From Z 0 to Zero: A Precise Measurement of the Running of the Weak Mixing Angle SLAC E158

From Z0 to Zero:A Precise Measurement of the Running of the Weak Mixing Angle

SLAC E158

Steve RockUniversity of Massachusetts

Zeuthen

For SLAC E158 Collaboration

E158 Steve Rock Nov 04

OutlineOutline• History• Physics Motivation• Technique for Precision

Measurement Beam LH2 Target Spectrometer Detectors Analysis

• Results • Outlook


Detectore

16 – 22 GeV

Liquid Deuterium

sin2W = 0.224 + 0.020

SLAC E122 (1978) e-D Inelastic Scattering

Measured Coupling of electron and nucleons

First definitive measurement of mixing between the weak and electromagnetic interaction

Help Establish the Standard Model


IMPROVEMENTS

• HIGHER POLARIZATION (NLC)

• BEAM STABILITY (SLC, FFTB)

• 50 GeV BEAM

• 5 LONGER TARGET

• RADIATION RESISTANT DETECTOR (LHC)

• LOW NOISE, HIGH RESOLUTION ELECTRONICS


Standard Model Electroweak Theory

Based on gauge group SU(2)L X U(1)with isotriplet field W and isosinglet field BSU(2)L coupling is g, U(1) coupling is g'After spontaneous symmetry breaking via Higgs mixing of W and B fields produces observed particles

Charged bosons

Neutral bosons

Vector couplings

Axial couplings

fermion weak isospinfermion charge q/e


Standard ModelStandard Model

WW and g determine ALL couplings and g determine ALL couplings

f

f

0Z )sin()'(

sin

2/

23

2/122W

ff

Wf

QIgg

gQ

g

LH coupling

)sin()'(

sin

0

22/122W

f

Wf

Qgg

gQ

RH coupling

eW

f

f


“High Energy” EW Data• Spectacular precision at LEP, SLD

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

Belle, BaBar, E158, g-2,…


Parity Violation at the Z-poleParity Violation at the Z-pole(SLD at SLAC)

]cos)(2

)cos1)(1[( cos

222

eef

eeff

PAA

APavd

d

2222

22 2

ff

ff

RL

RLf av

av

gg

ggA

0Z

e

e

f

f

sin2W at Z pole


Running of Weak Mixing Angle

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

3%


Beyond the Standard Model

Precision EW via PVNew physics ~|amplitude| Make high massexchange visible

APV ~

Find new physics by deviations from SM predictions

e e

e

?

e

Beat large against small to see new

+

e e

Z


γ-Z INTERFERENCEAT LOW Q2

• Interference proportional to Q2

• Sensitive to Z’

4Z

4

2X

2

2Z

2

2X

2Z

2 M

Q

M

Q

M

Q1Q

2

M

1

M

1

Q

1 4


Precise Low Q2 PV Experiments

e e

ee

Z

e e

Ze e

Z

e

Z

Moller- E158 DIS-Parity Q-Weak(JLAB) Atomic Parity V

Pure leptonic, Incoherent Coherent quarks Coherent quarks isoscalar quarks proton entire nucleus

This Experiment Well understood Results ~2008 Large syst. errors small corrections atomic, nuclear structure

Cs133

p

n

No quarks (2C1u-C1d)+Y(2C2u-C2d) -2(2C1u+C1d) -376C1u- 422C1d


Parity Violation in Møller Scattering

• Scatter polarized 50 GeV electrons off unpolarized atomic electrons• Measure

• Small tree-level asymmetry

1-4sin2w At tree level, (smaller after radiative effects)• Raw asymmetry about 130 ppb

– Measure it with precision of 10% – Most precise measurement of sin2W at low Q2

)2sin4

1(

2)2cos3(

2sin16

2 WG

mEAF

PV

LRPV ALR

LRA

910280 PV

A


E158 New Physics ReachE158 New Physics Reach

Compositeness ~ 10 TeV

Grand Unified TheoriesMZ’ ~ 0.8 TeV

~ 0.01GF 0.01


SLAC E-158SLAC E-158

2 GHz45 GeV

P=85%

5x1011 e-/pulse

L ~ 1038 cm-2s-1 integratingflux counter

LH24-7 mrad

End Station AEnd Station A


•UC Berkeley•Caltech•Jefferson Lab•Princeton•Saclay

•SLAC•Smith College•Syracuse•UMass•Virginia

7 Ph.D. Students60 physicists

Sep 97: EPAC approval1998-99: Design and Beam Tests2000: Funding and construction2001: Engineering run2002: Physics Runs 1 (Spring), 2 (Fall)2003: Physics Run 3 (Summer)

E158 CollaborationE158 Collaboration


TINY ASYMMETRY EXPRIMENT ( )

(STABILITY AND INTENSITY)

• HIGH INTENSITY POLARIZID e- BEAM

• HIGH DENSITY ELECTRON TARGET (LH2)

• BACKGROUND REMOVERS

• MOLLER ELECTRON SELECTORS

• DETECTORS

• MONITORS

910130 PV

A


• Scattering of polarized electrons off atomic electrons High cross section (14 Barn) High intensity electron beam, ~85% polarization 1.5m LH2 target

