Parity-Violating Electron Scattering Jeff Martin University of Winnipeg

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.

Electromagnetic Elastic Electron Scattering Scattering cross-section depends on two “form factors” G E (Q 2 ), G M (Q 2 ). At small Q 2, 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” A useful variable:

The charge and magnetization are carried by quarks We can do the same experiment for the neutron (udd) Relationship to Quarks isospin symmetry

The Extra Handle: Z 0 scattering  e p SpeciesChargeWeak Charge u d s

Parity Violating Asymmetry kinematical factors forward ep backward ep backward ed Note: Asymmetry is of order ppm

The Proton’s Weak Charge measures Q p – proton’s electric charge measures Q p weak – proton’s weak charge M EM M NC As Q 2  0 At tree level in the standard model: A sensitive, low-energy extraction of the weak mixing angle.

Physics: The Running of sin 2  W present: “d-quark dominated” : Cesium APV (Q A W ): SM running verified at ~ 4  level “pure lepton”: SLAC E158 (Q e W ): SM running verified at ~ 6  level future: “u-quark dominated” : Q weak (Q p W ): projected to test SM running at ~ 10  level “pure lepton”:12 GeV e2ePV (Q e W ): projected to test SM running at ~ 25  level 12 GeV Q W (e)

(published) ±0.006 (proposed) - Qweak measurement will provide a stringent stand alone constraint on Lepto-quark based extensions to the SM. Q p weak (semi-leptonic) and Moller (pure leptonic) together make a powerful program to search for and identify new physics. Q p weak & Q e weak – Complementary Diagnostics for New Physics JLab QweakSLAC E158 (complete) Erler, Kurylov, Ramsey-Musolf, PRD 68, (2003)

DHB, 17 June 2005 Summary of PV Electron Scattering Experiments K. Kumar publishing, running x2, publishing, running published x2, running 2008

G 0 Forward-Angle Measurements Elastic proton detection toroidal focusing spectrometer Time-of-flight distinguishes pions and protons pions inelastic protons elastic protons Det 8

G 0 beam monitoring girder superconducting magnet (SMS) detectors (Ferris wheel) cryogenic supply target service module G 0 Forward-Angle Configuration at Jefferson Lab Beam

Largest Systematic Effect: Backgrounds  Determined using fitting techniques  Large asymmetry from hyperon production, decay, rescattering detector 8

G E s +  G M s, Q 2 = GeV 2  2 test taking into account random and correlated errors: the non-vector-strangeness hypothesis is disfavored at 89% G 0 forward-angle experiment – final results

Comparison to World Data Q 2 =0.1 GeV 2 Q 2 =0.48 GeV % CL Q 2 =0.23 GeV 2

Compare G E s with G E n, and G M s with G M p Empirical Fit: G E s and G M s Separately -1/3  s /  p = -18% -1/3G E s (0.2)/G E n (0.2)~40%

Upcoming Data-Taking: The year of G 0 In coming years, G 0 will run at backward angles in order to truly separate the electric and magnetic form factors. March 15 – April 29, 2006: Q 2 = 0.6 GeV 2. July 21-Sept. 1, 2006: Q 2 = 0.23 GeV 2. Sept. 22-Dec : Q 2 = 0.6 GeV : finish low Q 2.

Backward-Angle Measurements E beam (MeV) Q 2 (GeV 2 ) 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 G M s, G A e

Recent progress: - Target installed - Beamline/Shielding in progress - Upstream Girder in progress - Cosmics testing ongoing

G 0 contribution = GEGE s = 0.13 GMGM s = 0.22 GAGA e Very soon – high precision data from Happex at 0.1 GeV 2 theory: Lewis, Wilcox, Woloshyn

Polarized Electron Beam 35cm Liquid Hydrogen Target Collimator with 8 openings θ= 8° ± 2° Region I GEM Detectors Region II Drift Chambers Toroidal Magnet Region III Drift Chambers and Quartz Scanner Elastically Scattered Electron Eight Fused Silica (quartz) Čerenkov Detectors Luminosity Monitors electronics

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  B  dl ~ 0.7 T-m bends elastic electrons ~ 10 o current ~ 9500 A Q p Weak Toroidal Magnet - QTOR

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 –Q 2 determination Similar technique used in both E158 and HAPPEx UWinnipeg RTI proposal to NSERC submitted Oct ” air-core light guide PMT quartz Pb pre-rad scattered beam Č

Q weak 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 G 0 First run : 8% → 4% More running : 4% → 2.5%

Summary PV electron scattering is a useful tool for: –strangeness form factor determination. –extraction of sin 2  W for standard model test. G 0 Forward angle results published. G 0 Backward angle running Qweak beginning in 2008.

Summary of Systematic Effects SourceUncertainty Electronics deadtime0.05 ppm Helicity-correlated differences in beam properties 0.01 ppm 499 MHz (2 ns) leakage beam0.14 ppm Beam polarization (Hall C Møller)1 % Transverse beam polarization0.01 ppm Inelastic background subtraction0.2-9 ppm Radiative corrections0.3 % Detector  Q 2  1 %

  A phys /A phys  Q p weak /Q p weak Statistical (2200 hours production) 1.8% 2.9% Systematic: Hadronic structure uncertainties % Beam polarimetry 1.0% 1.6% Absolute Q 2 determination 0.5% 1.1% Backgrounds 0.5% 0.8% Helicity-correlated Beam Properties 0.5%0.8% _________________________________________________________ Total 2.2% 4.1%   A phys /A phys  Q p weak /Q p weak Statistical (2200 hours production) 1.8% 2.9% Systematic: Hadronic structure uncertainties % Beam polarimetry 1.0% 1.6% Absolute Q 2 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, (2003)) Q p W =  , theoretical extrapolation from Z-pole 0.8% error comes from QCD uncertainties in box graphs, etc. (Erler, Kurylov, Ramsey-Musolf, PRD 68, (2003)) Q p W =  , theoretical extrapolation from Z-pole 0.8% error comes from QCD uncertainties in box graphs, etc. Anticipated Q p Weak Uncertainties 4% uncertainty on Q p W → 0.3% precision on sin 2  W at Q 2 ~ 0.03 GeV 2

G 0 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).