Lead ( P b) R adius Ex periment : PREX

1. Lead( Pb) Radius Experiment : PREX 208 Elastic Scattering Parity Violating Asymmetry E = 1 GeV, electrons on lead • Spokespersons • Paul Souder • Krishna Kumar • Robert Michaels • Guido Urciuoli 208Pb Hall A Collaboration Experiment

2. Electron - Nucleus Potential electromagnetic axial is small, best observed by parity violation neutron weak charge >> proton weak charge Proton form factor Neutron form factor Parity Violating Asymmetry APV~ 500 ±15ppb, Q2~ 0.01 GeV2

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

4. Z of weak interaction : sees the neutrons 0 Analysis is clean, like electromagnetic scattering: 1. Probes the entire nuclear volume 2. Perturbation theory applies

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

6. 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

7. 2 Measurement at one Q is sufficient to measure R Pins down the symmetry energy (1 parameter) N PREX accuracy PREX accuracy ( R.J. Furnstahl )

8. PREX & Neutron Stars ( C.J. Horowitz, J. Piekarweicz ) 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) 0.2 fm URCA probable, else not Crab Pulsar

9. Neutron EOS and Neutron Star Crust Liquid FP Solid TM1 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. Horowitz Fig. fromJ.M. Lattimer & M. Prakash, Science 304 (2004) 536.

10. Pb Radius vs Neutron Star Radius • The 208Pb radius constrains the pressure of neutron matter at subnuclear densities. • The NS radius depends on the pressure at nuclear density and above. • 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…

11. PREX Constrains Rapid Direct URCA Cooling of Neutron Stars • Proton fraction Yp for matter in beta equilibrium depends on symmetry energy S(n). • Rn in Pb determines density dependence of S(n). • The larger Rn in Pb the lower the threshold mass for direct URCA cooling. • If Rn-Rp<0.2 fm all EOS models do not have direct URCA in 1.4 M¯ stars. • If Rn-Rp>0.25 fm all models do have URCA in 1.4 M¯ stars. Rn-Rp in 208Pb Horowitz If Yp > red line NS cools quickly via direct URCA reaction n p+e+

12. Atomic Parity Violation • Low Q test of Standard Model • Needs R to make further progress. 2 Isotope Chain Experiments e.g. Berkeley Yb N APV

13. Neutron Skin and Heavy – Ion Collisions • Impact on Heavy - Ion physics: constraints and predictions • Imprint of the EOS left in the flow and fragmentation distribution. Danielewicz, Lacey, and Lynch, Science 298 (2002) 1592.

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

15. 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

16. Polarized e- Source Hall A Hall A at Jefferson Lab

17. Pol. Source Hall A CEBAF PREX in Hall A at JLab Spectometers Lead Foil Target

18. High Resolution Spectrometers Spectrometer Concept: Resolve Elastic Elastic detector Inelastic Left-Right symmetry to control transverse polarization systematic Quad target Dipole Q Q

19. Flux Integration Technique: HAPPEX: 2 MHz PREX: 850 MHz Experimental Method

20. Polarized Source High Pe High Q.E. Low Apower GaAS Photocatode • Optical pumping of solid-state photocathode • High Polarization • Pockels cell allows rapid helicity flip • Careful configuration to reduce beam asymmetries. • Slow helicity reversal to further cancel beam asymmetries controls effective analyzing power Tune residual linear pol. Slow helicity reversal Intensity Attenuator (charge Feedback)

21. P I T AEffect Important Systematic : Polarization Induced Transport Asymmetry Laser at Pol. Source Intensity Asymmetry where Transport Asymmetry drifts, but slope is ~ stable. Feedback on

22. Intensity Feedback Adjustments for small phase shifts to make close to circular polarization HAPPEX Low jitter and high accuracy allows sub-ppm Cumulative charge asymmetry in ~ 1 hour ~ 2 hours In practice, aim for 0.1 ppm over duration of data-taking.

