PREX

1
Lead ( Pb) Radius Experiment PREX
208
E 850 MeV, electrons on lead
Elastic Scattering Parity Violating Asymmetry
0
Z of Weak Interaction
Clean Probe Couples Mainly to Neutrons
( T.W. Donnelly, J. Dubach, I Sick )
In PWIA (to illustrate)
208Pb
w/ Coulomb distortions (C. J. Horowitz)
2
Impact on Nuclear PhysicsWhat is the
size of a nucleus ?
Is the size of a heavy nucleus determined
by neutrons or by protons ?
3
Reminder Electromagnetic Scattering
determines
(charge distribution)
208
Pb
1
2
3
4
Z of weak interaction sees the neutrons
0
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
5
Neutron Densities

• Proton-Nucleus Elastic
• Pion, alpha, d Scattering
• Pion Photoproduction
• Magnetic scattering
• Theory Predictions

Involve strong probes
Most spins couple to zero.
Fit mostly by data other than neutron
densities
Therefore, PREX is a powerful check of
nuclear theory.
6
Electron - Nucleus Potential
axial
electromagnetic
is small, best observed by
parity violation
neutron weak charge gtgt proton weak charge
Neutron form factor
Proton form factor
Parity Violating Asymmetry
7
PREX
2
Measurement at one Q is sufficient to
measure R
N
( R.J. Furnstahl )
Why only one parameter ? (next slide)
PREX error bar
8
PREX
pins down the symmetry energy (1 parameter)
energy cost for unequal protons
neutrons
PREX error bar
( R.J. Furnstahl )
208
Pb
PREX
9
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
Impact on Neutron Stars
What is the nature of extremely dense
matter ? Do collapsed stars form exotic
phases of matter ?
11
PREX Neutron Stars
( C.J. Horowitz, J. Piekarweicz )
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
12
Liquid/Solid Transition Density
Neutron EOS and Neutron Star Crust

• Thicker neutron skin in Pb means energy rises
  rapidly with density ? Quickly favors uniform
  phase.
• Thick skin in Pb ? low transition density in
  star.

Fig. from J.M. Lattimer M. Prakash,
Science 304 (2004) 536.
13
Pb Radius vs Neutron Star Radius
( C.J. Horowitz, J. Piekarweicz )

• 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

14
PREX Constrains Rapid Direct URCA Cooling of
Neutron Stars
( C.J. Horowitz, J. Piekarweicz )

• 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?
15
Impact on Atomic Parity
Measures atomic overlap with weak
charge. Neutrons carry most weak charge
16

• Atomic Parity Violation
• Low Q test of Standard Model
• Needs R to make further progress.

2
Isotope Chain Experiments e.g. Berkeley Yb
N
APV
17
Measured Asymmetry
PREX
Physics Impact
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
Heavy
Ions
R
n
18
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
19
PREX Experimental Issues
Spokespersons P.A. Souder, G.M. Urciuoli,
R. Michaels
Hall A Collaboration Experiment
20
PREX in Hall A at JLab
Spectometers
Lead Foil Target
21
Hall A at Jefferson Lab
22
High Resolution Spectrometers
Spectrometer Concept Resolve Elastic
Elastic
detector
Inelastic
Left-Right symmetry to control transverse
polarization systematic
Quad
target
Dipole
Q Q
23
Integrating Detection

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

- 4
Integrator
Calorimeter (for lead, fits in palm of hand)
ADC
PMT
electrons
24
Optimum Kinematics for Lead Parity E
850 MeV,
ltAgt 0.5 ppm. Accuracy in Asy 3
Fig. of merit
Min. error in R maximize
n
1 month run 1 in R
n
25
Optimization for Barium -- of possible
direct use for Atomic PV
1 GeV optimum
26
Beam Asymmetries
Araw Adet - AQ ??E ??i?xi

• natural beam jitter (regression)
• beam modulation (dithering)

Slopes from
27
Helicity Correlated Differences Position,
Angle, Energy
Scale /- 10 nm
BPM X1
slug
Spectacular results from HAPPEX-H show we
can do PREX.
BPM X2
slug

• Position Diffs average to 1 nm
• Good model for controlling laser
  systematics at source
• Accelerator setup (betatron matching, phase
  advance)

BPM Y1
slug
BPM Y2
slug
Energy BPM
slug 1 day running
28
Redundant Position Measurements at the 1
nm level
(Helicity correlated differences averaged
over 1 day)
X (cavity) nm
Y (cavity) nm
X (stripline) nm
Y (stripline) nm
29
Lead Target
208
Pb
Liquid Helium Coolant
12
beam
C
Diamond Backing

• High Thermal Conductivity

• Negligible Systematics

Beam, rastered 4 x 4 mm
30
208
Pb Elastic
Lead Target Tests
Data taken Nov 2005
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)
31
Polarimetry
PREX 1 desirable 2 required
Møller dPe/Pe 3 (limit foil
polarization) (a high field target ala
Hall C being considered) Compton
2 syst. at present
2 analyses based on either electron or photon
detection
Superlattice Pe86 !
32
Upgrade of Compton Polarimeter (Nanda,
Lhuillier)
in 1.5 years
electrons
To reach 1 accuracy

• Green Laser (increased sensitivity at low
  E)
• ? laser on-hand, being tested
• Integrating Method (removes some
  systematics of analyzing power)
• ? developed during HAPPEX in
  2006
• New Photon Detector

33
PREX Summary

• Fundamental Nuclear Physics with many
  applications
• HAPPEX test runs have demonstrated
  technical aspects
• Polarimetry Upgrade needed
• Will run 1 month, perhaps in 2008

34
Neutron Skin and Heavy Ion Collisions

• Impact on Heavy - Ion physics
  constraints and predictions
• Imprint of the EOS left in the flow
  and fragmentation distribution.

Danielewicz, Lacey, and Lynch, Science 298
(2002) 1592.
35
Example Recent Pion Photoproduction
B. Krusche arXivnucl-ex/0509003 Sept 2005
This paper obtains
!!
Proton Nucleus Elastic
Mean Field Theory
PREX accuracy
36
Transverse Polarization
Part I Left/Right Asymmetry
Transverse Asymmetry
Systematic Error for Parity
Theory est. (Afanasev)
Error in
Left-right apparatus asymmetry
Transverse polarization
Control w/ slow feedback on
polarized source solenoids.
measure in 1 hr ( 8 hr setup)
HRS-Left
HRS-Right
lt
lt
Need
correction
syst. err.
37
Transverse Polarization
Part II Up/Down Asymmetry
Vertical misalignment
Systematic Error for Parity
Horizontal polarization e.g. from (g-2)

• Measured in situ using 2-piece
  detector.
• Study alignment with tracking M.C.
• Wien angle feedback ( )

up/down misalignment
Need
HRS-Left
HRS-Right
lt
lt
)
( Note, beam width is very tiny
38
Noise

• Need 100 ppm per window pair
• Position noise already good enough
• New 18-bit ADCs
• ? Will improve BCM noise.
• Careful about cable runs, PMTs, grounds.
• ? Will improve detector noise.
• Plan Tests with Luminosity Monitor
• to demonstrate capability.

39
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
40
2
Measurement at one Q is sufficient to
measure R
Pins down the symmetry energy (1 parameter)
N
PREX accuracy

PREX accuracy
( R.J. Furnstahl )