Lead ( Pb) Radius Experiment : PREX Pb E = 1 GeV, electrons on lead Elastic Scattering Parity Violating Asymmetry Spokespersons Paul Souder Krishna Kumar Robert Michaels Guido Urciuoli G.M. Urciuoli Hall A Collaboration Experiment

neutron weak charge >> proton weak charge is small, best observed by parity violation Electron - Nucleus Potential electromagneticaxial Neutron form factor Parity Violating Asymmetry Proton form factor A PV ~ 500 ±15ppb, Q 2 ~ 0.01 GeV 2

G.M. Urciuoli Reminder: Electromagnetic Scattering determines Pb 208 (charge distribution) 123

G.M. Urciuoli Z of weak interaction : sees the neutrons 0 proton neutron Electric charge 1 0 Weak charge Analysis is clean, like electromagnetic scattering: 1. Probes the entire nuclear volume 2. Perturbation theory applies

G.M. Urciuoli Neutron Densities Proton-Nucleus Elastic Pion, alpha, d Scattering Pion Photoproduction Magnetic scattering Theory Predictions Fit mostly by data other than neutron densities Involve strong probes Most spins couple to zero. Therefore, PREX is a powerful check of nuclear theory.

G.M. Urciuoli 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.m. density ratio proton/neutrons Slope unconstrained by data Adding R from Pb will eliminate the dispersion in plot. N 208

G.M. Urciuoli ( R.J. Furnstahl ) Measurement at one Q is sufficient to measure R 2 N Pins down the symmetry energy (1 parameter) PREX accuracy

G.M. Urciuoli PREX & Neutron Stars Crab Pulsar ( C.J. Horowitz, J. Piekarweicz ) R calibrates EOS of Neutron Rich Matter Combine PREX R with Obs. Neutron Star Radii Some Neutron Stars seem too Cold N N Crust Thickness Explain Glitches in Pulsar Frequency ? Strange star ? Quark Star ? Cooling by neutrino emission (URCA) 0.2 fm URCA probable, else not Phase Transition to “Exotic” Core ?

G.M. Urciuoli FP TM1 Solid Liquid Liquid/Solid Transition Density Thicker neutron skin in Pb means energy rises rapidly with density  Quickly favors uniform phase. Thick skin in Pb  low transition density in star. Neutron EOS and Neutron Star Crust Fig. from J.M. Lattimer & M. Prakash, Science 304 (2004) 536. Horowitz

G.M. Urciuoli Pb Radius vs Neutron Star Radius The 208 Pb radius constrains the pressure of neutron matter at subnuclear densities. The NS radius depends on the pressure at nuclear density and above. 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…

G.M. Urciuoli PREX Constrains Rapid Direct URCA Cooling of Neutron Stars Proton fraction Y p for matter in beta equilibrium depends on symmetry energy S(n). R n in Pb determines density dependence of S(n). The larger R n in Pb the lower the threshold mass for direct URCA cooling. If R n -R p <0.2 fm all EOS models do not have direct URCA in 1.4 M ¯ stars. If R n -R p >0.25 fm all models do have URCA in 1.4 M ¯ stars. R n -R p in 208 Pb If Y p > red line NS cools quickly via direct URCA reaction n p+e+ Horowitz

G.M. Urciuoli Atomic Parity Violation Low Q test of Standard Model Needs R to make further progress. 2 N APV Isotope Chain Experiments e.g. Berkeley Yb

G.M. Urciuoli Neutron Skin and Heavy – Ion Collisions Danielewicz, Lacey, and Lynch, Science 298 (2002) Impact on Heavy - Ion physics: constraints and predictions Imprint of the EOS left in the flow and fragmentation distribution.

G.M. Urciuoli Measured Asymmetry Weak Density at one Q 2 Neutron Density at one Q 2 Correct for Coulomb Distortions Small Corrections for G n E G s E MEC Assume Surface Thickness Good to 25% (MFT) Atomic Parity Violation Mean Field & Other Models Neutron Stars R n PREX Physics Impact Heavy I ons

G.M. Urciuoli Corrections to the Asymmetry are Mostly Negligible Horowitz, et.al. PRC 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

G.M. Urciuoli Hall A at Jefferson Lab Polarized e - Source Hall A

G.M. Urciuoli PREX in Hall A at JLab CEBAF Hall A Pol. Source Lead Foil Target Spectometers

G.M. Urciuoli High Resolution Spectrometers Elastic Inelastic detector Q Dipole Quad Spectrometer Concept: Resolve Elastic target Left-Right symmetry to control transverse polarization systematic

Experimental Method Flux Integration Technique: HAPPEX: 2 MHz PREX: 850 MHz

G.M. Urciuoli controls effective analyzing power Tune residual linear pol. Slow helicity reversal Intensity Attenuator (charge Feedback) Polarized Source High P e High Q.E. Low A power Optical pumping of solid-state photocathode High Polarization Pockels cell allows rapid helicity flip Careful configuration to reduce beam asymmetries. Slow helicity reversal to further cancel beam asymmetries GaAS Photocatode

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

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 In practice, aim for 0.1 ppm over duration of data-taking. ~ 2 hours HAPPEX

G.M. Urciuoli Beam Asymmetries A raw = A det - A Q +  E +  i  x i natural beam jitter (regression) beam modulation (dithering) Slopes from

G.M. Urciuoli “Energy” BPM BPM Y2 BPM Y1 BPM X1 BPM X2 Scale +/- 10 nm Position Diffs average to ~ 1 nm Good model for controlling laser systematics at source Accelerator setup (betatron matching, phase advance) Helicity Correlated Differences: Position, Angle, Energy slug “slug” = ~1 day running Spectacular results from HAPPEX-H show we can do PREX.

