Lead 208 Pb Radius Experiment PREX Elastic Scattering

Lead (208 Pb) Radius Experiment : PREX Elastic Scattering Parity Violating Asymmetry E = 1 Ge. V, electrons on lead Spokespersons Kent Paschke, Guido Urciuoli, Paul Souder, Krishna Kumar, Robert Michaels (presenting) • PREX-I Results 2010 Run • Approved PREX-II Proposal • Future : PREX-N ? R. Michaels, Jlab PAVI -11 (N = III, IV, V …) PAVI – 11 Conference

Z 0 of weak interaction : sees the neutrons proton neutron Electric charge 1 0 Weak charge 0. 08 1 Neutron form factor Parity Violating Asymmetry R. Michaels, Jlab PAVI -11 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

Hall A at Jefferson Lab Hall A R. Michaels, Jlab PAVI -11

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) Slide adapted from C. Horowitz R. Michaels, Jlab PAVI -11 Neutron Stars Rn

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. m. density ratio proton/neutrons • Slope unconstrained by data 208 • Adding R N from Pb will eliminate the dispersion in plot. R. Michaels, Jlab PAVI -11 5

Thanks, Alex Brown PREX Workshop 2008 E/N R. Michaels, Jlab PAVI -11 Skx-s 15

Thanks, Alex Brown PREX Workshop 2008 E/N R. Michaels, Jlab PAVI -11 Skx-s 20

Thanks, Alex Brown PREX Workshop 2008 Skx-s 25 E/N R. Michaels, Jlab PAVI -11 8

Application: Atomic Parity Violation • Low Q 2 test of Standard Model • Needs RN (or APV measures RN ) Isotope Chain Experiments e. g. Berkeley Yb APV R. Michaels, Jlab PAVI -11 9

PREX & Neutron Stars C. J. Horowitz, J. Piekarewicz RN calibrates equation of state (pressure vs density) of Neutron Rich Matter Fig from: Dany Page. J. M. Lattimer & M. Prakash, Science 304 (2004) 536. 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) ? 0. 2 fm R. Michaels, Jlab PAVI Crab Pulsar -11 URCA probable, else not

PREX Overview Parity: “The entire lab is the experiment” Spectometers Lead Foil Target Hall A JLAB R. Michaels, Jlab PAVI -11 Pol. Source CEBAF

Parity Quality Beam ! Points: Not sign corrected ( why we love Jlab ! ) Helicity – Correlated Position Differences Average with signs = what exp’t feels < ~ 3 nm Units: microns Slug # R. Michaels, Jlab PAVI -11 ( ~ 1 day)

(NEW for PREX) 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 B 2). Flips spin without moving the beam ! Electron Beam SPIN Joe Grames, et. al. R. Michaels, Jlab PAVI -11 13

Hall A Compton Upgrade with Green Laser Sirish Nanda, et. al. (Megan Friend’s talk) 1 % Polarimetry at 1 Ge. V R. Michaels, Jlab PAVI -11

Magnet and Target Hall A Moller Upgrade Sasha Glamazdin, et. al. (talk Thurs) Superconducting Magnet from Hall C Saturated Iron Foil Targets < 1 % Polarimetry DAQ Upgrade (FADC) R. Michaels, Jlab PAVI -11

Hall A High Resolution Spectrometers • Resolve Elastic Scattering • Discriminate Excited States Elastic Inelastic detector Pure, Thin 208 Pb Target 2. 6 Me. V target Dipole DETECTOR footprint Quads Scattered Electron’s R. Michaels, Jlab PAVI -11 Momentum (Ge. V/c) 16

Lead / Diamond Target Diamond • Three bays • Lead (0. 5 mm) sandwiched by diamond (0. 15 mm) • Liquid He cooling (30 Watts) R. Michaels, Jlab PAVI -11 LEAD

Performance of Lead / Diamond Targets melted NOT melted Last 4 days at 70 u. A melted Targets with thin diamond backing (4. 5 % background) degraded fastest. Thick diamond (8%) ran well and did not melt at 70 u. A. Solution: Run with 10 targets. R. Michaels, Jlab PAVI -11

Beam-Normal Asymmetry in elastic electron scattering i. e. spin transverse to scattering plane y Possible systematic if small transverse spin component AT > 0 means + x - z New results PREX ry n tio ra a in prepa limn in e r io P ! at c bli Pu • Small AT for • AT for R. Michaels, Jlab PAVI -11 208 Pb is a big (but pleasant) surprise. 12 C qualitatively consistent with 4 He and available calculations (1) Afanasev ; (2) Gorchtein & Horowitz 19

