Parity-Violating Electron Scattering

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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.

k
q k k 4-momentum transfer
k
A useful variable
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Relationship to Quarks

• The charge and magnetization are carried by
  quarks
• We can do the same experiment for the neutron
  (udd)

isospin symmetry
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The Extra HandleZ0 scattering
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Parity Violating Asymmetry
forward ep
backward ep
backward ed
kinematical factors
Note Asymmetry is of order ppm
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The Protons Weak Charge
As Q2 ? 0
MNC
MEM
measures Qp protons electric charge
measures Qpweak protons weak charge
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 sin2?W
12 GeV QW (e)
present d-quark dominated Cesium APV (QAW)
SM running verified at 4? level pure lepton
SLAC E158 (QeW ) SM running verified
at 6? level
future u-quark dominated Qweak (QpW)
projected to test SM running at 10? level pure
lepton12 GeV e2ePV (QeW ) projected to test
SM running at 25 ? level
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Qpweak Qeweak Complementary Diagnostics for
New Physics
JLab Qweak
SLAC E158 (complete)
-
(proposed)
(published) 0.006

• 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.

Erler, Kurylov, Ramsey-Musolf, PRD 68, 016006
(2003)
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Summary of PV Electron Scattering Experiments
publishing, running
publishing, running
x2,
published x2, running
publishing, running
2008
K. Kumar
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G0 Forward-Angle Measurements

• Elastic proton detection
• toroidal focusing spectrometer
• Time-of-flight distinguishes pions and protons

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G0 Forward-Angle Configuration at Jefferson Lab
superconducting magnet (SMS)
cryogenic supply
G0 beam monitoring girder
detectors (Ferris wheel)
target service module
Beam
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Largest Systematic Effect Backgrounds
detector 8

• Determined using fitting techniques
• Large asymmetry from hyperon production, decay,
  rescattering

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G0 forward-angle experiment final results
GEs?GMs, Q2 0.12-1.0 GeV2
?2 test taking into account random and correlated
errors the non-vector-strangeness hypothesis is
disfavored at 89
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Comparison to World Data
Q20.23 GeV2
95.5 CL
Q20.1 GeV2
Q20.48 GeV2
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Empirical Fit GEs and GMs Separately

• Compare GEs with GEn, and GMs with GMp

-1/3?s/?p -18
-1/3GEs(0.2)/GEn(0.2)40
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Upcoming Data-TakingThe 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

• Electron detection (Note VERY different
  systematics)
• Add Cryostat Exit Detectors (CEDs) to define
  electron trajectory
• Add aerogel Cerenkov counter to reject ?-
• Measurements on H and D to separate GMs, GAe

FPD 16
CED 9
Cerenkov
magnet
elastic e-
inelastic e- or photo ?-
FPD 1
CED 1
beam
target
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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

• Very soon high precision data from Happex at
  0.1 GeV2

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Elastically Scattered Electron
Luminosity Monitors
Region III Drift Chambers and Quartz Scanner
Toroidal Magnet
Region II Drift Chambers
Region I GEM Detectors
Eight Fused Silica (quartz) Cerenkov Detectors
Collimator with 8 openings ? 8 2
electronics
35cm Liquid Hydrogen Target
Polarized Electron Beam
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QpWeak Toroidal Magnet - QTOR

• 8 toroidal coils, 4.5m long along beam
• Resistive, similar to BLAST magnet
• Pb shielding between coils
• Coil holders frame all Al
• ?B?dl 0.7 T-m
• bends elastic electrons 10o
• current 9500 A

beam
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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.

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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 sin2?W 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
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Anticipated QpWeak Uncertainties
? ?Aphys /Aphys
?Qpweak/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


4 uncertainty on QpW ? 0.3 precision on sin2?W
at Q2 0.03 GeV2
(Erler, Kurylov, Ramsey-Musolf, PRD 68, 016006
(2003)) QpW 0.0716 ? 0.0006, theoretical
extrapolation from Z-pole 0.8 error comes from
QCD uncertainties in box graphs, etc.
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G0 Backward AngleParasitic 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).