1. Thomas Jefferson National Accelerator Facility
Lead (208Pb) Radius Experiment : PREX
Elastic Scattering Parity Violating Asymmetry
E = 1 GeV, electrons on lead
PAVI – 11
Conference
Spokespersons
Kent Paschke, Paul Souder, Krishna Kumar,
Guido Urciuoli, Robert Michaels (presenting)
PREX-I Results Run
Approved PREX-II Proposal
Future : PREX-N ? (N = III, IV, V …)
Thomas Jefferson National Accelerator Facility

2. Z0 of weak interaction : sees the neutrons
T.W. Donnelly, J. Dubach, I. Sick
proton
neutron
Electric charge 1
Weak charge
0.08
Nucl. Phys. A 503, 589, 1989
C. J. Horowitz, S. J. Pollock, P. A. Souder, R. Michaels
Phys. Rev. C 63, , 2001
Neutron form factor
C.J. Horowitz
Parity Violating Asymmetry

3. Hall A at Jefferson Lab
Hall A

4. 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

5. 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 5

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

7. Thanks, Alex Brown
PREX Workshop 2008
Skx-s20
E/N

8. Thanks, Alex Brown
PREX Workshop 2008
Skx-s25
E/N 8

9. 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 9

10. PREX & Neutron Stars
C.J. Horowitz, J. Piekarewicz
RN calibrates equation of state (pressure vs density) of Neutron Rich Matter
Combine PREX RN with Observed Neutron Star Radii
Fig from: Dany Page J.M. Lattimer & M. Prakash, Science 304 (2004) 536.
Phase Transition to “Exotic” Core ?
Strange star ? Quark Star ?
Some Neutron Stars seem too cold
Explained by Cooling by neutrino emission (URCA process) ?
0.2 fm URCA probable, else not
Crab Pulsar

11. PREX Overview Spectometers Lead Foil Target Hall A JLAB CEBAF
Parity: “The entire lab is the experiment”
CEBAF
Hall A
JLAB
Pol. Source
Spectometers
Lead Foil Target

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

13. Flips spin without moving the beam !
Double Wien Filter
(NEW for PREX)
Crossed E & B fields to rotate the spin
Two Wien Spin Manipulators in series
Solenoid rotates spin +/-90 degrees (spin rotation as B but focus as B2).
Flips spin without moving the beam !
Electron Beam
SPIN
Joe Grames, et. al. 13

14. Hall A Compton Upgrade 1 % Polarimetry at 1 GeV with Green Laser
Sirish Nanda, et. al. (Megan Friend’s talk)
1 % Polarimetry
at GeV

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

16. Hall A High Resolution Spectrometers
Resolve Elastic Scattering
Discriminate Excited States
Elastic
detector
Inelastic
Pure, Thin Pb Target
2.6 MeV
target
Dipole
DETECTOR footprint
Quads
Scattered Electron’s Momentum (GeV/c) 16

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

18. 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.

19. + - 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
Preliminary !
Publication in preparation
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 19

20. PREX-I Result Statistics limited ( 9% )
Systematic Errors
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
Statistics limited ( 9% )
Systematic error goal achieved ! (2%)
(1) Normalization Correction applied
(2) Nonzero correction (the rest assumed zero) 20

21. PREX Asymmetry (Pe x A)
ppm
Slug ~ 1 day

22. * * Asymmetry leads to RN
Establishing a neutron skin at ~90 % CL *
Neutron Skin = RN - RP = fm
fig from C.J. Horowitz
PREX data *
Preliminary: Awaiting the “final” acceptance function:

23. PREX-I Result, cont. Neutron Skin = RN - RP = 0.31 + 0.15 - 0.17 fm
DATA
rN - rP (fm)
theory: P. Ring
rN = rP
Atomic Number, A
DATA
Preliminary: Awaiting the “final” acceptance function
A physics letter is in preparation for publication. 23

24. PREX-II Approved by PAC (Aug 2011)
“A” Rating days run in 2013 / 2014

25. Septum Magnet PREX Region After Target Tungsten Collimator & Shielding
Improvements for PREX-II
Tungsten Collimator & Shielding
HRS-L Q1
Septum Magnet
target
HRS-R Q1
Former O-Ring location which failed & caused
time loss during PREX-I
 PREX-II to use all-metal seals
Collimators

26. Geant 4 Radiation Calculations
PREX-II shielding strategies
J. Mammei, L. Zana
scattering chamber
shielding
Number of Neutrons per incident Electron
MeV
beamline
Energy (MeV)
PREX-I
PREX-II, no shield
PREX-II, shielded
MeV
Strategy
Tungsten ( W ) plug
Shield the W
x 10 reduction in
0.2 to 10 MeV neutrons
Energy (MeV)
MeV
Energy (MeV) 26

27. 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]

28. Possible Future PREX Program ?
Each point 30 days stat. error only
Nucleus
E (GeV)
dRN / RN
comment
208Pb 1
1 %
PREX-II (approved)
48Ca
2.2 (1-pass)
0.4 %
natural 12 GeV exp’t
2.6
2 %
surface thickness
40Ca
0.6 %
basic check of theory
tin isotope
1.8
apply to heavy ion
1.6 %
Not yet proposed.
Just a “what if ?”
Shufang Ban, C.J. Horowitz, R. Michaels
arXiv:   [nucl-th]

29. Future in Hall A at JLab PREX – II ? Early Experiments g2p/GEp
12 mo. Shutdown
no promised beam
Commissioning
SuperBigbite $
Beam 1st to Hall A
Moller
$$$
SOLID
$$$$
PREX – II ?
2011
2012
2013
2014
2015
2016
2017
2018

30. PREX : Summary Fundamental Nuclear Physics with many applications
PREX-I 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
PREX-II approved (runs in or we hope )

31. Extra Slides

32. How to Measure Neutron Distributions, Symmetry Energy
Proton-Nucleus Elastic
Pion, alpha, d Scattering
Pion Photoproduction
Heavy ion collisions
Rare Isotopes (dripline)
Magnetic scattering
PREX (weak interaction)
Theory
Involve strong probes
Most spins couple to zero.
MFT fit mostly by data other than neutron densities

33. 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

34. 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

35. 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

36. Pull Plot (example)
PREX Data

37. 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.

38. Backgrounds that might re-scatter into the detector ?
Detector cutoff
Run magnets down: measure inelastic region
Run magnets up : measure probability to rescatter
No inelastics observed on top of radiative tail. Small systematic for tail.

39. 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