Lead ( Pb) Radius Experiment : PREX

1
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 2010 in Hall A at
Jefferson Lab
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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
A piece of the weak interaction violates
parity (mirror symmetry) which allows to
isolate it.
Parity Transformation
1800 rotation
Negative spin
Positive spin
4
How to use the

• Parity Violating Asymmetry ?

2

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
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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6
Hall A at Jefferson Lab
Hall A
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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
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 ?
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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 0
Weak charge 0.08 1
Nucl. Phys. A 503, 589, 1989
C. J. Horowitz, S. J. Pollock, P. A.
Souder, R. Michaels
Phys. Rev. C 63, 025501, 2001
Neutron form factor
C.J. Horowitz
Parity Violating Asymmetry
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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
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Example Heavy Ions (adapted from Betty
Tsang, PREX Workshop, 2008)
Isospin Diffusion (NSCL)
Probe the symmetry energy in 124Sn 112Sn
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Using Parity Violation
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
2
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
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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
E/N
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Thanks, Alex Brown PREX Workshop 2008
Skx-s25
E/N
8
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• 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
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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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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.
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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
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PREX Setup
Parity The entire lab is the experiment
Spectometers
Lead Foil Target
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How to do a Parity Experiment
(integrating method)
Example HAPPEX
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Polarized Electron Source
Laser
GaAs Crystal
Pockel Cell flips helicity
Halfwave plate (retractable, reverses
helicity)
Gun
-
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

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P I T A Effect
Important Systematic
Polarization Induced Transport Asymmetry
Intensity Asymmetry
Laser at Pol. Source
where
Transport Asymmetry
drifts, but slope is stable. Feedback
on
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Methods to Reduce Systematics
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
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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
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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
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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 ! ( why we love
Jlab ! ) Helicity Correlated Position
Differences lt 3 nm
Points Not sign corrected
Average with signs what expt feels
Units microns
Slug ( 1 day)
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Compton Polarimeter
to measure electron beams 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

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PREX
Compton Polarimeter Results
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Upgraded for PREX
Moller Polarimeter Superconducting Magnet
from Hall C Saturated Iron Foil Targets
lt 1 Polarimetry
Magnet and Target
Electronics/DAQ Upgrade (FADC)
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Hall A High Resolution Spectrometers

• Resolve Elastic Scattering
• Discriminate Excited States

Elastic
detector
Inelastic
Pure, Thin 208 Pb Target
2.6 MeV
target
Dipole
DETECTOR footprint
Quads
Scattered Electrons Momentum (GeV/c)
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Measure ? from Nuclear Recoil
dEEnergy loss EBeam energy MANuclear
mass ?Scattering angle
Scattered Electron Energy (GeV)
Recoil is large for H, small for nuclei
(3X better accuracy than survey)
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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.
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PREX Integrating Detectors
Detector Package in HRS
UMass / Smith
DETECTORS
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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
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

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PREX-I Result
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
Physics Asymmetry

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

(1) Normalization Correction applied
(2) Nonzero correction (the rest assumed zero)
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PREX Asymmetry (Pe x A)
ppm
Slug 1 day
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Asymmetry leads to RN
Establishing a neutron skin at 95 CL

Neutron Skin RN - RP 0.34 0.16 -
0.18 fm
fig from C.J. Horowitz
PREX data

Interpretation requires the acceptance
function for spectrometer
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Neutron Skin RN - RP 0.34 0.16 -
0.18 fm
PREX-I Result, cont.
DATA
rN - rP (fm)
theory P. Ring
rN rP
Atomic Number, A
DATA
A physics letter is in preparation for
publication. (early 2012)
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PREX-II Approved by PAC (Aug 2011)
A Rating 35 days to run in 2013 or
2014
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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
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Improvements for PREX-II
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
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After PREX
Other Nuclei ?
RN
and Shape Dependence ?
Surface thickness
Parity Violating Electron Scattering Measurements
of Neutron Densities Shufang Ban, C.J.
Horowitz, R. Michaels
RN
Surface thickness
J. Phys. G39 014104 2012
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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 expt will propose _at_ next PAC
48Ca 2.6 2 surface thickness
40Ca 2.2 (1-pass) 0.6 basic check of theory
tin isotope 1.8 0.6 apply to heavy ion
tin isotope 2.6 1.6 surface thickness
Not proposed
J. Phys. G39 014104 2012
Shufang Ban, C.J. Horowitz, R. Michaels
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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 2013 or 2014 )

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Extra Slides
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Geant 4 Radiation Calculations PREX-II
shielding strategies
scattering chamber
shielding
Number of Neutrons per incident Electron
0 - 1 MeV
beamline
Energy (MeV)
--- PREX-I --- PREX-II, no shield ---
PREX-II, shielded
1 - 10 MeV

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

Energy (MeV)
10 - 1200 MeV
Energy (MeV)
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Pull Plot (example)
PREX Data
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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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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)
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Water Cell Measure
(agrees with survey)
Nilanga Liyanage, Seamus Riordan, Kiadtisak
Saenboonruang,
16O
Hydrogen
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Pockel Cell Related Systematic Error
integrate
wait
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
60
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
61
Collimators inside Q1
Symmetry in all dimensions to 1 mm
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Pb Radius vs Neutron Star Radius
(slide from C. Horowitz)

• 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

63
PREX Constrains Rapid Direct URCA Cooling of
Neutron Stars
(slide from C. Horowitz)

• 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-Rplt0.2 fm all EOS models do not have
  direct URCA in 1.4 M stars.
• If Rn-Rpgt0.25 fm all models do have URCA in
  1.4 M stars.

Rn-Rp in 208Pb
If Yp gt red line NS cools quickly via direct URCA
reaction n pe?
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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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Weak Interaction
1930s - The weak nuclear interaction was needed
to explain nuclear beta decay
Contact interaction with charge exchanged or,
mediated by a heavy, charged boson