1 May 16 2006The E158 Experiment A Precision Measurement of the Weak Mixing Angle in Fixed Target...

May 16 2006 The E158 Experiment 1

The E158 Experiment

A Precision Measurement of the Weak Mixing Angle in Fixed Target electron-electron (Møller) Scattering

Krishna Kumar

University of Massachusetts, Amherst

PAVI06, May 16, 2006


Outline

• Physics Motivation

• Evolution of the E158 Proposal

• Experimental Design

• Final Result & Implications

• Future Prospects with Møller Scattering


Beyond the Standard Model

•Rare or Forbidden Processes neutrinoless double-beta decay…

Low Q2 offers unique and complementary probes of “new physics”

Low Energy: Q2 << MZ2

•Symmetry Violations Neutrino oscillations, EDM searches…

•New Particle Searches•Rare or Forbidden Processes•Symmetry Violations•Electroweak One-Loop Effects

ComplementaryApproaches

High Energy CollidersSLC, LEP, LEPII, HERA, Tevatron, PEPII, LHC…

•Indirect Effects of “New Physics”

g-2 anomaly, CKM Unitarity weak neutral current interactions

(WNC)


Precision WNC Measurements @ Low Q2

•Precise predictions @ 0.1%•Indirect access to TeV scale•World electroweak data has marginal 2, but no

discernable pattern•Data used to put limits on energy scale of new physics effects•Parity-conserving contact interactions probed at 10 TeV level

•Parity-violating contact interactions probed at 1 TeV level


Evolution of an Idea

€

APV ∝me E lab (1− 4sin2 ϑ W )

€

σ ∝1

E lab

Figure of Merit rises linearly with Elab

€

δ(sin2 ϑ W )

sin2 ϑ W

≅ 0.05δ(APV )

APV

Depending on the day of the week, I get zero or 16/9ths times the e-p asymmetryPurely leptonic reaction


Anatomy of a SLAC Proposal

€

APV ≈ −4 ×10−9 × Ebeam × Pbeam•~ 10 ppb statistical error at highest Ebeam

•~ 0.4% error on weak mixing angle• 10 nm control of beam centroid on target

– R&D on polarized source laser transport elements

• 12 microamp beam current maximum– 1.5 meter Liquid Hydrogen target

• 20 Million electrons per pulse @ 120 Hz– 200 ppm pulse-to-pulse statistical fluctuations

• Electronic noise and density fluctuations < 10-4

• Pulse-to-pulse monitoring resolution ~ 1 micron• Pulse-to-pulse beam fluctuations < 100 microns

– 100 Mrad radiation dose from scattered flux• State-of-the-art radiation-hard integrating calorimeter

• Full Azimuthal acceptance with lab ~ 5 mrad– Quadrupole spectrometer– Complex collimation and radiation shielding issues


E158 Collaboration & Chronology

Parity-Violating Left-Right Asymmetry In Fixed Target Møller Scattering

•Berkeley•Caltech•Jefferson Lab•Princeton•Saclay

•SLAC•Smith •Syracuse•UMass•Virginia

8 Ph.D. Students60 physicists

E158 Collaboration

At the Stanford Linear Accelerator Center

Feb 96: Workshop at PrincetonSep 97: SLAC EPAC approvalMar 98: First Laboratory Review1999: Design and Beam tests2000: Funding and construction2001: Engineering run2002-2003: Physics2004: First PRL2005-2006: Final publications

E158 Chronology

Goal: error small enough to probe TeV scale physics


E158 New Physics Reach


Stanford Linear Accelerator Center


Polarized Beam

Laser Power (µJ)

Ele

ctro

ns

per

pu

lse

New cathode

Old cathode

No sign of No sign of charge limit!charge limit!

Low doping for most of Low doping for most of active layer yields high active layer yields high polarization.polarization.

High doping for 10-nm High doping for 10-nm GaAs surface GaAs surface overcomes charge limit.overcomes charge limit.


Systematic Control

CID Gun

CID Gun

Vault

Vault

Source

Source

Laser Room

Laser Room

IA Feedback LoopIA Feedback Loop

IA cell applies a helicity-correlated phase shift to the beam.

The cleanup polarizer transforms this into intensity asymmetry.

POS Feedback LoopPOS Feedback Loop

Piezomirror can deflect laser beam on a pulse-to-pulse basis.

Can induce helicity-correlated position differences.

““Double” Feedback LoopDouble” Feedback Loop

Adjusts CP, PS to keep IA & Piezo corrections small (~ ppm & ~100 nm).

Very slow feedback (n = 24k pairs).


