1. Lead ( Pb) Radius Experiment : PREX
Results from
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 (speaker)
Graduate Students
Ahmed Zafar, Chun Min Jen, Abdurahim Rakham (Syracuse)
Jon Wexler (UMass)
Kiadtisak Saenboonruang (UVa)
208Pb
Ran March – June
in Hall A at Jefferson Lab

2. Standard Electroweak Model
Left –handed fermion fields (quarks & leptons)
= doublets under SU(2)
Right-handed fields = singlets under SU(2)
The Glashow-Weinberg-Salam Theory unifies the electromagnetic and weak interactions.
Parity Violation
p, n
decay
Weak charge
208 Pb of

3. Parity Transformation
A piece of the weak interaction violates parity (mirror symmetry) which allows to isolate it.
Parity Transformation Pb p
1800
rotation
Positive spin
Negative spin

4. Parity Violating Asymmetry2 +
APV from interference
208Pb
208Pb
Applications of APV at Jefferson Lab
Nucleon Structure
Test of Standard Model of Electroweak
Nuclear Structure (neutron density) : PREX
Strangeness s s in proton (HAPPEX, G0 expts)
e – e (MOLLER) or e – q (PVDIS)
elastic e – p at low Q2 (QWEAK)
This talk

5. 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) : 5

6. Hall A at Jefferson Lab
Hall A

7. 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

8. 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 ?

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

10. 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 10

11. 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

12. Isospin Diffusion (NSCL)
Example: Heavy Ions
(adapted from Betty Tsang, PREX Workshop, 2008)
Isospin Diffusion (NSCL)
Probe the symmetry energy in 124Sn + 112Sn

13. Using Parity Violation
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

14. 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

15. 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 significantly reduce the dispersion in plot.
208 N 15

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

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

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

19. 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
Momentum transfer 19

20. 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)

21. Inputs: Eq. of state (EOS) PREX helps here Hydrostatics (Gen. Rel.)
pressure
density
Inputs:
Eq. of state (EOS)
PREX helps here
Hydrostatics (Gen. Rel.)
Astrophysics Observations
Luminosity L
Temp. T
Mass M from pulsar timing
(with corrections … )
Mass - Radius relationship
Fig from: Dany Page.
J.M. Lattimer & M. Prakash, Science 304 (2004) 536. 21

22. 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
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

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

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

25. Polarized Electron Source
Laser
GaAs Crystal
Pockel Cell flips helicity
Gun
Halfwave plate
(retractable, reverses helicity) -
e beam
Based on Photoemission from GaAs Crystal
Polarized electrons from polarized laser
Need :
Rapid, random helicity reversal
Electrical isolation from the rest of the lab
Feedback on Intensity Asymmetry

26. P I T A Effect Intensity Asymmetry
Important Systematic :
(Gordon Cates)
Polarization Induced Transport Asymmetry
Intensity Asymmetry
Laser at Pol. Source
where
Transport Asymmetry
drifts, but slope is ~ stable Feedback on 26

27. Methods to Reduce Systematics
(Gordon Cates, Kent Paschke, Mark Dalton, Rupesh Silwal )
A simplified picture: asymmetry=0 corresponds to minimized DoLP at analyzer
Perfect DoCP
Intensity Asymmetry (ppm)
Pockels cell voltage D offset (V)
Scanning the Pockels Cell voltage = scanning the residual linear polarization (DoLP)
A rotatable l/2 waveplate downstream of the P.C. allows arbitrary orientation of the ellipse from DoLP

28. Intensity Feedback Adjustments for small phase shifts to make close to
circular polarization
Low jitter and high accuracy allows sub-ppm
cumulative charge asymmetry in ~ 1 hour
~ 2 hours 28

29. Flips spin without moving the beam !
Double Wien Filter
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 29

30. 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 31

31. 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)

32. Compton Polarimeter Upgrade for 1% accuracy at 1 GeV electrons
to measure electron beam’s polarization (needed to normalize asymmetry)
electrons
Upgrade for 1% accuracy at 1 GeV
Green Laser (increased sensitivity at low E)
Integrating Method (removes some systematics of analyzing power)
New Photon & Electron Detectors

