Transcript Slide 1

Results from
Lead (
208
Pb) Radius Experiment :
PREX
Elastic Scattering
Parity Violating Asymmetry
E = 1 GeV,   5 0
electrons on lead
Spokespersons
Paul Souder,
Krishna Kumar
Guido Urciuoli, Robert Michaels
(speaker)
Graduate Students
Ahmed Zafar, Chun Min Jen,
Abdurahim Rakham (Syracuse)
208Pb
Jon Wexler (UMass)
Kiadtisak Saenboonruang (UVa)
Ran March – June 2010
in Hall A at Jefferson Lab
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Standard Electroweak Model
The Glashow-Weinberg-Salam Theory unifies the
electromagnetic and weak interactions.
Left –handed fermion fields (quarks & leptons)
= doublets under SU(2)
Right-handed fields

decay
= singlets under SU(2)
p, n
Weak charge
of
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Parity
Violation
208
Pb
A piece of the weak interaction
violates parity (mirror symmetry)
which allows to isolate it.
pPb
Pb
Pb
Pb
Pb
p
p
Pb
1800
rotation
Positive spin
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Negative spin
Parity Violating Asymmetry
APV
R L

~ 104  Q 2 ~ 106
R L
2

e


+
208Pb
e
Z0
APV from
interference
208Pb
Applications of APV at Jefferson Lab
• Nucleon Structure
Strangeness s s in proton (HAPPEX, G0 expts)
• Test of Standard Model of Electroweak sin2 W
e – e (MOLLER) or e – q (PVDIS)
elastic e – p at low Q2 (QWEAK)
This talk
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
• Nuclear Structure (neutron density) :
PREX
Idea behind
Z
0
PREX
of Weak Interaction :
Clean Probe Couples Mainly to Neutrons
( T.W. Donnelly, J. Dubach, I Sick 1989 )
In PWIA (to illustrate) :
 d 
 d 

 

GF Q 2
 d  R  d  L
A 

2 2
 d 
 d 

 

 d  R  d  L
2
F n (Q ) 

2
 1  4 sin W 

2
F
(
Q
)
P


0
w/ Coulomb distortions (C. J. Horowitz) :
dA
 3% 
A
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
dRn
 1%
Rn
5
Hall A at Jefferson Lab
Hall A
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
PREX
Physics
Output
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)
Stars
Slide adapted from
C. Horowitz
Rn
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Neutron
Fundamental Nuclear Physics
:
What is the size of a nucleus ?
Neutrons are thought to determine
the size of heavy nuclei like 208Pb.
Can theory predict it ?
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Reminder: Electromagnetic Scattering determines
 r 
(charge distribution)
d
d
 m b
208
Pb
 r 
d


d  str 
1
2
3
q  fm
1
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Z0 of weak interaction : sees the neutrons
proton
neutron
Electric charge
1
0
Weak charge
0.08
1
Neutron form factor
FN (Q 2 ) 
1
4
T.W. Donnelly, J. Dubach, I. Sick
Nucl. Phys. A 503, 589, 1989
C. J. Horowitz, S. J. Pollock,
P. A. Souder, R. Michaels
Phys. Rev. C 63, 025501, 2001
C.J. Horowitz
3
d
 r j0 (qr )  N (r )
Parity
Violating
Asymmetry
GF Q 2
A
2 2

FN (Q 2 ) 
2
 1  4 sin W 

FP (Q 2 ) 

0
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
10
How to Measure
Neutron Distributions, Symmetry Energy
•
•
•
•
•
Proton-Nucleus Elastic
Pion, alpha, d Scattering
Pion Photoproduction
Heavy ion collisions
Rare Isotopes (dripline)
•
Magnetic scattering
•
PREX
•
Theory
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Involve strong probes
Most spins couple to zero.
(weak interaction)
MFT fit mostly by data other
than neutron densities
Example:
Heavy Ions
(adapted from Betty Tsang, PREX Workshop, 2008)
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Isospin Diffusion
(NSCL)
Probe the symmetry
energy in 124Sn + 112Sn
Using Parity Violation
Vˆ (r )  V (r )   5 A(r )
Electron - Nucleus Potential
axial
electromagnetic
/
V (r )   d r Z  (r ) | r  r |
3 /
208
/
A(r ) 
d
d

| FP (Q 2 ) | 2
d d Mott
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
Pb is spin 0
Proton form factor
GF
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
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012

