Bound on $3+1$ Active-Sterile Neutrino Mixing from the First Four-Week Science Run of KATRIN

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Bound on $3 + 1$ Active-Sterile Neutrino Mixing from the First Four-Week Science Run of KATRIN

M. Aker et al. (KATRIN Collaboration)

Phys. Rev. Lett. 126, 091803 – Published 5 March 2021

Abstract

We report on the light sterile neutrino search from the first four-week science run of the KATRIN experiment in 2019. Beta-decay electrons from a high-purity gaseous molecular tritium source are analyzed by a high-resolution MAC-E filter down to 40 eV below the endpoint at 18.57 keV. We consider the framework with three active neutrinos and one sterile neutrino. The analysis is sensitive to the mass, $m_{4}$ , of the fourth mass state for $m_{4}^{2} ≲ 1000 {eV}^{2}$ and to active-to-sterile neutrino mixing down to $| U_{e 4} |^{2} ≳ 2 \times 10^{- 2}$ . No significant spectral distortion is observed and exclusion bounds on the sterile mass and mixing are reported. These new limits supersede the Mainz results for $m_{4}^{2} ≲ 1000 {eV}^{2}$ and improve the Troitsk bound for $m_{4}^{2} < 30 {eV}^{2}$ . The reactor and gallium anomalies are constrained for $100 < Δ m_{41}^{2} < 1000 {eV}^{2}$ .

Received 13 November 2020
Revised 6 January 2021
Accepted 22 January 2021

DOI:https://doi.org/10.1103/PhysRevLett.126.091803

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Sterile neutrinos

Particles & FieldsNuclear Physics

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Vol. 126, Iss. 9 — 5 March 2021

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Images

Figure 1
Components of the KATRIN experiment: (a) the rear section, (b) the windowless gaseous tritium source, (c) the pumping section, (d) the prespectrometer, (e) the main spectrometer, (f) the electron detector.
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Figure 2
(a) Electron spectrum of experimental data $R (⟨ q U ⟩)$ over the interval [ $E_{0} - 40$ to $E_{0} + 50 eV$ ] from all 274 tritium scans and the three-neutrino mixing best-fit model $R_{calc} (⟨ q U ⟩)$ (line). The integral $β$ -decay spectrum extends to $E_{0}$ on top of a flat background $R_{bg}$ . The rate is given in counts per second (cps). $1 σ$ errors are enlarged by a factor 50. (b) Simulation of an arbitrary sterile neutrino imprint on electron spectrum. The ratio of the simulated data without fluctuation, including a fourth neutrino of mass $m_{4} = 10 eV$ and mixing $| U_{e 4} |^{2} = 0.01$ , to the three-neutrino mixing model is shown (red solid line). (c) Integral measurement time distribution of all 27 HV set points.
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Figure 3
95% C.L. exclusion curves in the ( $| U_{e 4} |^{2}$ , $m_{4}^{2}$ ) plane obtained from this analysis. Both statistical and systematic uncertainties are included. The two solid lines show the expected sensitivity (light gray) and the associated exclusion (blue) for fixed $m_{ν}^{2} = 0 {eV}^{2}$ (case I). The dotted line in dark blue illustrates the exclusion curve obtained with a free $m_{ν}^{2}$ (case II). Last, the dot-dashed line in turquoise displays the intermediate exclusion curve with a free $m_{ν}^{2}$ constrained with an uncertainty $σ (m_{ν}^{2}) = 1 {eV}^{2}$ (case III). These results supersede the Mainz exclusion limit [44] for $m_{4}^{2} ≲ 1000 {eV}^{2}$ and improve the Troitsk bounds [45] for $m_{4}^{2} < 30 {eV}^{2}$ .
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Figure 4
95% C.L. exclusion curves in the ( $\sin^{2} (2 θ_{e e}), Δ m_{41}^{2}$ ) plane obtained from the analysis of KATRIN data with fixed $m_{ν} = 0$ . The green contour delimits the $3 + 1$ neutrino oscillations allowed at 95% C.L. by the reactor and gallium anomalies [4]. KATRIN data improve the exclusion of the high $Δ m_{41}^{2}$ values with respect to DANSS, PROSPECT, STEREO, Daya Bay, and Double Chooz reactor measurements [37, 39, 46, 48, 49]. Mainz [44] and Troitsk [45] exclusion curves [50] are also displayed for comparison. An estimation of KATRIN’s final sensitivity is represented by the dotted line. The light (dark) gray bands delimit the exclusions from $0 ν β β$ experiments, for the case of inverted and normal hierarchies (the extension of the bands reflects the uncertainties of the parameters of the Pontecorvo-Maki-Nakagawa-Sakata matrix [1]).
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