Bulletin of the American Physical Society
APS April Meeting 2019
Volume 64, Number 3
Saturday–Tuesday, April 13–16, 2019; Denver, Colorado
Session D14: Mini-Symposium:The Neutron Lifetime Anomaly - future directionsFocus
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Sponsoring Units: DNP Chair: Geoff Greene, University of Tennessee Room: Sheraton Plaza Court 3 |
Saturday, April 13, 2019 3:30PM - 4:06PM |
D14.00001: The neutron lifetime puzzle - Contributions from Europe Invited Speaker: Peter Geltenbort The best experiments in the world cannot agree on how long free neutrons live before decaying into other particles. Two main types of experiments are underway: bottle-like traps count the number of neutrons that survive after various intervals of storage time, while beam experiments look for one of the particles into which neutrons decay. Resolving this question is vital to answering a number of fundamental questions in physics. Past, present and future experimental efforts to measure the lifetime of the free neutron in Europe will be presented. |
Saturday, April 13, 2019 4:06PM - 4:18PM |
D14.00002: BL3 - A next generation "beam" experiment to measure the neutron lifetime to <0.3 s. Nadia Fomin Neutron beta decay is an archetype for all semi-leptonic charged-current weak processes. A precise value for the neutron lifetime is required for consistency tests of the Standard Model and is needed to predict the primordial 4He abundance from the theory of Big Bang Nucleosynthesis. An effort is under way for an in-beam measurement of the neutron lifetime with an projected ≤0.3s uncertainty. This effort is part of a phased campaign of neutron lifetime measurements based at the NIST Center for Neutron Research, using the Sussex-ILL-NIST technique. Recent advances in neutron fluence measurement techniques as well as new large area silicon detector technology address the two largest sources of uncertainty of in-beam measurements, paving the way for a new measurement. The experimental design, schedule, and projected uncertainties for the 0.3s measurement will be discussed. |
Saturday, April 13, 2019 4:18PM - 4:30PM |
D14.00003: Absolute Proton Detection Efficiency Determination in “Beam” Experiments Grant V Riley This talk introduces a new proposed method to measure the proton detector efficiency for use in ``beam'’ determinations of the free neutron lifetime. There is currently a 4 sigma disagreement between the ``beam'' and ``storage'' methods of measuring the lifetime of the neutron. A possible reason for this is a systematic uncertainty that is not properly accounted for in one or both types of experiments. Absolute proton counting is an essential facet of the ``beam'' experimental approach. The absolute detector efficiency is not currently known for existing experiments and could be a source of a hidden systematic error. This proposed calibration technique can also be extended to new, large area particle detectors designed for future experiments. |
Saturday, April 13, 2019 4:30PM - 4:42PM |
D14.00004: Ideas for a Superconducting Second-Generation Magneto-Gravitational Trap for Measurement of the Neutron Lifetime Peter Lowell Walstrom Ideas are presented for a second-generation UCN trap (UCNtau2) with the same basic geometry as the existing UCNtau trap at LANL, but with a trapping field produced by an array of side-by-side superconducting "banana" coils in place of the permanent magnets of UCNtau. The coils employ NbTi/Cu superconducting cables and operate at a current that is less than 50% of the critical current at the peak field in the windings. The effective trapping field of UCNtau is ~0.8 T and the trap depth approximately 0.47 m. If the trapping field were to be increased, for example, to 2 T, the trap depth could be increased to 1.2 m and simple scaling of the present trap geometry would give a trap volume of 15 times the present volume, or about 6.3 m3. The simplest filling scheme is to fill the trap with the trapping field off, containing the neutrons with a material liner, and then to ramp the current to the design value, after which the UCNs are contained by the trapping field. Alternative filling schemes that do not require coil ramping are also under consideration and will be described in other talks. |
Saturday, April 13, 2019 4:42PM - 4:54PM |
D14.00005: Tau2: A next generation neutron lifetime experiment based on UCN&tau Alexander Saunders The UCNτ experiment measures the free neutron lifetime by in situ counting of surviving ultracold neutrons (UCNs) after different storage times in an asymmetric magneto-gravitational storage volume. It has acquired sufficient data to measure the neutron lifetime with a statistical uncertainty of about 0.3 s and has demonstrated a systematic uncertainty of 0.28 s; it is expected to ultimately reach a total uncertainty of about 0.2 s, limited primarily by the efficiency with which UCNτ utilizes the neutrons from the Los Alamos UCN facility. In this talk, we will describe the conceptual Tau2 experiment, which is intended to use UCNτ techniques to minimize systematic uncertainties while maximizing the statistical reach possible with UCNs supplied by Los Alamos. Replacing the permanent magnet-based trap of UCNτ with a larger volume superconducting trap is expected to enable improving the total uncertainty on the neutron lifetime to 0.1 s and beyond, permitting investigation of the weak nuclear force, when taken with precision beta decay correlation experiments, with physics reach comparable to LHC-based high energy experiments. |
Saturday, April 13, 2019 4:54PM - 5:06PM |
D14.00006: Systematic effects in a next-generation magnetic trap-based neutron lifetime measurement Daniel J Salvat The UCNτ2 experiment at the Los Alamos Neutron Science Center (LANSCE) will measure the free neutron lifetime τn to the 0.01% level, leveraging techniques developed for the ongoing UCNτ effort. The current experiment employs a ∼1 T NdFeB magnet array to trap ultracold neutrons (UCN) from the LANSCE UCN source. UCN are filled into the trap, stored for hundreds of seconds, and the surviving neutrons are counted. We have developed several techniques to study and mitigate systematic effects related to determining the relative number of UCN per fill ("normalization"), removing untrappable high energy UCN ("cleaning"), and potential time-dependent modulations of the trapping potential ("heating"). The next-generation effort will use a superconducting magnetic trap with larger volume and at least two-fold greater magnetic field strength to more efficiently use the LANSCE source and improve statistical sensitivity. A successful experiment will require improved techniques for normalization, cleaning, and heating-related effects. Here we outline the UCN detector technology, neutron tracking simulations, and data-driven systematic studies needed to measure τn to unprecedented precision. |
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