FN (Q 2 ) 
2
 1  4 sin W 

2
F
(
Q
)
P


0
PREX:
2
Measurement at one Q is sufficient to measure R
N
( R.J. Furnstahl )
Why only one
parameter ?
(next slide…)
proposed error
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Slide adapted from J. Piekarewicz
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 significantly reduce
the dispersion in plot.
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
15
Thanks, Alex Brown
Skx-s15
PREX Workshop 2008
E/N
N
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Thanks, Alex Brown
Skx-s20
PREX Workshop 2008
E/N
N
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Thanks, Alex Brown
Skx-s25
PREX Workshop 2008
E/N
N
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
8
Application: Atomic Parity Violation
2
• Low Q test of Standard Model
• Needs RN
H PNC 
GF
2 2
Isotope Chain Experiments
e.g. Berkeley Yb
(or APV measures RN )
  N
 N (r )  Z (1  4 sin 2 W )  P (r )  e/  5  e d 3 r


0
APV
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Momentum transfer
19
Application :
Neutron
Stars
What is the nature
of extremely dense
matter ?
Do collapsed stars form
“exotic” phases of
matter ? (strange stars,
quark stars)
Crab Nebula
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
(X-ray, visible, radio, infrared)
Inputs:
Eq. of state (EOS)
P(  )
PREX helps here
Hydrostatics (Gen. Rel.)
Astrophysics Observations
Luminosity L
Temp. T
Mass M from pulsar timing
L  4 B R 2 T 4
(with corrections … )
Mass - Radius relationship
Fig from: Dany Page.
J.M. Lattimer & M. Prakash, Science 304 (2004) 536.
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
21
PREX & Neutron Stars
C.J. Horowitz, J. Piekarewicz
RN calibrates equation of
state (pressure vs density)
of Neutron Rich Matter
Combine PREX RN with
Observed Neutron Star Radii
Phase Transition to “Exotic” Core ?
Strange star ? Quark Star ?
Some Neutron Stars seem
too cold
Explained by Cooling by neutrino
emission (URCA process) ?
Rn  Rp  0.2 fm
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012Crab Pulsar
URCA probable, else not
PREX Setup
Parity: “The entire lab is
the experiment”
Spectometers
Lead Foil
Target
Hall A
JLAB
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Pol. Source
CEBAF
How to do a Parity Experiment
(integrating method)
Flux Integration Technique:
HAPPEX: 2 MHz
PREX: 500 MHz
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Example : HAPPEX
Polarized Electron Source
GaAs Crystal
Gun
Laser
Pockel
Cell flips
helicity
Halfwave plate
(retractable, reverses
helicity)
e - beam
• Based on Photoemission from GaAs Crystal
• Polarized electrons from polarized laser
• Need :
• Rapid, random helicity reversal
• Electrical isolation from the rest of the lab
• Feedback on Intensity Asymmetry
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Important Systematic :
P I T A Effect
Polarization Induced Transport Asymmetry
Intensity
Asymmetry
where

