Session DJ: Mini-Symposium on Parity Violation and Fundamental Symmetries I

Overview of Parity Violation and Fundamental Symmetries

The fields of nuclear and particle physics have undertaken extensive programs of research to search for evidence of new phenomena via the precision measurement of observables that are well predicted within the standard model of electroweak interaction. The standard model is known to be incomplete as it does not include gravity and dark matter/energy and therefore likely the low energy approximation of a more complex theory. This talk will be an overview of the motivation, experimental method and status of some of these efforts (past and future) related to precision in-direct searches that are complementary to the direct searches currently underway at the Large Hadron Collider.   

Improved experimental limit on the EDM of $^{225}$Ra

Searches for permanent electric dipole moments (EDMs) in fundamental and composite particles are sensitive probes of beyond-standard-model symmetry violation that could explain the dominance of matter over anti-matter. The $^{225}$Ra (t$_{1/2}=$15d, I$=$1/2) atom is a particularly attractive system to use for an EDM measurement because its large nuclear octupole deformation, closely spaced ground-state parity doublet, and large atomic mass make $^{225}$Ra uniquely sensitive to symmetry-violating interactions in the nuclear medium. We have developed an experiment to measure the EDM of $^{225}$Ra and demonstrated the first ``proof-of-principle'' measurement, giving a 95{\%} confidence upper limit of 5E-22 e-cm. After implementing a vacuum upgrade, we have observed nuclear spin coherence after 20 s of free evolution -- a factor of ten improvement over our earlier results - and have lowered the $^{225}$Ra EDM limit by over an order of magnitude. Upcoming experimental upgrades have the potential to further improve our EDM sensitivity by many orders of magnitude, allowing us to test symmetry violation at an unprecedented level.   

The full weak charge density distribution of $^{48}Ca$ from parity violating electron scattering

The ground state neutron density of a medium mass nucleus contains fundamental nuclear structure information and is at present relatively poorly known. We explore if parity violating elastic electron scattering can provide a feasible and model independent way to determine not just the neutron radius but the full radial shape of the neutron density and weak charge density of a nucleus. We expand the weak charge density of $^{48}Ca$ in a model independent Fourier Bessel series and calculate the statistical errors in the individual coefficients that might be obtainable in a model parity violating electron scattering experiment. We find that it is feasible to determine roughly six Fourier Bessel coefficients of the weak charge density of $^{48}Ca$ within a reasonable amount of beam time. To conclude, Parity violating elastic electron scattering can determine the full weak charge density of a medium mass nucleus in a model independent way. This weak density contains fundamental information on the size, surface thickness, shell oscillations, and saturation density of the neutron distribution in a nucleus. The measured weak charge density, together with the previous known charge density, will provide a detailed picture of where the neutrons and protons are located in an atomic nucleus.   

The time-reversal invariance violating nucleon-nucleon potential in the large $N_C$ expansion

The violation of time reversal invariance (T) in nuclear systems, such as a non-zero neutron electric dipole moment, can serve as a sensitive probe of beyond the standard model (BSM) physics. While the QCD $\theta$ term violates T, its effects are small, thus opening a potential window for the detection of BSM physics. In addition to the neutron electric dipole moment, T violation could also be observed in systems of two and more nucleons. We analyze the T-violating nucleon-nucleon potential in terms of the large $N_C$ expansion of QCD, and identify a hierarchy of terms. This hierarchy can then be mapped onto existing T-violating potentials, such as those derived from meson-exchange models, thereby delineating the terms that should be most important in phenomenological applications.   

Test of Time-Reversal Invariance Violation in Neutron Scattering At Spallation Neutron Sources

Time Reversal Invariant Violating effects in neutron transmission through a nuclear target are discussed. A class of free from false asymmetries experiments is presented, and a comparison of a sensitivity of these transmission experiments and electric dipole moment measurements to different mechanisms of CP-violation is discussed.   

Toward a measurement of weak magnetism in $^{6}$He decay

The simplicity of $^{6}$He beta decay has attracted considerable interest for the study of the weak interaction and searches for new physics beyond the standard model (SM). The comparisons between precision correlation measurements and SM predictions require an accurate determination of observables within the SM. At the level of sensitivity of new generation experiments, it is expected that recoil order terms in the hadronic weak current, such as weak magnetism, should have a sizable contribution. We have performed an exploratory experiment using a beam of $^{6}$He produced by projectile fragmentation of $^{18}$O, with the purpose to assess the conditions for a measurement of the shape of the beta energy spectrum in a geometry where the beta particles do not have to cross any interface and cannot escape from the detector. Particular attention has been devoted to identify possible beam contaminants as well as background produced by beam induced reactions in the detectors. This contribution will describe the experiment and present the status of the data analysis.   

Precision Measurements with UCNA

UCNA is the only experiment to use ultracold neutrons (UCN) to measure the free neutron $\beta$-decay correlation parameter, ``$A$", between the neutron spin and $\beta$ momentum. This parameter yields a sensitive measurement of the ratio of axial-vector and vector coupling constants, $\lambda = g_{A}/g_{V}$, and $\lambda$ can be combined with the neutron lifetime to give a value for the CKM matrix element $V_{ud}$. UCNA is operated at the Los Alamos Neutron Science Center, where UCN are produced in a solid deuterium source. The $\beta$-decays are observed in a solenoidal spectrometer with a combined multi-wire proportional chamber and plastic scintillator detector for position and energy reconstruction, respectively. Improvements to energy calibration, neutron polarimetry, and statistics are expected to improve the precision in the UCNA value for $A$ from 1\% to about 0.6\%.