Parity-Violation in Fixed Target

1
Electrons and Mirror Symmetry

• Parity-Violation in Fixed Target
• electron-electron (Møller) Scattering
• Krishna Kumar
• University of Massachusetts, Amherst

Fermilab Colloquium April 4, 2007
2
Acknowledgement
I have stolen numerous cartoons from the
following website at Lawrence Berkeley Laboratory
www.particleadventure.org
and a few pictures from Y. Kolomensky, W.J.
Marciano, M. J. Ramsey-Musolf C.Y. Prescott
Special thanks to my fellow band of
conspirators The E158 Collaboration
Apologies to my senior particle physics
colleagues The beginning of this talk is at a
level for graduate students You have to wait for
the last 5 slides (especially the theorists)!
3
Outline

• A story - historical context
• What? - the science
• Why? - probe TeV Scale
• Where? - SLAC E158
• How? - aspects of the technique
• Well? - the result
• So what? - implications
• What next? - possible new experiments

4
Electro-weak Interactions
By the late 1940s A consistent relativisitic
quantum theory of Electromagnetic Interactions
Electromagnetic interactions mediated by massless
photons
Charged particle scattering
5
Weak Force Mirror Asymmetry
parity transformation (reflection)
6
Weak Interaction




Left-
Right-
? Charge
W Charge
zero
CERN
The weak mixing angle introduced
How to measure the neutral weak force between two
charged particles?
Show it is not mirror-symmetric
7
A Classic, Prescient Paper
8
Parity Violation in Electron Scattering?
9
Observable Parity-Violating Asymmetry

• One of the incident beams longitudinally
  polarized
• Change sign of longitudinal polarization
• Measure fractional rate difference

10
SLAC E122 Experiment

• Parity Violation in Weak Neutral Current
  Interactions
• sin2?W 0.224 0.020 same as in neutrino
  scattering

11
21st Century Electroweak Physics
Many questions still unanswered.

• Why 3 generations of particles?
• Why is the weak boson mass 100 GeV?
• What is the origin of mass?
• How did matter dominate over anti-matter?
• Is there a single unifying force?
• Why are neutrinos so light?
• Are neutrinos Majorana particles?
• Why is the Top so heavy?
• What is the dark matter?
• .

12
Beyond the Electroweak Theory

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

High Energy Colliders
as well as
Low Energy Q2 ltlt MZ2
Low Q2 offers complementary probes of physics at
high energy scales

• Neutrino Physics
• Oscillations and the MSN matrix
• Tritium Beta Decay and Double Beta Decay
• Muon Physics
• g-2 anomaly
• Precision muon decay parameters
• Charged lepton number violation
• Semi-leptonic Weak Decays
• Standard Model CP Violation
• Tests of CKM unitarity
• Anomalous charged current interactions
• Search for Proton decay
• Dark Matter Searches
• Electric Dipole Moment Searches
• Table Top Searches for Time Reversal Violation
• Neutral Weak Interaction Studies

13
New Neutral Current Interactions
Many theories predict new forces that disappeared
when the universe cooled
14
High Q2 vs Low Q2
LEPII, Tevatron access scales Ls 10 TeV
e.g. Tevatron dilepton spectra, quark pair
production at LEPII
- L,R combinations accessed are parity-conserving
Window of opportunity for weak neutral current
measurements at Q2ltltMZ2
15
A New Idea at Low Q2 (mid-1990s)
By the mid-1990s, two promising techniques
well-established

• Atomic Parity Violation Expts. (Table Top)

• Neutrino-Nucleon Deep Inelastic Scattering
  (Fermilab)

• Parity-Violating Electron Scattering can compete!

End Station A at the Standord Linear Accelerator
Center (SLAC)
16
Anatomy of a SLAC Proposal

• 10 ppb statistical error
• 0.4 error on weak mixing angle

• High Peak Charge and High Beam Polarization
• RD on polarized electron source improvements
• 10 nm control of beam centroid on target
• RD on polarized source laser transport elements
  (E122 1 micron)
• 12 microamp beam current maximum
• 1.5 meter Liquid Hydrogen target (previous
  record 75 cm)
• 20 Million electrons per pulse _at_ 120 Hz
• 2x10-4 pulse-to-pulse statistical fluctuations
  (previous record 3x10-3)
• Electronic noise and density fluctuations lt 10-4
• Pulse-to-pulse monitoring resolution 1 micron
• Pulse-to-pulse beam fluctuations lt 100 microns
• 100 Mrad radiation dose from scattered flux
• State-of-the-art radiation-hard integrating
  calorimeter
• Full Azimuthal acceptance with ?lab 5 mrad
• Novel Quadrupole spectrometer
• Complex collimation and radiation shielding
  issues

17
E158 Collaboration Chronology
Parity-Violating Left-Right Asymmetry In Fixed
Target Møller Scattering
At the Stanford Linear Accelerator Center
Goal error small enough to probe TeV scale
physics
E158 Collaboration
E158 Chronology

