0Ze e

+

2

624 10~10~

QALR

LRPV

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