Transcript SLAC E158: Measuring Parity Violation in Fixed-Target Møller Scattering E158 Goal: dsin2qW = +/- 0.001 Best measurement of sin2qW away from the Z-pole David Relyea SLAC/Princeton University CIPANP 2003 20

SLAC E158:
Measuring Parity Violation in
Fixed-Target Møller Scattering
E158 Goal:
dsin2qW = +/- 0.001
Best measurement of sin2qW
away from the Z-pole
David Relyea
SLAC/Princeton University
CIPANP 2003
20 May, 2003
QC1B
Main acceptance collimator
Outline
•
•
•
•
•
Physics Motivation
Experimental Technique
Data Analysis
Results and Interpretations
Outlook
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E158 @ CIPANP2003
Parity Violation in Møller Scattering
• Scatter polarized 50 GeV electrons
off unpolarized atomic electrons
• Measure A  σR - σL
PV
σR  σL
• Small tree-level asymmetry
APV
GF
16sin2θ
1

2

 sin θW 
 mE
2
2 
2π (3  cos θ)  4

-9
A

320

10
• At tree level, PV
(at 90 degrees in CM frame)
• Raw expected asymmetry about 150 ppb


Goal is to measure it with precision of 8%
Most precise to date measurement of sin2qW at low Q2 with
s(sin2qW)=0.001
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E158: Physics Impact
• Establish running of sinqW to 8s level
• Sensitivity to new physics: compositeness up to 15 TeV, Z’
(GUTs) to ~1TeV

Complementary to collider limits, different couplings
LEPII
e
Compositeness
R R
e
e
e
+
e
E158
L L
e
e
e
e
e
e
e
R R
–
e
L L
e
e
  15 TeV
e
FNAL
Neutral currents
(GUTs)
q
q
Z´
e
l+
Z´
l
e
e
Scalar interactions (LFV)
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e
e
D 
e
e
e
M Z'  1 T eV
g2
 0.01Gf
2
2MΔ
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Experimental Technique
• Scatter polarized electrons off atomic electrons

High cross section (14 mBarn)
High intensity electron beam, ~84% polarization
0.18 r.l. LH2 target (1.5 m)

Luminosity 4*1038 cm-2s-1
High counting rates [ flux-integrating calorimeter


End Station A
• Principal backgrounds:
elastic and inelastic ep
• Main systematics:
Beam polarization
Helicity-correlated beam effects
Backgrounds

Source
Linac
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ESA
E158 @ CIPANP2003
Parity-Violating Asymmetry
1. Measure asymmetry for each pair of pulses, p,
A exp
σR - σL

σR  σL
2. Correct for difference in R/L beam properties:
charge, position, angle, energy
Araw  Aexp  ai Δxi
R-L differences

coefficients determined experimentally by regression or from dithering coefficients
3. Obtain physics asymmetry:
1 Araw  fbkg Abkg
APV 
Pb
1  fbkg
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beam polarization
backgrounds
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Experimental Challenges
1. Electron Beam
2. Electron Beam monitoring
i) high intensity
- 500 kWatt beam power
ii) stability
- intensity jitter <1%
- spotsize jitter <10%
- position jitter <10%
iii) small left-right asymmetries
- intensity
- position/angle
- energy
iv) high (>80%) polarization
v) Compatibility with PEPII operation
i) toroid resolution: < 30 ppm per pulse
ii) BPM resolution: < 1 mm per pulse
iii) energy resolution: < 50 ppm per pulse
AI 
I
I
 I
R
Ax  x
AE 
 I
R
E
E
R
R
R
L
L
 x
 E
 E
3. Liquid Hydrogen Target
4. Detectors
i) target density fluctuations: <10-4 per pulse
ii) 18% radiation length
- absorbs 500W beam power
iii) Safety (largest LH2 target in the world)
i)
ii)
iii)
iv)
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 2 10 7
L
 10 nm
L
 2 10 8
L
detector resolution: <100 ppm per pulse
multiple backgrounds
radiation damage
linearity < 1%
E158 @ CIPANP2003
Electron Beam: Systematics
• Beam helicity is chosen pseudo-randomly
by using electro-optical Pockels cells
in the Polarized Light Source
• Create pulse quadruplets at 30 Hz
• Beam asymmetries reduced by using feedback at the Source
• Control charge asymmetry and position asymmetry
• Physics Asymmetries can be reversed
• Insert a half-wave plate in the Source
• Change the (g-2) spin precession in the A-Line
(take 45 GeV and 48 GeV data)
•“Null Asymmetry” cross-check is provided by a Luminosity Monitor
• Measures very forward angle e-p (Mott) and Møller scattering
• False Asymmetries can also be reversed
• Insert the “-I/+I” Inverter in Polarized Light Source
• Reverses both false beam position and angle asymmetries
• Leaves physics asymmetry unchanged
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Experimental Layout: ESA
Target chamber
Quadrupoles
Concrete Shielding Detector Cart
Precision
Beam
Monitors
Dipoles
Main
Collimators
Drift pipe
Luminosity
Monitor
60 m
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Experimental Layout: Spectrometer
0m
Photon
Collimator Acceptance Collimator
58 m
30 cm
20 cm
0 cm
Target
Dipoles
Quadrupoles
• Dipole chicane allows clean collimation of
photons and positrons from target interactions
• Quadrupoles separate moller and ep flux
at detector face (see inset)
• Main acceptance collimator (upper right corner)
accepts mollers in desired momentum/radial range
• Synchrotron
collimators (not shown) block synchrotron radiationE158 @ CIPANP2003
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DRR
Experimental Layout: Detectors
Primary (Unscattered) Beam
Radius
(cm)
(z axis not to scale)
MOLLER, EP are copper/quartz fiber calorimeters
PION is a quartz bar Cherenkov
LUMINOSITY is an ion chamber with Al pre-radiator
MOLLER
q lab
 6.0mrad
LUMI
q lab
 1.5mrad
All detectors have azimuthal segmentation,
and have PMT readout to 16-bit ADC
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A exp 
σR - σL
σR  σL
Regression Analysis
A exp
σR σL
 QR QL
σR σL

