Lead ( Pb) Radius Experiment : PREX 208 Elastic Scattering Parity Violating Asymmetry E = 1 GeV, electrons on lead Spokespersons Paul Souder, Krishna Kumar Guido Urciuoli, Robert Michaels Graduate Students Ahmed Zafar, Chun Min Jen, Abdurahim Rakham (Syracuse) Jon Wexler (UMass) Kiadtisak Saenboonruang (UVa) 208Pb Ran March – June 2010 in Hall A

Idea behind PREX Clean Probe Couples Mainly to Neutrons Z of Weak Interaction : Clean Probe Couples Mainly to Neutrons ( T.W. Donnelly, J. Dubach, I Sick 1989 ) In PWIA (to illustrate) : w/ Coulomb distortions (C. J. Horowitz) :

PREX Physics Output Mean Field & Other Atomic Parity Violation Models Measured Asymmetry Physics Output Correct for Coulomb Distortions Weak Density at one Q 2 Mean Field Small Corrections for n s & Other Atomic Parity Violation G G MEC E E Models 2 Neutron Density at one Q Assume Surface Thickness Good to 25% (MFT) Neutron Stars Slide adapted from C. Horowitz R n

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 ?

Reminder: Electromagnetic Scattering determines (charge distribution) 208 Pb 1 2 3

Z of weak interaction : sees the neutrons Analysis is clean, like electromagnetic scattering: 1. Probes the entire nuclear volume 2. Perturbation theory applies proton neutron Electric charge 1 Weak charge 0.08 6

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

PREX: Measurement at one Q is sufficient to measure R 2 Measurement at one Q is sufficient to measure R N ( R.J. Furnstahl ) Why only one parameter ? (next slide…) proposed error

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 ratio proton/neutrons n.m. density Slope unconstrained by data Adding R from Pb will eliminate the dispersion in plot. 208 N

Thanks, Alex Brown PREX Workshop 2008 Skx-s15 E/N

Thanks, Alex Brown PREX Workshop 2008 Skx-s20

Thanks, Alex Brown PREX Workshop 2008 Skx-s25

APV Application: Atomic Parity Violation Low Q test of Standard Model Needs RN (or APV measures RN ) 2 Isotope Chain Experiments e.g. Berkeley Yb APV 13

Neutron Stars What is the nature of extremely dense matter ? Application : What is the nature of extremely dense matter ? Do collapsed stars form “exotic” phases of matter ? (strange stars, quark stars) Crab Nebula (X-ray, visible, radio, infrared)

PREX & Neutron Stars Crab Pulsar ( C.J. Horowitz, J. Piekarewicz ) R calibrates EOS of Neutron Rich Matter N Crust Thickness Explain Glitches in Pulsar Frequency ? Combine PREX R with Obs. Neutron Star Radii N Phase Transition to “Exotic” Core ? Strange star ? Quark Star ? Some Neutron Stars seem too Cold Cooling by neutrino emission (URCA) Crab Pulsar 0.2 fm URCA probable, else not

How to do a Parity Experiment (integrating method) Flux Integration Technique: HAPPEX: 2 MHz PREX: 500 MHz Example : HAPPEX

Pull Plot (example) PREX Data

Araw = Adet - AQ + E+ ixi Beam Asymmetries Araw = Adet - AQ + E+ ixi Slopes from natural beam jitter (regression) beam modulation (dithering) PAVI 09 18

Parity Quality Beam ! < ~ 3 nm Wien Flips helped ! Helicity – Correlated Position Differences < ~ 3 nm Wien Flips helped ! Points: Not sign corrected Average with signs = what exp’t feels Units: microns Slug # ( ~ 1 day)

Hall A Compton Upgrade 1 % Polarimetry at 1 GeV with Green Laser Sirish Nanda, et. al. 1 % Polarimetry at 1 GeV

1 % Polarimetry Hall A Moller Upgrade Magnet and Target Superconducting Magnet from Hall C Saturated Iron Foil Targets 1 % Polarimetry Sasha Glamazdin, et.al. DAQ Upgrade (FADC)

High Resolution Spectrometers Spectrometer Concept: Resolve Elastic 1st excited state Pb 2.6 MeV Elastic detector Inelastic Quad Left-Right symmetry to control transverse polarization systematic target Dipole Q Q

