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Electron Scattering at Jefferson Lab and The Lead Radius Experiment PREX

Electron Scattering at Jefferson Lab and The Lead Radius Experiment PREX. Robert Michaels. Jefferson Lab . (JLab). Thomas Jefferson National Accelerator Facility. Hall A at Jefferson Lab. Hall A. Hall A. B. C.

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Electron Scattering at Jefferson Lab and The Lead Radius Experiment PREX

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  1. Electron Scattering at Jefferson Lab and The Lead Radius Experiment PREX Robert Michaels Jefferson Lab . (JLab) Thomas Jefferson National Accelerator Facility

  2. Hall A at Jefferson Lab Hall A Hall A B C

  3. Jefferson Lab At-A-Glance

  4. Electron Scattering Known interaction (electromagnetic or weak) at Jefferson Lab Nucleon or Nucleus to probe Structure of the Nuclear Building Blocks How does the NN Force arise from the underlying quark and gluon structure of hadronic matter ? Understanding of confinement The Structure of Nuclei What is the structure of nuclear matter ? At what distance and energy scale does the underlying quark and gluon structure of nuclear matter become evident? Beyond the Standard Model (Electroweak Theory) High-precision frontier, complementary to high-energy. 4/53

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

  6. Standard Electroweak Model Left –handed fermion fields (quarks & leptons) = doublets under SU(2) Right-handed fields = singlets under SU(2) The Glashow-Weinberg-Salam Theory unifies the electromagnetic and weak interactions. Parity Violation p, n decay Weak charge 208 Pb of

  7. p Pb Pb Pb p p Pb Pb Pb A piece of the weak interaction violates parity(mirror symmetry) which allows to isolate it. Parity Transformation 1800 rotation Positive spin Negativespin

  8. Parity Violating Asymmetry 2 + APV from interference 208Pb 208Pb Applications of APV at Jefferson Lab • Nucleon Structure • Test of Standard Model of Electroweak • Nuclear Structure (neutron density):PREX Strangeness s s in proton (HAPPEX, G0 expts) e – e (MOLLER) or e – q (PVDIS) elastic e – p at low Q2 (QWEAK) This talk 8/53

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

  10. 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

  11. Fundamental Nuclear Physics:What is the size of a nucleus ? Neutrons are thought to determine the size of heavy nuclei like 208Pb. Can theory predict it ?

  12. Reminder: Electromagnetic Scattering determines (charge distribution) 208 Pb 1 2 3 12/53

  13. 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 10

  14. 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

  15. Using Parity Violation 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

  16. PREX: 2 Measurement at one Q is sufficient to measure R N ( R.J. Furnstahl ) Why only one parameter ? (next slide…) proposed error 16/53

  17. 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 significantly reduce the dispersion in plot. 208 N 15

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

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

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

  21. 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 Momentum transfer 21/53

  22. Neutron Stars Application : 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)

  23. pressure density Inputs: Eq. of state (EOS) PREX helps here Hydrostatics (Gen. Rel.) Astrophysics Observations Luminosity L Temp. T Mass M from pulsar timing (with corrections … ) Mass - Radius relationship Fig from: Dany Page. J.M. Lattimer & M. Prakash, Science 304 (2004) 536.

  24. 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 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

  25. Pol. Source Hall A JLAB CEBAF PREX Setup Parity: “The entire lab is the experiment” Spectometers Lead Foil Target 25/53

  26. Flux Integration Technique: HAPPEX: 2 MHz PREX: 500 MHz How to do a Parity Experiment (integrating method) Example : HAPPEX

  27. Polarized Electron Source Laser GaAs Crystal Pockel Cell flips helicity Halfwave plate (retractable, reverses helicity) Gun - e beam • Based on Photoemission from GaAs Crystal • Polarized electrons from polarized laser • Need : • Rapid, random helicity reversal • Electrical isolation from the rest of the lab • Feedback on Intensity Asymmetry