Tx  Ty
AI    sin( )
Laser at
Pol. Source
Tx  Ty
Transport Asymmetry
 drifts, but slope
is ~ stable.
Feedback on 
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
26
Methods to Reduce Systematics
Intensity Asymmetry (ppm)
Perfect
DoCP
Scanning the Pockels Cell voltage
= scanning the residual linear
polarization (DoLP)
Pockels cell voltage  offset (V)
A rotatable l/2
waveplate downstream
of the P.C. allows
arbitrary orientation of
the ellipse from DoLP
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
A simplified picture:
asymmetry=0 corresponds to
minimized DoLP at analyzer
Intensity Feedback
Adjustments
for small phase shifts
to make close to
circular polarization
Low jitter and high accuracy allows sub-ppm
cumulative charge asymmetry in ~ 1 hour
~ 2 hours
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
28
Double Wien Filter
Crossed E & B fields to rotate the spin
• Two Wien Spin Manipulators in series
• Solenoid rotates spin +/-90 degrees (spin rotation as B but focus as B2).
Flips spin without moving the beam !
Electron
Beam
SPIN
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
29
Beam Asymmetries
Araw = Adet - AQ + E+ ixi
Slopes from
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
•natural beam jitter (regression)
•beam modulation (dithering)
PAVI 09
31
Parity Quality
Beam !
Points: Not
sign corrected
( why we love Jlab ! )
Helicity – Correlated
Position Differences
Average with signs =
what exp’t feels
 X R  X L
< ~ 3 nm
for helicity L, R
Units: microns
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Slug #
( ~ 1 day)
Compton Polarimeter


e 
scattering
to measure electron beam’s polarization
(needed to normalize asymmetry)

Upgrade for 1% accuracy at 1 GeV
• Green Laser
(increased sensitivity at low E)
• Integrating Method
• New Photon
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
(removes some systematics of
analyzing power)
& Electron Detectors
PREX
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Compton
Polarimeter Results
Upgraded for PREX
Moller Polarimeter


e  e scattering
Superconducting Magnet from Hall C
Saturated Iron Foil Targets
1 % Accuracy in Polarization
Magnet and Target
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Electronics/DAQ
Upgrade (FADC)
Hall A
High Resolution Spectrometers
• Resolve Elastic Scattering
• Discriminate Excited States
Elastic
Inelastic
detector
Pure, Thin
208
Pb
Target
2.6
MeV
target
Dipole
DETECTOR footprint
Quads
Scattered Electron’s
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Momentum (GeV/c)
35
Measure θ from Nuclear Recoil
δE=Energy loss
E=Beam energy
MA=Nuclear mass
θ=Scattering angle
E  E

E 2 MA
2
(these data taken during HAPPEX)
Scattered Electron Energy (GeV)
Recoil is large for H, small for nuclei
(3X better accuracy than survey)
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Detector
cutoff
Backgrounds
that might re-scatter
into the detector ?
Run magnets
down: measure
inelastic region
Run magnets up :
measure probability
to rescatter
No inelastics observed on
top of radiative tail. Small
systematic for tail.
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Detector Package in HRS
PREX Integrating Detectors
UMass / Smith
DETECTORS
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Lead / Diamond Target
Diamond
• Three bays
• Lead (0.5 mm)
sandwiched by
diamond (0.15 mm)
R. Michaels, Jlab
• Liquid He
Seminar @ CUA
Feb 29, 2012
cooling (30 Watts)
LEAD
Performance of Lead / Diamond Targets
melted
NOT
melted
Last 4 days at 70 uA
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
melted
Targets with thin diamond
backing (4.5 % background)
degraded fastest.
Thick diamond (8%) ran well and
did not melt at 70 uA.
Solution: Run with 10 targets.
Beam-Normal Asymmetry in elastic electron
scattering
i.e. spin transverse to scattering plane



   
AT  
 S e  ( k e  k 'e )

 
Possible systematic if small
transverse spin component
New results PREX
208
12
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
AT > 0 means
+