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

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

25. Integrating Detection • Integrate in 30 msec helicity period. • Deadtime free. • 18 bit ADC with < 10-4 nonlinearity. • Backgrounds & inelastics separated (HRS). Integrator Calorimeter ADC PMT Actually two thin quartz detectors Attempt to improve resolution by replacing Alzak mirrors in light guide with anodized Al or Silver. The x, y dimensions of the quartz determined from beam test data and MC (HAMC) simulations. (11 x 14 cm) Quartz thickness to be optimized with MC. New HRS optics tune focuses elastic events both in x & y at the PREx detector location. electrons

27. Lead Target 208 Pb Liquid Helium Coolant 12 beam C Diamond Backing: • High Thermal Conductivity • Negligible Systematics Successfuly tested 5 days at 60 uA 1 shift at 80 uA 3 hrs at 100 uA Beam, rastered 4 x 4 mm

28. Polarimetry Upgrade of Compton Polarimeter electrons To reach 1% accuracy: • Green Laser  Green Fabry-Perot cavity (increased sensitivity at low E) • Integrating Method (removes some systematics of analyzing power) • New Photon and Electron Detectors (new GSO photon calorimeter, FADC based • photon integration DAQ) Upgrade Møller polarimeter: 4 Tesla field saturated iron foil, new FADC DAQ

30. Transverse Polarization Part I: Left/Right Asymmetry Transverse Asymmetry Systematic Error for Parity Theory est. (Afanasev) “Error in” Left-right apparatus asymmetry Transverse polarization Control w/ slow feedback on polarized source solenoids. measure in ~ 1 hr (+ 8 hr setup) HRS-Left HRS-Right < < Need correction syst. err.

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

33. A_T detector design Figure of Merit M = 1/E * 1/sqrt(R) * sqrt(1 + B/S)where,E = A_T enhancement for A_T hole events = 50.R = Ratio of A_T hole detector to main Pb detector event ratesB/S = Ratio of bkgd under the A_T hole events to A_T signal The optimum A_T detector dimension is ~7.6cm in x by 0.8cm in y. This gives Figure of Merit = 0.637 and error inflation ~1.186.

34. 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. • Tests with Luminosity Monitor to demonstrate capability.

35. Asymmetries in Lumi Monitors after beam noise subtraction ~ 50 ppm noise per pulse  milestone for electronics ( need < 100 ppm) Jan 2008 Data

36. PREX: Summary • PREX is an extremely challenging experiment: • APV ≈ 500 ± 15 ppb. • 1% polarimetry. • Helicity correlated beam asymmetry < 100 ± 10 ppb. • Beam position differences < 1 ± 0.1 nm. • Transverse beam polarization < 1%. • Noise < 100 ppm • (Not melting) Lead Target • Forward angle detection  Septum magnet • Precision measurement of Q2: ± 0.7%  ± 0.02° accuracy in spectrometer angles • However HAPPEX & test runs have demonstrated its feasibility. • It will run in March-May 2010 and will measure the lead neutron radius with an unprecedented accuracy (1%). This result will have an impact on many other Physics fields (neutron stars, APV, heavy ions …).

38. Spares

39. Optimum Kinematics for Lead Parity: E = 850 MeV, <A> = 0.5 ppm. Accuracy in Asy 3% Fig. of merit Min. error in R maximize: n 1 month run 1% in R n

40. Optimization for Barium -- of possible direct use for Atomic PV 1 GeV optimum

41. Redundant Position Measurements at the ~1 nm level (Helicity – correlated differences averaged over ~1 day) X (cavity) nm Y (cavity) nm X (stripline) nm Y (stripline) nm

42. Integrating Detection • Integrate in 30 msec helicity period. • Deadtime free. • 18 bit ADC with < 10-4 nonlinearity. • Backgrounds & inelastics must be separeted (HRS). Quartz / Tungsten Calorimeter Integrator ADC PMT (Also a thin quartz detector upstream of this) Attempt to improve resolution by replacing Alzak mirrors in light guide with anodized Al or Silver. The x, y dimensions of the quartz determined from beam test data and MC (HAMC) simulations. (11 x 14 cm) Quartz thickness to be optimized with MC. New HRS optics tune focuses elastic events both in x & y at the PREx detector location. electrons