Lead (
208
Pb) Radius Experiment :
PREX
Elastic Scattering
Parity Violating Asymmetry
0
E = 1 GeV,   5
electrons on
lead
Spokespersons
•
Paul Souder
•
Krishna Kumar
•
Robert Michaels
•
Guido Urciuoli
Hall A Collaboration Experiment
G.M. Urciuoli
208Pb
Electron - Nucleus Potential
Vˆ (r )  V (r )   5 A(r )
electromagnetic
axial
/
V (r )   d r Z  (r ) | r  r |
3
/
/
A(r ) 
GF
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
FP (Q 2 ) 
1
4
3
d
 r j0 (qr )  P (r )
1  4 sin 2  W  1 neutron weak
charge >> proton weak charge
Neutron form factor
FN (Q 2 ) 
1
4
d
3
r j 0 (qr )  N (r )
Parity Violating Asymmetry
 d 
 d 

 

GF Q 2
 d  R  d  L
A 

2 2
 d 
 d 

 

 d  R  d  L
APV~ 500 ±15ppb, Q2~ 0.01 GeV2

FN (Q 2 ) 
2
 1  4 sin W 

FP (Q 2 ) 

0
dA
 3% 
A
dRn
 1%
Rn
Reminder: Electromagnetic Scattering determines
 r 
(charge distribution)
208
Pb
 r 
d  mb 


d  str 
1
G.M. Urciuoli
2
q
 fm1
3
0
Z of weak interaction : sees the
neutrons
Analysis is clean, like electromagnetic scattering:
1. Probes the entire nuclear volume
2. Perturbation theory applies
G.M. Urciuoli
proton
neutron
Electric charge
1
0
Weak charge
0.08
1
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.
G.M. Urciuoli
Nuclear Structure: Neutron density is a fundamental
observable that remains elusive.
Reflects poor understanding of
symmetry energy of nuclear
matter = the energy cost of N  Z
E (n, x)  E (n, x  1 / 2)  S (n) (1  2 x 2 )
n  n.m. density
x  ratio
proton/neutrons
• Slope unconstrained by data
208
• Adding R N from
Pb
will eliminate the
dispersion in plot.
G.M. Urciuoli
2
Measurement at one Q is
sufficient to measure R N
Pins down the symmetry
energy (1 parameter)
PREX
accuracy
PREX
accuracy
( R.J. Furnstahl )
G.M. Urciuoli
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
G.M. Urciuoli
Rn  R p  0.2 fm
URCA probable, else not
Neutron EOS and Neutron Star Crust
Liquid/Solid Transition Density
Liquid
FP
Solid
TM1
Horowitz
Fig. from
J.M. Lattimer & M. Prakash,
Science 304 (2004) 536.
G.M. Urciuoli
• Thicker neutron skin in Pb means
energy rises rapidly with density 
Quickly favors uniform phase.
• Thick skin in Pb  low transition
density in star.
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.
• 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…
G.M. Urciuoli
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
Horowitz
If Yp > red line NS cools quickly via
direct URCA reaction n p+e+
G.M. Urciuoli
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
G.M. Urciuoli
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.
G.M. Urciuoli
PREX
Physics
Impact
Measured Asymmetry
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)
Rn
G.M. Urciuoli
Heavy
Ions
Neutron
Stars
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
G.M. Urciuoli
Horowitz, et.al. PRC 63 025501
Hall A at Jefferson Lab
Polarized eSource
G.M. Urciuoli
Hall A
PREX in Hall A at JLab
Spectometers
Lead Foil
Target
Pol. Source
Hall A
CEBAF
G.M. Urciuoli
High Resolution Spectrometers
Spectrometer Concept:
Resolve Elastic
Elastic
detector
Inelastic
Left-Right symmetry to
control transverse
polarization systematic
Quad
target
Dipole
G.M. Urciuoli
Q Q
Experimental Method
Flux Integration Technique:
HAPPEX: 2 MHz
PREX: 850 MHz
High Pe
High Q.E.
Low Apower
GaAS
Photocatode
Polarized
Source
•Optical pumping of
solid-state
photocathode
•High Polarization
controls
effective
analyzing
power
Intensity
Attenuator
(charge
Feedback)
G.M. Urciuoli
Slow helicity
reversal
Tune
residual
linear pol.
• Pockels cell
allows rapid
helicity flip
•Careful
configuration to
reduce beam
asymmetries.
•Slow helicity
reversal to further
cancel beam
asymmetries
Important Systematic :
PITA
Effect
Polarization Induced Transport
Asymmetry
Intensity Asymmetry AI    sin(  )
Tx  Ty
where  
Tx  Ty
Laser at
Pol.
Source
Transport Asymmetry
 drifts, but slope is
~ stable.
Feedback on 
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.
Beam Asymmetries
Araw = Adet - AQ + E+ ixi
Slopes from
G.M. Urciuoli
•natural beam jitter (regression)
•beam modulation (dithering)
Helicity Correlated Differences: Position, Angle, Energy
Scale +/- 10 nm
BPM X1
slug
Spectacular results
from HAPPEX-H
show we can do
BPM X2
PREX.
Position Diffs
average to ~ 1 nm
• Good model for
controlling laser systematics
at source
• Accelerator setup
(betatron matching, phase
advance)
G.M. Urciuoli
slug
BPM Y1
slug
BPM Y2
slug
“Energy”
BPM
“slug” = ~1 day running
Integrating Detection
• Integrate in 30 msec helicity period.
• Deadtime free.
• 18 bit ADC with < 10-4 nonlinearity.
•
Backgrounds & inelastics separated (HRS).
Integrator
Calorimeter
PMT
ADC
Actually two thin quartz detectors
G.M. Urciuoli
Attempt to improve resolution by replacing Alzak mirrors in light guide
with anodized Al or Silver.
The x, y dimensions of the quartz determined from beam test data and
MC (HAMC) simulations. (11 x 14 cm)
Quartz thickness to be optimized with MC.
New HRS optics tune focuses elastic events both in x & y at the PREx
detector location.
G.M. Urciuoli
Lead Target
208
Pb
Liquid Helium
Coolant
12
beam
C
Diamond Backing:
• High Thermal Conductivity
• Negligible Systematics
Successfuly tested
5 days at 60 uA
1 shift at 80 uA
3 hrs at 100 uA
G.M. Urciuoli
Polarimetry
Upgrade of Compton Polarimeter

