Lead ( Pb) Radius Experiment : PREX Elastic Scattering Parity Violating Asymmetry 5 E = 1 GeV, electrons on lead Spokespersons Paul Souder, Krishna Kumar Guido Urciuoli, Robert Michaels Graduate Students Ahmed.
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Lead ( 208 Pb) Radius Experiment : PREX Elastic Scattering Parity Violating Asymmetry 5 E = 1 GeV, electrons on lead 0 Spokespersons Paul Souder, Krishna Kumar Guido Urciuoli, Robert Michaels Graduate Students Ahmed Zafar, Chun Min Jen, Abdurahim Rakham (Syracuse) 208Pb Jon Wexler (UMass) Kiadtisak Saenboonruang (UVa) R. Michaels, Jlab UGM, June, 2011 Ran March – June 2010 in Hall A Idea behind Z 0 PREX of Weak Interaction : Clean Probe Couples Mainly to Neutrons ( T.W. Donnelly, J. Dubach, I Sick 1989 ) In PWIA (to illustrate) : d d GF Q 2 d R d L A 2 2 d d d R d L 2 F n (Q ) 2 1 4 sin W 2 F ( Q ) P 0 w/ Coulomb distortions (C. J. Horowitz) : dA 3% A R. Michaels, Jlab UGM, June, 2011 dRn 1% Rn PREX Physics Output Measured Asymmetry Correct for Coulomb Distortions Weak Density at one Q 2 Mean Field & Other Models Small Corrections for Atomic Parity Violation G n E s GE MEC 2 Neutron Density at one Q Assume Surface Thickness Good to 25% (MFT) Stars Slide adapted from C. Horowitz Rn R. Michaels, Jlab UGM, June, 2011 Neutron 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 ? R. Michaels, Jlab UGM, June, 2011 Reminder: Electromagnetic Scattering determines r (charge distribution) 208 d d m b d d str 1 R. Michaels, Jlab UGM, June, 2011 Pb r 2 q fm 1 3 Z 0 of weak interaction : sees the neutrons Analysis is clean, like electromagnetic scattering: 1. Probes the entire nuclear volume 2. Perturbation theory applies proton neutron Electric charge 1 0 Weak charge 0.08 1 R. Michaels, Jlab UGM, June, 2011 6 Electron - Nucleus Potential Vˆ (r ) V (r ) 5 A(r ) axial electromagnetic / V (r ) d r Z (r ) | r r | 3 / 208 / A(r ) d d | FP (Q 2 ) | 2 d d Mott FP (Q 2 ) 1 4 3 d r j0 (qr ) P (r ) 2 2 (1 4 sin 2 W ) Z P ( r ) N N ( r ) A(r ) is small, best observed by parity violation Pb is spin 0 Proton form factor GF 1 4 sin 2 W 1 neutron weak charge >> proton weak charge Neutron form factor FN (Q 2 ) 1 4 d 3 r j 0 (qr ) N (r ) Parity Violating Asymmetry d d GF Q 2 d R d L A 2 2 d d d R d L R. Michaels, Jlab UGM, June, 2011 FN (Q 2 ) 2 1 4 sin W 2 F ( Q ) P 0 PREX: 2 Measurement at one Q is sufficient to measure R N ( R.J. Furnstahl ) Why only one parameter ? (next slide…) proposed error R. Michaels, Jlab UGM, June, 2011 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 N Z E(n, x) E(n, x 1/ 2) S (n) (1 2 x 2 ) n n.m. density x ratio proton/neutrons • Slope unconstrained by data 208 • Adding R N from Pb will eliminate the dispersion in plot. R. Michaels, Jlab UGM, June, 2011 Thanks, Alex Brown Skx-s15 PREX Workshop 2008 E/N N R. Michaels, Jlab UGM, June, 2011 Thanks, Alex Brown PREX Workshop 2008 R. Michaels, Jlab UGM, June, 2011 Skx-s20 Thanks, Alex Brown PREX Workshop 2008 R. Michaels, Jlab UGM, June, 2011 Skx-s25 Application: Atomic Parity Violation 2 • Low Q test of Standard Model • Needs RN H PNC GF 2 2 N Isotope Chain Experiments e.g. Berkeley Yb (or APV measures RN ) N (r ) Z (1 4 sin 2 W ) P (r ) e/ 5 e d 3 r 0 APV R. Michaels, Jlab UGM, June, 2011 13 Application : Neutron Stars What is the nature of extremely dense matter ? Do collapsed stars form “exotic” phases of matter ? (strange stars, quark stars) Crab Nebula R. Michaels, Jlab UGM, June, 2011 (X-ray, visible, radio, infrared) PREX & Neutron Stars ( C.J. Horowitz, J. Piekarewicz ) R N calibrates EOS of Neutron Rich Matter Crust Thickness Explain Glitches in Pulsar Frequency ? Combine PREX R N with Obs. Neutron Star Radii Phase Transition to “Exotic” Core ? Strange star ? Quark Star ? Some Neutron Stars seem too Cold Cooling by neutrino emission (URCA) Crab Pulsar R. Michaels, Jlab UGM, June, 2011 Rn Rp 0.2 fm URCA probable, else not How to do a Parity Experiment (integrating method) Flux Integration Technique: HAPPEX: 2 MHz PREX: 500 MHz R. Michaels, Jlab UGM, June, 2011 Example : HAPPEX Pull Plot (example) R. Michaels, Jlab UGM, June, 2011 PREX Data ( A A)/ Beam Asymmetries Araw = Adet - AQ + E+ ixi Slopes from •natural beam jitter (regression) •beam modulation (dithering) PAVI 09 R. Michaels, Jlab UGM, June, 2011 18 Parity Quality Beam ! Points: Not sign corrected Helicity – Correlated Position Differences < ~ 3 nm Average with signs = what exp’t feels X R X L Wien Flips helped ! for helicity L, R Units: microns Slug # R. Michaels, Jlab UGM, June, 2011 ( ~ 1 day) Hall A Compton Upgrade with Green Laser Sirish Nanda, et. al. 1 % Polarimetry at 1 GeV R. Michaels, Jlab UGM, June, 2011 Magnet and Target Hall A Moller Upgrade Sasha Glamazdin, et.al. Superconducting Magnet from Hall C Saturated Iron Foil Targets 1 % Polarimetry DAQ Upgrade (FADC) R. Michaels, Jlab UGM, June, 2011 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 R. Michaels, Jlab UGM, June, 2011 Planned Tungsten Collimator and Shielding PREX Region After Target Top view HRS-L Septum Magnet Q1 target HRS-R Q1 Former O-Ring location which failed and caused significant time loss R. Michaels, Jlab UGM, June, 2011 Collimators PREX-II proposal Geant 4 Radiation Calculations J. Mammei, L. Zana • benchmarking against LD2 (ran at 100 uA) • exploring shielding strategies Number of Neutrons per incident Electron 0 - 1 MeV W LD2 tgt Pb tgt 10 m Z Strategy 1 - 50 MeV 0.70 30 50 - 500 MeV • x10 reduction in 0.2 to 10 MeV R. Michaels, Jlabneutrons (similar UGM, June, 2011 to LD2) (distance along beamline) PREX -I PREX -II • Tungsten ( W ) plug • Shield the W 1 m -1 m High Resolution Spectrometers Pure, Thin 208 Pb Lead 5State Target 2.6 MeV DETECTOR footprint Momentum (GeV/c) R. Michaels, Jlab UGM, June, 2011 25 Lead / Diamond Target Diamond • Three bays • Lead (0.5 mm) sandwiched by diamond (0.15 mm) • Liquid He cooling (30 Watts) R. Michaels, Jlab UGM, June, 2011 LEAD Performance of Lead / Diamond Targets melted NOT melted Last 4 days at 70 uA melted Targets with thin diamond backing (4.5 % background) degraded fastest. Thick diamond (8%) ran well and did not melt at 70 uA. Solution: Run with 10 