Pb Electroweak Asymmetry in Elastic Electron-Nucleus Scattering : A measure of the neutron distribution PREX and CREX 48 Ca Neutron Skin Horowitz Robert Michaels Jefferson Lab CIPANP /15

48 Ca 208 Pb Choice of Nuclear Targets First excited state far from elastic Neutron excess Doubly-magic Stable Hall A High Resolution ( ) Spectrometers Neutron form factor Robert Michaels, CIPANP 2015

Neutron Skin vs Mass Number A R N – R P (fm) A PREX-I PREX-II published Approved proposal C-REX Approved proposal 3/15 Robert Michaels, CIPANP 2015

Measured Asymmetry Weak Density at one Q 2 Correct for Coulomb Distortions Small Corrections for n E G s E MEC surface thickness Atomic Parity Violation Neutron Stars R N - R P Weak Interaction: Sees the Neutrons G Nuclear Theory (Symmetry Energy ) Skin proton neutron Electric charge 1 0 Weak charge 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

Input : Eq. of state (EOS) Deduce Mass-Radius relationship, and other properties PREX / CREX helps here pressure density Neutron skin is highly correlated with several neutron star properties J. Piekarewicz, arXiv Crab Pulsar Neutron Stars relationship to 6/15

Parity Experiment Method Flux Integration Technique: C-REX : 140 MHz PREX: 500 MHz Example : HAPPEX (integrating mode) Robert Michaels, CIPANP 2015

Hall A High Resolution Spectrometers Elastic Inelastic detector Dipole Quads target Resolve Elastic Scattering Discriminate Excited States Quad Quartz detectors Drift chambers scintillator Robert Michaels, CIPANP 2015

Physics Asymmetry CEBAF Hall A JLAB Pol. Source  Statistics limited ( 9% )  Systematic error goal achieved ! (2%) Septum Magnet HRS + septum Pb target HRS Pb target 5050 PREX-I PRL 108 (2012) Results 9/15

Slug # ( ~ 1 day) Units: microns 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 nm diffs. with pol. source setup & feedback A raw = A det - A Q +  E +  i  x i Plotted below Measured separately Sign flips using ½ wave plate & Wien filter This BPM, Average = nm PREX data 10/15 Robert Michaels, CIPANP 2015

PREX Asymmetry (P e x A) ppm Slug ~ 1 day 11/15

PRL 108 (2012) Asymmetry leads to R N PREX-I has established a neutron skin at ~95 % CL Neutron Skin = R N - R P = fm 12/15 Robert 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 Ca 208 Pb Robert Michaels, CIPANP 2015

Possible Future PREX Program ? Nucleus E (GeV) dR N / R N comment 208 Pb 1 1 %PREX-II approved by Jlab PAC, A rating 48 Ca 2.2 (1-pass) 0.4 % natural 12 GeV exp’t approved, A rating 48 Ca % surface thickness 40 Ca 2.2 (1-pass) 0.6 % basic check of theory tin isotope % apply to heavy ion tin isotope % surface thickness For reference, each point 30 days stat. error only J. Phys. G 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