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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
IMPROVEMENTS
• HIGHER POLARIZATION (NLC)
• BEAM STABILITY (SLC, FFTB)
• 50 GeV BEAM
• 5 LONGER TARGET
• RADIATION RESISTANT DETECTOR (LHC)
• LOW NOISE, HIGH RESOLUTION ELECTRONICS
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
“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,…
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
Running of Weak Mixing Angle
sin2W = e2/g2 → test gauge structure of SU(2)U(1)
3%
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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 Steve Rock Nov 04
E158 New Physics ReachE158 New Physics Reach
Compositeness ~ 10 TeV
Grand Unified TheoriesMZ’ ~ 0.8 TeV
~ 0.01GF 0.01
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
•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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
• 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
E158 Steve Rock Nov 04
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)
E158 Steve Rock Nov 04
# scattered e- per pulse 107/pulse
Rep rate (120 Hz) 109 /sec
Seconds/day 1014/day
100 days 1016
A ~ 10-8
Statistics
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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)
E158 Steve Rock Nov 04
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)
E158 Steve Rock Nov 04
Polarized Source
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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
E158 Steve Rock Nov 04
BEAM VARIATIONS
• LONG TERM DRIFTS CONTROLLED WITH FEEDBACK LASER OPTICS LASER INTENSITY POCKEL CELLS ACCELERATING CAVITY STEERING MAGNETS
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
End Station A
E158 Steve Rock Nov 04
Target chamber
Dipoles
PrecisionBeam
Monitors
Detector Cart
Drift pipe
Quadrupoles
LuminosityMonitorMain
Collimators
Concrete Shielding
60 m
Experimental Layout in Experimental Layout in ESAESA
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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, , …
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
3 EXPERIMENTAL RESULTS
• e e TRANSVERSE ASYMMETRY 2 photon exchange physics
• e p LONGITUDINAL ASYMMETRY Proton Structure
• e e LONGITUDINAL (main result)
E158 Steve Rock Nov 04
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 Steve Rock Nov 04
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)
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
Raw Asymmetry Statistics
Asymmetry pulls per 10k pair “chunks”
Asymmetry pulls per pulse pair: 150M pairs
(A-<A>)/
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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)
E158 Steve Rock Nov 04
Running of the Weak Mixing Angle
7
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
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
E158 Steve Rock Nov 04
CONCLUSIONS
sin2W RUNS
• STANDARD MODEL STILL OK
• MANY MORE TESTS IN FUTURE
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