Lead ( Pb) Radius Experiment : PREX

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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

208Pb
2
PREX in Hall A at JLab
Spectometers
Lead Foil Target
3
Idea behind PREX
0
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)
4
A piece of the weak interaction violates
parity (mirror symmetry) which allows to
isolate it.
Incident electron
S
(spin)
Target
Positive longitudinal spin
P
(momentum)
Parity Transformation
208
Pb
Negative longitudinal spin
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• Parity Violating Asymmetry

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Applications of PV at Jefferson Lab
Applications of PV at Jefferson Lab

• Nucleon Structure (strangeness) -- HAPPEX /
  G0
• Standard Model Tests ( ) --
  e.g. Qweak
• Nuclear Structure (neutron density) PREX

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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 nucleus 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
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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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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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Heavy Ions (adapted from Betty Tsang, PREX
Workshop)
Isospin Diffusion (NSCL)
Probe the symmetry energy in 124Sn 112Sn
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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)
PREX error bar
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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
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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 )

2
Isotope Chain Experiments e.g. Berkeley Yb
APV
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Neutron Stars
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 )
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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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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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

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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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PREX Experiment Design
Spokespersons K. Kumar Univ. Mass.
P.A. Souder Syracuse U. G.M.
Urciuoli INFN Rome R. Michaels JLab
Hall A Collaboration Experiment
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PREX in Hall A at JLab
Spectometers
Lead Foil Target
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Hall A at Jefferson Lab
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Hall A
Spectro SQQDQ
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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)
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50 Septum magnet augments the High
Resolution Spectrometers Increased Figure of
Merit
HRS-L
HRS-R
Septum Magnet
collimator
target
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Experimental Method
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Previous experience in Parity Violation
HAPPEX Results
Hydrogen target
Raw Parity Violating Asymmetry
Araw correction 11 ppb
Helicity Window Pair Asymmetry
Q2 0.1089 0.0011GeV2 Araw -1.418 ppm ?
0.105 ppm (stat)
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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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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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Polarized Electron Source
Laser
GaAs Crystal
Halfwave plate (retractable, reverses
helicity)
Pockel Cell flips helicity
Gun
-
e beam

• Rapid, random helicity reversal
• Electrical isolation from rest of 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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Intensity Feedback
Adjustments for small phase shifts to make close
to circular polarization
HAPPEX
Low jitter and high accuracy allows
sub-ppm cumulative charge asymmetry in 1 hour
2 hours
In practice, aim for 0.1 ppm over duration of
data-taking.
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Methods to Reduce Systematics
Scanning the Pockels Cell voltage scanning the
retardation phase scanning residual DoLP
Voltage change of 58 Volts, added to both the
and - voltages, would zero the asymmetry.
A rotatable l/2 waveplate downstream of the P.C.
allows arbitrary orientation of DoLP
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Rotatable Half-Wave Plate
Add ?/2 plate to minimize analyzing power
Electron beam intensity asymmetry (ppm)
4q term measures analyzing powerDoLP (from
Pockels cell)
Rotating waveplate angle
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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
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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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The Corrections Work !
Shown period of data during HAPPEX
(4He) when beam had a helicity-correlated
position due to a mistake in
electronics.
X Angle BPM
With corrections
micron
Helicity-corr. Position diff
ppm
Raw ALL Asymetry
Helicity signal to driver reversed
Helicity signal to driver removed
The mistake Helicity signal deflecting the
beam through electronics pickup
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Final Beam Position Corrections (HAPPEX-H)
Energy -0.25 ppb X Target 1 nm X Angle 2
nm Y Target 1 nm Y Angle lt1 nm
Beam Asymmetry Results
micron
Corrected and Raw, Left spectrometer arm alone,
Superimposed!
Total correction for beam position asymmetry on
Left, Right, or ALL detector 10 ppb
ppm
Spectacular results from HAPPEX-H show we
can do PREX.
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Redundant position measurements at the 1
nm level
(i) Stripline monitors (2 pairs of wires)
(ii) Resonant microwave cavities
(Helicity correlated differences averaged
over 1 day) HAPPEX 2005
X (cavity) nm
Y (cavity) nm
X (stripline) nm
Y (stripline) nm
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Integrating Detector

• Integrate in 30 msec helicity period.
• Deadtime free.
• 18 bit ADC with lt 10-4 nonlinearity.
• But must separate backgrounds inelastics
  ( HRS).

Quartz / Tungsten Calorimeter
Integrator
UMass / Smith
ADC
PMT
(Also a thin quartz detector upstream of
this.)
electrons
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Successful Results of beam tests Jan 2008
Good energy resolution
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Lead Target
(2 x I in proposal)
208
Pb
Liquid Helium Coolant
12
beam
C
Diamond Backing

• High Thermal Conductivity

• Negligible Systematics

Beam, rastered 4 x 4 mm
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Target Assembly
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Lead Target Tests
208
Pb Elastic
Low Rate at E 1.1 GeV
Detector
Num. events
1st Excited State (2.6 MeV)

• Check rates
• Backgrounds (HRS is clean)
• Sensitivity to beam parameters
• Width of asymmetry
• HRS resolution
• Detector resolution

Momentum (MeV)
Num. events
X (dispersive coord) (m)
Y (m)
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Noise (integrating at 30 Hz)

• PREX 120 ppm
• Want only counting statistics noise, all
  others ltlt 120 ppm
• New 18-bit ADCs
• ? improve beam noise.
• Careful about cable runs, PMTs,
  grounds loops.
• .

(HAPPEX data)
Test Use the Luminosity Monitor to
demonstrate noise (next slide)
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Luminosity Monitor
A source of extremely high rate ?
establish noise floor
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Asymmetries in Lumi Monitors after
beam noise subtraction
50 ppm noise per pulse ?
milestone for electronics
( need ltlt 120 ppm)
Jan 2008 Data
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Upgrade of Compton Polarimeter
S. Nanda
electrons
To reach 1 accuracy

• Green Laser (increased sensitivity at low
  E)
• Integrating Method (removes some
  systematics of analyzing power)
• New Photon and Electron Detectors

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Polarimetry Accuracy 1 from 2 methods
Møller (New High-field design ) Compton
HAPPEX Compton Polarimety Measuremens
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PREX Summary

• Fundamental Nuclear Physics with many
  applications
• HAPPEX test runs have demonstrated
  technical aspects
• Polarimetry Upgrade critical
• 1 month run in early 2010

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Extra Slides
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At 50 the new Optimal FOM is at 1.05 GeV
(/- 0.05)
(when accounting for collimating small
angle, not shown)
1 _at_ 1 GeV
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What if Scenarios
Assuming other systematics
are negligible
dPe/Pe 1 dPe/Pe 2
50 uA 30 days dRN/RN ? 1 1.2
100 uA 60 days dRN/RN -? 0.6 0.8
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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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Optimization for Barium -- of possible
direct use for Atomic PV
1 GeV optimum
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Warm Septum
Existing superconducting septum wont work
at high L
Warm low energy (1 GeV) magnet
designed. Grant proposal in preparation
(100 k) Syracuse / Smith College
TOSCA design P resolution ok