Transcript PREX : Pb Radius Experiment JLab Hall A Collaboration Proposal to measure the RMS radius of the neutron distribution of lead through parity violating elastic scattering. R.

PREX :
208
Pb Radius
Experiment
JLab Hall A Collaboration
Proposal to measure the
RMS radius of the
neutron distribution of
lead through parity
violating elastic scattering.
R. Michaels (JLab), P.A. Souder (Syracuse),
G.M. Urciuoli (INFN), C.J. Horowitz (Indiana)
http://hallaweb.jlab.org/parity/prex
Low Q
2
elastic e - nucleus scattering
 6
E = 850 MeV
Z
0
Couples Mainly to Neutrons
:
(weak interaction)
A 
 d 
 d 

  

 d  R
 d  L
 d 
 d 

  

 d  R
 d  L

2

1

4
sin
W

2 2 
GF Q
2
F n (Q2 ) 


2
FP (Q ) 
(T.W. Donnelly, J. Duback, I. Sick).
L & R Polarized Electrons
(C. J. Horowitz)
dRn
 1%
Rn
dA
 3% 
A
(C. J. Horowitz)
Applications :
•
Crab Nebula
(neutron star)
Fundamental Check of
Neutron
Star
Nuclear Theory
•
R N is necessary input to precision
atomic tests of the Standard
Electroweak Model.
(Horowitz, Piekarewicz, Carriere)
2
Meaurement at one Q constrains R N
Liquid
FP
Solid
Neutron Star Structure
•
Pb
R N constrains pressure of neutron
matter. Can predict radius of
neutron star and other properties.
Input to Atomic PV Expts
•
208
Neutron star has a solid crust over a
liquid core.
TM1
Thick neutron skin in Pb  Low
transition density in neutron star.
•
Heavy nuclei have a neutron skin
•
Both neutron stars and neutron skins are made
from neutron rich matter at similar
densities.
• Common unknown: Equation of state (EOS)
at subnuclear densities.
• Neutron radius measurement constrains URCA
cooling of NS by determining the density
dependence of the symmetry energy.
Measurement of R N determines the
symmetry energy and its density
dependence.
(R. J. Furnstahl)
Choice of Kinematics
Rn-Rp in 208Pb
Experimental Aspects
•
Spectrometer and Integrating DAQ
of HAPPEX
•
Septum magnet at 6 to increase
figure of merit.
•
New high power Lead Target
Proton fraction YP in neutron star
as constrained by R N - R P
Lead Target
Successfully tested at 80
208
A
Pb
Liquid Helium
Coolant
12
C
Electron Beam
Diamond Backing:
• High Thermal Conductivity
• Negligible Systematics