From Z0 to Zero: A Precise Measurement of the Weak Mixing Angle from SLAC E158

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From Z0 to ZeroA Precise Measurement of the
Weak Mixing Angle from SLAC E158

• Yury Kolomensky
• UC Berkeley
• For SLAC E158 Collaboration

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Outline

• Historical interlude
• SLAC E158
• Motivation
• Experimental technique
• New results
• Outlook

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End Station A
End Station A
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End Station A How It All Started
SLAC-R-090 (08/1968)
Experiment E-4 SLAC-MIT-CIT (precursor to
discovery of quarks) First high-power LH2 target
at SLAC !
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SLAC E122
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SLAC E122
Detector
e
16 22 GeV
Liquid Deuterium
Polarized GaAs source
High current
30 cm target
Dedicated run
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E122 Asymmetry
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SLAC E122 Result
(1978)
sin2qW 0.224 0.020
First definitive measurement of mixing between
the weak and electromagnetic interaction
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A-Line 50 GeV Upgrade
50 GeV capability In ESA 1995
Polarized structure Function experiments E154,E15
5,E155x
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E158 Heritage

• SLAC provides unique capabilities with
  high-intensity, high-energy, high-polarization
  beams
• We are building on the past experience and
  physics interests
• Electroweak physics
• Even tests of QED and QCD predictions (somewhat
  surprisingly)
• 3 fundamental interactions for the price of one
  experiment ! ?

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SLAC E158 Motivation
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High Energy Electroweak Data
(LEP EWWG)
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High Energy EW Data

• Spectacular precision
• Quantum loop level (LO to NNLO)
• Precise indirect constraints on top and Higgs
  masses
• General consistency with the Standard Model
• Few smoking guns
• Leptonic and hadronic Z couplings seem
  inconsistent ?
• Direct searches have not yielded new physics
  phenomena (so far)
• Complementary sensitivity at low energies
• Rare or forbidden processes
• Symmetry violations
• Precision measurements

BaBar and E158
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Direct vs Indirect Searches
(according to Grimm Brothers)
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Electroweak Physics Away from Z pole

• Precision Z observables establish anchor points
  for SM
• Low energy observables probe interference
  between SM and NP
• Current low energy experiments are accessing
  scales of beyond
• 10 TeV

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Electroweak Mixing Angle

• Mixing of neutral SU(2)?U(1) currents
• Mixing angle
• e g sinqW gcosqW
• At tree level sin2qW 1-MW2/MZ2

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Running of Weak Mixing Angle
sin2qW e2/g2 ? test gauge structure of
SU(2)?U(1)
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Status Before E158 (1997)
sin2qW
Q (GeV)
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Cesium Atomic Parity Violation Result vs. Time
(Colorado measurement)
sin2qw
0.240
Standard Model
0.238
Kuchiev Flambaum
0.236
Kozlov Porsev Tupitsyn
0.234
Johnson Bednyhakov Soff
Derevianko
Bennett Wieman
Dzuba Flambaum
0.232
Wieman et al.
0.230
2000
1999
1998
2001
2002
2003
1997
Modifications in the theoretical corrections to
the atomic structure
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(No Transcript)
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Status A Week Ago
sin2qw
Run I II
Q (GeV)
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The Experiment
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Weak-Electromagnetic Interference in Electron
Scattering
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Fixed Target Møller Scattering
Purely leptonic reaction gee 1 4 sin2?W
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Parity Violation in Møller Scattering

• Scatter polarized 50 GeV electrons
• off unpolarized atomic electrons
• Measure
• Small tree-level asymmetry
• At tree level,
• Raw asymmetry about 130 ppb
• Measure it with precision of 10
• Most precise measurement of sin2qW at low Q2

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E158 Physics Sensitivity

• Unique window of opportunity
• Complementary to collider searches

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E158 Collaboration
Institutions
Caltech Syracuse Princeton Jefferson
Lab SLAC UC Berkeley CEA Saclay UMass
Amherst Smith College U. of Virginia
60 physicists, 7 Ph.D. students
Chronology
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Run I Results Published
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Experimental Technique

