First direct constraints on Fierz interference in free neutron β decay

K. P. Hickerson , X. Sun , Y. Bagdasarova , D. Bravo-Bergu\~no , L. J. Broussard , M. A.-P. Brown , R. Carr , S. Currie

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X. Ding B. W. Filippone A. Garc\'ia P. Geltenbort J. Hoagland A. T. Holley R. Hong T. M. Ito A. Knecht C.-Y. Liu J. L. Liu M. Makela R. R. Mammei J. W. Martin D. Melconian M. P. Mendenhall S. D. Moore C. L. Morris R. W. Pattie Jr. A. P\'erez Galv\'an R. Picker M. L. Pitt B. Plaster J. C. Ramsey R. Rios A. Saunders S. J. Seestrom E. I. Sharapov W. E. Sondheim E. Tatar R. B. Vogelaar B. VornDick C. Wrede A. R. Young B. A. Zeck

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keywords betaneutronconstraintsdecaydirectenergyfierzfirst

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Precision measurements of free neutron $\beta$-decay have been used to precisely constrain our understanding of the weak interaction. However the neutron Fierz interference term $b_n$, 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 $b_n = 0.067 \pm 0.005_{\text{stat}} {}^{+0.090}_{- 0.061}{}_{\text{sys}}$, consistent with the Standard Model. The uncertainty is dominated by absolute energy reconstruction and the linearity of the beta spectrometer energy response.

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First direct constraints on Fierz interference in free neutron β decay

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