1. R. Michaels, Jlab UGM, June, 2011 Lead ( Pb) Radius Experiment : PREX 208 208 Pb E = 1 GeV, electrons on lead Elastic Scattering Parity Violating Asymmetry Spokespersons Paul Souder, Krishna Kumar Guido Urciuoli, Robert Michaels Ran March – June 2010 in Hall A Graduate Students Ahmed Zafar, Chun Min Jen, Abdurahim Rakham (Syracuse) Jon Wexler (UMass) Kiadtisak Saenboonruang (UVa)

2. R. Michaels, Jlab UGM, June, 2011 Idea behind PREX Z of Weak Interaction : Clean Probe Couples Mainly to Neutrons ( T.W. Donnelly, J. Dubach, I Sick 1989 ) 0 In PWIA (to illustrate) : w/ Coulomb distortions (C. J. Horowitz) :

3. R. Michaels, Jlab UGM, June, 2011 Measured Asymmetry Weak Density at one Q 2 Neutron Density at one Q 2 Correct for Coulomb Distortions Small Corrections for G n E G s E MEC Assume Surface Thickness Good to 25% (MFT) Atomic Parity Violation Mean Field & Other Models Neutron Stars R n PREX Physics Output Slide adapted from C. Horowitz

4. R. Michaels, Jlab UGM, June, 2011 Fundamental Nuclear Physics : What is the size of a nucleus ? Neutrons are thought to determine the size of heavy nuclei like 208 Pb. Can theory predict it ?

5. R. Michaels, Jlab UGM, June, 2011 Reminder: Electromagnetic Scattering determines Pb 208 (charge distribution) 123

6. R. Michaels, Jlab UGM, June, 2011 Z of weak interaction : sees the neutrons proton neutron Electric charge 1 0 Weak charge 0.08 1 0 Analysis is clean, like electromagnetic scattering: 1. Probes the entire nuclear volume 2. Perturbation theory applies 6

7. R. Michaels, Jlab UGM, June, 2011 neutron weak charge >> proton weak charge is small, best observed by parity violation Electron - Nucleus Potential electromagnetic axial Neutron form factor Parity Violating Asymmetry Proton form factor Pb is spin 0 208

8. R. Michaels, Jlab UGM, June, 2011 ( R.J. Furnstahl ) Measurement at one Q is sufficient to measure R 2 N proposed error Why only one parameter ? (next slide…) PREX:

9. R. Michaels, Jlab UGM, June, 2011 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 Adding R from Pb will eliminate the dispersion in plot. N 208 Slide adapted from J. Piekarewicz

10. R. Michaels, Jlab UGM, June, 2011 Skx-s15 Thanks, Alex Brown PREX Workshop 2008 E/N

11. R. Michaels, Jlab UGM, June, 2011 Skx-s20 Thanks, Alex Brown PREX Workshop 2008

12. R. Michaels, Jlab UGM, June, 2011 Skx-s25 Thanks, Alex Brown PREX Workshop 2008

13. R. Michaels, Jlab UGM, June, 2011 Application: Atomic Parity Violation Low Q test of Standard Model Needs R N (or APV measures R N ) 2 Isotope Chain Experiments e.g. Berkeley Yb 13 APV

14. R. Michaels, Jlab UGM, June, 2011 Neutron Stars 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) Application :

15. R. Michaels, Jlab UGM, June, 2011 PREX & Neutron Stars Crab Pulsar ( C.J. Horowitz, J. Piekarewicz ) R calibrates EOS of Neutron Rich Matter Combine PREX R with Obs. Neutron Star Radii Some Neutron Stars seem too Cold N N Crust Thickness Explain Glitches in Pulsar Frequency ? Strange star ? Quark Star ? Cooling by neutrino emission (URCA) 0.2 fm URCA probable, else not Phase Transition to Exotic Core ?

16. R. Michaels, Jlab UGM, June, 2011 How to do a Parity Experiment Flux Integration Technique: HAPPEX: 2 MHz PREX: 500 MHz (integrating method) Example : HAPPEX

17. R. Michaels, Jlab UGM, June, 2011 Pull Plot (example) PREX Data

18. R. Michaels, Jlab UGM, June, 2011 PAVI 09 Beam Asymmetries A raw = A det - A Q + E + i x i natural beam jitter (regression) beam modulation (dithering) Slopes from 18

19. R. Michaels, Jlab UGM, June, 2011 Slug # ( ~ 1 day) Units: microns Parity Quality Beam ! Helicity – Correlated Position Differences < ~ 3 nm Wien Flips helped ! Average with signs = what expt feels Points: Not sign corrected

20. R. Michaels, Jlab UGM, June, 2011 Hall A Compton Upgrade with Green Laser Sirish Nanda, et. al. 1 % Polarimetry at 1 GeV

21. R. Michaels, Jlab UGM, June, 2011 Hall A Moller Upgrade Superconducting Magnet from Hall C Saturated Iron Foil Targets 1 % Polarimetry Magnet and Target DAQ Upgrade (FADC) Sasha Glamazdin, et.al.

