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Register a new userNon-oscillation probes of neutrino masses
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C. Weinheimer
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The absolute scale of neutrino masses is very important for understanding the evolution and the structure formation of the universe as well as for nuclear and particle physics beyond the present Standard Model. Complementary to deducing statements on the neutrino mass from cosmological observations two different methods to determine the neutrino mass scale in the laboratory are pursued: the search for neutrinoless double beta decay and the direct neutrino mass search. For both methods currently experiments with a sensitivity of order 100 meV are being set up or commissioned.
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There is a renewed interest in constraining the sum of the masses of the three neutrino flavours by using cosmological measurements. Solar, atmospheric, and reactor neutrino experiments have confirmed neutrino oscillations, implying that neutrinos have non-zero mass, but without pinning down their absolute masses. While it is established that the effect of light neutrinos on the evolution of cosmic structure is small, the upper limits derived from large-scale structure could help significantly to constrain the absolute scale of the neutrino masses. It is also important to know the sum of neutrino masses as it is degenerate with the values of other cosmological parameters, e.g. the amplitude of fluctuations and the primordial spectral index. A summary of cosmological neutrino mass limits is given. Current results from cosmology set an upper limit on the sum of the neutrino masses of ~1 eV, somewhat depending on the data sets used in the analyses and assumed priors on cosmological parameters. It is important to emphasize that the total neutrino mass (`hot dark matter) is derived assuming that the other components in the universe are baryons, cold dark matter and dark energy. We assess the impact of neutrino masses on the matter power spectrum, the cosmic microwave background, peculiar velocities and gravitational lensing. We also discuss future methods to improve the mass upper limits by an order of magnitude.
The various experiments on neutrino oscillation evidenced that neutrinos have indeed non-zero masses but cannot tell us the absolute neutrino mass scale. This scale of neutrino masses is very important for understanding the evolution and the structure formation of the universe as well as for nuclear and particle physics beyond the present Standard Model. Complementary to deducing constraints on the sum of all neutrino masses from cosmological observations two different methods to determine the neutrino mass scale in the laboratory are pursued: the search for neutrinoless double $beta$-decay and the direct neutrino mass search by investigating single $beta$-decays or electron captures. The former method is not only sensitive to neutrino masses but also probes the Majorana character of neutrinos and thus lepton number violation with high sensitivity. Currently quite a few experiments with different techniques are being constructed, commissioned or are even running, which aim for a sensitivity on the neutrino mass of {cal O}(100) meV. The principle methods and these experiments will be discussed in this short review.
Neutrino oscillation results from several experiments and sources are discussed. Recent results from solar neutrino measurements by Super-Kamiokande and Borexino, atmospheric neutrino measurements from Super-Kamiokande, and accelerator neutrino measurements by MINOS and OPERA are the main topics of this document.
From 21 independent Baryon Acoustic Oscillation (BAO) measurements we obtain the following sum of masses of active Dirac or Majorana neutrinos: $sum m_ u = 0.711 - 0.335 cdot delta h + 0.050 cdot delta b pm 0.063 textrm{ eV,}$ where $delta h equiv (h - 0.678) / 0.009$ and $delta b equiv (Omega_b h^2 - 0.02226) / 0.00023$. This result may be combined with independent measurements that constrain the parameters $sum m_ u$, $h$, and $Omega_b h^2$. For $delta h = pm 1$ and $delta b = pm 1$, we obtain $m_ u < 0.43$ eV at 95% confidence.
The recent observation of neutrino oscillations with atmospheric and solar neutrinos, implying that neutrinos are not massless, is a discovery of paramount importance for particle physics and particle astrophysics. This invited lecture discusses - hopefully in a way understandable also for the non-expert - the physics background and the results mainly from the two most relevant experiments, Super-Kamiokande and SNO. It also addresses the implications for possible neutrino mass spectra. We restrict the discussion to three neutrino flavours (nu_e, nu_mu, nu_tau), not mentioning a possible sterile neutrino.
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