Lead ( Pb) Radius Experiment : PREX

1
Lead ( Pb) Radius Experiment PREX
208
Elastic Scattering Parity Violating
Asymmetry
E 1 GeV, electrons on lead
Spokespersons Paul Souder, Krishna Kumar
Guido Urciuoli, Robert Michaels
Graduate Students Ahmed Zafar, Chun Min
Jen, Abdurahim Rakham (Syracuse) Jon
Wexler (UMass) Kiadtisak Saenboonruang (UVa)
208Pb
Ran March June 2010 in Hall A
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Idea behind PREX
0
Z of Weak Interaction
Clean Probe Couples Mainly to Neutrons
( T.W. Donnelly, J. Dubach, I Sick 1989 )
In PWIA (to illustrate)
w/ Coulomb distortions (C. J. Horowitz)
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Measured Asymmetry
PREX
Physics Output
Correct for Coulomb
Distortions
2
Weak Density at one Q
Mean Field
Small Corrections for
s
n
Other
G
G
MEC
Atomic Parity Violation
E
E
Models
2
Neutron Density at one Q
Assume Surface Thickness Good to 25 (MFT)
Neutron Stars
Slide adapted from C. Horowitz
R
n
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Fundamental Nuclear Physics What is the
size of a nucleus ?
Neutrons are thought to determine the size
of heavy nuclei like 208Pb. Can
theory predict it ?
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Reminder Electromagnetic Scattering
determines
(charge distribution)
208
Pb
1
2
3
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Z of weak interaction sees the neutrons
0
Analysis is clean, like electromagnetic
scattering 1. Probes the entire nuclear
volume 2. Perturbation theory applies
proton neutron
Electric charge 1 0
Weak charge 0.08 1
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Electron - Nucleus Potential
axial
electromagnetic
is small, best observed by
parity violation
208
Pb is spin 0
neutron weak charge gtgt proton weak charge
Neutron form factor
Proton form factor
Parity Violating Asymmetry
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PREX
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Measurement at one Q is sufficient to
measure R
N
( R.J. Furnstahl )
Why only one parameter ? (next slide)
proposed error
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Slide adapted from J. Piekarewicz
Nuclear Structure Neutron density is
a fundamental observable that remains
elusive.
Reflects poor understanding of symmetry
energy of nuclear matter the energy
cost of
ratio proton/neutrons
n.m. density

• Slope unconstrained by data
• Adding R from Pb
  will eliminate the dispersion in plot.

208
N
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Thanks, Alex Brown PREX Workshop 2008
Skx-s15
E/N
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Thanks, Alex Brown PREX Workshop 2008
Skx-s20
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Thanks, Alex Brown PREX Workshop 2008
Skx-s25
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• Application Atomic Parity Violation
• Low Q test of Standard Model
• Needs RN (or APV measures RN )

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Isotope Chain Experiments e.g. Berkeley Yb
APV
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Neutron Stars
Application
What is the nature of extremely dense
matter ? Do collapsed stars form exotic
phases of matter ? (strange stars, quark
stars)
Crab Nebula (X-ray, visible, radio,
infrared)
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PREX Neutron Stars
( C.J. Horowitz, J. Piekarewicz )
R calibrates EOS of Neutron Rich Matter
N
Crust Thickness
Explain Glitches in Pulsar Frequency ?
Combine PREX R with Obs. Neutron Star
Radii
N
Phase Transition to Exotic Core ?
Strange star ? Quark Star ?
Some Neutron Stars seem too Cold
Cooling by neutrino emission (URCA)
0.2 fm URCA probable, else not
Crab Pulsar
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How to do a Parity Experiment
(integrating method)
Example HAPPEX
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Pull Plot (example)
PREX Data
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Beam Asymmetries
Araw Adet - AQ ??E ??i?xi

• natural beam jitter (regression)
• beam modulation (dithering)

Slopes from
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Parity Quality Beam ! Helicity
Correlated Position Differences lt
3 nm Wien Flips helped !
Points Not sign corrected
Average with signs what expt feels
Units microns
Slug ( 1 day)
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Hall A Compton Upgrade with Green
Laser Sirish Nanda, et. al.