• Feb 96 Workshop at Princeton
• Sep 97 SLAC EPAC approval
• Mar 98 First Laboratory Review
• Design and Beam tests
• Funding and construction
• Engineering run
• -2003 Physics
• 2004 First PRL
• 2005-2007 Final publications

• Berkeley
• Caltech
• Jefferson Lab
• Princeton
• Saclay

• SLAC
• Smith
• Syracuse
• UMass
• Virginia

8 Ph.D. Students 60 physicists
18
E158 New Physics Reach
19
Stanford Linear Accelerator Center
20
Polarized Electron Source
E158 benefited from Linear Collider RD
High doping for 10-nm GaAs surface overcomes
charge limit.
Low doping for most of active layer yields high
polarization.
T. Maruyama et al.
No sign of charge limit!
21
Systematic Beam Centroid Control
Helicity-correlated beam motion originates at the
laser beam
Source Laser Room
Several man-years of RD - Senior physicists and
two dedicated graduate students
22
Beam Monitoring
?energy ? 1 MeV
?toroid ? 30 ppm
?BPM ? 2 microns
Agreement (MeV)
Resolution 1.05 MeV
23
End Station A
24
Kinematics
Scattering angle of Moller electron for Ebeam50
GeV
25
Quadupoles and Dipoles
26
E158 Spectrometer
27
Downstream Configuration
28
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
ep ring
Møller ring
End plate
Light guide
Lead shield
PMT holder
Lead shield
29
E158 Analysis
electron flux
Radial and azimuthal segmentation

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

30
Physics Runs
APV Sign Flips
Run 1 Apr 23 1200 May 28 0000, 2002 Run 2
Oct 10 0800 Nov 13 1600, 2002 Run 3 July 10
0800 - Sep 10 0800, 2003
Run 1 Spring 2002 Run 2 Fall 2002 Run 3
Summer 2003
g-2 spin precession
45 GeV 14.0 revs
48 GeV 14.5 revs
1020 Electrons on Target
Data divided into 75 slugs - Wave plate
flipped few hours - Beam energy changed few
days
31
Beam Asymmetries
Charge asymmetry agreement at 45 GeV
Charge asymmetry at 1 GeV
Energy difference agreement in A line
Energy difference in A line
Position agreement 1 nm
Position differences lt 20 nm
32
Final Analysis of All 3 Runs
Phys. Rev. Lett. 95 081601 (2005)
33
Quantum Corrections
Electroweak Physics at 1-Loop

• All coupling constants run
• Predictions for electroweak processes need 3
  inputs
• Fine structure constant scale of electroweak
  interactions
• Fermi constant relative scale of weak
  interactions
• Z boson mass relative scale of the neutral weak
  interaction

sin2qW e2/g2 ? test gauge structure of
SU(2)?U(1)

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

3
34
Status in 1999
sin2qw
Q (GeV)
35
Current Status
36
Implications
sin2?eff 0.2397 0.0010 0.0008
Nature
Vol 435
NEWS AND VIEWS
26 May 2005

37
Weak Mixing Angle at HIGH Energy
Courtesy C. Kolda
The Average sin2?w 0.23122(17)
? mH 89 38-28 GeV ? S -0.13 0.10
W. Marciano, CIPANP06 EW BSM Session
Rules out Technicolor! Favors SUSY!
ALR
AFB (Z? bb)
(also APV in Cs)
(also Moller _at_ E158)
sin2?w 0.2322(3) ? mH 480 350-230 GeV S
0.55 17
sin2?w 0.2310(3) ? mH 35 26-17 GeV S -0.11
17
Rules out SUSY! Favors Technicolor!
Rules out the SM!

• sin2?W improvements at hadron colliders very
  challenging
• Not a fashionable LHC topic I hope some brave
  souls will try!
• Giga-Z option of ILC or neutrino factory very
  powerful
• Is there any other method in the next decade?

38
Møller Scattering at Jefferson Lab

• The 12 GeV upgrade project of Jefferson
  Laboratory is under way (0.25B)
• A Møller scattering experiment could reach
  ?(sin2?W) 0.00025 (on paper)
• Best low energy measurement until ILC or
  ?-Factory
• Could be launched 2013

Z pole asymmetries
Jefferson Lab
Address longstanding discrepancy between hadronic
and leptonic Z asymmetries
39
Ultrahigh Precision
Measure contribution from scalars to quantum loops
loop corrections
H
t
new
b
physics
Z
Suppose scalar(s) of specific mass are observed
at Tevatron/LHC
Critical self-consistency check of electroweak
theory requires that it be observed as a specific
deviation in the value of the weak mixing angle
(world average 0.0002)
Colliders will attempt this with ALR and MW but
Systematics are extremely challenging!
Energy scale to 10-4, polarimetry to 0.15
40
Fixed-Target 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
dALR (ppm) 0.015 0.008
dsin2(qW) 0.001 0.00006-8
K.K, Snowmass 96
Order of magnitude better than E158
Møller scattering could be the centerpiece of a
compelling fixed-target program at the ILC
41
Summary

