Otokar Dragoun

The Karlsruhe Tritium Neutrino (KATRIN) experiment aims at measuring the effective electron neutrino mass with a sensitivity of 0.2 eV/c², i.e., improving on previous measurements by an order of magnitude. Neutrino mass data taking with KATRIN commenced in early 2019, and after only a few weeks of data recording, analysis of these data showed the success of KATRIN, improving on the known neutrino mass limit by a factor of about two. This success very much could be ascribed to the fact that most of the system components met, or even surpassed, the required specifications during long-term operation. Here, we report on the performance of the laser Raman (LARA) monitoring system which provides continuous high-precision information on the gas composition injected into the experiment’s windowless gaseous tritium source (WGTS), specifically on its isotopic purity of tritium—one of the key parameters required in the derivation of the electron neutrino mass. The concentrations c_x for all six hydrogen isotopologues were monitored simultaneously, with a measurement precision for individual components of the order 10⁻³ or better throughout the complete KATRIN data taking campaigns to date. From these, the tritium purity, ε_T, is derived with precision of −3 and trueness of −3, being within and surpassing the actual requirements for KATRIN, respectively.

Although neutrinos are probably the most abundant fermions of the universe their mass is not yet known. Oscillation experiments have proven that at least one of the neutrino mass states hasm_i> 0.05 eV while various interpretations of cosmological observations yielded an upper limit for the sum of neutrino masses ∑m_i< (0.14 ‒ 1.7) eV. The searches for the yet unobserved 0νββ decay result in an effective neutrino massm_ββ< (0.2 ‒ 0.7) eV. The analyses of measured tritium β-spectra provide an upper limit for the effective electron neutrino massm(v_e) < 2 eV. In this review, we summarize the experience of two generations of β-ray spectroscopists who improved the upper limit ofm(v_e) by three orders of magnitude. We describe important steps in the development of radioactive sources and electron spectrometers, and recapitulate the lessons from now-disproved claims for the neutrino mass of 30 eV and the 17 keV neutrino with an admixture larger than 0.03%. We also pay attention to new experimental approaches and searches for hypothetical sterile neutrinos.

Although neutrinos are probably the most abundant particles of the universe their mass is not yet known. Oscillation experiments have proven that at least one of the neutrino mass states has m_{i}>0.05 eV while various interpretations of cosmological observations yielded an upper limit for the sum of neutrino masses \sum m_{i}<(0.14-1.7) eV. The searches for the yet unobserved 0\nu\beta\beta decay result in an effective neutrino mass m_{\beta\beta}<(0.2-0.7) eV. The analyses of measured tritium \beta-spectra provide an upper limit for the effective electron neutrino mass m(\nu_e)<2 eV. In this review, we summarize the experience of two generations of \beta-ray spectroscopists who improved the upper limit of m(\nu_e) by three orders of magnitude. We describe important steps in the development of radioactive sources and electron spectrometers, and recapitulate the lessons from now-disproved claims for the neutrino mass of 30 eV and the 17 keV neutrino with an admixture larger than 0.03 %. We also pay attention to new experimental approaches and searches for hypothetical sterile neutrinos.

Advantages and drawbacks of the kinematic methods of the neutrino mass determination are discussed. The meaning of the effective neutrino mass, resulting from measurements of the endpoint region of β-spectra is clarified. Current experimental constraints on the mass of active as well as sterile neutrinos are presented. Several new experiments are briefly outlined.