S
x

k
-
z
Pb: AT   0.13  0.19  0.36 ppm
C : AT   6.52  0.36  0.35 ppm
• Small AT for
• AT for
y
208Pb
is a big (but pleasant) surprise.
12C
qualitatively consistent with 4He and available
calculations (1) Afanasev ; (2) Gorchtein & Horowitz
41
PREX-I Result
Systematic Errors
Error Source
Absolute
(ppm)
Relative
( %)
Physics Asymmetry
Polarization (1)
0.0083
1.3
A  0.656 ppm
Beam Asymmetries (2)
0.0072
1.1
Detector Linearity
0.0076
1.2
BCM Linearity
0.0010
0.2
Rescattering
0.0001
0
Transverse Polarization
0.0012
0.2
Q2 (1)
0.0028
0.4
Target Thickness
0.0005
0.1
12C
0.0025
0.4
Inelastic States
0
0
TOTAL
0.0140
2.1
Asymmetry (2)
(1) Normalization Correction applied
(2) Nonzero correction (the rest assumed zero)
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
 0.060( stat)  0.014(syst )
 Statistics limited ( 9% )
 Systematic error goal
achieved ! (2%)
A physics letter was recently
accepted by PRL.
arXiv 1201.2568 [nucl-ex]
42
PREX Asymmetry
(Pe x A)
ppm
Slug ~ 1 day
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Asymmetry leads to RN
*
Establishing a neutron skin at ~95 % CL
Neutron Skin = RN - RP = 0.33 + 0.16 - 0.18 fm
fig from C.J. Horowitz
PREX data
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
*
Interpretation requires the acceptance
function for spectrometer:
 ( )
Neutron Skin = RN - RP = 0.33 + 0.16 - 0.18 fm
cont.
A  0.656 ppm
 0.060( stat)  0.014(syst )
DATA
rN - rP
(fm)
PREX-I Result,
theory: P. Ring
rN = rP
Atomic Number, A
DATA
A physics letter was recently
accepted by PRL.
arXiv 1201.2568 [nucl-ex]
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
46
PREX-II
Approved by PAC
“A” Rating 35 days
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
(Aug 2011)
to run in 2013 or 2014
Recent Rn Predictions Can Be Tested By PREX at Full Precision
PREX could provide an electroweak complement to Rn predictions
from a wide range of physical situations and model dependencies
Recent Rn predictions:
Hebeler et al. Chiral EFT
calculation of neutron matter.
Correlation of pressure with
neutron skin by Brown. Threeneutron forces!
Steiner et al. X-Ray n-star mass
and radii observation + Brown
correlation. (Ozel et al finds
softer EOS, would suggest
smaller Rn).
Hebeler
Steiner
Tamii
Tsang
Tamii et al. Measurement of
electric dipole polarizability of
208Pb + model correlation with
neutron skin.
These can be tested with
(APV)/APV ~ 3%
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
(Rn)/Rn ~ 1%
Tsang et al. Isospin diffusion in
heavy ion collisions, with Brown
correlation and quantum
molecular dynamics transport
model.
Improvements for PREX-II
Region downstream of target
Tungsten
Collimator
& Shielding
HRS-L
Septum
Magnet
Q1
target
HRS-R
Q1
Location of ill-fated O-Ring
which failed & caused significant
time loss during PREX-I
R. Michaels,
Jlab
 PREX-II
Seminar @ CUA
Feb 29, 2012
to use all-metal seals
Collimators
After PREX …
Other
Nuclei ?
RN
Surface
thickness
and Shape Dependence ?
each point 30 days
Parity Violating Electron Scattering
Measurements of Neutron Densities
Shufang Ban, C.J. Horowitz, R. Michaels
J. Phys. G39 014104 2012
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
RN
Surface
thickness
Possible
Future
PREX
Program ?
Each point 30 days
Nucleus
208Pb
stat. error only
E (GeV)
dRN / RN
comment
1
1%
PREX-II (approved by
Jlab PAC, A rating)
48Ca
2.2 (1-pass)
0.4 %
natural 12 GeV exp’t
will propose @ next PAC
48Ca
2.6
2%
surface thickness
40Ca
2.2 (1-pass)
0.6 %
basic check of theory
tin isotope
1.8
0.6 %
apply to heavy ion
tin isotope
2.6
1.6 %
surface thickness
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Shufang Ban, C.J. Horowitz, R. Michaels
J. Phys. G39 014104 2012
Not
proposed
PREX : Summary
• Fundamental Nuclear Physics with
many applications
• PREX-I achieved a 9% stat. error in Asymmetry
(original goal : 3 %)
• Systematic Error Goals Achieved !!
• Significant time-losses due to O-Ring problem
and radiation damage
• PREX-II approved
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
(runs in 2013 or 2014 )
Extra Slides
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Geant 4 Radiation Calculations
scattering chamber
PREX-II shielding strategies
shielding
Number of Neutrons per incident Electron
0 - 1 MeV
beamline
Energy (MeV)
1 - 10 MeV
Strategy
-------
PREX-I
PREX-II, no shield
PREX-II, shielded
• Tungsten ( W ) plug
Energy (MeV)
0.7    3
0
0
10 - 1200 MeV
• Shield the W
• x 10 reduction in
R.0.2
Michaels,
Jlab neutrons
to 10 MeV
Seminar @ CUA
Feb 29, 2012
Energy (MeV)
49
Pull Plot
(example)
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
PREX Data
( A A)/
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
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Horowitz, et.al. PRC 63 025501
Optimum Kinematics for Lead Parity:
<A> = 0.5 ppm.
E = 1 GeV if
Accuracy in Asy 3%
Fig. of merit
Min. error in R n
maximize:
1 month run
1% in R
PAVI 09
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
n
(2 months x
100 uA
 0.5% if no
systematics)
Source
Studies
Kent Paschke, Gordon
Charge
Asymmetry
~ 2000 ppm
 of waveplate
~ 0.5 um
Delta X
(um)
~ 0.5 um
Optimizing laser optics
to minimize helicitycorrelated systematics.
Hel. Correl. Diff
(X)
Hel. Correl. Diff
(Y)
Cates, Mark Dalton,
Rupesh Silwal
Transmission of Helicity-Correlated
Position DIffs
Hel. Correl. Diff
(X)
Delta Y
(um)
BPMs
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
in Injector Region
Hel. Correl. Diff
(Y)
Water Cell : Measure 

