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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) :

  2. Parity Violating Asymmetry 2 + Applications of PV at Jefferson Lab Applications of PV at Jefferson Lab • Nucleon Structure (strangeness) -- HAPPEX / G0 • Standard Model Tests ( ) -- e.g. Qweak • Nuclear Structure (neutron density): PREX

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

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

  5. 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

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

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

  8. 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

  9. 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

  10. Liquid FP Solid TM1 Liquid/Solid Transition Density Neutron EOS and Neutron Star Crust ( C.J. Horowitz, J. Piekarweicz ) • 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.

  11. 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…

  12. 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+

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

  14. 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

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

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

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

  18. High Resolution Spectrometers Spectrometer Concept: Resolve Elastic 1st excited state Pb 2.6 MeV Elastic detector Inelastic Quad Left-Right symmetry to control transverse polarization systematic target Dipole Q Q

  19. Polarized Electron Source Laser GaAs Crystal Pockel Cell flips helicity Halfwave plate (retractable, reverses helicity) Gun - e beam • Rapid, random helicity reversal • Electrical isolation from rest of lab • Feedback on Intensity Asymmetry

  20. 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

  21. 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.

  22. X Angle BPM Beam Position Corrections (HAPPEX) Beam Asymmetry Results Energy: -0.25 ppb X Target: 1 nm X Angle: 2 nm Y Target : 1 nm Y Angle: <1 nm micron Corrected and Raw, Left spectrometer arm alone, Superimposed! Total correction for beam position asymmetry on Left, Right, or ALL detector:10 ppb ppm Spectacular results from HAPPEX-H show we can do PREX.

  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

  24. Flux Integration Technique: HAPPEX: 2 MHz PREX: 850 MHz The Raw Asymmetry

  25. Application of Parity Violating Electron Scattering : HAPPEX & Strange Quarks Hall AProton Parity Experiment 4 Isolating the u, d, s quark structure in protons (and He) Electromagnetic Scattering: Parity Violation can Access:

  26. Asymmetry (ppm) Slug 1H Preliminary 2006 Results Raw Parity Violating Asymmetry Araw correction ~11 ppb Helicity Window Pair Asymmetry Q2 = 0.1089 ± 0.0011GeV2 Araw = -1.418 ppm 0.105 ppm (stat)

  27. Strange FF near ~0.1 GeV2 GMs = 0.28 +/- 0.20 GEs = -0.006 +/- 0.016 ~3% +/- 2.3% of proton magnetic moment ~0.2 +/- 0.5% of electric distribution Preliminary

  28. Hydrogen: 86.7% ± 2% Helium: 84.0% ± 2.5% PolarimetryAccuracy :2 % required, 1% desired Preliminary Møller : dPe/Pe ~ 3 %(limit: foil polarization) (a high field target ala Hall C being considered) Compton : 2% syst. at present Prelim. HAPPEX Results

  29. Upgrade of Compton Polarimeter 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

  30. 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

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

  32. PREX : Summary • Fundamental Nuclear Physics with many applications • HAPPEX & test runs have demonstrated technical aspects • Polarimetry Upgrade needed • Will run 1 month in 2008

  33. Extra Slides

  34. 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

  35. Asymmetry (ppm) Slug 4He Preliminary 2006 Results Raw Parity Violating Asymmetry Araw correction ~ 0.12 ppm Helicity Window Pair Asymmetry Q2 = 0.07725 ± 0.0007 GeV2 Araw = 5.253 ppm 0.191 ppm (stat)

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

  37. 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)

  38. 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.

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

  40. 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.

  41. 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

  42. 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.

  43. 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

  44. 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 )

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