22. R. Michaels, Jlab UGM, June, 2011 High Resolution Spectrometers Elastic Inelastic detector Q Dipole Quad Spectrometer Concept: Resolve Elastic target Left-Right symmetry to control transverse polarization systematic 1 st excited state Pb 2.6 MeV

23. R. Michaels, Jlab UGM, June, 2011 Collimators Septum Magnet Top view target HRS-L Q1 HRS-R Q1 PREX Region After Target Former O-Ring location which failed and caused significant time loss Planned Tungsten Collimator and Shielding

24. R. Michaels, Jlab UGM, June, 2011 10 m -1 m Geant 4 Radiation Calculations benchmarking against LD2 (ran at 100 uA) exploring shielding strategies J. Mammei, L. Zana Number of Neutrons per incident Electron Z (distance along beamline) 1 m LD2 tgt Pb tgt 1 - 50 MeV 50 - 500 MeV 0 - 1 MeV Strategy Tungsten ( W ) plug Shield the W x10 reduction in 0.2 to 10 MeV neutrons (similar to LD2) W PREX-II proposal PREX -I PREX -II

25. R. Michaels, Jlab UGM, June, 2011 Pure, Thin 208 Pb Target Momentum (GeV/c) Lead 5- State 2.6 MeV High Resolution Spectrometers DETECTOR footprint 25

26. R. Michaels, Jlab UGM, June, 2011 Diamond LEAD Three bays Lead (0.5 mm) sandwiched by diamond (0.15 mm) Liquid He cooling (30 Watts) Lead / Diamond Target

27. R. Michaels, Jlab UGM, June, 2011 Performance of Lead / Diamond Targets Last 4 days at 70 uA Targets with thin diamond backing (4.5 % background) degraded fastest. Thick diamond (8%) ran well and did not melt at 70 uA. melted NOT melted Solution: Run with 10 targets.

28. R. Michaels, Jlab UGM, June, 2011 y z x + - A T > 0 means Beam-Normal Asymmetry in elastic electron scattering i.e. spin transverse to scattering plane Possible systematic if small transverse spin component New results PREX Small A T for 208 Pb is a big (but pleasant) surprise. A T for 12 C qualitatively consistent with 4 He and available calculations (1) Afanasev ; (2) Gorchtein & Horowitz

29. R. Michaels, Jlab UGM, June, 2011 Error SourceAbsolute (ppm)Relative ( % ) Polarization (1) 0.00711.1 Beam Asymmetries (2) 0.00721.1 Detector Linearity0.00711.1 BCM Linearity0.00100.2 Rescattering0.00010 Transverse Polarization0.00120.2 Q 2 (1) 0.00280.4 Target Thickness0.00050.1 12 C Asymmetry (2) 0.00250.4 Inelastic States00 TOTAL0.01302.0 Systematic Errors (1) Normalization Correction applied (2) Nonzero correction (the rest assumed zero)

30. R. Michaels, Jlab UGM, June, 2011 PREX Physics Result ppm at Q 2 = 0.00906 GeV 2 Statistics limited ( 9% ) Systematic error goal achieved ! (2%) 9.2 % 2.0 %

31. R. Michaels, Jlab UGM, June, 2011 PREX Asymmetry (P e x A) ppm Slug ~ 1 day (blinded, raw)

32. R. Michaels, Jlab UGM, June, 2011 PREX Physics Interpretation

33. R. Michaels, Jlab UGM, June, 2011 Asymmetry leads to R N Neutron Skin = R N - R P = 0.34 + 0.15 - 0.17 fm arXiv:1010.3246 [nucl-th] Shufang Ban, C.J. Horowitz, R. Michaels Establishing a neutron skin at 95% CL

34. R. Michaels, Jlab UGM, June, 2011 PREX – II Proposal Future ?

35. R. Michaels, Jlab UGM, June, 2011 Future ?

36. R. Michaels, Jlab UGM, June, 2011 Other Nuclei ? Shape Dependence ? RNRN RNRN Surface thickness arXiv:1010.3246 [nucl-th] Parity Violating Electron Scattering Measurements of Neutron Densities Shufang Ban, C.J. Horowitz, R. Michaels

37. R. Michaels, Jlab UGM, June, 2011 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

38. R. Michaels, Jlab UGM, June, 2011 Extra Slides

39. R. Michaels, Jlab UGM, June, 2011 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

40. R. Michaels, Jlab UGM, June, 2011 PAVI 09 Optimum Kinematics for Lead Parity: E = 1 GeV if = 0.5 ppm. Accuracy in Asy 3% n Fig. of merit Min. error in R maximize: 1 month run 1% in R n (2 months x 100 uA 0.5% if no systematics) 5

41. R. Michaels, Jlab UGM, June, 2011 Liquid/Solid Transition Density Thicker neutron skin in Pb means energy rises rapidly with density Quickly favors uniform phase. Thick skin in Pb low transition density in star. FP TM1 Solid Liquid Neutron Star Crust vs Pb Neutron Skin 208 Pb Neutron Star C.J. Horowitz, J. Piekarawicz

42. R. Michaels, Jlab UGM, June, 2011 PREX: pins down the symmetry energy (1 parameter) ( R.J. Furnstahl ) energy cost for unequal # protons & neutrons PREX PREX error bar Pb 208 Actually, its the density dependence of a 4 that we pin down.