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Transcript of + 2 208 Pb Electroweak Asymmetry in Elastic Electron-Nucleus Scattering : A measure of the neutron...
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624 10~10~
QALR
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208Pb
Electroweak Asymmetry in Elastic Electron-Nucleus Scattering : A measure of the neutron distribution
PREX and CREX 48Ca
22pnpn rrRR
Neutron Skin
Horowitz
Robert Michaels Jefferson Lab
CIPANP 2015
1/15
48Ca
208Pb
Choice of Nuclear Targets• First excited state far from elastic• Neutron excess• Doubly-magic• Stable
Hall A
Hall AHigh Resolution ( 10-4 ) Spectrometers
)(
)(sin41
22 2
22
2
QF
QFQGA
P
NW
F
Neutron form factor
Robert Michaels, CIPANP 2015
Neutron Skin vs Mass Number AR N
– R
P (f
m)
A
PREX-I PREX-IIpublished Approved proposal
C-REXApproved proposal
3/15Robert Michaels, CIPANP 2015
Measured Asymmetry
Weak Density at one Q2
Correct for CoulombDistortions
Small Corrections fornEG
sE MEC
surface thickness
Atomic Parity Violation
Neutron
Stars RN - RP
Weak Interaction: Sees the Neutrons
G
Nuclear Theory (Symmetry Energy )
Skin
proton neutron
Electric charge 1 0
Weak charge 0.08 1
APPLICATIONS
Heavy
Ions
Ways to Find
Neutron Distribution and Symmetry Energy
• Proton-Nucleus Elastic• Pion, alpha, d Scattering• Pion Photoproduction• Heavy ion collisions• Rare Isotopes (dripline)• Electric Dipole Polarizabilities
• Magnetic scattering
• PREX / C-REX (weak interaction)
• Theory MFT fit mostly by data other than neutron densities
Involve strong probes
Most spins couple to zero.
5/15
)(PInput :
Eq. of state (EOS)
Deduce Mass-Radius relationship, and other properties
PREX / CREX helps here
pres
sure
dens
ity
Neutron skin is highly correlated with several neutron star properties
J. Piekarewicz, arXiv 1502.01559
Crab Pulsar
Neutron Starsrelationship to
6/15
Parity Experiment Method
Flux Integration Technique:C-REX : 140 MHzPREX: 500 MHz
Example : HAPPEX
(integrating mode)
Robert Michaels, CIPANP 2015
Hall A High Resolution Spectrometers
Elastic
Inelasticdetector
Dipole
Quads
target
• Resolve Elastic Scattering
• Discriminate Excited States
Quad
Quartz
detectors
Drift chambers
scintillator
Robert Michaels, CIPANP 2015
Physics Asymmetry
CEBAFHall AJLAB
Pol. Source Statistics limited ( 9% ) Systematic error goal
achieved ! (2%)
)(014.0)(060.0
656.0
syststat
ppmA
Septum Magnet
HRS + septum
Pb target
HRS
Pb target
50
PREX-I PRL 108 (2012) 112502Results
9/15
Slug # ( ~ 1 day)
Un
its:
m
icro
ns
RLhelicityforXX LR ,
Parity Quality Beam : Unique Strength of JLab
Helicity – Correlated Position Differences
Sign flips provide further suppression : Average with signs = what experiment feels
achieved < 5 nm
Points: Not sign-corrected. 20-50 nm diffs. with pol. source setup & feedback
Araw
= Adet
- AQ +
E+
i xi
Plotted below
Measured separately
Sign flips using ½ wave plate & Wien filter ++ -+ +- --
This BPM, Average = 2.4 3.1 nm
PREX data
10/15Robert Michaels, CIPANP 2015
PREX Asymmetry (Pe x A)
ppm
Slug ~ 1 day
11/15
PRL 108 (2012) 112502
Asymmetry leads to RNPREX-I has established a neutron skin at ~95 % CL
Neutron Skin = RN - RP = 0.33 + 0.16 - 0.18 fm
12/15Robert Michaels, CIPANP 2015
PREX-II and C-REX
Status
targets
Improve the shielding and vacuum seals
• Both experiments approved PREX-II A rating 35 days C-REX A- rating 45 days
• Systematic Error Goals for PREX-II demonstrated by PREX-I
spectrometers
4848Ca 208Pb
Robert Michaels, CIPANP 2015
Possible Future PREX Program ?
Nucleus E (GeV) dRN / RN comment
208Pb 1 1 % PREX-II approved by Jlab PAC, A rating
48Ca 2.2 (1-pass) 0.4 % natural 12 GeV exp’t approved, A rating
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
For reference, each point 30 days stat. error only
J. Phys. G39 014104 2012 Shufang Ban, C.J. Horowitz, R. Michaels
Not proposed
PREX, C-REX : Summary • Fundamental Nuclear Physics with
many applications
• PREX-I : 9% stat. error in Asymmetry ( PREX-II goal 3 % )
• Systematic Error Goals Achieved
• Main problems to be fixed: shielding and rad-hard vacuum seals.
• PREX-II and C-REX approved ( might run in 2017 )
Robert Michaels, CIPANP 2015