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 in Hall A

2. Idea behind PREX Clean Probe Couples Mainly to Neutrons
Z of Weak Interaction :
Clean Probe Couples Mainly to Neutrons
( T.W. Donnelly, J. Dubach, I Sick )
In PWIA (to illustrate) :
w/ Coulomb distortions (C. J. Horowitz) :

3. PREX Physics Output Mean Field & Other Atomic Parity Violation Models
Measured Asymmetry
Physics Output
Correct for Coulomb
Distortions
Weak Density at one Q 2
Mean Field
Small Corrections for n s
& Other
Atomic Parity Violation G G
MEC 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

4. 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 ?

5. Reminder: Electromagnetic Scattering determines
(charge distribution)
208 Pb 1 2 3

6. Z of weak interaction : sees the neutrons
Analysis is clean, like electromagnetic scattering:
1. Probes the entire nuclear volume
2. Perturbation theory applies
proton
neutron
Electric charge 1
Weak charge
0.08 6

7. Parity Violating Asymmetry
Electron - Nucleus Potential
electromagnetic
axial
is small, best observed by parity violation
208
Pb is spin 0
neutron weak charge >> proton weak charge
Proton form factor
Neutron form factor
Parity Violating Asymmetry

8. PREX: Measurement at one Q is sufficient to measure R2
Measurement at one Q is sufficient to measure R N
( R.J. Furnstahl )
Why only one parameter ?
(next slide…)
proposed error

9. 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

10. Thanks, Alex Brown
PREX Workshop 2008
Skx-s15
E/N

11. Thanks, Alex Brown
PREX Workshop 2008
Skx-s20

12. Thanks, Alex Brown
PREX Workshop 2008
Skx-s25

13. APV Application: Atomic Parity Violation Low Q test of Standard Model
Needs RN (or APV measures RN ) 2
Isotope Chain Experiments e.g. Berkeley Yb
APV 13

14. Neutron Stars What is the nature of extremely dense matter ?
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)

15. PREX & Neutron Stars Crab Pulsar
( 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)
Crab Pulsar
0.2 fm URCA probable, else not

16. How to do a Parity Experiment
(integrating method)
Flux Integration Technique:
HAPPEX: 2 MHz
PREX: MHz
Example : HAPPEX

17. Pull Plot (example)
PREX Data

18. Araw = Adet - AQ + E+ ixi
Beam Asymmetries
Araw = Adet - AQ + E+ ixi
Slopes from
natural beam jitter (regression)
beam modulation (dithering)
PAVI 09 18

19. Parity Quality Beam ! < ~ 3 nm Wien Flips helped !
Helicity – Correlated Position Differences
< ~ 3 nm
Wien Flips helped !
Points: Not sign corrected
Average with signs = what exp’t feels
Units: microns
Slug # ( ~ 1 day)

20. Hall A Compton Upgrade 1 % Polarimetry at 1 GeV with Green Laser
Sirish Nanda, et. al.
1 % Polarimetry
at GeV

21. 1 % Polarimetry Hall A Moller Upgrade Magnet and Target
Superconducting Magnet from Hall C
Saturated Iron Foil Targets
1 % Polarimetry
Sasha Glamazdin, et.al.
DAQ Upgrade (FADC)

22. High Resolution Spectrometers
Spectrometer Concept: Resolve Elastic
1st excited state Pb MeV
Elastic
detector
Inelastic
Quad
Left-Right symmetry to control transverse polarization systematic
target
Dipole
Q Q

23. Septum Magnet Planned Tungsten Collimator and Shielding
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

24. 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
MeV
1 m W
LD2 tgt
Pb tgt
10 m
Z (distance along beamline)
Strategy
Tungsten ( W ) plug
Shield the W
x10 reduction in
0.2 to 10 MeV neutrons (similar to LD2)
MeV
PREX -I
PREX -II
MeV
-1 m

25. High Resolution Spectrometers
Pure, Thin Pb Target
Lead 5- State
2.6 MeV
DETECTOR footprint
Momentum (GeV/c) 25

26. Lead / Diamond Target Diamond LEAD Three bays
Lead (0.5 mm) sandwiched by diamond (0.15 mm)
Liquid He cooling (30 Watts)

27. 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.

28. + - Beam-Normal Asymmetry in elastic electron scattering
i.e. spin transverse to scattering plane y z x + -
AT > 0 means
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

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

30. PREX Physics Result ppm Statistics limited ( 9% )
9.2 %
2.0 %
at Q2 = GeV2
Statistics limited ( 9% )
Systematic error goal achieved ! (2%)

31. PREX Asymmetry (Pe x A)
ppm
(blinded, raw)
Slug ~ 1 day

32. PREX Physics Interpretation

33. Asymmetry leads to RN Neutron Skin = RN - RP = 0.34 + 0.15 - 0.17 fm
Establishing a neutron skin at 95% CL
Neutron Skin = RN - RP = fm
Shufang Ban, C.J. Horowitz, R. Michaels
arXiv:   [nucl-th]

34. Future ?
PREX – II Proposal

35. Future ?

36. Other Nuclei ? RN Surface thickness RN Surface thickness
Shape Dependence ?
Surface thickness
Parity Violating Electron Scattering Measurements of Neutron Densities
Shufang Ban, C.J. Horowitz, R. Michaels RN
Surface thickness
arXiv:   [nucl-th]

37. 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

38. Extra Slides

39. 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

40. Optimum Kinematics for Lead Parity: E = 1 GeV if
<A> = 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 uA
 0.5% if no systematics) 5
PAVI 09

41. Liquid/Solid Transition Density
Neutron Star Crust vs Pb Neutron Skin FP
TM1
Solid
Liquid
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.

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