Luminosity 4*1038 cm-2s-1

High counting rates: flux-integrating calorimeter

• Principal backgrounds: elastic and inelastic ep• Main systematics: beam polarization, helicity-correlated beam effects, backgrounds

Experimental Technique


Major Challenges

• Statistics ! Need to accumulate ~1016 scattered electrons

• Suppress Random noise Want to be dominated by counting statistics Beam pulse to pulse “random” fluctuations are major

(potential) source of additional “statistical error

• Systematics False asymmetries Backgrounds (Need to measure in situ)


# scattered e- per pulse 107/pulse

Rep rate (120 Hz) 109 /sec

Seconds/day 1014/day

100 days 1016

A ~ 10-8

Statistics


Beam helicity is chosen pseudo-randomly at 120 Hz• sequence of pulse quadruplets• use electro-optical Pockels cell in Polarized Light Source

Reduce beam asymmetries by feedback • Control charge and position asymmetry at Source• Control Angle Asymmetry• Control Energy Asymmetry

Control of Beam Systematics


0.16%0.15%Energy Spread

85%85%e- Polarization

250 GeV45 GeVEnergy

120 Hz120 HzRepetition Rate

14.4 x 10115 x 1011Charge/Train

22%13%Beam Loading

1.4ns0.3nsMicrobunch spacing

267ns270nsTrain Length

NLC-500E-158Parameter

E-158 BeamE-158 Beam(and comparison with 500 GeV Linear Collider Design)


BEAM

• HIGH INTENSITY 6×1011 /burst• HIGH POLARIZATION• “HIGH ENERGY” (Asymmetry Q2 )• STABLE (Same Beam for L & R Polarization) INTENSITY (acceleration, Target) POSITION (Acceptance, backgrounds) ANGLE (σ depends on θ) ENERGY (σ depends on E) TRANSVERSE SIZE (Target effects)


Polarized Source


RF Cavity BPMPulse-to-pulse monitoring of beam asymmetries

and resolutions:

Agreement (MeV)

toroid 30 ppm BPM 2 microns energy 1 MeV

BPM

24

X (

MeV

)

BPM12 X (MeV)

Resolution 1.05 MeV

Energy dithering region

Beam Diagnostics

linac A-Line


BEAM VARIATIONS

• LONG TERM DRIFTS CONTROLLED WITH FEEDBACK LASER OPTICS LASER INTENSITY POCKEL CELLS ACCELERATING CAVITY STEERING MAGNETS


Beam Asymmetries

Position differences < 20 nm Position 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


BEAM VARIATIONS

• BEAM JITTER (OFFLINE CORRECTIONS) ENERGY ANGLES POSITION (and correlation with time) INTENSITY BEAM SIZE TARGET DENSITY

• MONITOR EFFECTS OF VARIATION “NATURAL JITTER” (Linear Regression) CONTROLLED VARIATION

• CORRECT PULSE BY PULSE


Eliminating Beam Jitter

1 - -202-128

i Determined by Linear Regressioni Determined by Dithering

Both Methods agree

E158 Steve Rock Nov 04In addition, independent analysis based on beam dithering

Moller DetectorMoller DetectorRegression CorrectionsRegression Corrections


Physics Asymmetry Reversals• Insertable Half-Wave Plate in Polarized Light Source• (g-2) spin precession in A-line (45 GeV and 48 GeV data)

Reverse special laser optics• 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

Reversals and Checks


End Station A


Target chamber

Dipoles

PrecisionBeam

Monitors

Detector Cart

Drift pipe

Quadrupoles

LuminosityMonitorMain

Collimators

Concrete Shielding

60 m

Experimental Layout in Experimental Layout in ESAESA


Liquid Hydrogen TargetLiquid Hydrogen Target

Refrigeration Capacity 1000WMax. Heat Load:

- Beam 500W- Heat Leaks 200W- Pumping 100W

Length 1.5 mRadiation Lengths 0.18Volume 47 litersFlow Rate 5 m/s

Disk 1 Disk 2 Disk 3 Disk 4

Wire mesh disks in target cell region to introduce turbulence at 2mm scale and a transverse velocity component.Total of 8 disks in target region.

CONSTANTDENSITY


SPECTROMETER & DETECTORS

• Separate Moller e- from other processes

• Accurately measure number of Moller e-

• Measure Q2• Measure Backgrounds Pions e- p e- p +? (BIGGIST CORRECTION) Neutrons, , …


Moller ring is 20 cm from the beam

Forward background of e+ e- pairsLine-of-sight shielding requires a “chicane”

E158 SpectrometerE158 Spectrometer

Emoller~25 GeV


MOLLER, ep are copper/quartz fiber calorimetersPION is a quartz bar CherenkovLUMI is an ion chamber with Al pre-radiator

All detectors have azymuthal segmentation, and PMT readout to 16-bit ADC

DetectorsDetectors

~1.5o~1.5 mr


Profile Detector: Profile Detector: Q²Q²

Bearing wheels

Linear drive

Cerenkov detector

4 Quartz Cherenkov detectors with PMT readoutinsertable pre-radiatorsinsertable shutter in front of PMTs

Radial and azymuthal scans collimator alignment, spectrometer tuning background determination Q2 measurement Monte Carlo Tune


e p Background

• MEASURE e p Asymmetry in e p Detector Shape of e p and e e in Profile Detector using Movable

