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Register a new userThe neutrino mass direct measurements
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Christian Weinheimer
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One of the most important tasks in neutrino physics is to determine the neutrino mass scale to distinguish between hierarchical and degenerate neutrino mass models and to clarify the role of neutrinos as dark matter particles in the universe. The current tritium beta decay experiments at Mainz and Troitsk are reaching their sensitivity limit. The different options for a next generation direct neutrino mass experiment with sub-eV sensitivity are discussed. The KATRIN experiment, which will investigate the tritium beta spectrum with an unprecedented precision, is being prepared to reach a sensitivity of 0.2 eV.
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With the compelling evidence for massive neutrinos from recent neutrino oscillation experiments, one of the most fundamental tasks of particle physics over the next years will be the determination of the absolute mass scale of neutrinos, which has crucial implications for cosmology, astrophysics and particle physics. Neutrino oscillation experiments can measure squared mass differences but not masses. The latter have to be determined in a different way. The direct mass experiments investigate -- besides time-of-flight measurements -- the kinematics of weak decays obtaining information on the neutrino mass without further requirements. Here the tritium beta decay experiments give the most stringent results. The current tritium beta decay experiments at Mainz and Troitsk are reaching their sensitivity limit. The different options for a next generation direct neutrino mass experiment with sub-eV sensitivity are discussed. The KATRIN experiment, which will investigate the tritium beta spectrum with a MAC-E-Filter of 1 eV resolution, is being prepared to reach a sub-eV sensitivity.
The investigation of the endpoint region of the tritium beta decay spectrum is still the most sensitive direct method to determine the neutrino mass scale. In the nineties and the beginning of this century the tritium beta decay experiments at Mainz and Troitsk have reached a sensitivity on the neutrino mass of 2 eV/c^2 . They were using a new type of high-resolution spectrometer with large sensitivity, the MAC-E-Filter, and were studying the systematics in detail. Currently, the KATRIN experiment is being set up at Forschungszentrum Karlsruhe, Germany. KATRIN will improve the neutrino mass sensitivity by one order of magnitude down to 0.2 eV/c^2, sufficient to cover the degenerate neutrino mass scenarios and the cosmologically relevant neutrino mass range.
We report the results of the second measurement campaign of the Karlsruhe Tritium Neutrino (KATRIN) experiment. KATRIN probes the effective electron anti-neutrino mass, $m_{ u}$, via a high-precision measurement of the tritium $beta$-decay spectrum close to its endpoint at $18.6,mathrm{keV}$. In the second physics run presented here, the source activity was increased by a factor of 3.8 and the background was reduced by $25,%$ with respect to the first campaign. A sensitivity on $m_{ u}$ of $0.7,mathrm{eV/c^2}$ at $90,%$ confidence level (CL) was reached. This is the first sub-eV sensitivity from a direct neutrino-mass experiment. The best fit to the spectral data yields $m_{ u}^2 = (0.26pm0.34),mathrm{eV^4/c^4}$, resulting in an upper limit of $m_{ u}<0.9,mathrm{eV/c^2}$ ($90,%$ CL). By combining this result with the first neutrino mass campaign, we find an upper limit of $m_{ u}<0.8,mathrm{eV/c^2}$ ($90,%$ CL).
First on-line mass measurements were performed at the SHIPTRAP Penning trap mass spectrometer. The masses of 18 neutron-deficient isotopes in the terbium-to-thulium region produced in fusion-evaporation reactions were determined with relative uncertainties of about $7cdot 10^{-8}$, nine of them for the first time. Four nuclides ($^{144, 145}$Ho and $^{147, 148}$Tm) were found to be proton-unbound. The implication of the results on the location of the proton drip-line is discussed by analyzing the one-proton separation energies.
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 endpoint at 18.57 keV gives an effective neutrino mass square value of $(-1.0^{+0.9}_{-1.1})$ 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 two and provides model-independent input to cosmological studies of structure formation.
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