Parity – Violating Neutron Density Measurements : PREX, C-REX Robert Michaels (On Behalf of the PREX Collaboration ) Elastic electron scattering from a nucleus PREX, C-REX 208Pb Concept Experiments Flux Integration Technique PREX: 500 MHz C-REX : 80 MHz 4 SSF are needed to characterize the spin dependent properties. While g1p has been extensively mapped out, g2p remains largely unmeasured, esp at low Q2. The main goal of this exp is to fill in the gap of knowledge of this spin structure, in order to gain a more complete understanding of the proton. We will perform a definitive measurement of g2p this fundamental quantity in the low Q2 region, which will help to clarify several outstanding puzzles. BC: g2p will address the intriguing discrepancy between data and theory for BC sum rule. Polarizability: this new data will be invaluable in establishing the reliability and range of PT Additionally, this QCD quantity also affect QED calculations in bound system. Lack of knowledge of the g2p structure function at low Q2 is also one of the leading uncertainties in calculations of the hyperfine splitting of the hydrogen atom, and the proton charge radius from Lamb shift. In particular, due to the kinematic weighting, the gp2 contribution to these calculations is dominated by the low Q2 region where E08-027 will measure. contributions to structure dependent effect in/bound state QED calculations ep scattering determine the radius by measuring the cs to get Form Factor, then partial differential of FF is root-mean-square <r2> largest CODATA compilation of physical constants: middle Proton polarizability is one of leading uncertainties in determining the proton charge radius from Lamb shift of hydrogen atom: least g2p is an important input of the hydrogen hyperfine energy corrections, QED LT spin polarizability is the clean channel to study g2 (in comparison to forward polarizability gamma0) In order to minimize… we will work at low beam current, hence we upgrade the existing beam diagnostics, and use the slow raster chicane (2 dipoles) to compensate the bending effect local dump since beam will not make it to hall dump In order to reach the lowest possible Q2 in the resonance region, we use the RT sep magnets to detect the forward scattering, which used in PREX, modified with new coils ------------ Previous measurements of g2p: SLAC Q2=5GeV2, rrs Q2=1.3GeV2 Z0 is a Clean Probe that couples mainly to Neutrons CEBAF Hall A JLAB Pol. Source HRS spectrometers target p, n w/ Coulomb distortions (C. J. Horowitz) : proton neutron Electric charge 1 Weak charge 0.08 Weak charge of 208Pb Nuclear Structure: Neutron density is a fundamental observable that remains elusive. Septum Magnet HRS Pb target HRS spectrometers fig: C.J. Horowitz Reflects poor understanding of symmetry energy of nuclear matter . = the energy cost of Lead / diamond sandwich, cryo-cooled 50 target Neutron Density Ratio Neutron / Proton Applications Nuclear Structure Results, New Proposals fig: B. A. Brown E/N PREX-I Result PRL 108 (2012) 112502 PRC 85 (2012) 03250(R) PREX & Neutron Stars C.J. Horowitz, J. Piekarewicz Asymmetry leads to RN RN calibrates equation of state (pressure vs density) of Neutron Rich Matter Establishing a neutron skin at ~95 % CL Neutron Skin = RN - RP = 0.33 + 0.16 - 0.18 fm C-REX Proposal (conditionally approved) Application: Atomic Parity Violation . Low Q2 test of Standard Model . Needs RN Combine PREX RN with Observed Neutron Star Radii Phase Transition to “Exotic” Core ? theory: P. Ring , et.al Strange star ? Quark Star ? published approved to run RN - RP Some Neutron Stars seem too cold Might be explained by cooling by neutrino emission (URCA process) depending on symmetry energy. A