Collimators

• MODEL ASYMMETRY(Q2,W2) Monte Carlo using Detector Acceptance Tuned with Profile Detector

Quads on/off

Collimator Positions

• CORRECTION ~ -25 ±5 ppb


AAPV PV MeasurementMeasurement1. Measure asymmetry for each pair of pulses

LR

LRpexp σ σ

σ -σ A

coefficients determined experimentally by regression or from dithering coefficients

2. Correct for difference in R/L beam properties,

charge, position, angle, energy R-L differences

iipexp

praw ΔxaAA

3. Sum over all pulse pairs, prawraw AA

norm

bkgbkgraw

bPV f

AfA

P

1A

4. Obtain physics asymmetry:

backgrounds

beam polarizationdilutions


3 EXPERIMENTAL RESULTS

• e e TRANSVERSE ASYMMETRY 2 photon exchange physics

• e p LONGITUDINAL ASYMMETRY Proton Structure

• e e LONGITUDINAL (main result)


e e Transverse Asymmetries

)'()(2

eeeT kkSd

dA

Beam-Normal Asymmetry in elastic electron scattering Beam Polarization Perpendicular to Direction of motion Select by beam energy (number of spin rotations)Azimuthal Dependence of Asymmetry

Interference between one- and two-photon exchange

)( ppmOs

mA e

T


E158 e- e- Transverse Results

43 and 46 GeVee eeLH2 target

~ 24 hrs of data

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

(Azimuthal angle)


ep Detector Data ep Detector Data (Run 1)(Run 1)

Ratio of asymmetries:APV(48 GeV) /APV(45 GeV) = 1.25 ± 0.08 (stat) ± 0.03 (syst)

ARAW(45 GeV) = -1.36 ± 0.05 ppm (stat. only)ARAW(48 GeV) = -1.70 ± 0.08 ppm (stat. only)

Consistent with expectations for inelastic ep asymmetry


Møller Asymmetry (Blind Analysis)• Over 330M pulse pairs over 3 separate runs (2002-2003) at

Ebeam=45 and 48 GeV• Passively flip helicity of electrons wrt source laser light

~every day to suppress spurious helicity-correlated biases


Raw Asymmetry Statistics

Asymmetry pulls per 10k pair “chunks”

Asymmetry pulls per pulse pair: 150M pairs

(A-<A>)/


2-80.0070.058ep elastic

6-220.0030.009ep inelastic

100.0020.005Brem and Compton electrons

110.0010.001Pions

8-270.0090.082TOTAL

1-10.00020.0006Neutrons

200.00050.0015Synchrotron photons

3+30.0020.004High energy photons

2-4--Transverse asymmetry

10--Beam spotsize

4---Beam asymmetries

Acorr) (ppb)Acorr (ppb)fbkg)fbkgCorrection

Asymmetry Corrections and Systematics

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


from Afrom APVPV to sin to sin22WWeffeff

eff

WbremF

PV Fyy

yQGA

2

44

2

sin4111

1

24

where:

44

2

11

1

24 yy

yQGF

is an analyzing power factor; depends on kinematics and experimental geometry. Uncertainty is 1.7%. (y = Q2/s)


Running of the Weak Mixing Angle

7


The Weak Mixing Angle

• General agreement between low Q2 experiments, although NuTeV is still 3 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


Preliminary Results

Significance of parity non-conservation in Møller scattering: 8

APV (e-e- at Q2=0.026 GeV2) = 128 14 (stat) 12 (syst)

sin2eff (Q2=0.026 GeV2) = 0.2403 ± 0.0010 (stat) ±0.0009 (syst)

Most precise measurement at low Q2

Significance of running of sin2W: 7

sin2WMS(MZ) = 0.2330 ± 0.0011 (stat) ±0.0010 (syst)

Standard Model pull: +1.2


Outlook• Next set of precision measurements on the horizon

Neutrino-electron scattering Reactor experiments (in conjunction with 13): cross section

measurements to 0.7-1.3% would translate in (sin2W) 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 (sin2W) ~ 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 (sin2W) ~ 0.001 per experiment

ee and eeat the Linear Collider


Selected Future Measurements

1GeV

0.238

0.236

0.234

0.232

10GeV 100GeV 1TeV0.1GeV0.01GeV

133Cs PNCSingle Isotope

168Yb - 176Yb PNCIsotopic Chain

SLAC E158Møller Scattering

JLab ProtonWeak Charge

NuTev Neutrino-Nucleon Scattering

LEP + SLACMeasurements

At Z Pole

Linear ColliderIn e+ e- and e- e-

Modes

sin2(W)

Q

JLab DIS-parity


CONCLUSIONS

sin2W RUNS

• STANDARD MODEL STILL OK

• MANY MORE TESTS IN FUTURE

Documents

From Z 0 to Zero: A Precise Measurement of the Running of the Weak Mixing Angle SLAC E158