• Scattering of polarized electrons off atomic
  electrons
• High cross section (14 mBarn)
• High intensity electron beam, 80 polarization
• 1.5m LH2 target
• Luminosity 41038 cm-2s-1
• High counting rates flux-integrating
  calorimeter
• Principal backgrounds elastic and inelastic ep
• Main systematics beam polarization,
• helicity-correlated beam effects, backgrounds

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Major Challenges

• Statistics !
• Need to accumulate 1016 electrons
• Suppress other sources of noise to be dominated
  by counting statistics
• Beam monitoring and resolution
• Major (potential) source of additional jitter
• Beam systematics
• False asymmetries
• Backgrounds
• Need to measure in situ

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Key Ingredients

• High beam polarization and current
• Largest high-power LH2 target in the world
• Spectrometer optimized for Møller kinematics
• Stringent control of helicity-dependent
  systematics. Passive asymmetry reversals

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Parity-Violating Asymmetry
Rapidly flip electron helicity (120 Hz) and form
pulse pairs of opposite helicity Measure
pulse-pair flux asymmetry
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Statistics
electrons per pulse 107 Rep rate
(120 Hz) 109 Seconds/day
1014 100 days 1016
DA 10-8
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E158 Runs
Run 1 Spring 2002 Run 2 Fall 2002 Run 3
Summer 2003
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Eliminating Beam Jitter
Integrate Detector response Flux Counting
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Polarized Beam
High doping for 10-nm GaAs surface overcomes
charge limit.
Low doping for most of active layer yields high
polarization.
No sign of charge limit!
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Control of Beam Systematics

• Beam helicity is chosen pseudo-
• randomly at 120 Hz
• use electro-optical Pockels cell in Polarized
  Light Source
• sequence of pulse quadruplets
• Reduce beam asymmetries by feedback
• at the Source
• Control charge asymmetry and position asymmetry

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Passive Reversals and Checks

• Physics Asymmetry Reversals
• Insertable Half-Wave Plate in Polarized Light
  Source
• (g-2) spin precession in A-line (45 GeV and 48
  GeV data)
• False Asymmetry Reversals
• Reverse false beam position and angle
  asymmetries
• physics asymmetry unchanged
• Insertable -I/I Inverter in Polarized Light
  Source
• Null Asymmetry Cross-check is provided by a
• Luminosity Monitor
• measure very forward angle e-p (Mott) and Møller
  scattering

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Polarized Source
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Beam Diagnostics
Energy dithering region
A-Line
linac
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Beam Asymmetries
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Møller Polarimetry

• Average polarization
• 85 5 in Run I
• 84 5 in Run II
• 91 5 in Run III
• New superlattice !

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Liquid Hydrogen Target
Refrigeration Capacity 1 kW Operating
Temperature 20 K Length
1.5 m Flow Rate 5
m/s Vertical Motion 6 inches
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Kinematics
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Spectrometer
x (cm)
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Setup in ESA
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(No Transcript)
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Detector Concept
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MOLLER Detector
electron flux
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Luminosity Monitor
more than 108 scattered electrons per spill at
?lab 1 mrad

• Null asymmetry test

• Density fluctuations monitor

• Enhanced sensitivity
• to beam fluctuations

Parallel plates
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Profile Detector

• 4 Quartz Cherenkov detectors with
• PMT readout
• insertable pre-radiators
• insertable shutter in front of PMTs
• Radial and azimuthal scans

• collimator alignment, spectrometer tuning
• background determination
• Q2 measurement

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Scattered Flux Profile
Møller peak scan data vs Monte Carlo
Møller scattering kinematics 0.026
GeV-2 0.6
Data Monte Carlo

• 2 mm geometry
• 1 energy scale
• Radiative tail

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MOLLER Statistics and Fluctuations
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Raw Asymmetry Statistics
Asymmetry pulls per pulse pair
150M pairs
Asymmetry pulls per 10k pair chunks
(A-)/s
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Raw Asymmetry Systematics

