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Lead ( 208 P b) R adius Ex periment : PREX

Lead ( 208 P b) R adius Ex periment : PREX. Elastic Scattering Parity Violating Asymmetry . E = 1 GeV , electrons on lead. PAVI – 11 Conference. Spokespersons Kent Paschke, Paul Souder, Krishna Kumar,

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Lead ( 208 P b) R adius Ex periment : PREX

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  1. Lead(208Pb) Radius Experiment : PREX Elastic Scattering Parity Violating Asymmetry E = 1 GeV, electrons on lead PAVI – 11 Conference Spokespersons Kent Paschke, Paul Souder, Krishna Kumar, Guido Urciuoli, Robert Michaels (presenting) • PREX-I Results 2010 Run • Approved PREX-II Proposal • Future : PREX-N ? (N = III, IV, V …) Thomas Jefferson National Accelerator Facility

  2. Z0 of weak interaction : sees the neutrons T.W. Donnelly, J. Dubach, I. Sick Nucl. Phys. A 503, 589, 1989 C. J. Horowitz, S. J. Pollock, P. A. Souder, R. Michaels Phys. Rev. C 63, 025501, 2001 Neutron form factor C.J. Horowitz Parity Violating Asymmetry

  3. Hall A at Jefferson Lab Hall A

  4. Measured Asymmetry PREX Physics Output 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 Slide adapted from C. Horowitz R n

  5. 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 5

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

  7. Thanks, Alex Brown PREX Workshop 2008 Skx-s20 E/N

  8. Thanks, Alex Brown PREX Workshop 2008 Skx-s25 E/N 8

  9. 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 9

  10. PREX & Neutron Stars C.J. Horowitz, J. Piekarewicz RN calibrates equation of state (pressure vs density) of Neutron Rich Matter Combine PREX RN with Observed Neutron Star Radii Fig from: Dany Page.J.M. Lattimer & M. Prakash, Science 304 (2004) 536. Phase Transition to “Exotic” Core ? Strange star ?Quark Star ? Some Neutron Stars seem too cold Explained by Cooling by neutrino emission (URCA process) ? 0.2 fm URCA probable, else not Crab Pulsar

  11. Pol. Source Hall A JLAB CEBAF PREX Overview Parity: “The entire lab is the experiment” Spectometers Lead Foil Target

  12. Parity Quality Beam ! ( why we love Jlab ! ) Helicity – Correlated Position Differences < ~ 3 nm Points: Not sign corrected Average with signs = what exp’t feels Units: microns Slug # ( ~ 1 day)

  13. Double Wien Filter (NEW for PREX) Crossed E & B fields to rotate the spin • Two Wien Spin Manipulators in series • Solenoid rotates spin +/-90 degrees (spin rotation as B but focus as B2). • Flips spin without moving the beam ! Electron Beam SPIN Joe Grames, et. al. 13

  14. Hall A Compton Upgrade with Green Laser Sirish Nanda, et. al. (Megan Friend’s talk) • 1 % Polarimetry • at 1 GeV

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

  16. Hall A High Resolution Spectrometers • Resolve Elastic Scattering • Discriminate Excited States Elastic detector Inelastic Pure, Thin 208PbTarget 2.6 MeV target Dipole DETECTOR footprint Quads Scattered Electron’s Momentum (GeV/c) 16

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

  18. 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.

  19. y AT > 0 means - x + z Beam-Normal Asymmetry in elastic electron scattering i.e. spin transverse to scattering plane Possible systematic if small transverse spin component New results PREX Preliminary ! Publication in preparation • 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 19

  20. PREX-I Result Systematic Errors • Statistics limited ( 9% ) • Systematic error goal achieved ! (2%) (1) Normalization Correction applied (2) Nonzero correction (the rest assumed zero) 20

  21. PREX Asymmetry (Pe x A) ppm Slug ~ 1 day

  22. Asymmetry leads to RN Establishing a neutron skin at ~90 % CL * Neutron Skin = RN - RP = 0.31 + 0.15 - 0.17 fm fig from C.J. Horowitz PREX data * Preliminary: Awaiting the “final” acceptance function:

  23. Neutron Skin = RN - RP = 0.31 + 0.15 - 0.17 fm PREX-I Result, cont. DATA rN - rP (fm) theory: P. Ring rN = rP Atomic Number, A DATA Preliminary: Awaiting the “final” acceptance function A physics letter is in preparation for publication. 23

  24. PREX-II Approved by PAC (Aug 2011) “A” Rating 35 days run in 2013 / 2014

  25. PREX Region After Target Improvements for PREX-II Tungsten Collimator & Shielding HRS-L Q1 Septum Magnet target HRS-R Q1 Former O-Ring location which failed & caused time loss during PREX-I  PREX-II to use all-metal seals Collimators

  26. Geant 4 Radiation Calculations PREX-II shielding strategies J. Mammei, L. Zana scattering chamber shielding Number of Neutrons per incident Electron 0 - 1 MeV beamline Energy (MeV) --- PREX-I --- PREX-II, no shield --- PREX-II, shielded 1 - 10 MeV • Strategy • Tungsten ( W) plug • Shield the W • x 10 reduction in • 0.2 to 10 MeV neutrons Energy (MeV) 10 - 1200 MeV Energy (MeV) 26

  27. Other Nuclei ? RN 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]

  28. Possible Future PREX Program ? Each point 30 days stat. error only Not yet proposed. Just a “what if ?” Shufang Ban, C.J. Horowitz, R. Michaels arXiv:1010.3246  [nucl-th]

  29. Future in Hall A at JLab Early Experiments g2p/GEp 12 mo. Shutdown Commissioning no promised beam SuperBigbite $ Beam 1st to Hall A Moller $$$ SOLID $$$$ PREX – II ? 2014 2012 2017 2018 2011 2013 2015 2016

  30. PREX : Summary • Fundamental Nuclear Physics with many applications • PREX-I 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 • PREX-II approved (runs in 2013 or 2014 we hope )

  31. Extra Slides

  32. How to MeasureNeutron Distributions, Symmetry Energy • Proton-Nucleus Elastic • Pion, alpha, d Scattering • Pion Photoproduction • Heavy ion collisions • Rare Isotopes (dripline) • Magnetic scattering • PREX(weak interaction) • Theory Involve strong probes Most spins couple to zero. MFT fit mostly by data other than neutron densities

  33. Electron - Nucleus Potential axial electromagnetic 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

  34. 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

  35. 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

  36. Pull Plot (example) PREX Data

  37. Liquid FP Solid TM1 Liquid/Solid Transition Density Neutron Star Crust vs Pb Neutron Skin C.J. Horowitz, J. Piekarawicz • 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 Star 208Pb

  38. Backgrounds that might re-scatter into the detector ? Detector cutoff Run magnets down: measure inelastic region Run magnets up : measure probability to rescatter No inelastics observed on top of radiative tail. Small systematic for tail.

  39. PREX: pins down the symmetry energy (1 parameter) 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

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