Fundamental Interactions  Neutrino Physics  Conclusion and Prospect

  
Outlook and recommendations

Neutrino physics is presently focused on the crucial problem of neutrino mass. If at least one neutrino has non-zero mass, this would mean evidence for physics beyond the Standard Model. In addition, massive neutrinos would play an important role in cosmic structure formation. Direct measurements provided until now only upper limits: several eV for $\nu_e$, 0.16 MeV for $\nu_\mu$, 24 MeV for $\nu_\tau$. The double beta decay experiments have given an upper limit of 1.-1.5 eV for the average mass of Majorana neutrinos. At present, the most promising way to obtain evidence for non-zero neutrino masses is to search for neutrino oscillations. There are three indications of neutrino oscillations: they have been obtained by the experiment LSND and by the study of Solar neutrinos and Atmospheric neutrinos. It is mandatory to continue the experimental activities in these three fields to clarify the situation (LSND) and obtain direct evidences of the neutrino oscillations, if they exist (Solar and Atmospheric neutrinos).

i. Solar neutrinos.

It is imperative to study the energy dependence of the solar neutrino flux and to this purpose a direct measurement of the ⁷Be component is necessary. Borexino is the only planned experiment capable of such a measurement. It is also important to independently demonstrate that the solar neutrino deficit results from neutrino oscillations. This can be achieved in three different ways:

• by detecting time variations of the solar neutrino flux. Seasonal variations, as expected from vacuum oscillations, can be observed with high sensitivity by Borexino, while day-night effects resulting from matter effects when neutrinos traverse the Earth can be observed by Superkamiokande, SNO and Borexino.
• by detecting distortions of the expected energy spectrum, as planned by SNO and by Superkamiokande.
• by measuring the total neutrino flux independently of neutrino flavour, through the detection of neutral current interactions. Such a measurement is planned by SNO at a later phase of the experiment.

Of the three experiments just mentioned, only Borexino is installed in a European laboratory (LNGS). This experiment should receive full support in order to start data taking as early as possible. Other experiments aimed at improving existing measurements must also be encouraged, such as Supermunu and Hellaz, which are designed for the spectroscopy of pp, ⁷Be and pep neutrinos. GNO will monitor the solar neutrino flux as a function of time over many years, thus detecting, for instance, slow variations associated with possible changes of Sun properties.

ii. Atmospheric neutrinos. The anomalies of the atmospheric neutrinos need a clarification for the multi-GeV data. In addition the hypothesis that the Atmospheric Neutrino problem could be explained by the oscillation mechanism needs a direct investigation, independent from the uncertainties in the predicted atmospheric fluxes. This is the case of the Long Baseline experiments. A long baseline experiment in Japan will start data taking in 1999 using neutrinos produced by the KEK 12 GeV proton synchrotron and the Superkamiokande detector (the energy of these neutrinos is below threshold for $\tau$ production). Another experiment (MINOS), with a total detector mass of 10 KTon, will use a beam from Fermilab at a distance of 730 km and will probably start data taking in the first years of the next century. Long baseline experiments have been proposed in Europe, using neutrino beams from CERN to LNGS at a distance of 732 km. At present the only approved experiment which could use such a beam is ICARUS, with a mass of only 600 Tons, but the construction of the beam is not yet approved. This programme must receive full support. We also recommend that any new detector for long baseline experiment be built with good electron identification capability in order to be simultaneously sensitive to $\nu_{\mu} -\nu_{e}$ and $\nu_{\mu} - \nu_{\tau}$ oscillations.

iii. Short baseline experiment. The LSND result need an independent confirmation. It will only be possible to plan a detailed strategy for a next round of short baseline experiments when the results from the upgraded KARMEN experiment and new data from LSND become available. But also CHOOZ, even less sensitive, is in condition to explore the $\Delta m^2$ region 10^-1-10^-2 eV² interesting for the atmospheric neutrinos. Therefore both KARMEN and CHOOZ have to be encouraged and supported. New more sensitive short base-line experiment are surely welcome. We should like to note that the $\Delta m^2$ region above 0.1 eV² is the region of cosmological importance, because neutrinos with such mass values would represent an important component of dark matter in the Universe. Another important way to search for neutrino physics beyond the Standard Model is to study the possible neutrino magnetic moment. MUNU is now installed at Bugey and it is expected to have a sensitivity better than the previous experiments by an order of magnitude. This experiment has to be fully supported. Upgradings and developments in cryogenic detectors have to be strongly encouraged in various fields as: beta decay, double beta decay, dark matter and X-ray spectroscopy. This technique may provide a very powerful tool for opening a new range of sensitivity in detector technologies with respect to energy resolution and detection threshold. Experiments on dark matter, using scintillators (as NaI) have to encouraged too; they have a good chance to reach large volumes in relative cheap conditions. Concerning double beta decay it will be desirable to push the sensitivity below $\sim$0.3 eV, which is the limit of experiments presently running or being planned. To achieve a sensitivity of $\sim$0.1 eV, a source mass of $\sim$ 100 Kg of enriched material, possibly ¹³⁶Xe, is required. This does not seem unrealistic. Improved detection techniques, methods to further reduce the background, must however be developed. R$\&$D in that direction should be encouraged. Experiments for precision measurements of scattering are sensitive to fundamental neutrino properties, and of high importance in elementary and particle astrophysics. An improvement of experiments in this field is highly recommended. The sensitivity of these experiments depends strongly on the detection threshold for the electron energy and background. Presently, cryogenic detectors and driftchambers seem to be promising technologies.