Precision measurements of free-neutron β decay have been used to precisely constrain our understanding of the weak interaction. However, the neutron Fierz interference term bn, which is particularly sensitive to beyond-standard-model tensor currents at the TeV scale, has thus far eluded measurement. Here we report the first direct constraints on this term, finding bn = 0.067 ± 0.005stat+0.090-0.061sys, consistent with the standard model. The uncertainty is dominated by absolute energy reconstruction and the linearity of the β spectrometer energy response.

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Published in Physical Review C, v. 96, issue 4, 042501, p. 1-6.

©2017 American Physical Society

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Due to the large number of authors, only the first 30 and the authors affiliated with the University of Kentucky are listed in the author section above. For the complete list of authors, please download this article or visit: https://doi.org/10.1103/PhysRevC.96.042501

This group of authors is collectively known as the UCNA Collaboration.

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This work is supported in part by the US Department of Energy, Office of Nuclear Physics (DE-FG02-08ER41557, DE-SC0014622, DE-FG02-97ER41042) and the National Science Foundation (Grants No. NSF-0653222, No. NSF-0700491, No. NSF-0855538, No. NSF-1002814, No. NSF-1005233, No. NSF-1205977, No. NSF-1306997, No. NSF-1307426, No. 1506459, and No. NSF-1615153).