Improved Upper Limit on the Neutrino Mass from a Direct Kinematic Method by KATRIN

We report on the neutrino mass measurement result from the first four-week science run of the Karlsruhe Tritium Neutrino experiment KATRIN in spring 2019. Beta-decay electrons from a high-purity gaseous molecular tritium source are energy analyzed by a high-resolution MAC-E filter. A fit of the integrated electron spectrum over a narrow interval around the kinematic end point at 18.57 keV gives an effective neutrino mass square value of $(-{1.0}_{-1.1}^{+0.9}){\mathrm{eV}}^2$. From this, we derive an upper limit of 1.1 eV (90% confidence level) on the absolute mass scale of neutrinos. This value coincides with the KATRIN sensitivity. It improves upon previous mass limits from kinematic measurements by almost a factor of 2 and provides model-independent input to cosmological studies of structure formation.

Figure 1

The major components of the KATRIN beam line consist of (a) the rear section for diagnostics, (b) the windowless gaseous tritium source WGTS, c) the pumping section with the DPS and CPS cryostats, and a tandem setup of two MAC-E-filters: (d) the smaller prespectrometer and (e) the larger main spectrometer with its surrounding air coil system. This system transmits only the highest-energy $\beta$ decay electrons onto (f) the solid-state detector where they are counted.

Figure 2

(top) Measured and calculated response functions $f(E-qU)$ for electron surplus energies $E-qU$ at different $\rho d$ values of $T_2$. Measured $f(E-qU)$ for a narrow-angle photoelectron source close to $\rho d_{\mathrm{nom}}$ and fit (cyan line); and calculated $f_{\mathrm{calc}}(E-qU)$ for isotropically emitted $\beta$ decay electrons up to ${\theta }_{\max }$ at $\rho d_{\exp }$ ($1.11\times {10}^{17}{\mathrm{cm}}^{-2}$), the set point of our scans (red line), and in the limit of vanishing $\rho d=0$ (grey, dashed-dotted line). (bottom) Differential distributions of energy losses $\delta E$ from the MAC-E-TOF mode after a selection $35\mu \mathrm{s}\le \mathrm{TOF}\le 50\mu \mathrm{s}$ at $\rho d\approx \rho d_{\mathrm{nom}}$ and fit (cyan line). The “no loss” peak at $\delta E=E-qU=0$ is followed by peaks with $s=2$ ($s=3$) scattering at twice (triple) the $\delta E$ value of $s=1$. The energy loss function $ϵ(\delta E)$ for $s=1$ is obtained by deconvolution (orange line).

Figure 3

(a) Spectrum of electrons $R(⟨qU⟩)$ over a 90 eV-wide interval from all 274 tritium scans and best-fit model $R_{\mathrm{calc}}(⟨qU⟩)$ (line). The integral $\beta$ decay spectrum extends up to $E_0$ on top of a flat background $R_{\mathrm{bg}}$. Experimental data are stacked at the average value $⟨qU{⟩}_l$ of each HV set point and are displayed with $1\sigma$ statistical uncertainties enlarged by a factor of 50. (b) Residuals of $R(⟨qU⟩)$ relative to the $1\sigma$ uncertainty band of the best fit model. (c) Integral measurement time distribution of all 27 HV set points.

Figure 4

Scatter plot of fit values for the mass square $m_{\nu }^2$ and the effective $\beta$-decay end point $E_0$ together with $1\sigma$ (black) and $2\sigma$ (blue) error contours around the best fit point (cross). It follows from a large set of pseudoexperiments emulating our experimental data set and its statistical and systematical uncertainties.