To reach 1% accuracy:
• Green Laser  Green Fabry-Perot cavity (increased sensitivity at low E)
•
Integrating Method
(removes some systematics of analyzing power)
• New Photon and Electron Detectors (new GSO photon calorimeter, FADC based
photon integration DAQ)
Upgrade Møller polarimeter: 4 Tesla field saturated iron foil, new FADC DAQ
G.M. Urciuoli
Transverse Polarization
Part I: Left/Right Asymmetry
Transverse Asymmetry
AT  AT0 PT sin 
Theory est. (Afanasev)
Systematic Error for Parity

A  5  1 ppm
0
T
Transverse polarization
PT  P sin 
 3o

HRS-Left

P

 A   AT0  PT 
“Error in”
 
Left-right apparatus
asymmetry
Control  w/ slow feedback on
polarized source solenoids.
 AT0   1 ppm
HRS-Right
Need
 PT << 10 3  10 3
correction
G.M. Urciuoli
measure in ~ 1 hr
(+ 8 hr setup)
syst. err.
Transverse Polarization
Part II: Up/Down Asymmetry
Vertical misalignment
Horizontal polarization
e.g. from (g-2)
 cos    0

Systematic Error for Parity
 A   AT0 PT  cos   
• Measured in situ using
2-piece detector.
PT  P sin 
up/down
misalignment
HRS-Left

P
• Study alignment with
tracking & M.C.
• Wien angle feedback (  )

Need
HRS-Right
PT  cos   << 10 3  10 3

G.M. Urciuoli
( Note, beam width is very tiny
~ 100  m )
G.M. Urciuoli
A_T detector design
Figure of Merit M = 1/E * 1/sqrt(R) * sqrt(1 + B/S)
where,
E = A_T enhancement for A_T hole events = 50.
R = Ratio of A_T hole detector to main Pb detector event rates
B/S = Ratio of bkgd under the A_T hole events to A_T signal
The optimum A_T detector dimension is
~7.6cm in x by 0.8cm in y. This gives
Figure of Merit = 0.637 and error inflation
~1.186.
G.M. Urciuoli
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.
•
Tests with Luminosity Monitor to demonstrate capability.
G.M. Urciuoli
Asymmetries in Lumi
Monitors
after beam noise
subtraction
~ 50 ppm noise per pulse
 milestone for
electronics
( need < 100 ppm)
Jan
2008
Data
PREX Workshop Aug 08
PREX: Summary
• PREX is an extremely challenging experiment:
–
–
–
–
–
–
–
–
–
APV ≈ 500 ± 15 ppb.
1% polarimetry.
Helicity correlated beam asymmetry < 100 ± 10 ppb.
Beam position differences < 1 ± 0.1 nm.
Transverse beam polarization < 1%.
Noise < 100 ppm
(Not melting) Lead Target
Forward angle detection  Septum magnet
Precision measurement of Q2: ± 0.7%  ± 0.02° accuracy in
spectrometer angles
• However HAPPEX & test runs have demonstrated its
feasibility.
• It will run in March-May 2010 and will measure the lead
neutron radius with an unprecedented accuracy (1%). This
result will have an impact on many other Physics fields
(neutron stars, APV, heavy ions …).
G.M. Urciuoli
Spares
G.M. Urciuoli
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
G.M. Urciuoli
Optimization for Barium
-- of possible direct use for Atomic PV
1 GeV optimum
G.M. Urciuoli
Redundant Position Measurements at the ~1 nm level
Y (cavity)
X (cavity)
nm
nm
(Helicity – correlated differences averaged over ~1 day)
X (stripline)
G.M. Urciuoli
nm
Y (stripline)
nm
Integrating Detection
• Integrate in 30 msec helicity period.
• Deadtime free.
• 18 bit ADC with < 10-4
•
nonlinearity.
Backgrounds & inelastics must be separeted (HRS).
Quartz / Tungsten Calorimeter
Integrator
PMT
ADC
(Also a thin quartz detector upstream of this)
electrons
Attempt to improve resolution by replacing Alzak mirrors in light guide with anodized
Al or Silver.
The x, y dimensions of the quartz determined from beam test data and MC (HAMC)
simulations. (11 x 14 cm)
Quartz thickness to be optimized with MC.
New HRS optics tune focuses elastic events both in x & y at the PREx detector
location.