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Physics Interpretation of PREX
Elastic Scattering Parity Violating Asymmetry E = 1 GeV, electrons on lead Physics Analysis Can one Q2 determine RN ? Follow-up measurements other Q2 , other nuclei Theory Questions 208Pb Trento PREX Workshop 09
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Parity Violating Asymmetry
Electron - Nucleus Potential electromagnetic axial 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 Trento PREX Workshop 09
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PREX Physics Analysis Mean Field & Other Atomic Parity Violation
Measured Asymmetry Physics Analysis Correct for Coulomb Distortions Weak Density at one Q 2 Mean Field Small Corrections for n s & Other Atomic Parity Violation G G MEC E E Models 2 Neutron Density at one Q Assume Surface Thickness Good to 25% (MFT) Neutron Stars from C.J. Horowitz R n Trento PREX Workshop 09
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Reminder: Electromagnetic Scattering determines
(charge distribution) 208 Pb 1 2 3 Trento PREX Workshop 09
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PREX: Measurement at one Q is sufficient to measure R
2 Measurement at one Q is sufficient to measure R N ( R.J. Furnstahl ) Why only one parameter ? (next slide…) proposed error * * 2/3 this error if uA, dPe/Pe = 1% Trento PREX Workshop 09
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Pb PREX: pins down the symmetry energy (1 parameter)
energy cost for unequal # protons & neutrons ( R.J. Furnstahl ) Actually, it’s the density dependence of a4 that we pin down. 208 Pb PREX Trento PREX Workshop 09
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Skx-s15 Thanks, Alex Brown PREX Workshop 2008 E/N
Trento PREX Workshop 09
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Thanks, Alex Brown PREX Workshop 2008 Skx-s20 Trento PREX Workshop 09
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Thanks, Alex Brown PREX Workshop 2008 Skx-s25 Trento PREX Workshop 09
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Relationship of Measured Asymmetry to Theory
PREX Expt provides these Differential Cross Section Acceptance Function will be measured, presently simulated Theoretical Asymmetry to be compared Trento PREX Workshop 09
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Simulated Acceptance It will also be measured !
(integrated over azimuth) Trento PREX Workshop 09
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Examples of Theoretical Models
Calculations by C. J. Horowitz Acceptance Trento PREX Workshop 09
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Simulation of Observed Asymmetry for various Models
Thanks, C.J. Horowitz Trento PREX Workshop 09
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Application: Atomic Parity Violation
Low Q test of Standard Model Needs RN (or APV measures RN ) 2 Isotope Chain Experiments e.g. Berkeley Yb Shape dependence enters APV similarly to PVES. Nearly a direct application. Analysis by Steve Pollock PRC 63, (2001). (lucky accident of PVES kinematics) Trento PREX Workshop 09
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Theory Questions : Corrections to the Asymmetry
Coulomb Distortions ~20% = the biggest correction. Under control Transverse Asymmetry (to be measured) Two – photon exchange effects -- including Dispersion Corrections (intermediate excited state) Strangeness Electric Form Factor of Neutron Parity Admixtures Meson Exchange Currents Shape Dependence Isospin Corrections Radiative Corrections Excited States Target Impurities How well do we really know these ? Horowitz, et.al. PRC Trento PREX Workshop 09
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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 uA 0.5% if no systematics) 5 Trento PREX Workshop 09
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Fine-Tuning: At 50 the Optimal FOM is at 1.05 GeV (+/- 0.05)
Trento PREX Workshop 09
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Optimization for Barium -- of possible direct use for Atomic PV
1 GeV optimum Trento PREX Workshop 09
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Here ? Higher Q2 Point ? ( e.g. E = 1.3 GeV, 80, q = 0.92 fm-1 )
For Lead Here ? 5 Trento PREX Workshop 09
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Time to make measurements
JLab Spectrometers (2 HRS) For PREX (optimal) % in RN days Higher Q2 point ( E = 1.3 GeV, q = 0.92 fm-1 ) 2% in RN days Trento PREX Workshop 09
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Estimates for Other Nuclei: 48Ca 120sn
Forward angle use 50 septum Want thick target % X0 = practical max Radiative losses estimated ~ ~ Tlen (mm) 48Ca 120Sn 208Pb Trento PREX Workshop 09
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Each a 2 month run E (GeV) Rate % Error 48Ca 1.7 2.0 3.4 % 25 3.5
For effectively 1% in RN 48Ca sn Each a month run max E (GeV) Rate (MHz / arm) Sensitivity to RN % Error ppm 48Ca % 120Sn % These should match for 1% in RN Trento PREX Workshop 09
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PREX : Summary of Future Options
Do PREX better RN to ~ 0.6 % ?! Higher Q2 point Pb marginal 138 Ba 48 Ca 120 Sn ~ 2 month beam (each) Thanks: C. J. Horowitz, P. Souder, K. Kumar Trento PREX Workshop 09
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