The Qpweak Experiment:

1
The Qpweak Experiment A Search for Physics
beyond the Standard Model via parity-violating
e-p scattering at low Q2 S. Page, PAVIO6, Milos,
Greece
2
Parity violating scattering asymmetry
Q2 Dependence
3
What is the protons weak charge and why are we
measuring it?

• QpWeak 1 4sin2 ?W , which has never been
• directly measured in a precision experiment, sets
  
• the scale of the protons coupling to W and Z
  bosons
• that mediate the weak interaction
• The weak mixing angle, sin2 ?W, is predicted to
  vary with the energy scale (Q)
• at which it is measured, due to electroweak
  radiative corrections that are
• sensitive to the possible existence of
  additional force carriers beyond the Standard
  Model -- e.g. right handed Z bosons etc.
• sin2 ?W is well determined at the Z-pole from
  high energy collider experiments but lower energy
  precision measurements have not yet been made
• The Qweak experiment at Jefferson Lab will enable
  us to check the predicted running of sin2 ?W,
  by making a precision measurement at low energy,
  placing limits on physics beyond the Standard
  Model.

4
Energy Scale dependence of the weak mixing angle
Significance planned error bar corresponds to
a 10? measurement of the Standard Model
prediction
5
Comparison of proton and electron weak charge
sensitivities
JLab Qweak
SLAC E158
-
(proposed)

• Qweak measurement will provide a stringent
  stand alone constraint
• on Lepto-quark based extensions to the SM.
• Qpweak (semi-leptonic) and E158 (pure
  leptonic) together make a
• powerful program to search for and identify
  new physics.

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Impact via Model-independent Semi-Leptonic
Analysis
Effective electron-quark neutral current
Lagrangian
DC1u ? C1u(exp) ? C1u(SM) DC1d ? C1d(exp) ?
C1d(SM)
Large ellipse (existing data) SLAC e-D
(DIS) MIT-Bates 12C (elastic) Cesium atomic
parity violation
Red ellipse Impact of QpWeak measurement (centroi
d assumes agreement with the Standard Model)
Erler, Kurylov Ramsey-Musolf
Phys.Rev.D68016006,2003
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Experimental sensitivity

• Experimental considerations
• need high statistics ? integrating detector
  system
• measured asymmetry is P A ? beam polarization
  P should be large well measured
• somebody else has to measure B(Q2) for us (done
  ? ) so that we can subtract it
• we need to know the detector-response-weighted
  and to interpret the data
• helicity correlated systematic errors must be
  kept below 5 x 10-9

8
Anticipated QpWeak Uncertainties


? ?Aphys /Aphys
?Qpweak/Qpweak Statistical (2200 hours
production) 1.8
2.9 Systematic Hadronic structure
uncertainties --
2.2 Beam polarimetry 1.0
1.6 Absolute Q2 determination
0.7
1.1 Backgrounds 0.5
0.8 Helicity-correlated Beam
Properties 0.5 0.8 ________________
_________________________________________
Total
2.3 4.3
An additional uncertainty associated with QCD
corrections applied to the extraction of sin2?W
it raises ?sin2?W / sin2?W from 0.2 to 0.3.
9
Hadronic Form Factors
Hadronic FFs at Q2 0.1 GeV2 ? extrapolation to
Q2 0.03 GeV2 gives contribution to Qweak
(see Ross Youngs talk !)
10
Beam Property Requirements
?
?
?
?
close
on track
85
85 87 Achieved at injector
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Main apparatus target plus toroidal magnetic
spectrometer
inelastic electrons
quartz detectors for elastic e-
LH2 target
photons hit here
beam
double collimator system selects scattering
angle/ accepted Q2 range
toroidal magnetic spectrometer elastically
scattered electrons are bent away from the
beamline and focused onto the detector plane
12
Qweak LH2 target
Requirements

• 2500 W cooling power !
• raster size 4 x 4 mm2
• ??/?

13
Layout drawing main asymmetry
plus tracking apparatus for ,
Quartz Cerenkov Bars (insensitive to
non-relativistic particles)
Region 2 Horizontal drift chamber location
Region 1 GEM Gas Electron Multiplier
Mini-torus
e- beam
Ebeam 1.165 GeV Ibeam 180 µA Polarization
85 Target 2.5 kW
Lumi Monitors
QTOR Magnet
Region 3 Vertical Drift chambers
Collimator System
Trigger Scintillator
(We will turn down the beam current and track
particles to determine , )
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View of QpWeak Apparatus collapsed along beam
direction - Simulated Events
Central scattering angle 8
2 Phi Acceptance
50 of 2? Average Q²
0.027 GeV2 Acceptance averaged asymmetry
0.29 ppm Integrated Rate (per detector)
900 MHz Inelastic/Elastic ratio
0.01
Elastic e-p envelope
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Main Detector and Electronics System

• Focal plane detector requirements
• Insensitivity to background ?, n, ?.
• Radiation hardness (expect 300 kRad).
• Operation at counting statistics
• nonlinearity less than 1
• Fused Silica (synthetic quartz) Cerenkov
  detector.
• Plan to use 18 cm x 200 cm x 1.25 cm quartz
• bars read out at both ends by S20
• photocathode PMTs (expect 50 pe/event)
• n 1.47, ?Cerenkov47, total internal
  reflection ?tir43
• reflectivity 0.997

16
Determination of Average Q2
distribution 0.03 GeV2

• reduce beam current
• use tracking system
• read out quartz light yield

17
Precision Polarimetry

• See talks on Saturday
• Jurgen Diefenbach (Compton)
• Dave Mack (Moller)

• Present limitations existing Moller polarimeter
• IMax 10 ?A.
  
