Recent Results in Parity-Violating Electron Scattering at Jefferson Lab: PREX and HAPPEX-III Kent Paschke APS Spring Meeting Anaheim, CA May 1, 2011 HAPPEX Collaboration PREX Collaboration

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 The Neutral Current and Nucleon Vector Form Factors Strange quarks exist in the nucleon at short distance scales. Do they play a role in elastic proton scattering? ? Assume 3 quark flavor contributions to vector form factors: G u, G d, G s Charge Symmetry + 3 equations with 3 unknowns

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Measuring Strange Vector Form Factors ~ few parts per million For a proton: Forward angle Backward angle For a spin=0,T=0 4 He: G s E only! For deuterium: Enhanced G A  Z0Z0  2 “Anapole” radiative corrections are problematic

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Experimental Overview SAMPLE HAPPEX HAPPEX-3: G E s G M s at Q 2 = 0.62 GeV 2 Precision spectrometer, integrating A4 open geometry, integrating, back-angle only Open geometry Fast counting calorimeter for background rejection Forward and Backward angles G0 Open geometry Fast counting with magnetic spectrometer + TOF for background rejection Forward and Backward angles over a range of Q 2 Forward angle, also 4 He at low Q 2

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 World data on G s “Form Factor” error: precision of EMFF (including 2 γ) and Anapole correction Significant systematic uncertainty in higher Q 2 points At Q 2 =~0.1 GeV 2, G s < few percent of G p all forward-angle proton data all low Q 2 data

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Global fit of all world data Data set appears to show consistent preference for positive effect Significant contributions at higher Q 2 are not ruled out. Fit includes all world data Q 2 < 0.65 GeV 2 G0 Global error allowed to float with unit constraint Simple fit: G E s = ρ s *τ G M s = μ s

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Lead - Lucite Cerenkov Shower Calorimeter phototube current integrated over fixed time periods Integrating in the High Resolution Spectrometers parts per million Psuedo-random, rapid helicity flip Very clean separation of elastic events by HRS optics no PID needed; detector sees only elastic events Elastic Inelastic detector Q Dipole Quad target

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 HAPPEX-III Error Budget δA PV (ppm) δA PV / A PV Polarization % Q 2 Measurement % Backgrounds % Linearity % Finite Acceptance % False Asymmetries % Total Systematic % Statistics % Total Experimental % Compton + Moller polarimeters more later from Megan Friend, CMU Spectrometer Calibration more later from Kiadtisak Saenboonruang, UVa Linearity Studies HRS Backgrounds more later from Rupesh Silwal, UVa Systematic uncertainties are well controlled - experiment is statistics dominated

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 HAPPEX-III Measurement of A PV A RAW =  (stat) ppm Additional corrections are then applied: backgrounds (1.1%) acceptance averaging (0.5%) beam polarization (11%) This includes beam asymmetry correction (-0.01 ppm) charge normalization (0.20 ppm) 3.26% (stat)± 1.49% (syst) total correction ~2.5% + polarization Analysis Blinded ± 2.5 ppm parts per million data “slug” combined 2-arm data Trajectory at target averaged to <3nm,<0.5nrad OUT / IN from “slow” spin reversals to cancel systematics slug micron Position Differences

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 HAPPEX-III Results A PV =  (stat)  (syst) ppm Q 2 = ± (GeV/c) 2

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 HAPPEX-III Results A PV =  (stat)  (syst) ppm Q 2 = ± (GeV/c) 2 A(G s =0) = ppm ± ppm

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 HAPPEX-III Results A PV =  (stat)  (syst) ppm Q 2 = ± (GeV/c) 2 A(G s =0) = ppm ± ppm G s E G s M = ± (stat) ± (syst) ± (FF)

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Fit, World Data G s E G s M = ± (stat) ± (syst) ± (FF) Wtih HAPPEX-3 result at Q 2 = GeV 2 :

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Considering only the 4 HAPPEX measurements High precision Small systematic error Clean theoretical interpretation

