1. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Lead ( Pb) Radius Experiment : PREX Z of Weak Interaction : Clean Probe Couples Mainly to Neutrons ( T.W. Donnelly, J. Dubach, I Sick ) 0 In PWIA (to illustrate) : w/ Coulomb distortions (C. J. Horowitz) : 208 208 Pb E = 850 MeV, electrons on lead Elastic Scattering Parity Violating Asymmetry

2. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Impact on Nuclear Physics : What is the size of a nucleus ? Is the size of a heavy nucleus determined by neutrons or by protons ?

3. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Reminder: Electromagnetic Scattering determines Pb 208 (charge distribution) 123

4. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Z of weak interaction : sees the neutrons 0 proton neutron Electric charge 1 0 Weak charge 0.08 1 Analysis is clean, like electromagnetic scattering: 1. Probes the entire nuclear volume 2. Perturbation theory applies

5. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Neutron Densities Proton-Nucleus Elastic Pion, alpha, d Scattering Pion Photoproduction Magnetic scattering Theory Predictions Fit mostly by data other than neutron densities Involve strong probes Most spins couple to zero. Therefore, PREX is a powerful check of nuclear theory.

6. PREX PAVI06 May 2006 R. Michaels Jefferson Lab 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

7. PREX PAVI06 May 2006 R. Michaels Jefferson Lab ( R.J. Furnstahl ) Measurement at one Q is sufficient to measure R 2 N PREX error bar Why only one parameter ? (next slide…) PREX:

8. PREX PAVI06 May 2006 R. Michaels Jefferson Lab PREX: pins down the symmetry energy (1 parameter) ( R.J. Furnstahl ) energy cost for unequal # protons & neutrons PREX PREX error bar Pb 208

9. PREX PAVI06 May 2006 R. Michaels Jefferson Lab 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

10. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Impact on Neutron Stars What is the nature of extremely dense matter ? Do collapsed stars form “exotic” phases of matter ?

11. PREX PAVI06 May 2006 R. Michaels Jefferson Lab PREX & Neutron Stars Crab Pulsar ( C.J. Horowitz, J. Piekarweicz ) 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 ?

12. PREX PAVI06 May 2006 R. Michaels Jefferson Lab FP TM1 Solid Liquid 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. Neutron EOS and Neutron Star Crust Fig. from J.M. Lattimer & M. Prakash, Science 304 (2004) 536.

13. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Pb Radius vs Neutron Star Radius The 208 Pb radius constrains the pressure of neutron matter at subnuclear densities. The NS radius depends on the pressure at nuclear density and above. Most interested in density dependence of equation of state (EOS) from a possible phase transition. Important to have both low density and high density measurements to constrain density dependence of EOS. –If Pb radius is relatively large: EOS at low density is stiff with high P. If NS radius is small than high density EOS soft. –This softening of EOS with density could strongly suggest a transition to an exotic high density phase such as quark matter, strange matter, color superconductor, kaon condensate… ( C.J. Horowitz, J. Piekarweicz )

14. PREX PAVI06 May 2006 R. Michaels Jefferson Lab PREX Constrains Rapid Direct URCA Cooling of Neutron Stars Proton fraction Y p for matter in beta equilibrium depends on symmetry energy S(n). R n in Pb determines density dependence of S(n). The larger R n in Pb the lower the threshold mass for direct URCA cooling. If R n -R p <0.2 fm all EOS models do not have direct URCA in 1.4 M ¯ stars. If R n -R p >0.25 fm all models do have URCA in 1.4 M ¯ stars. R n -R p in 208 Pb If Y p > red line NS cools quickly via direct URCA reaction n p+e+ ( C.J. Horowitz, J. Piekarweicz )

15. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Impact on Atomic Parity Measures atomic overlap with weak charge. Neutrons carry most weak charge

16. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Atomic Parity Violation Low Q test of Standard Model Needs R to make further progress. 2 N APV Isotope Chain Experiments e.g. Berkeley Yb

17. PREX PAVI06 May 2006 R. Michaels Jefferson Lab 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 Impact Heavy I ons

18. PREX PAVI06 May 2006 R. Michaels Jefferson Lab 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

19. PREX PAVI06 May 2006 R. Michaels Jefferson Lab PREX: Experimental Issues Spokespersons: P.A. Souder, G.M. Urciuoli, R. Michaels Hall A Collaboration Experiment

20. PREX PAVI06 May 2006 R. Michaels Jefferson Lab PREX in Hall A at JLab CEBAF Hall A Pol. Source Lead Foil Target Spectometers

21. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Hall A at Jefferson Lab Polarized e - Source Hall A

22. PREX PAVI06 May 2006 R. Michaels Jefferson Lab High Resolution Spectrometers Elastic Inelastic detector Q Dipole Quad Spectrometer Concept: Resolve Elastic target Left-Right symmetry to control transverse polarization systematic

23. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Integrating Detection PMT Calorimeter (for lead, fits in palm of hand) ADC Integrator electrons Integrate in 30 msec helicity period. Deadtime free. 18 bit ADC with < 10 nonlinearity. But must separate backgrounds & inelastics ( HRS). - 4

24. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Optimum Kinematics for Lead Parity: E = 850 MeV, = 0.5 ppm. Accuracy in Asy 3% n Fig. of merit Min. error in R maximize: 1 month run 1% in R n

25. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Optimization for Barium -- of possible direct use for Atomic PV 1 GeV optimum

26. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Beam Asymmetries A raw = A det - A Q +  E +  i  x i natural beam jitter (regression) beam modulation (dithering) Slopes from

27. PREX PAVI06 May 2006 R. Michaels Jefferson Lab “Energy” BPM BPM Y2 BPM Y1 BPM X1 BPM X2 Scale +/- 10 nm Position Diffs average to ~ 1 nm Good model for controlling laser systematics at source Accelerator setup (betatron matching, phase advance) Helicity Correlated Differences: Position, Angle, Energy slug “slug” = ~1 day running Spectacular results from HAPPEX-H show we can do PREX.

28. PREX PAVI06 May 2006 R. Michaels Jefferson Lab X (cavity) nmY (cavity) nm X (stripline) nmY (stripline) nm Redundant Position Measurements at the ~1 nm level (Helicity – correlated differences averaged over ~1 day)

29. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Lead Target Liquid Helium Coolant Pb C 208 12 Diamond Backing: High Thermal Conductivity Negligible Systematics Beam, rastered 4 x 4 mm beam

30. PREX PAVI06 May 2006 R. Michaels Jefferson Lab X (dispersive coord) (m) Y (m) Momentum (MeV) Detector Pb Elastic 208 1 st Excited State (2.6 MeV) Lead Target Tests Check rates Backgrounds (HRS is clean) Sensitivity to beam parameters Width of asymmetry HRS resolution Detector resolution Num. events Data taken Nov 2005

31. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Polarimetry Electron only Photon only Preliminary: 2.5% syst (  only) Møller :  P e /P e ~ 3 % (limit: foil polarization) (a high field target ala Hall C being considered) Compton : 2% syst. at present 2 analyses based on either electron or photon detection Superlattice: P e =86% ! PREX: 1 % desirable 2 % required

32. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Upgrade of Compton Polarimeter (Nanda, Lhuillier) To reach 1% accuracy: Green Laser (increased sensitivity at low E)  laser on-hand, being tested Integrating Method (removes some systematics of analyzing power)  developed during HAPPEX & in 2006 New Photon Detector electrons in ~ 1.5 years

33. PREX PAVI06 May 2006 R. Michaels Jefferson Lab PREX : Summary Fundamental Nuclear Physics with many applications HAPPEX & test runs have demonstrated technical aspects Polarimetry Upgrade needed Will run 1 month, perhaps in 2008

34. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Neutron Skin and Heavy – Ion Collisions Danielewicz, Lacey, and Lynch, Science 298 (2002) 1592. Impact on Heavy - Ion physics: constraints and predictions Imprint of the EOS left in the flow and fragmentation distribution.

35. PREX PAVI06 May 2006 R. Michaels Jefferson Lab B. Krusche arXiv:nucl-ex/0509003 Sept 2005 Example : Recent Pion Photoproduction This paper obtains Mean Field Theory PREX accuracy !! Proton – Nucleus Elastic:

36. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Transverse Polarization HRS-LeftHRS-Right Transverse AsymmetrySystematic Error for Parity “Error in” Left-right apparatus asymmetry Need < measure in ~ 1 hr (+ 8 hr setup) Theory est. (Afanasev) Transverse polarization Part I: Left/Right Asymmetry correctionsyst. err. < Control w/ slow feedback on polarized source solenoids.

37. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Transverse Polarization HRS-LeftHRS-Right Vertical misalignment Systematic Error for Parity Horizontal polarization e.g. from (g-2) Part II: Up/Down Asymmetry ( Note, beam width is very tiny up/down misalignment Measured in situ using 2 -piece detector. Study alignment with tracking & M.C. Wien angle feedback ( ) Need ) <<

38. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Noise Need 100 ppm per window pair Position noise already good enough New 18-bit ADCs  Will improve BCM noise. Careful about cable runs, PMTs, grounds.  Will improve detector noise. Plan: Tests with Luminosity Monitor to demonstrate capability.

39. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Warm Septum Existing superconducting septum won’t work at high L Warm low energy (1 GeV) magnet designed. Grant proposal in preparation (~100 k$) [ Syracuse / Smith College] TOSCA design P resolution ok

40. PREX PAVI06 May 2006 R. Michaels Jefferson Lab ( R.J. Furnstahl ) Measurement at one Q is sufficient to measure R 2 N Pins down the symmetry energy (1 parameter) PREX accuracy