Comments on the results presented in Neutrino 2000, Sudbury

The XVIII Conference on Neutrino Physics and Astrophysics (Neutrino 2000) on the Laurentian University of Sudbury, Canada. The results showed a steady progress of the field, with no surprises. No tantalizing discoveries were reported, no new anomalies were shown. Instead many old anomalies are being settled down.

Atmospheric neutrinos

SuperKamiokande has gathered data for more than 1000 days, and the newest results are in agreement withe previous results. Both the statistical and systematic errors get smaller. Also Soudan 2 has measured the energy spectrum and angular distribution, and MACRO reports nice results, both supporting SK results. On the other hand, BUST in Baksan is in discrepancy.

The results are in excellent agreement with the hypothesis of neutrino oscillation. The favoured channel is the oscillation from muon neutrinos to tau neutrinos. Pure transformation to electron neutrino is both a bad fit and in disagreement with reactor experiments. Sterile neutrinos fit also poorly. However, 3 or 4 neutrino scenarios allow a substantial mixing between all four neutrinos, as long as the mu to tau oscillation is dominant.

Alternative solutions are in bad shape, although neutrino decay gives formally a satisfactory fit. It is, however, much more difficult to build an acceptable model predicting such a rapid neutrino decay.

New experiments have been proposed, but in the very near future there may not be dramatic progress.

Solar neutrinos

Everybody was anxious to hear the results of SNO. However, it was already expectable that the experiment might not get enough data to be published, which turned out to be true. As a hint of what to expect, McDonald showed the measured energy spectrum of electrons, which looked quite flat, with large error bars. However, he refused to give neither the scale of the plot nor any numbers referring to the measured neutrino flux. Anyway, SNO is running smoothly, and the first results will be reported within the next few years.

The other running experiments gave results basically confirming the previous results. SuperKamiokande converges to flat energy spectrum and no time variation. Although the new data of SuperKamiokande are not dramatically different from previous data, some people made quite strong conclusions about them. Now the combined data favors the large mixing angle MSW from electron neutrino to muon (or tau) neutrino. All solutions with a clear signature are in bad shape, including sterile neutrinos.

Measurements of neutrino mass

There are two ongoing experiments measuring directly the mass of the electron neutrino. Both Mainz and Troitsk groups study the beta decay of tritium. Mainz gives physically acceptable results (i.e. no negative mass squared), and shows no time variation. The results of Troitsk still show some strange time variation, with a half year cycle, that have not been explained.

Several experiments measure the double beta decay. The most stringent bound for the effective Majorana mass is 0.2 eV.

Supernova neutrinos

Now new supernovae were reported (surprise?). Several experiments are ready to measure the next neutrino burst. Neither has there been any essential breakthrough in theoretical understanding of supernova explosions.

Neutrino oscillation experiments

The most de-exciting news was that the time anomalies of Karmen have diappeared. No clear explanation for their appearand and disappearance was given, other than statistical fluctuation.

The origin of the oscillation events in LSND has not yet been clarified. Karmen gives no support for neutrino oscillation, but there is still a small range of parameters (shifted from some previous reports) compatible with both experiments. The best hope to solve the question is the MiniBooNe experiment, under construction in Femrilab.

K2K has measured accelerator neutrino flux. They see 17 muon neutrino events, while the expected number is 29 ± 3. This disfavors null oscillations at 2 sigma level. The preliminary results are compatible with the results of the atmospheric neutrinos. However, the experiment has not gathered enough data to make any decisive conclusion, and no plot was given.

Other accelerator neutrino experiments

NUTEV at Fermilab searches direct evidence for tau neutrinos, by detecting the produced taus with a huge detector consisting of successive layers of iron target and emulsion. They reported 5 candidates for taus, which agrees with the expected number. Complete results will be announced within a few weeks.


The last years brought extra-dimensional phenomena to neutrino physics. Several theorists got exited about them, and these superstring-related models found their way for explaining neutrino masses. So far no superior model has been found, but at least it keeps many theorists occupied.

Neutrinos and cosmology

The role of neutrinos in cosmology was not challenged in this conference. We had already experienced a huge progress in cosmology during the last years, giving us strong evidence for a cosmological constant. The dominant opinion among the cosmologists favors a scenario with insignificant neutrino masses. Models with neutrino mass dominating the universe are in clear disagreement with observations (including Sciama's neutrinos that were ruled out by satellite measurements already some time ago.)

The quest of sterile neutrinos

The interest for sterile neutrinos has decreased but not disappeared since the last neutrino conference. The sterile neutrinos are hypothetical particles that are like neutrinos but do not have ordinary weak interactions. They can be added to the theory as the right-handed neutrinos missing from the minimal standard model, or they can have a completely different origin. Apart from the heavy-right handed neutrinos present in see-saw model, the present interest focuses on light sterile neutrinos.

There are several facts speaking for or against light sterile neutrinos. They are summarized below.

  • The observations of solar and atmospheric neutrinos give strong evidence for neutrino mixing. The fits to present experimental results saturate the parameters. To solve the LSND results one needs more free paramaters, conveniently brought in by introducing sterile neutrinos.
  • Even solving simultaneously the solar and atmospheric neutrino problems with three neutrinos requires some fine tuning (fine tune factor about 0.05 for the best fits). [If you do not believe, play with the Mass Toy].
  • Sterile neutrinos help to avoid the limits for neutrino mixing by supernova nucleosynthesis. They can even help to make the conditions for the r-process more favourable (Peltoniemi95, Nunokawa et al 97).
  • Some models with extra dimensions predict sterile neutrinos.
  • A conversion to sterile neutrinos fits the solar neutrino data worse than conversions to muon neutrinos.
  • Sterile neutrinos fit badly to atmospheric neutrino data.
  • The observations of Karmen exclude most of the parameter range needed for LSND solution.
  • Recent observations (esp distant supernovae) for cosmological parameters suggest a large cosmological constant and no hot dark matter.
  • In most GUTs the right-handed neutrinos are heavy, with no natural possibility to make them light.
  • Big Bang Nucleosynthesis hardly allows any additional neutrino species.
  • "A model explaining all the observations must be wrong."


This conference was much less dramatic than the previous neutrino conferences. There was no big and really new result, although many experiments are making steady progress in understanding the neutrino nature. It may even appear that the neutrino physics has matured as a routine research. However, something may happen in the near future, even before the next galactic supernova.

More information

The transparencies are in the net:


These comments are based on my personal views, and may not reflect those of the speakers. Some of the claims may be incorrect, because of my bad memory or erratic notes. If you disagree with something, please mail me your opinion.
Last modified the 21th July 2000 Juha Peltoniemi (