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 World Data on Strange FF HAPPEX-III provides a clean, precise measure of A PV at Q 2 =0.62 GeV 2, and finds that it is consistent with no strangeness contribution. Recent lattice results indicate values smaller than these FF uncertainties Further improvements in precision would require additional theoretical and empirical input for interpretation Q 2 = 0.62 GeV 2

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Weak Charge Distribution of Heavy Nuclei Nuclear theory predicts a neutron “skin” on heavy nuclei 208 Pb Neutron distribution is not accessible to the charge-sensitive photon.  Q 2 ~ 0.01 GeV 2 A PV ~ 0.6 ppm Rate ~1.5 GHz 5 o scattering angle PREX (Pb-Radius EXperiment) protonneutron Electric charge10 Weak charge~0.081

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 MFT fit mostly by data other than neutron densities Involve strong probes Most spins couple to zero. Proton-Nucleus Elastic Pion, alpha, d Scattering Pion Photoproduction Heavy ion collisions Rare Isotopes (dripline) Magnetic scattering PREX Theory Measurements of neutron skin Electroweak probe

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 A crucial calibration point for nuclear theory ( R.J. Furnstahl ) …and measuring r N pins down the symmetry energy The single measurement of F n translates to a measurement of R n via mean-field nuclear models

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Nuclear Structure: Symmetry energy variation with 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/neutro ns Slope unconstrained by data Adding R n from 208 Pb will eliminate the dispersion in the plot. Slide adapted from J. Piekarewicz

From 208 Pb to a Neutron Star R n calibrates the equation of state of neutron rich matter Crust Thickness Explain Glitches in Pulsar Frequency ? Combine PREX R n with observed neutron star radii Some neutron stars seem too cold Strange star ? Quark Star ? Cooling by neutrino emission (URCA) 0.2 fm URCA probable, else not Phase Transition to “Exotic” Core ? Crab Nebula pressure density

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 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 Output Slide adapted from C. Horowitz 20% corrections, calculated to precision by multiple groups see later talk in last session

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Challenging Experiment Compton Polarimeter 10X more precise than any previous e - -nucleus scattering! Similar to the HAPPEX measurements Use Hall A spectrometers integrating technique Injector magnetic spin manipulation Source optimization - reduce position difference and spot-size asymmetry Precise kinematics calibration Low energy electron beam polarimetry Target survivability Electronics noise new low-noise ADCs Integrating photon detection Beam False Asymmetries New modulation system for calibrating corrections Water cell calibration High rate tracking with GEMS Low current beam position monitors 20 ppb absolute measurement 3% relative error Ultimate goal: upgrade to SC magnet FADC DAQ upgrade upgrade IR to Green light Moller Polarimeter  (A PV )/A PV ~ 3%  (R n )/R n ~ 1% see later talk by Zafar Ahmed see later talk by Luis Mercado Transverse Asymmetry see later talk by Bob Michaels

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 target HRS-L HRS-R collimator Septum Magnet collimator calibration collimators 5 o Septum to augment the HRS

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Detector integrates the elastic peak. Backgrounds from inelastics are suppressed Momentum (GeV/c) C 1st excited state Pb excited states PbC Ground States Carbon Ground State High Resolution Spectrometer 2.6 MeV Negligible contributions from inelastic events rescattering in spectrometer p (GeV/c) Lead Carbon

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Slug # ( ~ 1 day) microns Average with signs = what exp’t feels Points: Not sign corrected microns Parity Quality Beam Helicity – Correlated Position Differences < ~ 4 nm Injector spin manipulation proved important for cancellation

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Error SourceAbsolute (ppm)Relative ( % ) Polarization (1) Beam Asymmetries (2) Detector Linearity BCM Linearity Rescattering Transverse Polarization Q 2 (1) Target Thickness C Asymmetry (2) Inelastic States00 TOTAL Systematic Errors (1) Normalization Correction applied (2) Nonzero correction (the rest assumed zero)

