Bulletin of the American Physical Society
85th Annual Meeting of the APS Southeastern Section
Volume 63, Number 19
Thursday–Saturday, November 8–10, 2018; Holiday Inn at World’s Fair Park, Knoxville, Tennessee
Session J01: Fundamental Symmetry Studies with Neutrons |
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Chair: Nadia Fomin, University of Tennessee, Knoxville Room: Holiday Inn Knoxville Downtown Cumberland |
Saturday, November 10, 2018 8:30AM - 9:00AM |
J01.00001: UCNτ: The Neutron Lifetime via Bottled Ultracold Neutrons Invited Speaker: Adam Holley The mean lifetime for β-decay of a free neutron is an interesting empirical target because it is an input for calculation of light element abundances in the early universe, affects predictions from cosmic microwave background data about the number of effective neutrino species, and in combination with other β-decay observables can provide sensitive tests for physical phenomena not currently included in the Standard Model. Theoretical uncertainties and uncertainties in astrophysical measurements make the interesting precision band for measurements of τn between 0.1% and 0.01%. Current experimental techniques have the capability to reach precisions in this range, but a clear application of the resulting measurements is hampered by a nearly four-sigma discrepancy between the two prevalent methods, called the "beam" and "bottle" techniques. The recent UCNτ result (τn = 877.7 ± 0.7(stat) +0.4/-0.2(sys)) is of the latter type, and maintains the discrepancy between beam and bottle determinations despite using a large-volume magneto-gravitational trap which eliminates the dominant systematic effect in material bottle experiments and features new detection techniques that allow direct empirical characterization of residual systematic effects. This result has led to renewed interest in possible exotic explanations for the beam/bottle discrepancy. I will describe the experimental developments leading to our new result, the current status of the UCNτ experiment, and our plans for future measurements. |
Saturday, November 10, 2018 9:00AM - 9:30AM |
J01.00002: UCNA: Measuring the β-asymmetry in neutron decay with ultracold neutrons Invited Speaker: Robert Pattie The neutron provides a simple yet dynamic nuclear system to study the Standard Model of Particle Physics. The process of transforming a neutron into a proton, an electron, and an anti-neutrino contains a wealth of information in the decay rate and the kinematics of the emitted particles. Precision measurements of angular correlations in neutron β-decay can lend insight to the structure of the weak interaction and probe for physics beyond the Standard Model. The UCNA experiment at the Los Alamos Neutron Science Center is the only experiment to use ultracold neutrons to perform such a measurement, determining the β-asymmetry parameter A0= -0.12054(44)stat(68)sys. The β-asymmetry describes the correlation between the neutron's spin and the emission direction of the out-going electron. To leading order A0 is defined by the ratio of the axial-vector and vector weak couplings constants λ ≡ gA / gV. In this presentation I will discuss the final results from UCNA, their implications for searches of beyond the standard model physics, and prospects for a future UCNA+ experiment capable of a ≤0.2% precision. |
Saturday, November 10, 2018 9:30AM - 10:00AM |
J01.00003: A Cryogenic Search for the Neutron Electric Dipole Moment at the Spallation Neutron Source Invited Speaker: Alina Aleksandrova It is a well known fact that the visible universe is made almost entirely of baryonic matter. Yet, this is also one of the greatest puzzles that physicists are trying to solve: Where did all of this matter come from in the first place? The Standard Model (SM) of particle physics predicts a baryon asymmetry that is much smaller than what is observed in nature. In order to try and explain this discrepancy, Sakharov (1967) postulated three necessary conditions for baryogenesis in the early universe. One of these is the requirement that charge conjugation (C) and the product of C and parity (P) symmetries are violated. Because the SM fails to meet the Sakharov conditions, additional sources of CP violation are needed in order to help reconcile theory and observation. Thus, physicists have been looking for extensions to the SM in search of an answer. The presence of a neutron Electric Dipole Moment (nEDM) would signal a new source of CP violation. A non-vanishing nEDM would provide evidence for the breaking of both parity (P) and time-reversal symmetry (T). Because CPT symmetry is assumed to be conserved and has not been found to be broken, this would signal CP violation. |
Saturday, November 10, 2018 10:00AM - 10:30AM |
J01.00004: Narrowing in on the Hadronic Weak Interaction: The Final Results of the NPDGamma Experiment Invited Speaker: Jason A Fry The weak interaction between nucleons has been of interest for nearly 50 years but is still not well described. The electroweak sector of the Standard Model describes the weak interaction of W and Z bosons with quarks and hadrons. This weak interaction causes small, but observable parity-violating signals in certain observables, such as spin-momentum correlations. But since the weak interaction between hadrons contains both the strong and weak interaction, the unsolved non-perturbative nature of QCD makes a direct calculation impossible. Instead, many theories have parametrized the Hadronic Weak Interaction (HWI) in terms of meson exchange degrees of freedom or S-P amplitudes as low energy constants. The theoretical difficulties have motivated few-body, low energy experiments, where there is little or no nuclear structure error. The NPDGamma experiment has completed one of these experiments and, for the first time, leads to a determination of one of the coupling constants as well as a constraint on others. The NPDGamma experiment and analysis will be discussed as well as the landscape and future of the HWI. |
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