Transcript Parity – Violating Neutron Density Measurements (On Behalf of the PREX Collaboration ) Robert Michaels Concept PREX, C-REX : 208Pb Experiments Elastic electron scattering from a nucleus Flux Integration.

Parity – Violating Neutron Density Measurements
(On Behalf of the PREX Collaboration )
Robert Michaels
Concept
PREX, C-REX
:
208Pb
Experiments
Elastic electron scattering from a nucleus
Flux Integration Technique
PREX: 500 MHz
C-REX : 80 MHz
Z0 is a Clean Probe that couples mainly to Neutrons
 d 
 d 

 

2
GF Q
 d  R  d  L
A 

2 2
 d 
 d 

 

 d  R  d  L
p, n
2

FN (Q ) 
2
 1  4 sin W 

2
FP (Q ) 

HRS
spectrometers
target
w/ Coulomb distortions (C. J. Horowitz) :
proton
Weak charge
208Pb
of
neutron
Electric charge
1
0
Weak charge
0.08
1
dA
 3% 
A
dRn
 1%
Rn
Pol. Source
Hall A
JLAB
CEBAF
Nuclear Structure: Neutron density is a fundamental observable that remains elusive.
HRS
spectrometers
Pb target
fig: C.J. Horowitz
Reflects poor understanding of
symmetry energy of nuclear matter
= the energy cost of N  Z
HRS
Septum Magnet
Lead / diamond sandwich, cryo-cooled
.
50
target
E(n, x)  E(n, x  1/ 2)  S (n) (1  2 x )
2
n 
Applications
x
Neutron
Density
Ratio
Neutron / Proton
Nuclear Structure
fig: B. A. Brown
Results, New Proposals
E/N
PREX-I Result
PREX & Neutron Stars
PRL 108 (2012) 112502
PRC 85 (2012) 03250(R)
N
A  0.656 ppm
 0.060( stat)  0.014(syst )
C.J. Horowitz, J. Piekarewicz
Asymmetry leads to RN
Establishing a neutron skin at ~95 % CL
RN calibrates equation of state
(pressure vs density) of
Neutron Rich Matter
Neutron Skin = RN - RP = 0.33 + 0.16 - 0.18 fm
.
Low Q2 test of Standard Model
.
Needs
H PNC 
Some Neutron Stars
seem too cold
Might be explained by
cooling by neutrino
emission (URCA process)
depending on symmetry
energy.


2
GF
2
RN
theory: P. Ring , et.al



2
 N  N (r )  Z (1 4 sin W )  P (r )  e/  5  e d 3 r
published
approved
to run
RN - RP
Strange star ? Quark Star ?
dRN
 0.9 %
RN
(conditionally approved)
Application: Atomic Parity Violation
Combine PREX RN with
Observed Neutron Star Radii
Phase Transition to “Exotic” Core ?
C-REX Proposal
A