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 PREX Asymmetry A raw (ppm) Slug ~ 1 day A RAW =  (stat) ppm This includes beam asymmetry correction (-40 ppb) charge normalization (96 ppb) OUT / IN, L/R from “slow” spin reversals to cancel systematics Additional corrections are then applied: backgrounds (1.1%) acceptance averaging (0.3%) beam polarization (11%) Analysis Blinded ± 200 ppb

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 PREX Asymmetry A raw (ppm) Slug ~ 1 day A RAW =  (stat) ppm This includes beam asymmetry correction (-40 ppb) charge normalization (96 ppb) OUT / IN, L/R from “slow” spin reversals to cancel systematics Additional corrections are then applied: backgrounds (1.5%) acceptance averaging (0.3%) beam polarization (11%) Analysis Blinded ± 200 ppb at Q 2 = GeV 2  Statistics limited ( 9% )  Systematic error goal achieved ! (2%) ppm 9.2 % 2.0 %

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 ( theory curve, not integrated over acceptance ) arXiv: [nucl-th] Shufang Ban, C.J. Horowitz, R. Michaels Neutron Skin = R N - R P = fm Preliminary estimate from C.J. Horowitz Asymmetry leads to R n

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 PREX Interpetation

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 PREX Interpetation First electroweak observation of the neutron skin of a heavy nucleus (CL =95%)

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Future Run can reach physics goal Pb Major experimental questions have been answered! Lead sandwich target Precision polarimetry at 1 GeV HRS optics optimization Source performance Transverse Asymmetry Must address: Beam vacuum issues Neutron radiation in the Hall (shielding, reduced beam collimation) Proposal for a 2nd run is under development

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 RNRN Surface thickness RNRN Options for other nuclei E (GeV) Rate 50 μA) APV (ppm) days to 1% on R n 208 Pb Sn Ca arXiv: [nucl-th] Parity Violating Electron Scattering Measurements of Neutron Densities Shufang Ban, C.J. Horowitz, R. Michaels Additional measurements could address R n in other nuclei shape dependence? isotope dependence? Plans to pursue 48 Ca far from 208 Pb closer comparison to microscopic calculation compatible with 12 GeV! 5 o scattering at 2 GeV : new septum required

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 The most precise measurement of electron- nuclear scattering asymmetry yet: 62 ppb! Fundamental Nuclear Physics with many applications Achieved a 9% stat. error in Asymmetry: 2σ electroweak observation of neutron skin Success in experimental technique: systematic errors can be controlled Proposal for a second run is in preparation PREX : Summary

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Backup

G0 Backward Scattering, PRL 104, (2010) Young et al., Phys.Rev.Lett. 97 (2006) , nucl-ex/ Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Form Factor Separation E.J. Beise et al., Prog Nuc Part Phys 54 (2005) SAMPLE QCD lattice suggests very small effects

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Model guidance is unclear: kaon loops, vector dominance, Skyrme model, chiral quark model, dispersion relations, NJL model, quark-meson coupling model, chiral bag model, HBChPT, chiral hyperbag, QCD equalities, … - Dong, Liu, Williams PRD 58(1998) Lewis, Wilcox, Woloshyn PRD 67(2003) Leinweber, et al.,PRL 94(2005) ; 97 (2006) Lin, arXiv:0707: Wang et al, Phys.Rev. C79 (2009) Doi et al., Phys.Rev. D80 (2009) QCD models Recent significant progress in Lattice QCD: these all suggest very small effects

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 HAPPEX Results ep at Q 2 =0.5 (GeV/c) 2, 12.3 degrees Phys. Rev. Lett. 82: ,1999; Phys. Lett. B509: ,2001; Phys. Rev. C 69, (2004) G s E G s M = ± (exp) ± (FF) A PV = ppm ± 0.98 (stat) ppm ± 0.56 (syst) ppm 0.6 Statistics limited. Leading systematic is polarimetry