QR QL
In addition, independent analysis based on beam dithering
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Raw Asymmetry Statistics
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Detector Asymmetry Dipoles
Detector divided into 3
rings of PMTs: Inner,
Middle and Outer
Each ring manifests an
asymmetry dipole
Asymmetry dipoles can be
used to study systematics
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Beam Systematics
Beam systematics are small
But, some detector ‘monitors’ show poor c2 and non-zero mean values.
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Beam Systematics
Use radial and azimuthal segmentation
of Moller detector to construct ‘monitors’
that have much larger sensitivity to beam
parameters than the Moller ‘monopole’
Beam
Parameter
Detector Monitor
Monitor slope /
Moller monopole slope
E
(OUT-MID) monopole
11
X
MID xdipole
20
Y
MID ydipole
35
X’
OUT xdipole
37
Y’
(OUT-MID) ydipole
52
Current systematic error: 18 ppb
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Should reduce to 10 ppb or less
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“ep” Detector Data
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APV Corrections and Backgrounds
APV 
1 Araw  fbkg Abkg

, Acorr  fbkg Abkg
Pb
1 fbkg
•Run I systematic error should reduce from 24 to less than 15 ppb
•Run
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DRRII corrections will be on the order of 25 ppb
E158 @ CIPANP2003
Normalization Errors
APV
1 Araw  fbkg Abkg
 