(agrees with
survey)
Nilanga Liyanage, Seamus Riordan,
Kiadtisak Saenboonruang,
16O
Hydroge
n
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Pockel Cell Related Systematic
Error
wait
integrate
wait
An instability in Pockel Cell “bleeds”
into the itegration gate. It depends on
helicity.
Beam Current
Detector (1 of 4)
Response to
pulsed
beam
time
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
time
Want small time constants, and same for detectors and bcm
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
( 1 )
208
Actually, it’s the
density dependence of
a4 that we pin down.
Pb
PREX
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Collimators inside
Q1
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Symmetry in all dimensions to 1
mm
(slide from C.
Horowitz)
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…
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
(slide from C. Horowitz)
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.
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
Rn-Rp in 208Pb
If Yp > red line NS cools quickly via
direct URCA reaction n p+e+
Neutron Star Crust vs
Pb Neutron Skin
Liquid/Solid Transition Density
C.J. Horowitz, J. Piekarawicz
Liquid
FP
Neutron
Star
208Pb
Solid
• 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, Jlab
Seminar @ CUA
Feb 29, 2012
TM1
Weak Interaction
1930’s - The weak nuclear interaction was needed to explain nuclear beta decay
Contact interaction with charge exchanged
or, mediated by a heavy, charged boson
1950’s - Discovery of parity-violation by the weak interaction
60Ni
60Co
Weak decay of
60Co Nucleus
V-A theory described W’s as only
interacting with left-handed particles!
60Co
observed
R. Michaels, Jlab
Seminar @ CUA
Feb 29, 2012
60Ni
L
R
right-handed
anti-neutrino
Right
Left
W Charge
T 
1
2
zero
60Ni
60Co

not observed
R
L
left-handed
anti-neutrino