Improved limits on Fierz Interference using asymmetry measurements from the UCNA experiment

Abstract

The Ultracold Neutron Asymmetry (UCNA) experiment was designed to measure the $\beta$-decay asymmetry parameter, $A_0$, for free neutron decay. In the experiment, polarized ultracold neutrons are transported into a decay trap, and their $\beta$-decay electrons are detected with $\approx 4\pi$ acceptance into two detector packages which provide position and energy reconstruction. The experiment also has sensitivity to $b_n$, the Fierz interference term in the neutron $\beta$-decay rate. In this work, we determine $b_n$ from the energy dependence of $A_0$ using the data taken during the UCNA 2011-2013 run. In addition, we present the same type of analysis using the earlier 2010 $A$ dataset. Motivated by improved statistics and comparable systematic errors compared to the 2010 data-taking run, we present a new $b_n$ measurement using the weighted average of our asymmetry dataset fits, to obtain $b_n=0.066\pm 0.041_{\text{stat}}\pm 0.024_{\text{syst}}$ which corresponds to a limit of $-0.012<b_n<0.144$ at the 90% confidence level.

6 pages, 3 figures. Submission to PRC

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• Constraining beyond the Standard Model nucleon isovector charges
• Opportunities and open questions in modern $\beta$ decay
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• Exploring Four Fermion Contact Couplings of a Dark Fermion and an Electron at Hadron Colliders and Direct Detection Experiments
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• Improving of ultracold neutron traps coated with liquid helium using capillarity and electric field
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• Fundamental Neutron Physics: a White Paper on Progress and Prospects in the US
• Study of silicon photomultipliers for use in neutron decay experiments
• Numerical Calculation of Electric Field Enhancement in Neutron Traps with Rough Walls Coated with Superfluid Helium