Pb
1  fbkg
•Beam polarization measured using polarized foil target
•Same spectrometer used with dedicated movable detector
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Raw Asymmetry Result
(blinded; before corrections and normalization)
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Moller Physics Asymmetry
(unblinded; with corrections and normalization)
APV(e-e- at Q2 = 0.027 GeV2):
-151.9  29.0 (stat)  32.5 (syst)
parts per billion
(preliminary)
Significance of parity nonconservation in Møller scattering: 3.6s
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Weak Mixing Angle
sin2q W(Q2=0.027 GeV2) = 0.2371 ± 0.0025 (stat) ±0.0027 (syst)
(preliminary)
Convert to sin 2 qWMS (M Z2  for comparison with
other experiments:
E158 projected
sin 2 qWMS (M Z2 
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Implications
• Parity is violated in Møller scattering
• Limit on LL at the level of 5-6 TeV (90% C.L.)
• Limits on extra Zs at the level of 300 GeV
• Limit on lepton-flavor violating coupling ~ 0.02-0.03 GF
These numbers are competitive with collider limits
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Conclusions
A very challenging experiment is producing physics results
• Parity is violated in Møller scattering
• Inelastic e-p asymmetry at low Q2 measured;
consistent with quark picture
• First measurement of e-e transverse asymmetry
• Run II data are being analyzed; will double statistics
• Final Run III in July-August 2003
Preliminary Run 1 results
APV (Moller) = -151.9 ± 29.0 ±32.5 ppb
sin2qWeff= 0.2371 ± 0.0025 ±0.0027
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Polarimetry
84.9 +/- 4.4 % polarization throughout Run I
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Beam Delivery for E158
(April 20 - May 27)
~84% Electron Polarization
Beam Delivery Efficiency (120Hz running)
72% for 48 GeV, 3.5 x 1011
65% for 45 GeV, (5-6) x 1011
48 GeV
3.5 • 1011 / Pulse
45 GeV
(5-6) • 1011 / Pulse
104,000 Peta-Electrons
(16.6 Coulombs)
232 Million Spills
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E158 @ CIPANP2003
LH2 Target Chamber
Quadrupoles
Dipoles
BPM
LH2
Scattering Chamber
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2 Spectrometer Collimators
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Charge Asymmetry
-85 ± 344 ppb
Avg. correction = -44 ± 332 ppb
The double-feedback keeps the average
charge asymmetry corrections tiny!
-341 ± 334 ppb
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2a-3a = 2.10 ± 5.72 ppb
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Energy Asymmetry
4.9 ± 15.7 ppb
Nulling
at 1.2
indeed
energy
the charge asymmetry
GeV region does
seem to help zero the
asymmetry.
The two energy BPM’s agree
to very high precision!
1.71 ± 2.60 ppb
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POS Loop Performance
Feeding back on
ASSET BPM’s,
after ~30M pairs:
2.2 ± 2.5 nm
0.9 ± 5.0 nm
Some A-line BPM’s:
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Polarized Source Laser System
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Beam Asymmetry Feedbacks
Item
Control
Intensity
Position
Diagnostic
IA Pockels Cell
Piezo Mirror
Toroid (@ 1 GeV)
BPM (@ 1 GeV)
Algorithm: - measure asymmetry on a run with
Flash:Ti
Piezomirror
L3
IA
Intensity 1
PD
N pulses (typically 1-30K pulses)
- induce asymmetry on next run to
cancel measured asymmetry on
current run
Helicity Control Bench
Helicity /2
filter plate
Cleanup
polarizer
-2I
PS CP
+2I
remotely
insertable
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Asymmetry
inverter
Gives better than 1/sqrt(N) scaling
of charge asymmetry, position difference
Intensity 2
PD
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Results: Pion Asymmetry
ALR(p) = 1.6 ppm
Estimated correction
to ALR ~2 ppb
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Electron Beam: Results
Beam property
beamA
Intensity
225 +/- 320 ppb
Energy
-0.1 +/- 1.4 keV
(1.1 +/- 16 ppb)
X Position
-16.9 +/- 5.6 nm
Y Position
-3.3 +/- 4.0 nm
X Angle
0.41 +/- 0.23 nrad
Y Angle
0.12 +/- 0.07 nrad
Polarization
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LR
84.9 +/- 4.4%
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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:
<Q2> = 0.0266 GeV2
<y> = 0.6
Data
Monte Carlo
• ~2 mm geometry
• 1% energy scale
• Radiative tail
• <1% background
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Pion Detector
•~ 0.5 % pion flux
•~ 1 ppm asymmetry
•< 5 ppb correction
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Collimators
Acceptance Collimator
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Luminosity Monitor Data
•Null test at level of 20 ppb
• Target density fluctuations small
• Limits on second order effects
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Luminosity Monitor
Segmented ion chamber detector with
Aluminum preradiator.
500W incident power (50W from
synchrotron radiation)
Signal: Motts and high energy Mollers
350M electrons per pulse; <E>~40 GeV