• SLAC E158s 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 6?
• sin2?eff 0.2397 0.0010 0.0008
• New constraints on TeV scale physics
• Next publications (by late 2007)
• 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!
• A future 12 GeV JLab measurement factor of 6
  improvement
• An ultimate measurement could be done at the
  ILC, if a fixed-target beam can be run
  simultaneous with collisions

42
Detector Concept
Data from Profile Detectors
43
Systematic Control (II)
44
Neutrino DIS
45
Atomic Parity Violation Boulder Expt
Power build-up cavity ( F100 000 )
E
Bp
B
xp
xex
polarizes the atoms ?F,mFgt
Reexcitation of the depleted HF levell
depletes one HF level
diode laser, tuned to the depleted HF level
dye laser beam
I fluo
APV signal odd in E, xex, B, Bp, xp
46
E158 Final Physics Result
sin2?WMS(MZ)
sin2?eff 0.2397 0.0010 0.0008

47
Backgrounds Normalization
Integrating calorimeter background dilutions
and asymmetries must be separately measured or
bounded.

• Elastic and inelastic e-p scattering and
  radiative tail
• High energy pions
• High and low energy photons
• Neutrons
• Synchrotron radiation

Total dilution 9.3 in Run I, 7.6 in Run II
III

• Beam polarization measured using polarized foil
  target
• - Same spectrometer used with dedicated movable
  detector
• Energy scale and spectrometer alignment to
  determine ltQ2gt
• Linearity of PMTs

• Largest systematic errors
• Inelastic ep -22 ? 4 ppb
• Beam polarization 0.89 0.04

48
Liquid Hydrogen Target
Refrigeration Capacity 1 kW Operating
Temperature 20 K Length
1.5 m Flow Rate 5
m/s Vertical Motion 6 inches
49
Spectrometer Collimation
Precision Collimators Critical for the Control
of Backgrounds
Significant Simulation, Design and
Fabrication Effort
50
Detector Cart
Profile Detector wheel
PMT Lead Holder/shield
Profile Detector wheel
PMT Lead Holder/shield
Luminosity Monitor region
51
Raw Asymmetry Statistics
?i 200 ppm
?i 600 ppb
N 818
N 85 Million
52
Summary of Corrections
Correction fbkg s(fbkg) Acorr (ppb) s(Acorr) (ppb)
Beam first order - - -10 1
Beam higher orders - - 0 3
Beam spotsize - - 0 1
Transverse asymmetry - - -4 2
High energy photons 0.004 0.002 3 3
Synchrotron photons 0.002 0.001 0 1
Neutrons 0.003 0.001 -1 1
ep elastic ep inelastic 0.056 0.009 0.007 0.001 -7 -22 1 4
Pions 0.001 0.001 1 1
TOTAL 0.075 0.008 -40 6

• Scale factors
• Average Polarization 89 4 ? New NLC cathode
  !
• Linearity 99 1
• Radiative corrections 1.01 0.01

53
ep Detector Data

• Radiative tail of elastic ep scattering is
  dominant background
• 8 under Moller peak
• Additional 1 from inelastic e-p scattering
• Coupling is large similar to 3 incoherent
  quarks 0.8 x 10-4 x Q2
• Background reduced in Run II III with
  additional collimation

54
Transverse Asymmetry
Asymmetry vs ?
Ebeam 46 GeV
Two-photon exchange QED effect
Flips sign at 43 GeV
?
for Møller scattering at 46 GeV
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
Schreiber (2004)
-3.5 ppm sin f
55
SUSY and QeW, QpW
SUSY effects in oblique corrections highly
suppressed
No electroweak nondecoupling
SUSY provides a potential dark matter candidate
Kurylov, Ramsey-Musolf, Su

• Stable, lightest SUSY particle if baryon (B) and
  lepton (L) numbers are conserved

SUSY loops
??? SUSY dark matter

• However, B and L need not be conserved in
  SUSY, leading to neutralino decay (RPV)

ee and ep
QeW and QpW would have new contributions from RPV
RPV 95 CL
56
Scattered Flux Profile

• 2 mm geometry
• 1 energy scale
• Radiative tail
• lt 1 background

57
Data Monte Carlo Comparison
58
Backgrounds
59
Pion Detector

• 0.5 pion flux
• 1 ppm asymmetry
• lt 5 ppb correction

60
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
61
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
62
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 ?

63
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
64
Luminosity Monitor
more than 108 scattered electrons per spill at
?lab 1 mrad

• Density fluctuations monitor

• Null asymmetry test

• Enhanced sensitivity to beam fluctuations

Parallel plates
65
Luminosity Monitor Data

• Null test at level of 20 ppb

• Density fluctuations small
• Limits on second order effects