Session HC: Electroweak Interactions II

Overview of the Parity Violating Neutron Spin Rotation Measurement in Liquid $^{4}$He

We present an overview of the physics and experimental design of an experiment to measure the parity violating (PV) neutron spin rotation of polarized neutrons propagating through liquid-$^{4}$He. This spin rotation is a PV observable that can be used to study the hadronic weak interaction, which is poorly understood. A previous measurement gave a result of (8.0$\pm $14(stat)$\pm $2.2(syst))x10$^{-7}$ rad/m[1]. The new measurement has sensitivity goal of 3x10$^{-7}$ rad/m. [1] PhD thesis: Measurement of the Parity Nonconserving Spin-Rotation of Transmitted Cold Neutrons Through a Liquid Helium Target; D.M Markoff   

Calibration of Apparatus for Parity-Violating Neutron Spin Rotation in $^{4}$He Using Heavy Nuclei and Small Angle Scattering Standards

A measurement of parity-violating (PV) neutron spin rotation in liquid $^{4}$He, is being prepared at the NIST Center for Neutron Research (NCNR). To test the apparatus and amplify certain possible systematic effects we plan to conduct spin rotation measurements in the nuciei $^{139}$La, $^{81}$Br, and $^{35}$Cl. Large PV spin rotation effects have been seen in the past in $^{139}$La and $^{81}$Br, and $^{35}$Cl possesses a large P-odd gamma asymmetry. We also plan to use D$_{2}$O, whose small angle neutron scattering is well-known, to verify our estimates of systematic effects from small angle scattering and longitudinal magnetic fields. I will talk about our choices of targets and the design of target system. Work supported in part by NSF PHY-0457219.   

Measurements of Polarized Neutron Beam Properties at NG-6 NIST and Performance of Polarized Neutron Optical Devices for a Precision Measurement of Parity-Violating Neutron Spin Rotation in $^{4}$He

A measurement of parity-violating (PV) neutron spin rotation in $^{4}$He to learn about NN weak interactions is now in preparation at the NIST Center for Neutron Research (NCNR). Because the expected magnitude of the PV signal is $\sim $ 10$^{-7}$ rad/m, and our sensitivity goal is 3$\times $10$^{-7}$ rad/m, knowledge of certain beam properties is essential to interpret the data properly and set limits on possible sources of systematic errors. I will present measurements of the beam intensity profile, wavelength distribution, flux, and the product of the polarizing power of the polarizer and the analyzing power of the polarization analyzer as a function of wavelength, position, and angle. Work supported in part by NSF PHY-0457219.   

Magnetic field compensation for $n-^4$He parity-violating spin-rotation experiment at NIST

A high precision measurement of the parity-violating spin rotation $\phi_{pv}(n,\alpha)$ for transversely polarized neutrons passing through $^4$He is currently underway at the NIST Center for Neutron Research (NCNR). Reducing parity conserving rotations due to ambient magnetic fields is the primary experimental challenge and is being met through the use of magnetic shielding, movable targets, four separate target locations, and neutron energy detection. External coils are used to stabilize the ambient field in the longitudinal direction at a predefined value. The system is based on measuring the magnetic field outside the shielding and using a proportional-integral-derivative(PID) feedback loop to control the current through the external coil system, suppressing any change of the ambient field by a factor of 10-20. The presentation will also include an internal coil system designed within magnetic shielding.   

The Liquid Helium Target for the Neutron Spin-Rotation Experiment

We are performing a new precision measurement of the parity-violating neutron spin-rotation of polarized neutrons that propagate through liquid helium with a sensitivity goal of 3 $\times $ 10$^{-7}$ rad / m at the NCNR. We describe the design, operation, and evaluation of systematic effects of a new liquid helium target that consists of a pair of target chambers located upstream and downstream of a vertically-aligned spin-precession coil. A system for changing target states by moving liquid helium between the front and back chambers allows one to isolate the parity-violating spin-rotation signal from much larger parity-conserving rotations that are primarily due to background magnetic fields. The targets are further split into left and right chambers, where the transfer of liquid helium between the front and back targets occurs in the opposite sense for left and right sides, thus allowing simultaneous measurements of opposite target states.   

A segmented $^3$He ion chamber for n-spin rotation experiment

Searches for parity violation effects in few nucleon systems often require current-mode detectors. We describe a high efficiency $^3$He/Ar ion chamber designed to operate in current mode at a CW cold n source, It uses $^3$He to absorb the neutrons and Ar to limit the range of the ions from the n+$^3$He reaction. It is longitudinally partitioned to gain information on the n velocity spectrum and transversely segmented to monitor the spatial distribution of the n beam. We adjust the $^3$He and Ar density to obtain an approximately even absorption of the beam in the 4 longitudinal partitions. A similar detector is described by Penn et al$^{[1]}$ and was tested at the LANSCE for possible use in the NPD$\gamma$ experiment$^{[2]}$. We will present details on the design and performance as measured on the NG6 beam at NIST. [1] S.D. Penn et al, Nucl. Instr. and Meth. A 457 (2001) 332-337. [2] C. Blessinger, PhD thesis, Indiana Univ. 2000   

Measurement of Cold Neutron Depolarization in Liquid and Solid Deuterium

A proposed experiment to measure the spin rotation, originating from the parity violating weak interaction, of polarized cold neutrons as they traverse through a deuterium target will yield further information on the weak meson coupling constants. However, neutron depolarization from a relatively large scattering cross section of a few barns could severely dilute the already small spin rotation signal. Likewise another proposed experiment, also designed to shed light on the weak meson couplings by measuring the parity violating gamma asymmetry from neutron capture on deuterium, could be compromised by the same neutron depolarizing scattering process. To explore the feasibility of these proposed measurements, an experiment was recently performed at the FUNSPIN beamline at the Paul Scherrer Institut to measure the depolarization of neutrons transmitted through liquid and solid ortho-deuterium as a function of neutron energy. Preliminary results will be presented.   