PREX-I Result Systematic Errors Error Source Absolute (ppm) Relative ( %) Polarization (1) 0. 0071 1. 1 Beam Asymmetries (2) 0. 0072 1. 1 Detector Linearity 0. 0071 1. 1 BCM Linearity 0. 0010 0. 2 Rescattering 0. 0001 0 Transverse Polarization 0. 0012 0. 2 Q 2 (1) 0. 0028 0. 4 Target Thickness 0. 0005 0. 1 12 C 0. 0025 0. 4 Inelastic States 0 0 TOTAL 0. 0130 2. 0 Asymmetry (2) Statistics limited ( 9% ) Systematic error goal achieved ! (2%) (1) Normalization Correction applied (2) Nonzero correction (the rest assumed zero) R. Michaels, Jlab PAVI -11 20

PREX Asymmetry (Pe x A) ppm Slug ~ 1 day R. Michaels, Jlab PAVI -11

Asymmetry leads to RN * Establishing a neutron skin at ~90 % CL Neutron Skin = RN - RP = 0. 31 + 0. 15 - 0. 17 fm fig from C. J. Horowitz PREX data * Preliminary: R. Michaels, Jlab PAVI -11 Awaiting the “final” acceptance function:

Neutron Skin = R N - RP = 0. 31 + 0. 15 - 0. 17 fm cont. DATA r. N - r P (fm) PREX-I Result, theory: P. Ring r N = r. P Atomic Number, A DATA Preliminary: Awaiting the “final” acceptance function A physics letter is in preparation for publication. R. Michaels, Jlab PAVI -11 23

PREX-II Approved by PAC “A” Rating 35 days R. Michaels, Jlab PAVI -11 (Aug 2011) run in 2013 / 2014

PREX Region After Target Improvements for PREX-II Tungsten Collimator & Shielding HRS-L Septum Magnet Q 1 target HRS-R Q 1 Former O-Ring location which failed & caused time loss during PREX-II R. Michaels, Jlab PAVI -11 to use all-metal seals Collimators

Geant 4 Radiation Calculations J. Mammei, L. Zana scattering chamber PREX-II shielding strategies shielding Number of Neutrons per incident Electron 0 - 1 Me. V beamline Energy (Me. V) 1 - 10 Me. V Strategy ------- PREX-II, no shield PREX-II, shielded • Tungsten ( W ) plug Energy (Me. V) 10 - 1200 Me. V • Shield the W • x 10 reduction in 0. 2 to 10 Me. V neutrons R. Michaels, Jlab PAVI -11 Energy (Me. V) 26

Other Nuclei ? RN Shape Dependence ? Surface thickness Parity Violating Electron Scattering Measurements of Neutron Densities Shufang Ban, C. J. Horowitz, R. Michaels ar. Xiv: 1010. 3246 [nucl-th] R. Michaels, Jlab PAVI -11 RN Surface thickness

Possible Future PREX Program ? Each point 30 days Nucleus 208 Pb Not yet proposed. Just a “what if ? ” stat. error only E (Ge. V) d. RN / RN comment 1 1% PREX-II (approved) 48 Ca 2. 2 (1 -pass) 0. 4 % natural 12 Ge. V exp’t 48 Ca 2. 6 2% surface thickness 40 Ca 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 Shufang Ban, C. J. Horowitz, R. Michaels, Jlab PAVI -11 ar. Xiv: 1010. 3246 [nucl-th]

Future in Hall A at JLab Early Experiments g 2 p/GEp 12 mo. Shutdown no promised beam Super. Bigbite Commissioning $ Beam 1 st to Hall A Moller $$$ SOLID $$$$ PREX – II ? 2011 2012 R. Michaels, Jlab PAVI -11 2013 2014 2015 2016 2017 2018

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 PAVI -11 (runs in 2013 or 2014 we hope )

Extra Slides R. Michaels, Jlab PAVI -11

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 PAVI -11 Involve strong probes Most spins couple to zero. (weak interaction) MFT fit mostly by data other than neutron densities

Electron - Nucleus Potential electromagnetic 208 Pb is spin 0 axial is small, best observed by parity violation neutron weak charge >> proton weak charge Proton form factor Parity Violating Asymmetry R. Michaels, Jlab PAVI -11 Neutron form factor

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 PAVI -11 Horowitz, et. al. PRC 63 025501

Optimum Kinematics for Lead Parity: <A> = 0. 5 ppm. E = 1 Ge. V if Accuracy in Asy 3% Fig. of merit Min. error in R n maximize: 1 month run 1% in R 5 PAVI 09 R. Michaels, Jlab PAVI -11 n (2 months x 100 u. A 0. 5% if no systematics)

Pull Plot (example) R. Michaels, Jlab PAVI -11 PREX Data

Neutron Star Crust vs Pb Neutron Skin Liquid/Solid Transition Density C. J. Horowitz, J. Piekarawicz Liquid FP Neutron Star 208 Pb 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 PAVI -11 TM 1

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 PAVI -11

208 Pb PREX: pins down the symmetry energy (1 parameter) energy cost for unequal # protons & neutrons ( R. J. Furnstahl ) PREX error bar R. Michaels, Jlab PAVI -11 Actually, it’s th density depend a 4 that we pin PREX