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Lead ( P b) R adius Ex periment : PREX

Lead ( P b) R adius Ex periment : PREX. 208. E = 850 MeV , electrons on lead. Elastic Scattering Parity Violating Asymmetry. 0. Z of Weak Interaction :. Clean Probe Couples Mainly to Neutrons. ( T.W. Donnelly, J. Dubach, I Sick ).

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Lead ( P b) R adius Ex periment : PREX

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

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

  3. Reminder: Electromagnetic Scattering determines (charge distribution) 208 Pb 1 2 3 R. Michaels Jefferson Lab

  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 R. Michaels Jefferson Lab

  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. R. Michaels Jefferson Lab

  6. Electron - Nucleus Potential axial electromagnetic is small, best observed by parity violation neutron weak charge >> proton weak charge Proton form factor Neutron form factor Parity Violating Asymmetry R. Michaels Jefferson Lab

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

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

  9. 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 R. Michaels Jefferson Lab

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

  11. 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 R. Michaels Jefferson Lab

  12. 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. Fig. fromJ.M. Lattimer & M. Prakash, Science 304 (2004) 536. R. Michaels Jefferson Lab

  13. Pb Radius vs Neutron Star Radius ( C.J. Horowitz, J. Piekarweicz ) • The 208Pb 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… R. Michaels Jefferson Lab

  14. 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. ( C.J. Horowitz, J. Piekarweicz ) Rn-Rp in 208Pb If Yp > red line NS cools quickly via direct URCA reaction n p+e+ R. Michaels Jefferson Lab

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

  16. 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 R. Michaels Jefferson Lab

  17. 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 R. Michaels Jefferson Lab

  18. 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 R. Michaels Jefferson Lab

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

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

  21. Polarized e- Source Hall A Hall A at Jefferson Lab R. Michaels Jefferson Lab

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

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

  24. 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 R. Michaels Jefferson Lab

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

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

  27. 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 R. Michaels Jefferson Lab

  28. 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 R. Michaels Jefferson Lab

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

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

  31. Electron only Photon only Preliminary: 2.5% syst (g only) Polarimetry PREX: 1 % desirable 2 % required Møller : dPe/Pe ~ 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: Pe=86% ! R. Michaels Jefferson Lab

  32. Upgrade of Compton Polarimeter (Nanda, Lhuillier) in ~ 1.5 years electrons 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 R. Michaels Jefferson Lab

  33. 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 R. Michaels Jefferson Lab

  34. 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. R. Michaels Jefferson Lab

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

  36. 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. R. Michaels Jefferson Lab

  37. 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 R. Michaels Jefferson Lab

  38. 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. R. Michaels Jefferson Lab

  39. 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 R. Michaels Jefferson Lab

  40. 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 ) R. Michaels Jefferson Lab

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