• First order systematic effects
• False asymmetry in electronics
• Measured to be smaller than 1 ppb
• Errors in correction slopes
• Measured by comparing two timeslots
• Beam-induced asymmetries of 1 ppm corrected to
  below stat errors of 50 ppb in multiple data
  samples
• Higher-order corrections
• Beam size fluctuations
• Measured by wire array
• Correlation between beam asymmetry and pulse
  length (intra-spill asymmetries)
• New electronics in Run III

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SLICES Temporal Beam Profile

• SLICES readout in 10 bit ADCs
• Q bpm31Q (4)
• E bpm12X (3)
• X bpm41X (4)
• Y bpm41Y (4)
• dX bpm31X (4)
• dY bpm31Y (4)

BPM 12X Real Waveform
Integration time
S1 0 -100 ns S2 100-200 ns S3 200-300
ns S3 300-1000 ns
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Additional Corrections

• OUT detector at edge of Møller acceptance most
  sensitive to beam systematics
• Use it to set limits on the grand asymmetry

OUT detector asymmetry vs sample
OUT asymmetry with SLICE correction
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Møller Asymmetry

• Over 330M pulse pairs over 3 separate runs
  (2002-2003) at Ebeam45 and 48 GeV
• Passively flip helicity of electrons wrt source
  laser light every day to suppress spurious
  helicity-correlated biases

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Backgrounds

• Electron-proton elastic scattering
• Well-understood at our kinematics
• Radiative electron-proton inelastic scattering
• PV asymmetry unknown at our kinematics
• Naïve quark model prediction O(1 ppm)
• Pion production
• Two-photon exchange events with transverse
  polarization
• A bit of a surprise
• Other contributions at O(0.1) level

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EP Detector Data
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EP Sample Summary
Preliminary (raw asymmetries)
ARAW(45 GeV) -1.36 0.05 ppm (stat.
only) ARAW(48 GeV) -1.70 0.08 ppm (stat. only)

• Ratio of asymmetries
• APV(48 GeV) /APV(45 GeV) 1.25 0.08 (stat)
  0.03 (syst)
• Consistent with expectations for inelastic ep
  asymmetry,
• but hard to interpret in terms of
  fundamental parameters
• 3510 ppb correction to Møller asymmetry in Run
  I, below
• 20 ppb for Run II
• Test of strong interactions in E158 ?

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Transverse Asymmetries
Beam-Normal Asymmetry in elastic electron
scattering

• Electron beam polarized transverse to beam
  direction

Interference between one- and two-photon exchange
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AT in Møller Scattering
e
e
e
e
e
e
e
e
Theory References 1. A. O. Barut and C.
Fronsdal, (1960) 2. L. L. DeRaad, Jr. and
Y. J. Ng (1975) 3. Lance Dixon and Marc
Schreiberhep/ph-0402221 (Included bremsstrahlung
corrections few percent)
Prediction for 46 GeV -3.5 ppm
E158 acceptance
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E158 Transverse Results
24 hrs of data
43 and 46 GeV ee ? ee LH2 target
f
(Azimuthal angle)

• Raw asymmetry!
• Publication by Fall
• Crosscheck E158 polarimetry at 3?
• O(a3) test of QED in E158 ?
• 5 residual transverse polarization in
• longitudinal data carefully combine
• detector channels to suppress this effect

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ATep at E158

• Raw asymmetry!
• Has the opposite sign! (preliminary!)
• Polarization background corrections
• 25 inelastic ep
• Few percent pions (asymmetry small)
• Proton structure at E158 !

Moller ring
ep ring
f
(Azimuthal angle)
43 46 GeV ep ? ep
24 hrs of data
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Asymmetry Corrections and Systematics

• Scale factors
• Average Polarization 88 5
• Linearity 99 1
• Radiative corrections 1.016 0.005

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Preliminary Results
APV (e-e- at Q20.026 GeV2) -128 ? 14 (stat) ?
12 (syst)

• Significance of parity non-conservation in
  Møller scattering 8 ?

sin2?eff (Q20.026 GeV2) 0.2403 0.0010 (stat)
0.0009 (syst)

• Most precise measurement at low Q2
• Significance of running of sin2qW 7 ?

sin2?WMS(MZ) 0.2330 0.0011 (stat) 0.0010
(syst)

• Standard Model pull 1.2 ?

