Lead ( Pb) Radius Experiment : 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

• Parity Violating Asymmetry

2

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

3
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
4
Reminder Electromagnetic Scattering
determines
(charge distribution)
208
Pb
1
2
3
5
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
6
PREX
2
Measurement at one Q is sufficient to
measure R
N
( R.J. Furnstahl )
Why only one parameter ? (next slide)
PREX error bar
7
PREX
pins down the symmetry energy (1 parameter)
energy cost for unequal protons
neutrons
PREX error bar
( R.J. Furnstahl )
208
Pb
PREX
8
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
9
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
10
Liquid/Solid Transition Density
Neutron EOS and Neutron Star Crust
( C.J. Horowitz, J. Piekarweicz )

• 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.
11
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

12
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?
13

• Impact on 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
14
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
15
PREX Experimental Issues
Spokespersons P.A. Souder, G.M. Urciuoli,
R. Michaels
Hall A Collaboration Experiment
16
PREX in Hall A at JLab
Spectometers
Lead Foil Target
17
Hall A at Jefferson Lab
18
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
19
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

20
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
21
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.
22
Beam Position Corrections (HAPPEX)
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.
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
The Raw Asymmetry
25
Application of Parity Violating Electron
Scattering
HAPPEX Strange Quarks
Hall A Proton Parity Experiment
4
Isolating the u, d, s quark structure
in protons (and He)
Electromagnetic Scattering
Parity Violation can Access
26
1H Preliminary 2006 Results
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)
27
Strange FF near 0.1 GeV2
GMs 0.28 /- 0.20 GEs -0.006 /- 0.016 3
/- 2.3 of proton magnetic moment 0.2 /- 0.5
of electric distribution
Preliminary
28
Polarimetry Accuracy 2 required, 1
desired
Preliminary
Møller dPe/Pe 3 (limit foil
polarization) (a high field target ala Hall
C being considered) Compton
2 syst. at present
Prelim. HAPPEX Results
29
Upgrade of Compton Polarimeter
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

30
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
31
Lead Target
208
Pb
Liquid Helium Coolant
12
beam
C
Diamond Backing

• High Thermal Conductivity

• Negligible Systematics

Beam, rastered 4 x 4 mm
32
PREX Summary

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

33
Extra Slides
34
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
35
4He Preliminary 2006 Results
Raw Parity Violating Asymmetry
Araw correction 0.12 ppm
Helicity Window Pair Asymmetry
Q2 0.07725 0.0007 GeV2 Araw 5.253 ppm ?
0.191 ppm (stat)
36
Optimization for Barium -- of possible
direct use for Atomic PV
1 GeV optimum
37
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)
38
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.
39
Example Recent Pion Photoproduction
B. Krusche arXivnucl-ex/0509003 Sept 2005
This paper obtains
!!
Proton Nucleus Elastic
Mean Field Theory
PREX accuracy
40
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.
41
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
42
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.

43
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
44
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 )