  28. P I T AEffect Important Systematic : Polarization Induced Transport Asymmetry Intensity Asymmetry Laser at Pol. Source where Transport Asymmetry drifts, but slope is ~ stable. Feedback on 28/53

  29. Intensity Feedback Adjustments for small phase shifts to make close to circular polarization Low jitter and high accuracy allows sub-ppm cumulative charge asymmetry in ~ 1 hour ~ 2 hours

  30. Perfect DoCP Intensity Asymmetry (ppm) A simplified picture: asymmetry=0 corresponds to minimized DoLP at analyzer Pockels cell voltage D offset (V) Methods to Reduce Systematics Scanning the Pockels Cell voltage = scanning the residual linear polarization (DoLP) A rotatable l/2 waveplate downstream of the P.C. allows arbitrary orientation of the ellipse from DoLP

  31. Double Wien Filter 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 31/53

  32. Beam Asymmetries Araw = Adet - AQ + E+ ixi • natural beam jitter (regression) • beam modulation (dithering) Slopes from 31

  33. 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)

  34. Compton Polarimeter to measure electron beam’s polarization (needed to normalize asymmetry) electrons Upgrade for 1% accuracy at 1 GeV • Green Laser (increased sensitivity at low E) • Integrating Method (removes some systematics of analyzing power) • New Photon & Electron Detectors 34/53

  35. PREX Compton Polarimeter Results

  36. Upgraded for PREX Moller Polarimeter Superconducting Magnet from Hall C Saturated Iron Foil Targets 1 % Accuracy in Polarization Magnet and Target Electronics/DAQ Upgrade (FADC)

  37. 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)

  38. Measure θ from Nuclear Recoil δE=Energy loss E=Beam energy MA=Nuclear mass θ=Scattering angle (these data taken during HAPPEX) Scattered Electron Energy (GeV) Recoil is large for H, small for nuclei (3X better accuracy than survey) 38/53

  39. 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.

  40. Detector Package in HRS PREX Integrating Detectors UMass / Smith DETECTORS

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

  42. 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.

  43. 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 43/53

  44. PREX-I Result Systematic Errors Physics Asymmetry • Statistics limited ( 9% ) • Systematic error goal achieved ! (2%) A physics letter was recently accepted by PRL. arXiv 1201.2568 [nucl-ex] (1) Normalization Correction applied (2) Nonzero correction (the rest assumed zero)

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

  46. Asymmetry leads to RN Establishing a neutron skin at ~95 % CL * Neutron Skin = RN - RP = 0.33 + 0.16 - 0.18 fm fig from C.J. Horowitz PREX data * Interpretation requires the acceptance function for spectrometer:

  47. Neutron Skin = RN - RP = 0.33 + 0.16 - 0.18 fm PREX-I Result, cont. DATA rN - rP (fm) theory: P. Ring rN = rP Atomic Number, A DATA A physics letter was recently accepted by PRL. arXiv 1201.2568 [nucl-ex] 47/53

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

  49. Recent Rn Predictions Can Be Tested By PREX at Full Precision d(APV)/APV ~ 3% These can be tested with d(Rn)/Rn ~ 1% PREX could provide an electroweak complement to Rn predictions from a wide range of physical situations and model dependencies Recent Rn predictions: Hebeleret al. Chiral EFT calculation of neutron matter. Correlation of pressure with neutron skin by Brown. Three-neutron forces! Steineret al. X-Ray n-star mass and radii observation + Brown correlation. (Ozel et al finds softer EOS, would suggest smaller Rn). Tamiiet al. Measurement of electric dipole polarizability of 208Pb + model correlation with neutron skin. Tsanget al. Isospin diffusion in heavy ion collisions, with Brown correlation and quantum molecular dynamics transport model. PREX-II proposed Hebeler Steiner Tamii Tsang

  50. Improvements for PREX-II Region downstream of target Tungsten Collimator & Shielding HRS-L Q1 Septum Magnet target HRS-R Q1 Location of ill-fated O-Ring which failed & caused significant time loss during PREX-I  PREX-II to use all-metal seals Collimators 50/53

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