absolute Q2 determination 0.5% 1.1% Backgrounds 0.5% 0.8% Helicity-correlated Beam Properties 0.5% 0.8%_________________________________________________________ Total 2.2% 4.1%
(Erler, Kurylov, Ramsey-Musolf, PRD 68, 016006 (2003))Qp
W = 0.0716 0.0006, theoretical extrapolation from Z-pole0.8% error comes from QCD uncertainties in box graphs, etc.
(Erler, Kurylov, Ramsey-Musolf, PRD 68, 016006 (2003))Qp
W = 0.0716 0.0006, theoretical extrapolation from Z-pole0.8% error comes from QCD uncertainties in box graphs, etc.
Anticipated QpWeak Uncertainties
4% uncertainty on QpW → 0.3% precision on sin2W at Q2 ~ 0.03 GeV2
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G0 Backward Angle:Parasitic Physics
• Axial structure of the nucleon and the anapole moment.
• Parity-violation in electro and photo excitation of the Delta resonance (inelastic electron and photopion asymmetries).
• Beam normal asymmetries and two-photon exchange for form factor systematics (theory: Blunden et al).