G.M. Urciuoli Integrating Detection PMT Calorimeter ADC Integrator electrons Integrate in 30 msec helicity period. Deadtime free. 18 bit ADC with < nonlinearity. Backgrounds & inelastics separated (HRS). Attempt to improve resolution by replacing Alzak mirrors in light guide with anodized Al or Silver. The x, y dimensions of the quartz determined from beam test data and MC (HAMC) simulations. (11 x 14 cm) Quartz thickness to be optimized with MC. New HRS optics tune focuses elastic events both in x & y at the PREx detector location. Actually two thin quartz detectors

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Lead Target Liquid Helium Coolant Pb C Diamond Backing: High Thermal Conductivity Negligible Systematics Beam, rastered 4 x 4 mm beam 5 days at 60 uA 1 shift at 80 uA 3 hrs at 100 uA Successfuly tested

Upgrade of Compton Polarimeter To reach 1% accuracy: Green Laser  Green Fabry-Perot cavity (increased sensitivity at low E) Integrating Method (removes some systematics of analyzing power) New Photon and Electron Detectors (new GSO photon calorimeter, FADC based photon integration DAQ) electrons Upgrade Møller polarimeter: 4 Tesla field saturated iron foil, new FADC DAQ Polarimetry

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Transverse Polarization HRS-LeftHRS-Right Transverse AsymmetrySystematic Error for Parity “Error in” Left-right apparatus asymmetry Need < measure in ~ 1 hr (+ 8 hr setup) Theory est. (Afanasev) Transverse polarization Part I: Left/Right Asymmetry correctionsyst. err. < Control w/ slow feedback on polarized source solenoids.

G.M. Urciuoli Transverse Polarization HRS-LeftHRS-Right Vertical misalignment Systematic Error for Parity Horizontal polarization e.g. from (g-2) Part II: Up/Down Asymmetry ( Note, beam width is very tiny up/down misalignment Measured in situ using 2 -piece detector. Study alignment with tracking & M.C. Wien angle feedback ( ) Need ) <<

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Figure of Merit M = 1/E * 1/sqrt(R) * sqrt(1 + B/S) where, E = A_T enhancement for A_T hole events = 50. R = Ratio of A_T hole detector to main Pb detector event rates B/S = Ratio of bkgd under the A_T hole events to A_T signal The optimum A_T detector dimension is ~7.6cm in x by 0.8cm in y. This gives Figure of Merit = and error inflation ~ A_T detector design

G.M. Urciuoli 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. Tests with Luminosity Monitor to demonstrate capability.

PREX Workshop Aug 08 ~ 50 ppm noise per pulse  milestone for electronics Asymmetries in Lumi Monitors after beam noise subtraction Jan 2008 Data ( need < 100 ppm)

PREX is an extremely challenging experiment: –A PV ≈ 500 ± 15 ppb. –1% polarimetry. –Helicity correlated beam asymmetry < 100 ± 10 ppb. –Beam position differences < 1 ± 0.1 nm. –Transverse beam polarization < 1%. –Noise < 100 ppm –(Not melting) Lead Target –Forward angle detection  Septum magnet –Precision measurement of Q 2 : ± 0.7%  ± 0.02° accuracy in spectrometer angles However HAPPEX & test runs have demonstrated its feasibility. It will run in March-May 2010 and will measure the lead neutron radius with an unprecedented accuracy (1%). This result will have an impact on many other Physics fields (neutron stars, APV, heavy ions …). PREX: Summary

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Spares G.M. Urciuoli

Optimum Kinematics for Lead Parity: E = 850 MeV, = 0.5 ppm. Accuracy in Asy 3% n Fig. of merit Min. error in R maximize: 1 month run 1% in R n

G.M. Urciuoli Optimization for Barium -- of possible direct use for Atomic PV 1 GeV optimum

G.M. Urciuoli X (cavity) nmY (cavity) nm X (stripline) nmY (stripline) nm Redundant Position Measurements at the ~1 nm level (Helicity – correlated differences averaged over ~1 day)

Integrating Detection Integrate in 30 msec helicity period. Deadtime free. 18 bit ADC with < nonlinearity. Backgrounds & inelastics must be separeted (HRS). electrons ADC Integrator PMT Quartz / Tungsten Calorimeter (Also a thin quartz detector upstream of this) Attempt to improve resolution by replacing Alzak mirrors in light guide with anodized Al or Silver. The x, y dimensions of the quartz determined from beam test data and MC (HAMC) simulations. (11 x 14 cm) Quartz thickness to be optimized with MC. New HRS optics tune focuses elastic events both in x & y at the PREx detector location.