Bulletin of the American Physical Society
APS April Meeting 2019
Volume 64, Number 3
Saturday–Tuesday, April 13–16, 2019; Denver, Colorado
Session H14: Mini-Symposium:The Neutron Lifetime Anomaly - possible explanationsFocus
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Sponsoring Units: DNP Chair: Chris Wrede, Michigan State University Room: Sheraton Plaza Court 3 |
Sunday, April 14, 2019 10:45AM - 11:21AM |
H14.00001: Neutron Dark Decay Invited Speaker: Bartosz Fornal I will discuss the recently proposed dark matter interpretation of the neutron lifetime discrepancy. The difference between bottle and beam neutron lifetime measurements is explained by the existence of a neutron dark decay channel with a branching fraction 1%. Phenomenologically consistent particle physics models of this type can be constructed; they involve either a strongly self-interacting dark sector or a repulsive dark matter-baryon interaction. I will elaborate on the theoretical developments around this idea and describe the efforts undertaken to verify it experimentally. In general, the proposed neutron dark decay need not be linked to the neutron lifetime discrepancy and can occur at a smaller rate, giving rise to new theoretical and experimental avenues of investigation. |
Sunday, April 14, 2019 11:21AM - 11:33AM |
H14.00002: Search for anomalous decay of the free neutron using the UCNA experiment: n -> $\chi + e^+ + e^-$ Xuan Sun The neutron lifetime is currently measured by two different types of experiments: 'beam' and 'bottle', which have a 4$\sigma$ discrepancy in measured lifetime. In January 2018, Fornal and Grinstein proposed to resolve this issue by introducing a dark sector particle, $\chi$, that could offer an alternative decay channel for the neutron. The proposed theory allows for a $e^+ e^-$ pair produced in addition to the hypothesized $\chi$. The UCNA (Ultra Cold Neutron Asymmetry) experiment is particularly sensitive to this decay signature. In this experiment, polarized neutrons decay in a trap, and their charged products are guided by a 1 T magnetic field to detectors on either side. Timing information and energy reconstruction is done on each beta decay. We use results from the UCNA experiment's 2012-2013 dataset to set limits on the branching fraction of this decay channel, as a function of the mass of the $\chi$. In this talk, we present the analysis and set limits on this neutron dark matter decay channel. |
Sunday, April 14, 2019 11:33AM - 11:45AM |
H14.00003: Exotic decay modes in Be-11 Bruno Olaizola, Yassid Ayyad, Wolfgang J Mittig, Saul Beceiro In a recent letter [1] a novel explanation was proposed for the so-called neutron lifetime anomaly. To explain the difference in the rate of disappearing neutrons and appearing protons, they suggested that neutrons could decay into dark sector particles ~1% of the time. In [1], they also calculated that a few selected nuclei, as Be-11, haven an open energy window for this hypothetical dark decay. Be-11 is the only isotope for which indirect and ambiguous evidence of beta-delayed proton emission decay exists [2]. They implanted Be-11 in a foil and then, using mass spectroscopy, measured the amount of Be-10 created, thus measuring a combination of all possible decay modes leading to Be-10. We performed an experiment at TRIUMF using the prototype Active Target Time Projection Chamber (pATTPC) and the high purity of the ISAC Be beams to directly observe the beta-p decay channel for the first time. The use of the pATTPC allowed for the unambiguous observation of the proton emission. We will present preliminary results on the measured branching ratio, where any difference with that measured in [2] would hint to the presence of a dark decay branch in Be-11. [1] B. Fornal and B. Grinstein. Phys. Rev. Lett., 120:191801 (2018) [2] K. Riisager, et al. Physics Letters B, 732:305 (2014) |
Sunday, April 14, 2019 11:45AM - 11:57AM |
H14.00004: Search for novel decay modes using $^{11}$Be Jason Surbrook, Tamas A Budner, Moshe Friedman, Cathleen E Fry, Brent E Glassman, Molly A Janasik, Emmanuel Pollacco, Michael J Roosa, Jordan Stomps, Christopher Lars Henrik Wrede In 2014, the transmutation of $^{11}$Be to $^{10}$Be was observed using accelerator mass spectrometry and attributed to a previously unobserved mode of radioactive decay: $\beta^{-}$-delayed proton ($\beta^{-}p$) emission. The branching ratio of this decay channel was deduced to be nearly two orders of magnitude larger than expectations based on theoretical models. Confirmation of this discrepancy, through detection of emitted protons, would yield valuable input for these models and potentially relate the $\beta^{-}p$ rate to the neutron halo in $^{11}$Be. A more exotic decay-process was proposed in early 2018. The neutron lifetime anomaly might be explained by a small decay branch to a ``dark neutron’’ that cannot be observed directly. If this were true, then a neutron could spontaneously disappear from a weakly neutron-bound nuclide such as $^{11}$Be, converting it to $^{10}$Be, without proton emission. We have performed a measurement of the charged particle radiations from $^{11}$Be at the National Superconducting Cyclotron Laboratory. The measurement employed the recently-commissioned GADGET system, a gas-filled detector based on a MICROMEGAS structure for charge amplification surrounded by the SeGA array of high purity Germanium detectors for gamma-ray detection. |
Sunday, April 14, 2019 11:57AM - 12:09PM |
H14.00005: Ultra-Cold Neutron measurement of Proton branching ratio in neutron Beta decay (UCNProBe) Zhaowen Tang, Chris Cude-Woods, Steven Clayton, Christopher Lee Morris, Takeyasu M Ito, Mark F Makela, Alexander Saunders, Jin Ha Choi, Jared Lambert, Chen-Yu Liu, Albert Young, Deion Fellers, Kevin P Hickerson, Erik Watkins, Robert W Pattie, Bryan A Zeck, Christopher Martin O'Shaughnessy The free neutron lifetime can be measured using one of two methods: measuring the decay products of neutrons in a well calibrated neutron beam (beam experiment), or counting the number of surviving neutrons stored in a UCN trap over time (bottle experiment). The lifetime results from the two different methods differ by 10 seconds or five standard deviations. Our goal is to resolve the difference between the two measurements by measuring the proton branching ratio of neutron decay using UCNs. Detecting a proton branching ratio of less than one will indicate new physics beyond the Standard Model of particle physics. The experiment is realized by storing the neutrons in a material trap made from deuterated scintillators. To measure the beta decay lifetime, we will attempt to measure the absolute number of UCNs inside the trap and the absolute number of electrons from beta decay to 0.1% precision. In this talk, we will describe the concept of the experiment and its systematic error. We will also report recent characterization measurements performed on the scintillator. |
Sunday, April 14, 2019 12:09PM - 12:21PM |
H14.00006: UCNbX:a high-precision ultracold neutron decay branching ratio experiment. Kevin Peter Hickerson It is now established that the neutron lifetime has a discrepancy between counting decay protons, and bottled ultracold neutrons (UCN) disappearance. |
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