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The Weak Mixing Angle

• General agreement between low Q2
• experiments, although NuTeV is still
• 3s high compared to SM fit
• Stringent limits on new interactions at
• multi-TeV scales
• Parameterize as limit on 4-fermion
• contact term ?LL 6-14 TeV limits for
• E158 alone (95 C.L.)
• Limit on SO(10) Z at 900 GeV

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Running of the Weak Mixing Angle
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Summary

• Most precise measurement of parity
  non-conservation in scattering experiments
• Running of the weak mixing angle established at
  level of 7s
• Stringent constraints on new physics at multi-TeV
  scale
• Results for three out of four fundamental
  interactions !

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Outlook

• Next set of precision measurements on the horizon
  
• Neutrino-electron scattering
• Reactor experiments (in conjunction with q13)
  cross section measurements to 0.7-1.3 would
  translate in s(sin2qW) down to 0.001
• Ultimate measurements at the neutrino factory
• Atomic parity violation
• Ratios of APV in isotopes and hydrogenic ions
  could reach sensitivity of s(sin2qW) 0.001
• PV in electron scattering
• Active program planned for JLab PV in elastic ep
  scattering (2007), Møller scattering, and DIS eD
  scattering (2010) could reach below s(sin2qW)
  0.001 per experiment
• ee- and e-e- at the Linear Collider

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Selected Future Measurements
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Luminosity Monitor Data

• Null test at level of 20 ppb

• Density fluctuations small
• Limits on second order effects

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Neutrino-Nucleon Scattering
Charged-Current(CC)
Neutral-Current(NC)
NC coupling T3- Q sin2qW
CC coupling T3

• Measure n NC/CC ratio to extract ratio of weak
  couplings
• Experimental and theoretical uncertainties for
  sin2qW suppressed in the ratio
• NuTeV uses both neutrino and anti-neutrino beams
  form

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NuTeV Detector
690 ton n-target
Target / Calorimeter
Toroidal Spectrometer

• 168 Fe plates provide mass
• 84 liquid scintillation counters
• Trigger the detector
• Measure Visible energy, n interaction point,
  Event length
• 42 drift chambers
• Localize transverse vertex

• Solid Fe magnet
• Measures m momentum/charge

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NuTeV Result
NuTeV actually measures two ratios
Quote result in terms of sin2qWon-shell
0.2277 0.0013 (stat) 0.0009 (syst) or
sin2qWMS (MZ) 0.2361 0.0017 (3s
SM pull)
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New or Old Physics ?

• Hard to explain NuTeV results with popular NP
  models
• SUSY loops or RPV SUSY do not quite work
• Hard to fit with leptoquarks
• Designer Z is possible (need gL
• Possible Old Physics Explanations
• Electroweak corrections
• New calculations (hep-ph/0310364) claim
  significant shift in the result and
  underestimated uncertainties being checked
• QCD effects
• Isospin violation (up ? dn) plausible, but large
  effect needed (O(5) to move NuTeV result to
  Standard Model)

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Atomic Parity Violation

• Weak neutral currents induce mixing of
  opposite-parity states
• Ya. Zeldovich (1956)
• Look for forbidden transitions
• E.g. 1Sg2S, caused by 2S-2P mixing
• Effect too small in Hydrogen atom but enhanced by
  Z3 in heavy elements
• Atomic theory simplest for alkali atoms
• High-level transitions accessible by lasers
• 6S g7S in Cs

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Boulder Cs PNC Experiment
1982-1999

• P-odd, T-even correlation ? E ? B (Stark
  interference)
• 5 reversals to distinguish PNC from systematics

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APV Results

• APV measures the weak charge (neutral current
  vector coupling) of the nucleus
• QW r Z(1-4sin2qW)-N
• Standard Model QW(133Cs) -73.19 0.03
• Experiment QW(133Cs) -72.69 0.48
• 0.4 experimental and 0.5 theoretical
  uncertainty, primarily from Cs atomic wave
  function
• Equivalent to sin2qW(MZ) 0.2292 0.0019 (-1s
  SM pull)
• Future improvements
• Isotope measurements (e.g. Yb)
• 0.3 on QW(Yb) or 0.001 on sin2qW