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 HAPPEX-III Configuration: 25 cm cryogenic Hydrogen Target 100 μA 89% polarization Kinematics: E = GeV, θ=13.7 o, E’ = 3.14 GeV, Q 2 = GeV 2 A PV (assuming no strange vector FF): A PV NS = ppm ± 0.66 ppm Challenges similar to original HAPPEX, but seeking higher precision precision alignment for Q 2 uncertainty 1% polarimetry backgrounds linearity Sensitive to

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Q 2 = 0.62 GeV 2 in combination Combined fit includes form-factor uncertainties, experimental bands do not Zhu constraint is used for axial form-factor preliminary

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 The Axial Term and the Anapole Moment Anapole Moment Correction: Multiquark weak interaction in R A (T=1), R A (T=0) Axial form-factors G A p, G A n Determined at Q 2 =0 from neutron and hyperon decay parameters (isospin and SU(3) symmetries) Q 2 dependence often assumed to be dipole form, fit to ν DIS and π electroproduction Includes also Δs, fit from ν-DIS data Zhu, Puglia, Holstein, Ramsey-Musolf, Phys. Rev. D 62, Model dependent calculation with large uncertainty Model dependent calculation with large uncertainty Uncertainty dominates axial term Uncertainty dominates axial term Difficult to achieve tight experimental constraint

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Backgrounds Rescattering probability measured during H-I backgroundfA Net Correction Net Uncertainty Aluminum (target window) 1.15% (30%) ppm (30%) 126 ppb127 ppb Rescattering 0.3% (25%) -63 ppm (25%) 114 ppb55 ppb Aluminum from target windows Signal from inelastic electrons scattering inside spectrometer

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Beam Asymmetries Trajectory at target averages to <3nm,<0.5nrad Charge asymmetry (with feedback) averages to 200 parts per billion Implies energy asymmetry at 3 ppb Total Correction: ppm (0.05%) Individual detector response measured to be at the level of 5 ppb/nm

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Q 2 = 0.62 GeV 2 in combination Combined fit includes form-factor uncertainties, experimental bands do not Zhu constraint is used for axial form-factor

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Charge Symmetry Breaking Recent calculations suggest that effects could be large as the statistical error on HAPPEx-II data χ PBT, B. Kubis & R. Lewis Phys. Rev. C 74 (2006) HAPPEX-II: G s E G s M = / / / (FF) Contribution from ~ Contributions at higher Q 2 is not determined Most theoretical CSB estimates indicate <1% violations Miller PRC 57, 1492 (1998) Lewis & Mobed, PRD 59, (1999)

Arrington and Sick, Phys.Rev. C76 (2007) , nucl-th/ Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 EMFF GEpGEp 0.3% GMpGMp 1.1% GEnGEn 1.6% GMnGMn 1.4% σ red 1.1% R Ana 0.6% Total2.7%

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 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. PAVI 09 FP TM1 Solid Liquid Neutron Star Crust vs Pb Neutron Skin 208 Pb Neutron Star C.J. Horowitz, J. Piekarawicz

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 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. PAVI 09 R n -R p in 208 Pb If Y p > red line NS cools quickly via direct URCA reaction n p+e+  (slide from C. Horowitz)

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 High Resolution Spectrometers Momentum (GeV/c) 4.4 MeV 12 C

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Pure, Thin 208 Pb Target Momentum (GeV/c) Lead 5- State 2.6 MeV

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Detector integrates the elastic peak. Backgrounds from inelastics are suppressed. Momentum (GeV/c)

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 PREX – II Proposal Future ?

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Backgrounds that might re-scatter into the detector ? Run magnets down : measure inelastic region Run magnets up : measure probability to rescatter No inelastics observed on top of radiative tail. Small systematic for tail. Detector cutoff

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Diamond Pb Three bays Lead (0.5 mm) sandwiched by diamond (0.15 mm) Liquid He cooling (30 Watts) Lead / Diamond Target Day 1 Y Y X Day 7 Sudden change after ~1 50 μA Not all targets are created equal! Best was intact after 4 70 μA

Kent Paschke APS April Meeting, Anaheim, CA - May 1, 2011 Pull Plot (example) PREX Data