σtoroid 30 ppmσBPM 2 micronsσenergy 1 MeV

Beam Monitoring

Agreement (MeV)

Resolution 1.05 MeV

Event by event monitoring at 1 GeV and 45 GeV


Liquid Hydrogen Target

Refrigeration Capacity 1 kWOperating Temperature 20 KLength 1.5 mFlow Rate 5 m/sVertical Motion 6 inches


Kinematics


Quadupoles and Dipoles

Major Breakthrough:7 magnetic elements from historic 8 and 20 GeV spectrometer elements


E158 Spectrometer


Downstream Configuration


Detector Concept

Data from Profile Detectors


Integrating Calorimeter

•20 million 17 GeV electrons per pulse at 120 Hz•100 MRad radiation dose: Cu/Fused Silica Sandwich

-State of the art in ultra-high flux calorimetry-Challenging cylindrical geometry

Single Cu plate

End plate

“ep” ring

“Møller” ring

Lead shield

Lead shield

PMT holder

Light guide


ProfileDetectorwheel

LuminosityMonitorregion

PMT LeadHolder/shield

Detector Cart

ProfileDetectorwheel

PMT LeadHolder/shield


E158 Analysis

electron flux

Basic Idea:

: quartz: copper

light guide

PMTshielding

air

Radial and azimuthal segmentation

•Corrections for beam fluctuations•Average over runs•Statistical tests•Beam polarization and other normalization


Run 1: Spring 2002Run 2: Fall 2002Run 3: Summer 2003

Physics Runs

1020 Electrons on Target

Run 1: Apr 23 12:00 – May 28 00:00, 2002Run 2: Oct 10 08:00 – Nov 13 16:00, 2002Run 3: July 10 08:00 - Sep 10 08:00, 2003

45 GeV: 14.0 revsg-2 spin precession

48 GeV: 14.5 revs

Data divided into 75 “slugs”: - Wave plate flipped ~ few hours - Beam energy changed ~ few days

APV Sign Flips


Beam Asymmetries

Position differences < 20 nmPosition agreement ~ 1 nm

Charge asymmetry at 1 GeV

Charge asymmetry agreement at 45 GeV

Energy difference in A line

Energy difference agreement in A line


Raw Asymmetry Statistics

€

Ai − A

σ i

€

Ai − A

σ i

σi ≈ 200 ppm

N = 85 Million

σi ≈ 600 ppbN = 818


APV = (-131 ± 14 ± 10) x 10-9

€

APV ≈ −1×10−7 × Ebeam × Pbeam × (1− 4sin2 ϑ W )

≈ 250ppb

Final Analysis of All 3 Runs

Phys. Rev. Lett. 95 081601 (2005)


Electroweak Physics

sin2W = e2/g2 → test gauge structure of SU(2)U(1)

3%

•Czarnecki and Marciano•Erler and Ramsey-Musolf•Sirlin et. al.•Zykonov


Status in 1997

Q (GeV)

sin2w


NatureVol 435

26 May 2005

NEWS AND VIEWS

Status since 2005

* Limit on LL ~ 7 or 16 TeV

* Limit on SO(10) Z’ ~ 1.0 TeV* Limit on lepton flavor violating coupling ~ 0.01GF

(95% confidence level)

sin2eff = 0.2397 ± 0.0010 ± 0.0008

6σ

PRL 95 081601 (2005)


Møller Scattering with JLab Upgrade

Address longstanding discrepancy between hadronic and leptonic Z asymmetries

Z pole asymmetries

•Comparable to single Z pole measurement: shed light on disagreement•Best low energy measurement until ILC or -Factory •Could be done ~ 2012-13

Jefferson Lab


Ultrahigh Precision

loop corrections

(world average ~0.0002)

Compare with mass observed at the LHC:

Measure contribution from scalars to quantum loops

Colliders will attempt this with ALR and MW but…

Systematics are extremely challenging!

t

Z

H

b

new

physics

Critical crosscheck of electroweak theory

Energy scale to 10-4, polarimetry to 0.15%

€

δmH

mH

≈10% for δ sin2 θW ≈ 0.00004


Møller Scattering at the ILC

E158 LC

Energy (GeV) 48 250-500

Intensity/pulse 4.5 1011 14 1011

Pulse Rate (Hz) 120 120

Pe 85% 90%

Time (s) 5 106 2 107

ALR (ppm) 0.15 1-2

δALR (ppm) 0.015 0.008

δsin2(W) 0.001 0.00008

Compton Polarimetry

K.K, Snowmass 96

€

δPe (syst) ≈ 0.25% (projected)

≈ 0.50% (SLD)

€

δ sin2 θW

sin2 θW

≈1

20

δPe

Pe

⇒ δ sin2 θW ≈ 0.00003

•Order of magnitude better•Parasitic “exhaust” beam•Collider measurements unlikely to do better•Need 3 or 4 such independent

measurements


Summary

•SLAC E158’s main physics result has been published:•Parity is violated in Møller scattering•Final result with all data: APV: -131 ± 14 ± 10 ppb•Running of weak mixing angle established at 6σ•sin2eff = 0.2397 ± 0.0010 ± 0.0008•New constraints on TeV scale physics

•Next publications (by late 2006):•Inelastic e-p asymmetry at low Q2

•First measurement of e-e transverse asymmetry analyzing power

•This experiment could not be done elsewhere in the world•Last Fixed Target Experiment at Historic SLAC End Station A!

•Future experiments could improve sensitivity by ~ 2 to 6•An “ultimate” measurement could be done at an LC

1 May 16 2006The E158 Experiment A Precision Measurement of the Weak Mixing Angle in Fixed Target...

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