1. 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: MHz
C-REX : 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 E 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 Pb
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 (2012)
PRC 85 (2012) (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 = 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