Differential spectrum modeling and sensitivity for keV sterile neutrino search at KATRIN

Starting in 2026, the KATRIN experiment will conduct a high-statistics measurement of the differential tritium $\beta$-spectrum to energies deep below the kinematic endpoint. This enables the search for keV sterile neutrinos with masses less than the kinematic endpoint energy $m_\mathrm{4} \leq E_0 = 18.6\,\mathrm{keV}$, aiming for a statistical sensitivity of $|U_\mathrm{e4}|^2=\sin^2\theta\sim 10^{-6}$ for the mixing amplitude. The differential spectrum is obtained by decreasing the retarding potential of KATRIN's main spectrometer, and by determining the $\beta$-electron energies by their energy deposition in the new TRISTAN SDD array. In this mode of operation, the existing integral model of the tritium spectrum is insufficient, and a novel differential model is developed in this work.

The new model (TRModel) convolves the differential tritium spectrum using responese matrices to predict the energy spectrum of registered events after data acquisition. Each response matrix encodes the spectral spectral distrortion from individual experimental effects, which depend on adjustable systematic parameters. This approach allows to efficiently assess the sensitivity impact of each systematics individually or in combination with others. ... mehrThe response matrices are obtained from monte carlo simulations, numerical convolution, and analytical computation.

In this work, the sensitivity impact of 20 systematic parameters is assessed for the TRISTAN Phase-1 measurement for which nine TRISTAN SDD modules are integrated into the KATRIN beamline. Furthermore, it is demonstrated that the sensitivity impact is significantly mitigated with several beamline field adjustments and minimal hardware modifications.