• 1 Polarimetry
• at 1 GeV

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Magnet and Target
Hall A Moller Upgrade Superconducting
Magnet from Hall C Saturated Iron Foil
Targets 1 Polarimetry
Sasha Glamazdin, et.al.
DAQ Upgrade (FADC)
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High Resolution Spectrometers
Spectrometer Concept Resolve Elastic
1st excited state Pb 2.6 MeV
Elastic
detector
Inelastic
Quad
Left-Right symmetry to control transverse
polarization systematic
target
Dipole
Q Q
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PREX Region After Target
Planned Tungsten Collimator and Shielding
Top view
HRS-L Q1
Septum Magnet
target
HRS-R Q1
Former O-Ring location which failed and
caused significant time loss
Collimators
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• Geant 4 Radiation Calculations
• benchmarking against LD2 (ran at 100 uA)
• exploring shielding strategies

PREX-II proposal
J. Mammei, L. Zana
Number of Neutrons per incident Electron
0 - 1 MeV
1 m
LD2 tgt
W
Pb tgt
10 m
Z (distance along beamline)
1 - 50 MeV

• Strategy
• Tungsten ( W ) plug
• Shield the W
• x10 reduction in
• 0.2 to 10 MeV neutrons (similar to LD2)

PREX -I
PREX -II
50 - 500 MeV
-1 m
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High Resolution Spectrometers
Pure, Thin 208 Pb Target
Lead 5- State
2.6 MeV
DETECTOR footprint
Momentum (GeV/c)
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Lead / Diamond Target
Diamond
LEAD

• Three bays
• Lead (0.5 mm) sandwiched by diamond (0.15
  mm)
• Liquid He cooling (30 Watts)

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Performance of Lead / Diamond Targets
melted
melted
Targets with thin diamond backing (4.5
background) degraded fastest. Thick diamond (8)
ran well and did not melt at 70 uA.
NOT melted
Last 4 days at 70 uA
Solution Run with 10 targets.
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Beam-Normal Asymmetry in elastic electron
scattering
i.e. spin transverse to scattering plane
Possible systematic if small transverse
spin component
New results PREX

• Small AT for 208Pb is a big (but
  pleasant) surprise.
• AT for 12C qualitatively consistent with
  4He and available calculations (1) Afanasev
  (2) Gorchtein Horowitz

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Systematic Errors
Error Source Absolute (ppm) Relative ( )
Polarization (1) 0.0071 1.1
Beam Asymmetries (2) 0.0072 1.1
Detector Linearity 0.0071 1.1
BCM Linearity 0.0010 0.2
Rescattering 0.0001 0
Transverse Polarization 0.0012 0.2
Q2 (1) 0.0028 0.4
Target Thickness 0.0005 0.1
12C Asymmetry (2) 0.0025 0.4
Inelastic States 0 0
TOTAL 0.0130 2.0
(1) Normalization Correction applied
(2) Nonzero correction (the rest assumed zero)
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PREX Physics Result
ppm
9.2
2.0
at Q2 0.00906 GeV2

• Statistics limited ( 9 )
• Systematic error goal achieved ! (2)

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PREX Asymmetry (Pe x A)
ppm
(blinded, raw)
Slug 1 day
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PREX Physics Interpretation
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Asymmetry leads to RN
Establishing a neutron skin at 95 CL
Neutron Skin RN - RP 0.34 0.15 -
0.17 fm
arXiv1010.3246  nucl-th
Shufang Ban, C.J. Horowitz, R. Michaels
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Future ?
PREX II Proposal
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Future ?
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Other Nuclei ?
RN
Shape Dependence ?
Surface thickness
Parity Violating Electron Scattering Measurements
of Neutron Densities Shufang Ban, C.J.
Horowitz, R. Michaels
RN
Surface thickness
arXiv1010.3246  nucl-th
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PREX Summary

• Fundamental Nuclear Physics with
  many applications
• Achieved a 9 stat. error in Asymmetry
  (original goal 3 )
• Systematic Error Goals Achieved !!
• Significant time-losses due to O-Ring problem
  and radiation damage
• Proposal for PREX-II in preparation

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Extra Slides
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Corrections to the Asymmetry are Mostly
Negligible

• Coulomb Distortions 20 the biggest
  correction.
• Transverse Asymmetry (to be measured)
• Strangeness
• Electric Form Factor of Neutron
• Parity Admixtures
• Dispersion Corrections
• Meson Exchange Currents
• Shape Dependence
• Isospin Corrections
• Radiative Corrections
• Excited States
• Target Impurities

Horowitz, et.al. PRC 63 025501
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Optimum Kinematics for Lead Parity E 1
GeV if
ltAgt 0.5 ppm. Accuracy in Asy 3
Fig. of merit
Min. error in R maximize
n
1 month run 1 in R
n
(2 months x 100 uA ? 0.5 if
no systematics)
5
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Liquid/Solid Transition Density
Neutron Star Crust vs Pb Neutron Skin
C.J. Horowitz, J. Piekarawicz
Neutron Star
208Pb

• Thicker neutron skin in Pb means energy rises
  rapidly with density ? Quickly favors uniform
  phase.
• Thick skin in Pb ? low transition density in
  star.

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PREX
pins down the symmetry energy (1 parameter)
energy cost for unequal protons
neutrons
PREX error bar
( R.J. Furnstahl )
Actually, its the density dependence of a4
that we pin down.
208
Pb
PREX