33. Compton Polarimeter Results
PREX
Compton Polarimeter Results

34. 1 % Accuracy in Polarization
Upgraded for PREX
Moller Polarimeter
Superconducting Magnet from Hall C
Saturated Iron Foil Targets
1 % Accuracy in Polarization
Magnet and Target
Electronics/DAQ Upgrade (FADC)

35. 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) 35

36. Measure θ from Nuclear Recoil
( Nilanga Liyanage, Kiadtisak Saenboonruang)
δE=Energy loss
E=Beam energy
MA=Nuclear mass
θ=Scattering angle
(these data taken during HAPPEX)
Scattered Electron Energy (GeV)
Recoil is large for H, small for nuclei
(3X better accuracy than survey)

37. 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.

38. PREX Integrating Detectors
Detector Package in HRS
PREX Integrating Detectors
UMass / Smith
DETECTORS

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

40. 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.

41. + - 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 41

42. PREX-I Result Statistics limited ( 9% )
Systematic Errors
Error Source
Absolute (ppm)
Relative ( % )
Polarization (1)
0.0083
1.3
Beam Asymmetries (2)
0.0072
1.1
Detector Linearity
0.0076
1.2
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
0.0140
2.1
Physics Asymmetry
Statistics limited ( 9% )
Systematic error goal achieved ! (2%)
A physics letter was recently accepted by PRL.
arXiv [nucl-ex]
(1) Normalization Correction applied
(2) Nonzero correction (the rest assumed zero) 42

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

44. * * Asymmetry leads to RN
Establishing a neutron skin at ~95 % CL *
Neutron Skin = RN - RP = fm
fig from C.J. Horowitz
PREX data *
Interpretation requires the acceptance function for spectrometer:

45. PREX-I Result, cont. Neutron Skin = RN - RP = 0.33 + 0.16 - 0.18 fm
DATA
rN - rP (fm)
theory: P. Ring
rN = rP
Atomic Number, A
DATA
A physics letter was recently accepted by PRL.
arXiv [nucl-ex] 46

46. PREX-II Approved by PAC (Aug 2011)
“A” Rating days to run in 2013 or 2014
PREX-II: Kent Paschke, Krishna Kumar, Paul Souder, Guido Urciuoli, Robert Michaels

47. Recent Rn Predictions Can Be Tested By PREX at Full Precision
PREX could provide an electroweak complement to Rn predictions from a wide range of physical situations and model dependencies
Recent Rn predictions:
Hebeler et al. Chiral EFT calculation of neutron matter. Correlation of pressure with neutron skin by Brown. Three- neutron forces!
Steiner et al. X-Ray n-star mass and radii observation + Brown correlation. (Ozel et al finds softer EOS, would suggest smaller Rn).
Tamii et al. Measurement of electric dipole polarizability of 208Pb + model correlation with neutron skin.
Tsang et al. Isospin diffusion in heavy ion collisions, with Brown correlation and quantum molecular dynamics transport model.
PREX-II
proposed
Hebeler
Steiner
Tamii
Tsang
d(APV)/APV ~ 3%
d(Rn)/Rn ~ 1%
These can be tested with

48. Septum Magnet Improvements for PREX-II Tungsten Collimator & Shielding
Region downstream of target
Tungsten Collimator & Shielding
HRS-L Q1
Septum Magnet
target
HRS-R Q1
Location of ill-fated O-Ring
which failed & caused significant
time loss during PREX-I
 PREX-II to use all-metal seals
Collimators

49. Other Nuclei ? RN Surface thickness RN Surface thickness After PREX …
and Shape Dependence ?
each point 30 days
Parity Violating Electron Scattering Measurements of Neutron Densities
Shufang Ban, C.J. Horowitz, R. Michaels RN
Surface thickness
J. Phys. G