APV ~ -10ppb
Null asymmetry measurement
Enhanced sensitivity to beam fluctations
and target density fluctuations.
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Statistical and Systematic
Fluctuations
Integrate
Detector response:
Flux Counting
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Electron Beam:
Delivery Summary
ITEM
Goal
Run I (2002)
Beam Charge
6 x 1011
6 x 1011
Intensity Jitter
2% rmsa
0.5% rms
Position Jitter
<10% of spotsize
5% of spotsize
Spotsize Jitter
<10% of spotsize
5% of spotsize
Energy Spread 0.3% rms
Energy Jitter
0.2% rms
0.03% rms
Polarization
75%
~85%
a2%
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0.1% rms
required for physics measurement; 1% for accelerator operation
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Electron Beam:
Monitor Resolutions
Device
Goal
Tests
Run I
resultsa
Target BPM x,y
1 mm
0.5 mm
2 mm
Target BPM x’,y’
0.4 mrad 0.03 mrad
0.1 mrad
Energy BPMb
30 ppm
40 ppm
Target Toroid
30 ppm
60 ppm
sBPM  2 microns
goals for Run I (due
to statistics)
bEnergy goal ignores detector
calorimetric compensation for
1/E – dependence of Møller
cross section
Resolution goals are to achieve
1ppb error after 600M pulses for
each of x, x’, y, y’, E, I
senergy  1 MeV
BPM24 X (MeV)
storoid  30 ppm
30 ppm
aRelaxed
Agreement (MeV)
Resolution
1.05 MeV
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@
BPM12 XE158
(MeV)
CIPANP2003
Moller Asymmetry (Blinded)
Based on analysis of 146M spills collected in April-May 2002
Asymmetry blinded to avoid bias (expect ~ 150 ppb)
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Results: Systematic Check
Experiment run at 2 energies (for g-2 asymmetry flip)
Equal data samples taken at both half-wave plate settings
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Results: Asymmetry Pulls Per Run
Pull 
Arun  Arun
σ run
Expect a mean of 0
and an RMS of 1
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Results: Statistics and Systematics
Asymmetry pulls
per event pair: 17M spills
(about 2 days of data)
Average asymmetry
width: 195 ppm
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Physics Runs
Energy
#days
@120Hz
# Peta-Electron
#spills
Average
Charge
Production
Efficiency*
Run I
45.6 GeV
19.2
67K
125M
5.5 x 1011
63%
Run I
48.8 GeV
14.8
37K
105M
3.5 x 1011
69%
Run II
45.6 GeV
15.2
56K
113M
5.2 x 1011
72%
Run II
48.8 GeV
19.0
63K
153M
4.3 x 1011
78%
*Efficiency is avg. delivered rate normalized to 119Hz
Run I: April 23 12:00 – May 28 00:00 (this result)
Run II: October 10 08:00 – November 13 16:00
• Run I with PEPII, Run II dedicated
• One g-2 flip in each run
• /2 flip roughly once in two days
• Asymmetry inverter flip once a week
• Run I data divided into 24 “slugs”
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1020 Electrons on Target
Run 2
Run 1
E158 @ CIPANP2003
E158 Collaboration
Institutions
Caltech
Princeton
SLAC
Saclay
Smith College
Syracuse
Jefferson Lab
UC Berkeley
UMass Amherst
U. of Virginia
65 physicists
5 grad students
Sept 1997:
1998:
1999:
2000:
2001:
Spring 2002:
Fall 2002:
Summer 2003:
05/20/2003, DRR
PAC approval
Polarized Beam Instrumentation R&D
Spectrometer and Detector Design
Construction Funds and Test Beams
Commissioning Run
Physics Run I
Physics Run II
Physics Run III (final statistics)
E158 @ CIPANP2003
Polarized Beam
High doping for 10-nm
GaAs surface
overcomes charge limit.
Electrons per pulse
Low doping for most of
active layer yields high
polarization.
New cathode
No sign of
charge limit!
Old cathode
Laser Power (µJ)
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E158 @ CIPANP2003
Experimental Layout:
Liquid Hydrogen Target
Refrigeration Capacity
Max. Heat Load:
- Beam
- Heat Leaks
- Pumping
Length
Radiation Lengths
Volume
Flow Rate
Reynolds number in target cell
Disk 1
Disk 2
Disk 3
1000W
500W
200W
100W
1.5 m
0.18
47 liters
10 m/s
106
Disk 4
Wire mesh disks in the target introduce
turbulence at the 2mm scale and a transverse
velocity component. Total of 8 disks in the target.
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MOLLER
Detector
Basic Idea:
electron flux
light guide
: quartz
: copper
air
shielding
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PMT
E158 @ CIPANP2003
Electron Beam: Diagnostics
Accelerator
Thermionic
Gun
BPMs (3)
Polarized
Gun
Toroids (2)
1 GeV
Dithering Coils
for x, x’, y, y’
48 GeV
Momentum
Defining
Slits
Angle
BPMs
(2)
Dispersive
(Energy)
BPMs
(2)
Position
BPMs
(2)
Toroids
(2 pair)
Wire
Array
Not shown:
• Møller Polarimeter in ESA
• Synchrotron Light Monitor before momentum slits
• Energy dithered by using sub-booster phases for Sectors 27, 28
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Transverse Asymmetry
Asymmetry vs 
Flips sign with g-2
precession → physics !
~ 3 ppm up-down
asymmetry with 85%
transverse polarization
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Two-photon exchange
QED effect:
Calculation does not
exist in the literature
Data carefully re-weighted to
maintain azimuthal symmetry
E158 @ CIPANP2003