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**Lead ( Pb) Radius Experiment : PREX**

208 E = 850 MeV, electrons on lead Elastic Scattering Parity Violating Asymmetry 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) : PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Parity Transformation**

A piece of the weak interaction violates parity (mirror symmetry) which allows to isolate it. Incident electron S (spin) Target Positive longitudinal spin P (momentum) Parity Transformation 208 Pb Negative longitudinal spin PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Parity Violating Asymmetry**

2 + Applications of PV : Nucleon Structure (strangeness) -- HAPPEX / G0 Standard Model Tests ( ) -- e.g. Qweak Nuclear Structure (neutron density) : PREX PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Physics Impact PREX Mean Field & Other Atomic Parity Violation Models**

Measured Asymmetry PREX Physics Impact 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 R n PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**PREX in Hall A at JLab Spectometers Lead Foil Target Hall A CEBAF**

Pol. Source Spectometers Lead Foil Target PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Impact on Nuclear Physics: What is the size of a nucleus ?**

Is the size of a heavy nucleus determined by neutrons or by protons ? PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Reminder: Electromagnetic Scattering determines**

(charge distribution) 208 Pb PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Z of weak interaction : sees the neutrons**

Z 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 Weak charge 0.08 PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Parity Violating Asymmetry**

Electron - Nucleus Potential electromagnetic axial is small, best observed by parity violation neutron weak charge >> proton weak charge Proton form factor Neutron form factor Parity Violating Asymmetry PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Neutron Densities Proton-Nucleus Elastic Pion, alpha, d Scattering**

Pion Photoproduction Magnetic scattering Theory Predictions Involve strong probes Most spins couple to zero. Fit mostly by data other than neutron densities Therefore, PREX is a powerful check of nuclear theory. PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**!! Example : Recent Pion Photoproduction This paper obtains**

B. Krusche arXiv:nucl-ex/ Sept 2005 This paper obtains !! Proton – Nucleus Elastic: Mean Field Theory PREX accuracy PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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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…) PREX error bar PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Pb PREX: pins down the symmetry energy (1 parameter) PREX error bar**

energy cost for unequal # protons & neutrons ( R.J. Furnstahl ) PREX error bar 208 Pb PREX PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Impact on Atomic Parity**

Measures atomic overlap with weak charge. Neutrons carry most weak charge PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**APV Atomic Parity Violation Low Q test of Standard Model**

Needs R to make further progress. 2 Isotope Chain Experiments e.g. Berkeley Yb N APV PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Impact on Neutron Stars**

What is the nature of extremely dense matter ? Do collapsed stars form “exotic” phases of matter ? PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Inputs: Eq. of state (EOS) PREX constraint Hydrostatics (Gen. Rel.)**

pressure density Inputs: Eq. of state (EOS) PREX constraint Hydrostatics (Gen. Rel.) Typ. Astro. Observations Luminosity L Temp. T Mass M from pulsar timing (with corrections … ) Mass - Radius relationship Fig. from J.M. Lattimer & M. Prakash, Science 304 (2004) 536. PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**PREX & Neutron Stars Crab Pulsar**

( 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) Crab Pulsar 0.2 fm URCA probable, else not PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Liquid/Solid Transition Density**

FP TM1 Solid Liquid Neutron Star Crust vs Pb Neutron Skin Neutron Star 208Pb PREX calibrates the EOS at subnuclear densities. Thicker neutron skin in Pb means energy rises rapidly with density Quickly favors uniform phase. Thick skin in Pb low transition density in star. PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Pb Radius vs Neutron Star Radius**

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… PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**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. Rn-Rp in 208Pb If Yp > red line NS cools quickly via direct URCA reaction n p+e+ PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**PREX: Experiment Design**

Spokespersons: P.A. Souder, G.M. Urciuoli, R. Michaels Hall A Collaboration Experiment PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Hall A at Jefferson Lab Polarized e- Source Hall A R. Michaels**

PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Hall A Compton Moller Polarimeters Target Spectro: SQQDQ Cherenkov**

cones PMT Compton Moller Polarimeters Target Spectro: SQQDQ PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**High Resolution Spectrometers**

Spectrometer Concept: Resolve Elastic Elastic detector Inelastic Quad target Dipole Q Q PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**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 PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Corrections to the Asymmetry are Mostly Negligible**

Horowitz, et.al. PRC Coulomb Distortions ~20% = the biggest correction. Strangeness Electric Form Factor of Neutron Parity Admixtures Dispersion Corrections Meson Exchange Currents Shape Dependence Isospin Corrections Radiative Corrections Excited States Target Impurities PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Septum Magnets (INFN) Superconducting magnets Commissioned 2003-4**

Electrons scattered at 6 deg sent to the HRS at 12.5 deg. PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Integrating Detection**

Integrate in 30 msec helicity period. Deadtime free. 18 bit ADC with < nonlinearity. But must separate backgrounds & inelastics ( HRS). - 4 Integrator Calorimeter (for lead, fits in palm of hand) ADC PMT electrons PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Lead Target Pb C 208 12 Diamond Backing: High Thermal Conductivity**

Liquid Helium Coolant 12 beam C Diamond Backing: High Thermal Conductivity Negligible Systematics Beam, rastered 4 x 4 mm PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Polarized Electron Source**

Laser GaAs Crystal Gun Pockel Cell flips helicity Halfwave plate (retractable, reverses helicity) - e beam Rapid, random helicity reversal Electrical isolation from rest of lab Feedback on Intensity Asymmetry PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**P I T A Effect at Polarized Source**

Polarization Induced Transport Asymmetry (G. D. Cates) Intensity Asymmetry Laser at Pol. Source where Transport Asymmetry drifts, but slope is ~stable. Feedback on PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Araw = Adet - AQ + E+ ixi**

Beam Asymmetries Araw = Adet - AQ + E+ ixi natural beam jitter (regression) beam modulation (dithering) Slopes from PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Helicity Correlated Differences: Position, Angle, Energy**

BPM X1 Scale +/- 10 nm slug Position Diffs avg ~ 1 nm Redundant Monitors Stripline Monitors Resonant Cavities Negligible Systematic Error BPM X2 slug BPM Y1 slug BPM Y2 slug “Energy” BPM “slug” = ~1 day running PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Polarimetry Pe=86% ! Møller : dPe/Pe ~ 3 % (limit: foil polarization)**

PREX: 1 % desirable 2 % required Møller : dPe/Pe ~ 3 % (limit: foil polarization) Compton : 2% syst. at present Electron only Photon only Preliminary: 2.5% syst (g only) 2 analyses based on either electron or photon detection Superlattice: Pe=86% ! PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Upgrade of Compton Polarimeter (Nanda, Lhuillier)**

electrons To reach 1% accuracy: Green Laser (increased sensitivity at low E) Integrating Method (removes some systematics of analyzing power) PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**Moller Polarimetry with Atomic Hydrogen Target**

( E. Chudakov, V. Luppov, D. Crabb) H atoms Ultra Cold Traps Polarization ~ 100% Density Lifetime > 10 min Solenoid 8T Trap Polarimetry beam 1% stat. err. in 30 min at A Low background High beam currents allowed (100 A) Goal: ~ 0.5 % systematic error PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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**PREX : Summary Fundamental Nuclear Physics HAPPEX to demonstrate most**

technical aspects Polarimetry Upgrade needed PREX test run Nov (this weekend !) Experiment Runs in ? PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab

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