50. Possible Future PREX Program ?
Each point 30 days stat. error only
Nucleus
E (GeV)
dRN / RN
comment
208Pb 1
1 %
PREX-II (approved by Jlab PAC, A rating)
48Ca
2.2 (1-pass)
0.4 %
natural 12 GeV exp’t
will next PAC
2.6
2 %
surface thickness
40Ca
0.6 %
basic check of theory
tin isotope
1.8
apply to heavy ion
1.6 %
Not proposed
Shufang Ban, C.J. Horowitz, R. Michaels
J. Phys. G

51. UVa Participants in Jlab Parity-Violation & PREX
Gordon Cates
Kent Paschke
Nilanga Liyanage
Xiaochao Zheng
PREX-II spokesperson
Mark Dalton
Diancheng Wang
Rupesh Silwal
Kiadtisak Saenboonruang
Thesis on PREX-I
Also: Chao Gu, Xiaoyan Deng, Ge Jin, Richard Lindgren, Vladimir Nelyubin, Seamus Riordan,
Ramesh Subedi, Al Tobias

52. 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 )

53. Extra Slides

54. Geant 4 Radiation Calculations PREX-II shielding strategies
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) 49

55. Pull Plot (example)
PREX Data

56. 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

57. 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

58. Source Studies
Charge Asymmetry
~ 2000 ppm
Kent Paschke, Gordon Cates, Mark Dalton, Rupesh Silwal
Optimizing laser optics to minimize helicity-correlated systematics.
~ 0.5 um
Hel. Correl. Diff (X)
Transmission of Helicity-Correlated Position DIffs
Hel. Correl. Diff (X)
Hel. Correl. Diff (Y)
Delta X (um)
~ 0.5 um
BPMs in Injector Region
Hel. Correl. Diff (Y)
Delta Y (um)

59. 16O Water Cell : Measure Hydrogen (agrees with survey)
Nilanga Liyanage, Seamus Riordan, Kiadtisak Saenboonruang,
16O
Hydrogen

60. Pockel Cell Related Systematic Error
wait
integrate
wait
An instability in Pockel Cell “bleeds” into the itegration gate. It depends on helicity.
Beam Current
Detector (1 of 4)
Response to pulsed beam
time
time
Want small time constants, and same for detectors and bcm

61. 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

62. Collimators inside Q1
Symmetry in all dimensions to 1 mm

63. Pb Radius vs Neutron Star Radius
(slide from C. Horowitz)
Pb Radius vs Neutron Star Radius
The 208Pb radius constrains the pressure of neutron matter at subnuclear densities.
The NS radius depends on the pressure at nuclear density and above.
Most interested in density dependence of equation of state (EOS) from a possible phase transition.
Important to have both low density and high density measurements to constrain density dependence of EOS.
If Pb radius is relatively large: EOS at low density is stiff with high P. If NS radius is small than high density EOS soft.
This softening of EOS with density could strongly suggest a transition to an exotic high density phase such as quark matter, strange matter, color superconductor, kaon condensate…

64. PREX Constrains Rapid Direct URCA Cooling of Neutron Stars
(slide from C. Horowitz)
PREX Constrains Rapid Direct URCA Cooling of Neutron Stars
Proton fraction Yp for matter in beta equilibrium depends on symmetry energy S(n).
Rn in Pb determines density dependence of S(n).
The larger Rn in Pb the lower the threshold mass for direct URCA cooling.
If Rn-Rp<0.2 fm all EOS models do not have direct URCA in 1.4 M¯ stars.
If Rn-Rp>0.25 fm all models do have URCA in 1.4 M¯ stars.
Rn-Rp in 208Pb
If Yp > red line NS cools quickly via direct URCA reaction n p+e+

65. 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.

66. Weak Interaction observed not observed
1930’s - The weak nuclear interaction was needed to explain nuclear beta decay
Contact interaction with charge exchanged
or, mediated by a heavy, charged boson
1950’s - Discovery of parity-violation by the weak interaction
Weak decay of
60Co Nucleus
60Co
60Ni
Left
Right
W Charge
zero
V-A theory described W’s as only interacting with left-handed particles!
observed
60Co
60Ni L R
right-handed
anti-neutrino
left-handed
not observed