Lead (
208
Pb) Radius Experiment :
E = 850 MeV,   6 0
electrons on lead
PREX
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) :
 d 
 d 

 

GF Q 2
 d  R  d  L
A 

2 2
d

d






 

 d  R  d  L
F n (Q 2 )


2
1

4
sin




W
FP (Q 2 ) 

0
208Pb
w/ Coulomb distortions (C. J. Horowitz) :
dA
 3% 
A
PREX
UVa Seminar, Nov 2005
dRn
 1%
Rn
R. Michaels
Jefferson Lab
A piece of the weak interaction
violates parity (mirror symmetry)
which allows to isolate it.
Incident electron
Positive
longitudinal spin
S (spin)
Target
P
(momentum)
208
Pb
Negative
longitudinal spin
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Parity Violating Asymmetry APV
R L

~ 10 6
R L
2


e
+
Z0
e
Applications of PV :
• Nucleon Structure (strangeness) -- HAPPEX / G0
• Standard Model Tests ( sin 2 W ) -- e.g. Qweak
• Nuclear Structure (neutron density) : PREX
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Measured Asymmetry
PREX
Physics
Impact
Correct for Coulomb
Distortions
Weak Density at one Q 2
Mean Field
& Other
Models
Small Corrections for
Atomic
Parity
Violation
G
n
E
s
GE
MEC
2
Neutron Density at one Q
Assume Surface Thickness
Good to 25% (MFT)
Neutron
Stars
Rn
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
PREX in Hall A at JLab
Spectometers
Lead Foil
Target
Pol. Source
Hall A
CEBAF
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Impact on Nuclear Physics:
What is the size of a
nucleus ?
Is the size of a heavy nucleus
determined by neutrons or by protons ?
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Reminder: Electromagnetic Scattering determines
 r 
(charge distribution)
208
Pb
 r 
d  mb 


d  str 
q
PREX
UVa Seminar, Nov 2005
 fm1
R. Michaels
Jefferson Lab
0
Z of weak interaction : sees the
neutrons
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
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Electron - Nucleus Potential
Vˆ (r )  V (r )   5 A(r )
axial
electromagnetic
/
V (r )   d r Z  (r ) | r  r |
3
/
/
A(r ) 
FP (Q 2 ) 
1
4
3
d
 r j0 (qr )  P (r )
2 2
(1  4 sin
2
 W ) Z  P (r )  N  N (r )
A(r ) is small, best observed
by parity violation
d d

| FP (Q 2 ) | 2
d d Mott
Proton form factor
GF
1  4 sin 2  W  1 neutron weak
charge >> proton weak charge
Neutron form factor
FN (Q 2 ) 
1
4
3
d
 r j0 (qr )  N (r )
Parity Violating Asymmetry
 d 
 d 

 

GF Q 2
d


 R  d  L
A 

2 2
 d 
 d 

 

 d  R  d  L
PREX
UVa Seminar, Nov 2005

FN (Q 2 ) 
2
 1  4 sin W 

2
F
(
Q
)
P


0
R. Michaels
Jefferson Lab
Neutron Densities
• Proton-Nucleus Elastic
Involve strong probes
• Pion, alpha, d Scattering
• Pion Photoproduction
Most spins couple to zero.
• Magnetic scattering
• Theory Predictions
Fit mostly by data other than
neutron densities
Therefore, PREX is a powerful
check of nuclear theory.
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Example : Recent Pion Photoproduction
B. Krusche
arXiv:nucl-ex/0509003
Sept 2005
This paper obtains
RN
RP !!
Proton – Nucleus Elastic:
0.083  RN  RP  0.111 fm
Mean Field Theory
0.05  RN  RP  0.35 fm
PREX accuracy
 RN   0.05 fm
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
PREX:
2
Measurement at one Q is sufficient to measure R
N
( R.J. Furnstahl )
Why only one
parameter ?
(next slide…)
PREX
error bar
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
PREX: pins down the symmetry energy
E
 N Z 
  av  a 4 

A
A


( R.J. Furnstahl )
2
 as / A
1/ 3
 ...
(1 parameter)
energy cost for unequal #
protons & neutrons
PREX
error
bar
208
Pb
PREX
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Impact on Atomic Parity
Measures atomic overlap with weak charge.
Neutrons carry most weak charge
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Atomic Parity Violation
2
• Low Q test of Standard Model
Isotope Chain Experiments
e.g. Berkeley Yb
• Needs R N to make further progress.
H PNC 
 N

