The G0 Experiment: A First Look at the Data

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The G0 Experiment A First Look at the Data
Jeff Secrest
May 16, 2005
19 institutions with 100 physicists CalTech,
CMU, William Mary, Hampton, IPN-Orsay,
ISN-Grenoble, JLab, Kentucky, LaTech, NMSU,
TRIUMF, U Conn, UIUC, U Manitoba, U Maryland, U
Mass, UNBC, VPI, Yerevan
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Outline

• Physics motivation
• Strange quarks in the proton
• Some formalism
• Parity-violating electron scattering
• The apparatus
• All new equipment
• Look at some the Data

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What do we really know about the proton?

• Static properties of the proton
• charge e
• magnetic moment µp 2.79 µN
• spin ½ h
• mass 938 MeV
• But what is going on inside?
• How do we describe these static properties in
  terms of the internal degrees of freedom?
• Maybe use the quark model

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There are strange quarks in the proton !?!

• Violation of Ellis-Jaffe (spin) sum rule
• Ellis-Jaffe computed 0.17 /- 0.01 with the
  assumption only u and d quarks
• EMC measured 0.126 /- 0.010 (syst) /-
  0.015(stat)
• Maybe we should not be so fast in assuming no
  strange quarks!
• ?We should consider QCD!

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Returning to the static properties of the proton

• Mass (pN S-term)
• Spin (polarized
  DIS)
• Charge and Current

Goal of the G0 Experiment
Determine the contributions from the strange
quark sea to the electromagnetic properties of
the proton
For the experts
To separate the strange electric and magnetic
form factors over a Q2 ranging from 0.3 to 0.8
(GeV/c)2
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Form factors
Form factors parameterize the structure of the
nucleon
GE(Q2) ? Fourier transform of the charge
distribution GM(Q2) ? Fourier transform of the
current distribution
Nucleon electromagnetic form factors
Measured with a precision of 2-4 in the 0.1-1.0
GeV2 range
Proton weak form factors

• Projected precision of 10
• in the 0.1-1.0 GeV2 range

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How will we measure the strange electric and
magnetic form factors?

• Were scattering polarized electrons off of an
  unpolarized hydrogen target
• 2 types of interactions
• Electromagnetic interaction (parity-conserving)
  via ?
• Weak interaction (parity-violating) via Z0
• Measure the quantum interference between the two
  interactions
• leading to a parity-violating asymmetry
• This is a small asymmetry on the order of
  parts-per-million (ppm)
• Must be careful to guard against false
  asymmetries at this level
• The asymmetry can be rewritten in terms of known
  electromagnetic form factors and the unknown
  strange form factors

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Parity-violation asymmetry
Assuming SU(3) flavor decomposition
Assuming charge symmetry
Thus Up and Down quark contributions can be
eliminated
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What drives the design of the G0 apparatus?

• Ameas -3 to -40 ppm with a precision ?Ameas
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• High statistics experiment ? N1013-1014 events
• High count rate capability for detectors/electroni
  cs
• High luminosity 2.1 ? 1038 cm-2 s-1
• High beam current I40 µA
• Large acceptance ?? 0.9 sr
• High electron polarization Pbeam77
• Long target L20 cm

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The G0 experiment in Jefferson Lab Hall C
All new Hall C equipment Plastic scintillator
detector array Superconducting toroidal
magnet High powered 20 cm LH2 target Custom high
rate electronics 3 GeV polarized beam in 32 ns
pulses
G0 timeline Design and construction (1993
2001) Installation (Fall 2001- Fall 2002) Forward
angle program (completed) ?1st Commissioning
run (Oct 2002 - Jan 2003) 2nd Commissioning run
(Nov 2003 - Feb 2004) Production run (Feb 2004
May 2004)
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Jefferson Lab Newport News VA
Hall C
Injector
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G0 magnet

• Superconducting toroidal magnet
• Purpose the magnet sorts elastic protons
• by Q2 in the Focal Plane Detectors

• 8 coils
• Maximum 1.6 T field

Initial manufacturing defects were repaired in
early 2002 Ran at 4500 A initially
(Aug.-Dec. 2002) Ran at full design current
during Jan 2003 run
(UIUC)
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G0 Focal Plane Detectors
16 pairs of arc-shaped plastic scintillators
? shaped to focal plane and acceptance
of the spectrometer Back Front coincidence
eliminates neutrals Divided into 8 octants (not
very different) - 4 North American (NA) - 4
French (Fr) Detects recoil elastic protons 62
lt ?p lt 78 corresponding to 15 lt ?e lt 5
electrons
(CMU, TRIUMF, Manitoba, Maryland, WM, NMSU,
Grenoble, Orsay)
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G0 target

• 20 cm LH2 cell
• 250 W heat load at I 40 µA
• High flow rate to minimize target density
  fluctuations
• ? target density fluctuations
  increase statistical
  width s
• Magnet focal planes are independent of
  interaction along target length

s2meas s2stat s2noise
Well come back to this!
(CalTech)
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G0 custom electronics

NA Electronics 1 ns bins highly modular Fr
Electronics 0.25 ns bins highly compact
2 separate systems allows for a cross-check
Resulting histograms (our data!) ?
(CMU, Orsay, Grenoble)
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G0 beam

• Recall asymmetry 10-6 ? precision 10-7

• Must minimize false asymmetries
• source of false asymmetries helicity-correlated
  beam properties

• G0 uses a polarized beam that is different than
  the typical CEBAF beam
• Time Structure 31 MHz not 499 MHz
• High Peak Current 40 µA at 31 MHz 640 µA at 499
  MHz

• What to do?
• Minimize helicity-correlated beam properties
• ?Position and Intensity feedback systems
• Measure detector sensitivities to
  helicity-correlated beam properties

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G0 beam
G0 beam structure
Some important definitions
Must minimize these helicity-correlated beam
parameters!
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Minimizing helicity-correlated beam parameters by
active feedback systems
Charge feedback feedback from Hall C current
monitors to the IA cell ? electro-optic
intensity modulator
?
Schematic of the laser table in the injector
Position feedback feedback from Hall C position
monitors to PZT mirror
?
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Schematic of the injector
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G0 beam injector DAQ system
The G0 injector DAQ allows for monitoring beam
parameters from position and current monitors in
the injector
During the engineering run, the IA cells
calibration constant seemed to change with beam
current
?
The origin helicity-correlated scraping at
apertures in the injector
Apertures
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From raw asymmetry to physics results
Form raw asymmetry from detector yields
Correct for false asymmetries from
helicity-correlated beam properties
Correct for the background and its asymmetry
Correct for beam polarization and radiative
corrections
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Correcting systematic helicity-correlated beam
parameters
Beam properties
Advantage to azimuthal symmetry!
IHWP OUT AFalse -0.2 /- 0.6 ppm IHWP IN
AFalse 0.0 /- 0.5 ppm
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Inelastic background asymmetries
Ael (1 b)Acorr bAback
Asymmetries
Yield
Aback determined from a linear interpolation of
the side bands asymmetries
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Inelastic background
Ael (1 b) Acorr b Aback
elastic protons
pions
Events
inelastics

• 3 Gaussian fit
• numerical integration
• to find inelastic and elastic
• contributions within the
• cut window

Time (1/4 ns)
Background 10 30 depending on
detector
Simulation important contribution to background
comes from the downstream aluminum window
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Behavior of raw asymmetry under halfwave plate
reversal, Jan. 2003
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Counting statistics how much noise is in the
data?
G0 is a counting experiment---is it dominated by
counting statistics or is there some noise in
the system?
Compare to
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Asymmetry results from January 2003 Engineering
Run

• Based on 50 Hours
• of data
• (note full production run
• was 700 hours)
• Includes
• False asymmetry corrections
• Deadtime corrections
• Background corrections
• Beam polarization correction

Consistent results from both sets of
electronics Sign of the asymmetry
expected Expected Q2 behavior
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Backward Angle Running
Electron detection Add Cryostat Exit Detectors
(CEDs) to define electron trajectory Add
aerogel Cerenkovs to reject pions
Begin Backward Angle installation in 2005
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Expected Results
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Summary

• Learned a lot from the First Engineering Run
• The G0 apparatus was successfully commissioned
• The data shows good statistical properties
• Parity-violating asymmetries behave as expected
• Improvements for the Production Run
• Improved helicity-correlated beam properties
• Reduce target exit window thickness
• Improvements in the electronics
• Develop techniques for studying the background
• G0 Forward Angle Production complete
• Analysis is currently underway
• Backward Angle installation Fall 2005
• In a few years separated strange form factors of
  the proton vs. Q2 !

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Electron-nucleon axial form factor

• The weak interaction is a V-A current
• ?Both vector and axial form factors

• Tree level Z exchange
• Measured in ß decay for Q20
• Q2 evolution from ?-scattering

• Anapole moment Parity violating
• coupling of ? to the nucleon
• Unique to PV interactions
• w/ charged particles

• Radiative corrections

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Results from previous experimentsGMs vs. GEs for
3 values of Q2
HAPPEX I
Q20.477 (GeV/c)2
Q20.1 (GeV/c)2
PVA4
Q20.230 (GeV/c)2