KATRIN Sensitivity to keV Sterile Neutrinos with the TRISTAN Detector Upgrade

Abstract

Sterile neutrinos in the keV mass range are a well-motivated extension of the Standard Model and viable dark matter candidates. Their existence can be probed in laboratory experiments, as the admixture of a sterile state would induce a characteristic kink-like distortion in the $β$-decay electron energy spectrum. The KATRIN experiment is designed to measure the effective electron neutrino mass with sub-eV sensitivity by analyzing the endpoint region of the tritium $β$-decay spectrum. Following the completion of its neutrino mass program, KATRIN will extend its physics reach to the search for keV-scale sterile neutrinos. This effort will be enabled by the TRISTAN detector, a newly developed silicon drift detector array optimized for differential measurements at high rates and energies well below the endpoint. In this article, we present the projected sensitivity of KATRIN to keV-scale sterile neutrinos using a dedicated simulation framework. With four months of detector livetime, KATRIN has the statistical power to probe mixing amplitudes at the level of $|U_{e4}|^2 \sim 10^{-6}$ for sterile neutrino masses in the (4$-$13) keV range, significantly extending the reach of previous laboratory searches. The major experimental systematic uncertainties investigated in this work reduces the sensitivity by a factor of 10$-$50 over the same mass range.

Figures (13)

• Figure 1: Illustration of a sterile neutrino signature in the theoretical tritium $\upbeta$-decay spectrum. The black curve shows the standard spectrum, while the blue curve includes a sterile state with $m_4=10$ keV and an unphysically large mixing amplitude of $\sin^2\!\theta=0.2$ for visibility. The kink appears at $E=E_0-m_4$. For clarity, no experimental effects are included in this plot.
• Figure 2: Schematic view of the KATRIN beamline equipped with the TRISTAN detector. The magnetic-field setting corresponds to the reference configuration used in this analysis. The dominant systematic effects relevant for the keV-sterile neutrino search are illustrated: final-state distribution (a), $\mathrm{T_2}$ scattering (b), magnetic traps (c), rear-wall backscattering (d) and decay (e), magnetic reflection (f), magnetic collimation (g), spectrometer transmission (h), spectrometer adiabaticity (i), post-acceleration (j), synchrotron energy loss (k), detector backscattering and back-reflection (l), charge collection and backscattering escape (m), and charge sharing (n).
• Figure 3: Comparison of the on-axis magnetic field and electric potential for the nominal KATRIN neutrino measurement campaign and for the keV sterile neutrino search using the TRISTAN detector.
• Figure 4: Schematic of the iterative structure of the convolution model used to predict the observable $\upbeta$-decay tritium spectrum. Light-cyan blocks marked with a database symbol denote groups of systematic effects whose responses are implemented using precomputed response matrices stored in dedicated databases.
• Figure 5: Predicted tritium $\upbeta$-decay spectrum measured by the TRISTAN detector at KATRIN for four months of data taking, in the absence of any keV sterile-neutrino spectral distortion. The tritium $\upbeta$-decay spectrum is shifted by 20 keV due to the acceleration of electrons by the PAE before the detector. Partial energy depositions from charge sharing and backscattering are responsible for the low energy contribution below 20 keV. The non-zero contribution above the tritium energy endpoint is due to unresolved pileup events. In this configuration, because of the $-3.5$ kV retarding potential, the accessible sterile neutrino mass range is reduced to 15 keV.