2
GF
2
 N (r )  Z (1  4 sin 2  W )  P (r )  e/  5  e d 3 r


0
APV
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Impact on
Neutron Stars
What is the nature of extremely dense matter ?
Do collapsed stars form “exotic” phases of matter ?
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Inputs:
Eq. of state (EOS)
P(  )
PREX constraint
Hydrostatics (Gen. Rel.)
Typ. Astro. Observations
Luminosity L
Temp. T
Mass M from pulsar timing
L  4 B R 2 T 4
(with corrections … )
Mass - Radius relationship
Fig. from
J.M. Lattimer & M. Prakash,
Science 304 (2004) 536.
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
PREX & Neutron Stars
( C.J. Horowitz, J. Piekarweicz )
R N calibrates EOS of
Neutron Rich Matter
Crust Thickness
Explain Glitches in Pulsar Frequency ?
Combine PREX R N with
Obs. Neutron Star Radii
Phase Transition to “Exotic” Core ?
Strange star ? Quark Star ?
Some Neutron Stars
seem too Cold
Cooling by neutrino emission (URCA)
Crab Pulsar
PREX
UVa Seminar, Nov 2005
Rn  R p  0.2 fm
URCA probable, else not
R. Michaels
Jefferson Lab
Neutron Star Crust vs
Pb Neutron Skin
Liquid/Solid Transition Density
Liquid
FP
Neutron
Star
208Pb
PREX calibrates the EOS at
subnuclear densities.
PREX
UVa Seminar, Nov 2005
Solid
TM1
• Thicker neutron skin in Pb means
energy rises rapidly with density 
Quickly favors uniform phase.
• Thick skin in Pb  low transition
density in star.
R. Michaels
Jefferson Lab
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…
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
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+
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
PREX: Experiment Design
Spokespersons:
P.A. Souder, G.M. Urciuoli, R. Michaels
Hall A Collaboration Experiment
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Hall A at Jefferson Lab
Polarized eSource
PREX
UVa Seminar, Nov 2005
Hall A
R. Michaels
Jefferson Lab
Hall A
Cherenkov
cones
PMT
Polarimeters
Compton
Moller
Target
PREX
UVa Seminar, Nov 2005
Spectro: SQQDQ
R. Michaels
Jefferson Lab
High Resolution Spectrometers
Spectrometer Concept:
Resolve Elastic
Elastic
detector
Inelastic
Quad
target
Dipole
PREX
UVa Seminar, Nov 2005
Q Q
R. Michaels
Jefferson Lab
Optimum Kinematics for Lead Parity:
<A> = 0.5 ppm.
E = 850 MeV,
Accuracy in Asy 3%
Fig. of merit
Min. error in Rn
maximize:
1 month run
1% in R n
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Corrections to the Asymmetry are
Mostly Negligible
Horowitz, et.al. PRC 63 025501
• Coulomb Distortions ~20% = the biggest correction.
• Strangeness
• Electric Form Factor of Neutron
• Parity Admixtures
• Dispersion Corrections
• Meson Exchange Currents
• Shape Dependence
• Isospin Corrections
• Radiative Corrections
• Excited States
• Target Impurities
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Septum Magnets (INFN)
•Superconducting magnets
•Commissioned 2003-4
Electrons scattered
at 6 deg sent to the
HRS at 12.5 deg.
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Integrating Detection
• Integrate in 30 msec helicity period.
• Deadtime free.
• 18 bit ADC with < 10
•
-4
But must separate backgrounds & inelastics (
HRS).
Integrator
Calorimeter (for lead, fits in palm of hand)
PMT
PREX
UVa Seminar, Nov 2005
nonlinearity.
ADC
R. Michaels
Jefferson Lab
Lead Target
208
Pb
Successful ly tested at 80 A
Liquid Helium
Coolant
12
beam
C
Diamond Backing:
• High Thermal Conductivity
• Negligible Systematics
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Polarized Electron Source
Laser
GaAs Crystal
Gun
Pockel Cell
Halfwave plate
flips helicity
(retractable, reverses helicity)
e - beam
• Rapid, random helicity reversal
• Electrical isolation from rest of lab
• Feedback on Intensity Asymmetry
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
PITA
Effect at Polarized Source
Polarization Induced Transport Asymmetry
Intensity Asymmetry AI    sin(  )
Tx  Ty
where  
Tx  Ty
(G. D. Cates)
Laser at
Pol. Source
Transport Asymmetry
 drifts, but slope is ~stable.
Feedback on 
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Beam Asymmetries
Araw = Adet - AQ + E+ ixi
Slopes from
PREX
UVa Seminar, Nov 2005
•natural beam jitter (regression)
•beam modulation (dithering)
R. Michaels
Jefferson Lab
Helicity Correlated Differences: Position, Angle, Energy
Scale +/- 10 nm
BPM X1
slug
Position Diffs avg
~ 1 nm
Redundant Monitors
•
Stripline Monitors
•
Resonant Cavities
BPM X2
slug
BPM Y1
slug
BPM Y2
slug
Negligible
Systematic Error
PREX
UVa Seminar, Nov 2005
“Energy”
BPM
“slug” = ~1 day running
R. Michaels
Jefferson Lab
Polarimetry
Møller : Pe/Pe ~ 3 %
Compton
2 analyses based
on either electron
or photon detection
e 
(limit: foil polarization)
: 2% syst. at present
PREX:
1 % desirable
2 % required
Electron only
Photon only
Superlattice:
Pe=86% !
PREX
UVa Seminar, Nov 2005
Preliminary: 2.5% syst ( only)
R. Michaels
Jefferson Lab
Upgrade of Compton Polarimeter
(Nanda, Lhuillier)

To reach 1% accuracy:
• Green Laser
(increased sensitivity at low E)
• Integrating Method
PREX
UVa Seminar, Nov 2005
(removes some systematics of
analyzing power)
R. Michaels
Jefferson Lab
Moller Polarimetry with
Atomic Hydrogen Target
( E. Chudakov, V. Luppov, D. Crabb)
H atoms
Ultra Cold Traps
• Polarization ~ 100%
• Density 3  1015 cm 3
Solenoid 8T
• Lifetime > 10 min
Polarimetry
beam
Trap
• 1% stat. err. in 30 min at 30 A
• Low background
• High beam currents allowed (100 A)
• Goal: ~ 0.5 % systematic error
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
PREX : Summary
• Fundamental Nuclear Physics
• HAPPEX to demonstrate most
technical aspects
• Polarimetry Upgrade needed
• PREX test run Nov 2005
(this weekend !)
• Experiment Runs in 2007 ?
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab