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.

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

• 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

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

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

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.

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)

(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)

DHB, 17 June 2005

Summary of PV Electron Scattering Experiments

K. Kumar

publishing, running

x2,

publishing, running

publishing, running

published x2, running

2008

G0 Forward-Angle Measurements

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

pions

inelastic protons

elastic protons

Det 8

G0 beammonitoring girder

superconducting magnet (SMS)

detectors (Ferris wheel)

cryogenic supply

target service module

G0 Forward-Angle Configurationat Jefferson Lab

Beam

Largest Systematic Effect: Backgrounds

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

detector 8

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

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

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%

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.

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

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

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

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

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

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

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%

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.

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 %

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

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).