Development of an Ultracold Neutron Source at the NC State PULSTAR Reactor

Development of an ultracold neutron (UCN) source is underway at the North Carolina State University 1 MW PULSTAR reactor facility. Fast neutrons from the reactor core are moderated first by heavy water at room temperature and then by solid methane at a temperature of approximately 25 K. The cold neutrons emerging from the methane are then down-scattered to lower energies ($<$ 300 neV) by phonons in the 5 K solid ortho-deuterium converter. The UCN emerging from the deuterium source are guided to the experimental area through diamond-like carbon-coated quartz. The anticipated UCN densities in an experimental volume connected to the end of the guide should exceed densities presently obtained at the UCN source at the Institute Laue--Langevin. The current status of the design and construction will be discussed, with an emphasis on the thermal modeling of the cryostat cooling system. This work is supported in part by NSF grant {\#}0314114 and funds from the DOE INIE program.   

Measuring the Neutron Lifetime using Magnetically Trapped Ultracold

The neutron lifetime plays an important role in the test of standard model and big bang nucleosynthesis. Our collaboration has successfully demonstrated the feasiblity of a neutron lifetime measurement using magnetically trapped ultracold neutrons, which has the potential to improve the current experimental limit. In this experiment, ultracold neutrons are loaded into an Ioffe-type superconducting magnetic trap through inelastic scattering of 0.89 nm neutrons with phonons in superfluid helium-4. Trapped neutrons are detected via the scintillation light of decay electrons in liquid helium. The primary advantages of this technique are continuous detection of decay events and the elimination of wall losses. We are currently upgrading the experiment to incorporate a larger and deeper magnetic trap, that can reduce the statistical uncertainty in the measurement to 1-3 s. The apparatus upgrade and studies of systematic uncertainties such as above-threshold neutrons and Helium-3 impurities will be discussed.   

Laser trapping of Ra-225 and Ra-226 and progress towards an electric dipole moment measurement

Permanent electric dipole moments (EDMs) in atoms or molecules are signatures of Time (T)-and Parity (P)-violation and represent an important window onto physics beyond the Standard Model. We are developing a next generation EDM search around laser-cooled and trapped Ra-225 (t$_{1/2}$ = 15 d). Due to octupole deformation of the nucleus, Ra-225 is predicted to be two to three orders of magnitude more sensitive to T-violating interactions than Hg-199, which currently sets the most stringent limits in the nuclear sector. We will discuss our progress, including the successful laser cooling and trapping of Ra-225 and Ra-226 atoms. We have demonstrated transverse cooling, Zeeman slowing, and capture of Ra-225 and Ra-226 atoms in a magneto-optical trap (MOT). By driving a second atomic transition, we have extended the lifetime of the trap from milliseconds to seconds and performed necessary spectroscopic measurements.   

Current-current correlators as a probe of the chirality-flip scale in QCD

In the chiral limit, vector and axial vector current-current correlators are identical, and quark chirality is preserved, to any finite order of perturbation theory in QCD. The difference of such correlators, as distance increases from \mbox{$x=0$}, probes the scale for the onset of the leading non-perturbative phenomena in QCD. We examine the influence of dynamical chiral symmetry breaking upon such a correlator difference and deduce the characteristic distance scale for the onset of this non-perturbative phenomena. The analysis is fully covariant and uses ladder-rainbow dynamics constrained by the quark condensate. Comparison is made with the non-perturbative distance scale deduced some years ago in a similar analysis based on the Instanton Liquid model.   

Transverse Beam Spin Asymmetries in the G0 Forward-Angle Measurement

Although neglected historically, two-photon exchange contributions have become increasingly important as electron-scattering experiments push to higher precision measurements of nucleon structure. The transverse beam spin asymmetry measured in the elastic scattering of transversely polarized electrons from unpolarized nucleons provides a direct probe of the imaginary component of the two-photon exchange amplitude. Transverse beam spin asymmetries for 3 GeV electrons were measured during the forward-angle run of the G0 experiment in Jefferson Lab's Hall C, with center of mass scattering angles ranging from 19 to 37 degrees. A description of the analysis of the data and the results of the measurements will be presented.   

The Qweak Experiment : Design of Quartz \v{C}erenkov Detectors for a Measurement of the Proton Weak Charge

The Qweak experiment at Jefferson Lab aims to make a 4\% measurement of the parity-violating asymmetry in elastic scattering at very low $Q^2$ of a longitudinally polarized electron beam on a proton target. The experiment will measure the weak charge of the proton, and thus the weak mixing angle at low energy scale, providing a precision test of the Standard Model. The experiment is currently being constructed and is scheduled for a 2200 hour measurement starting in 2009, employing: an 80\% polarized, 180 $\mu$ A, 1.2 GeV electron beam; a 35 cm liquid hydrogen target; and a toroidal magnet to focus electrons scattered at 9$^{\circ}$ forward angle, corresponding to $Q^2 = 0.03~{\rm (GeV/c)^2}$. The experiment will run at an event rate of over 6 GHz. This requires current mode detection of the scattered electrons, using synthetic quartz \v{C}erenkov detectors. In this talk we will present a brief introduction to the experiment, with a focus on the design and status of the main \v{C}erenkov detectors.