Septum Magnet Planned Tungsten Collimator and Shielding PREX Region After Target Planned Tungsten Collimator and Shielding Top view HRS-L Q1 Septum Magnet target HRS-R Q1 Former O-Ring location which failed and caused significant time loss Collimators

Geant 4 Radiation Calculations benchmarking against LD2 (ran at 100 uA) exploring shielding strategies PREX-II proposal J. Mammei, L. Zana Number of Neutrons per incident Electron 0 - 1 MeV 1 m W LD2 tgt Pb tgt 10 m Z (distance along beamline) Strategy Tungsten ( W ) plug Shield the W x10 reduction in 0.2 to 10 MeV neutrons (similar to LD2) 1 - 50 MeV PREX -I PREX -II 50 - 500 MeV -1 m

High Resolution Spectrometers Pure, Thin 208 Pb Target Lead 5- State 2.6 MeV DETECTOR footprint Momentum (GeV/c) 25

Lead / Diamond Target Diamond LEAD Three bays Lead (0.5 mm) sandwiched by diamond (0.15 mm) Liquid He cooling (30 Watts)

Performance of Lead / Diamond Targets melted melted Targets with thin diamond backing (4.5 % background) degraded fastest. Thick diamond (8%) ran well and did not melt at 70 uA. NOT melted Last 4 days at 70 uA Solution: Run with 10 targets.

+ - Beam-Normal Asymmetry in elastic electron scattering i.e. spin transverse to scattering plane y z x + - AT > 0 means Possible systematic if small transverse spin component New results PREX Small AT for 208Pb is a big (but pleasant) surprise. AT for 12C qualitatively consistent with 4He and available calculations (1) Afanasev ; (2) Gorchtein & Horowitz

Systematic Errors Beam Asymmetries (2) Error Source Absolute (ppm) Relative ( % ) Polarization (1) 0.0071 1.1 Beam Asymmetries (2) 0.0072 Detector Linearity BCM Linearity 0.0010 0.2 Rescattering 0.0001 Transverse Polarization 0.0012 Q2 (1) 0.0028 0.4 Target Thickness 0.0005 0.1 12C Asymmetry (2) 0.0025 Inelastic States TOTAL 0.0130 2.0 (1) Normalization Correction applied (2) Nonzero correction (the rest assumed zero)

PREX Physics Result ppm Statistics limited ( 9% ) 9.2 % 2.0 % at Q2 = 0.00906 GeV2 Statistics limited ( 9% ) Systematic error goal achieved ! (2%)

PREX Asymmetry (Pe x A) ppm (blinded, raw) Slug ~ 1 day

PREX Physics Interpretation

Asymmetry leads to RN Neutron Skin = RN - RP = 0.34 + 0.15 - 0.17 fm Establishing a neutron skin at 95% CL Neutron Skin = RN - RP = 0.34 + 0.15 - 0.17 fm Shufang Ban, C.J. Horowitz, R. Michaels arXiv:1010.3246  [nucl-th]

Future ? PREX – II Proposal

Future ?

Other Nuclei ? RN Surface thickness RN Surface thickness Shape Dependence ? Surface thickness Parity Violating Electron Scattering Measurements of Neutron Densities Shufang Ban, C.J. Horowitz, R. Michaels RN Surface thickness arXiv:1010.3246  [nucl-th]

PREX : Summary Fundamental Nuclear Physics with many applications 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 Proposal for PREX-II in preparation

Extra Slides

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 Horowitz, et.al. PRC 63 025501

Optimum Kinematics for Lead Parity: E = 1 GeV if <A> = 0.5 ppm. Accuracy in Asy 3% Fig. of merit Min. error in R maximize: n 1 month run 1% in R n (2 months x 100 uA  0.5% if no systematics) 5 PAVI 09

Liquid/Solid Transition Density Neutron Star Crust vs Pb Neutron Skin FP TM1 Solid Liquid C.J. Horowitz, J. Piekarawicz Neutron Star 208Pb Thicker neutron skin in Pb means energy rises rapidly with density  Quickly favors uniform phase. Thick skin in Pb  low transition density in star.

Pb PREX: pins down the symmetry energy (1 parameter) PREX error bar energy cost for unequal # protons & neutrons PREX error bar ( R.J. Furnstahl ) Actually, it’s the density dependence of a4 that we pin down. 208 Pb PREX