targets. R. Michaels, Jlab UGM, June, 2011 Beam-Normal Asymmetry in elastic electron scattering i.e. spin transverse to scattering plane AT S e ( k e k 'e ) Possible systematic if small transverse spin component New results PREX 208 12 R. Michaels, Jlab UGM, June, 2011 AT > 0 means + S x k - z Pb: AT 0.13 0.19 0.36 ppm C : AT 6.52 0.36 0.35 ppm • Small AT for • AT for y 12C 208Pb is a big (but pleasant) surprise. qualitatively consistent with 4He and available calculations (1) Afanasev ; (2) Gorchtein & Horowitz Systematic Errors Error Source Absolute (ppm) Relative ( % ) 0.0071 1.1 Beam Asymmetries (2) 0.0072 1.1 Detector Linearity 0.0071 1.1 BCM Linearity 0.0010 0.2 Rescattering 0.0001 0 Transverse Polarization 0.0012 0.2 Q2 (1) 0.0028 0.4 Target Thickness 0.0005 0.1 0.0025 0.4 Inelastic States 0 0 TOTAL 0.0130 2.0 Polarization (1) 12C Asymmetry (2) (1) Normalization Correction applied R. Michaels, Jlab UGM, June, 2011 (2) Nonzero correction (the rest assumed zero) PREX Physics Result APV R L R L 0.6571 0.0604(stat) 0.0130(syst ) ppm 9.2 % 2.0 % at Q2 = 0.00906 GeV2 Statistics limited ( 9% ) Systematic error goal achieved ! (2%) R. Michaels, Jlab UGM, June, 2011 PREX Asymmetry ppm (Pe x A) (blinded, raw) Slug ~ 1 day R. Michaels, Jlab UGM, June, 2011 PREX Physics Interpretation R. Michaels, Jlab UGM, June, 2011 Asymmetry leads to RN Establishing a neutron skin at 95% CL Neutron Skin = RN - RP = 0.34 + 0.15 - 0.17 fm R. Michaels, Jlab UGM, June, 2011 Shufang Ban, C.J. Horowitz, R. Michaels arXiv:1010.3246 [nucl-th] Future ? R. Michaels, Jlab UGM, June, 2011 PREX – II Proposal Future ? R. Michaels, Jlab UGM, June, 2011 Other Nuclei ? RN Shape Dependence ? Surface thickness Parity Violating Electron Scattering Measurements of Neutron Densities Shufang Ban, C.J. Horowitz, R. Michaels arXiv:1010.3246 [nucl-th] R. Michaels, Jlab UGM, June, 2011 RN Surface thickness PREX : Summary • Fundamental Nuclear Physics with many applications • 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 • Proposal for PREX-II in preparation R. Michaels, Jlab UGM, June, 2011 Extra Slides R. Michaels, Jlab UGM, June, 2011 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 R. Michaels, Jlab UGM, June, 2011 Horowitz, et.al. PRC 63 025501 Optimum Kinematics for Lead Parity: <A> = 0.5 ppm. E = 1 GeV if Accuracy in Asy 3% Fig. of merit Min. error in R n maximize: 1 month run 1% in R PAVI 09 R. Michaels, Jlab UGM, June, 2011 n (2 months x 100 uA 0.5% if no systematics) Neutron Star Crust vs Pb Neutron Skin Liquid/Solid Transition Density C.J. Horowitz, J. Piekarawicz Liquid FP Neutron Star 208Pb Solid • Thicker neutron skin in Pb means energy rises rapidly with density Quickly favors uniform phase. • Thick skin in Pb low transition density in star. R. Michaels, Jlab UGM, June, 2011 TM1 PREX: pins down the symmetry energy E N Z av a 4 A A ( R.J. Furnstahl ) 2 as / A 1/ 3 ... (1 parameter) energy cost for unequal # protons & neutrons PREX error bar ( 1 ) 208 Actually, it’s the density dependence of a4 that we pin down. Pb PREX R. Michaels, Jlab UGM, June, 2011