• At higher currents the Fe target depolarizes.
• Measurement is destructive
• Møller upgrade
• Measure Pbeam at 100 ?A or higher,
  quasi-continuously
• Trick kicker strip or wire target
• Schematic of planned new Hall C Compton
  polarimeter.

Existing Hall C Møller can achieve 1
(statistics) in a few minutes.
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Summary and Outlook

• We will measure the protons weak charge, QWp
  to ? 4 and hence determine
• sin2?W to ? 0.3 from low Q2 parity-violating
  elastic scattering at Jefferson Lab.
• Experiment approved with A rating, 2002
  reaffirmed 2005
• JLab schedule installation in 2009 and 18
  months on the floor before 12 GeV upgrade
• Progress on experiment design, simulations,
  prototyping and construction is underway

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Region 1 GEM prototype
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The Qweak Collaboration
www.jlab.org/qweak/
Keppel, Cynthia - Hampton University Khol,
Michael - Massachusetts Institute of
Technology Korkmaz, Elie - University of Northern
British Columbia Lee, Lawrence - TRIUMF Liang,
Yongguang - Ohio University Lung, Allison -
Thomas Jefferson National Accelerator
Facility Mack, David - Thomas Jefferson National
Accelerator Facility Majewski, Stanislaw - Thomas
Jefferson National Accelerator Mammei, Juliette -
Virginia Polytechnic Inst. State Univ. Mammei,
Russell - Virginia Polytechnic Inst. State
Univ. Martin, Jeffery W. University of
Winnipeg Meekins, David Thomas Jefferson
National Accelerator Facility Mkrtchyan, Hamlet -
Yerevan Physics Institute Morgan, Norman -
Virginia Polytechnic Inst. State Univ. Myers,
Catherine George Washington University Opper,
Allena George Washington Univ. Penttila, Seppo
- Los Alamos National Laboratory Pitt, Mark -
Virginia Polytechnic Inst. State Univ. Poelker,
B. (Matt) - Thomas Jefferson National Accelerator
Facility Prok, Yelena Massachusetts Institute
of Technology Ramsay, Des - University of
Manitoba and TRIUMF Ramsey-Musolf, Michael -
California Institute of Technology Roche, Julie -
Thomas Jefferson National Accelerator
Facility Simicevic, Neven - Louisiana Tech
University Smith, Gregory - Thomas Jefferson
National Accelerator Facility Smith, Timothy -
Dartmouth College Souder, Paul Syracuse
University Suleiman, Riad Virginia Polytechnic
State Univ. Tsentalovich, Evgeni -
Massachusetts Institute of Technology van Oers,
W.T.H. - University of Manitoba Wang, Jie Pan
University of Winnipeg Wang, Peiqing University
of Manitoba Wells, Steven - Louisiana Tech
University Wood, Stephen Thomas - Jefferson
National Accelerator Facility Zhu, Hongguo -
University of New Hampshire Ziskin, Vitaly MIT
Bates Linear Accelerator Laboratory Zorn, Carl -
Thomas Jefferson National Accelerator
Facility Zwart, Townsend - Massachusetts
Institute of Technology
Qweak Collaboration Spokespersons Carlini, Roger
(Principal Investigator) - Thomas Jefferson
National Accelerator Facility Finn, J. Michael -
College of William and Mary Kowalski, Stanley -
Massachusetts Institute of Technology Page,
Shelley - University of Manitoba Qweak
Collaboration Members Armstrong, David - College
of William and Mary Averett, Todd - College of
William and Mary Birchall, James - University of
Manitoba Bosted, Peter Thomas Jefferson
National Accelerator Facility Botto, Tancredi -
Massachusetts Institute of Technology Bowman,
David Los Alamos National Laboratory Bruell,
Antje - Thomas Jefferson National Accelerator
Facility Cates, Gordon University of
Virginia Chattopadhyay, Swapan - Thomas Jefferson
National Accelerator Facility Davis, Charles -
TRIUMF Doornbos, J. - TRIUMF Dow, Karen -
Massachusetts Institute of Technology Dunne,
James - Mississippi State University Ent, Rolf -
Thomas Jefferson National Accelerator
Facility Erler, Jens - University of Mexico Falk,
Willie - University of Manitoba Farkhondeh,
Manouchehr - Massachusetts Institute of
Technology Forest, Tony - Louisiana Tech
University Franklin, Wilbur - Massachusetts
Institute of Technology Gaskell, David - Thomas
Jefferson National Accelerator Facility Gericke,
Michael University of Manitoba and
TRIUMF Grimm, Klaus - College of William and
Mary Hersman, F. W. - University of New
Hampshire Holtrop, Maurik - University of New
Hampshire Johnston, Kathleen - Louisiana Tech
University Jones, Richard - University of
Connecticut Joo, Kyungseon - University of
Connecticut
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The Qpweak Luminosity Monitor

• Luminosity monitor ? Symmetric array of 8
  quartz Cerenkov detectors
• instrument with rad hard vacuum photo
  diodes
• 
  integrating readout at small ? ( 1.2?).
• Low Q2, high rates 29 GHz/octant.
• Expected signal components 12 GHz e-e
  Moeller, 11 GHz e-p elastic,
• EM showers 6 GHz.
• Expected lumi monitor asymmetry detector asymmetry.
• Expected lumi monitor statistical error
  (1/6) main detector statistical error.
• Useful for
• Sensitive check on helicity-correlated
• beam parameter corrections procedure.
• Regress out target density fluctuations.

MAMI A4 LUMI Monitor
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