Comments on the results presented in Neutrino 98, June 4 - 9, Takayama, Japan

The XVII Conference on Neutrino Physics and Astrophysics (Neutrino 98) was held in Takayama Cultural Center, close to Kamioka. Several new results were presented, some of which are quite amazing, though not necessarily surprizing. As conclusion, we have much stronger reasons to assume that neutrinos have a non-zero mass.

Atmospheric neutrinos

The most important results of the conference were reported by professor Kajita (ICRR). These were related on the measurement of the atmoshperic neutrinos in Superkamiokande which is a 50 kton water Cerenkov detector located in the Kamioka zinc mine in the Japansese Alps. It has been operating since April 1997, and the reported data is based on 500 days of operation.

Atmospheric neutrinos are secondary particles produced by cosmic rays (protons) hitting the atmosphere of the earth. The flux of atmospheric neutrinos can be calculated, when the proton flux is known. Although the calculations for absolute neutrino fluxes contain lots of uncertainties, the ratio of electron neutrinos to muon neutrinos can be predicted with much better accuracy.

Superkamiokande measured the muon neutrino to electron neutrino ratio to be 63 % of the predicted ratio. The apparent deficit of muon neutrinos is consistent with previous measurements, like Kamiokande III and Soudan 2. The news is that Superkamiokande has measured the angular dependence of the neutrino events more accurately than before. They see that the flux of muon neutrinos going up is smaller than that of downgoing neutrinos. In other words, muon neutrinos coming from the other side of the earth tend to disappear more than those produced above the detector.

The most obvious solution to the observed discrepancy is neutrino flavor oscillation from muon neutrinos to tau neutrinos. This fits well to the angular distribution, since neutrinos traversing a longer way have more time to oscillate. Oscillation from muon neutrinos to sterile neutrinos is as good, but oscillation from muon neutrinos to electron neutrinos does not fit, and would also conflict laboratory measurements (CHOOZ). Apart from neutrino oscillation, no other consistent explanation has been proposed.

The results have been interpreted as an evidence for non-zero neutrino mass. Within the standard neutrino theory only massive neutrinos can oscillate. Although models for the oscillation of massless neutrinos have been proposed, (e.g. by Sheldon Glashow in the same conference) such models would require so many speculative features that they are not taken seriously by most scientists.

The Superkamiokande experiment does not tell directly the value for any neutrino mass. Instead it can be taken as a measurement of the difference of squared masses between neutrino flavors. The best fit is obtained for squared mass differences in the range 10-2 - 10-3 eV2, and very large mixing. (Plot of the oscillation parameters will be soon included.) Unless there is no fine tuning, this suggests a neutrino mass of the order of 0.1 eV. Such a mass implies the neutrino energy density in the universe to be 0.001 of the critical density which is too small to have cosmological consequences. Specific models, however, may allow larger neutrino masses quite naturally.

For a summary of the situation, see the data page for atmospheric neutrinos.

Solar neutrinos

The scientific sessions started with reports from solar neutrino experiment. Concerning solar models and predictions for solar neutrino fluxes, professor John Bahcall (IAS) presented the new calculations (referred later as BP98). Although the predictions for the experiments were slightly modified from the previous ones, nothing dramatic has happened. The biggest sources for errors are two nuclear cross sections that are poorly known for the relevant energies. According to Bahcall, measuring these reactions in laboratory conditions is as important for the solar neutrino puzzle as another solar neutrino experiment. There was no serious critic against the models of Bahcall, and no competing model was presented.

We heard the talks of Lande, Gavrin and Kirsten reporting the Chlorine and Gallium experiments. There were no essential changes to the previous situation: all see less neutrinos than predicted. Although some results of gallium experiments vary significantly from run to run, there is no evidence for regular time variation. Sage will continue until 2006, at least, while Gallex has been terminated. Its work will be continued by GNO.

The results of Superkamiokande were reported by Y. Suzuki. They have analyzed data for 504 effective days, observing 6823 +148130+238-198 neutrino events. This is 47 % of the predicted value. (Previously 36 %, the prediction has also changed.) They see no essential difference between the neutrino fluxes in night and day, neither do they see any seasonal variation beyond the geometrical effect due to the variation of the Earth-Sun distance. They have also been able to measure the energy spectrum of neutrinos (from 6.5 MeV to 20 MeV), and after calibrations by a linac they have reduced correlated systematic errors. A flat deficit gives a poor fit, rather the spectrum appears to be distorted so that at high energies they see more neutrinos than at low energies.

The only known consistent solutions to the observed deficits of neutrinos are transitions of neutrino type. No known astrophysical explanation can explain the data. Previously the small angle MSW conversion from electron neutrinos to muon neutrinos has been the most preferred solution, particularly from theoretical point of view. Indeed, it can solve the puzzle, although it fits less well to the high energy tail of the SK spectrum. I would not be too worried of two data points, however. The large angle MSW solution seems to be out. The vacuum oscillation is also a good solution, and it actually fits better to the SK energy spectrum. The best fit, however, seems to be MSW conversion to sterile neutrinos. The resonant spin-flavor precession has so many free or unknown parameters that it can be fitted to any observation.

For a summary of the measurements, see the data page of solar neutrinos. Plots of the fits are to be included as soon as they come available.

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. Previously most of the measurements of neutrino mass have reported negative value for the mass squared.

The Mainz experiment has made lot of effort to analyze the systematics of the measurement. They report now for the mass squared a value - 9 +- 8 +-2 eV2. Although this is still negative, it is less negative than before. They see some kind of bump at the endpoint. Fitting a monoenergetic line at the endpoint, they obtain an upper limit 3.4 eV. Without the extra degree of freedom one has the limit 5 eV. Until the origin of the endpoint anomaly is cleared, this is to be regarded as the most reliable limit for the electron neutrino mass.

The most surprising results of the conference were reported by Lobashev. The Troitsk experiment has a very good energy resolution for the electron spectrum near the endpoint. Their data can be interpreted as consisting of an ordinary beta decay spectrum and a monoenergetic peak. Assuming that the monoenergetic peak is of other origin, it can be detracted from the spectrum. With this procedure, one obtains for neutrino mass an upper limit of 2.7 eV (95 % CL). The most weirdest observation is, however, that the position of the monoenergetic peak is not constant, but changes from one measurement to another. Analyzing the time dependence of the peak one finds that it has a periodicity of half a year!

For a long time physicists have speculated that the tritium endpoint anomalies may be caused by a neutrino capture. A relic neutrino hitting the target might cause a reaction

nu + 3H --> 3He + e- + Q.

For ordinary weak interactions the neutrino density at earth should be 5*1022 1/m3, which is many orders of magnitude larger than the predicted neutrino density. To obtain such densities, there should be a new long range force acting on neutrinos. This force might then coalesce neutrinos to a cloud of diameter about 1-3 AU, with degenerate neutrinos having Fermi energy of 5 eV. Moreover, the neutrino cloud should be asymmetric, to cause the half a year periodicity.

Needless to say, this solution would upset everything we know about cosmology.

Several groups are working with the double beta decay measurements. The most stringent bound (0.46 eV) was claimed by prof. Klapdor-Kleingrothaus of the Moscow-Heidelberg germanium experiment. The results of this and other experiments are summarized at the data page for neutrino mass.

Supernova neutrinos

Since there are no observations of supernova neutrinos since 1987, the focus of the reports was rather theoretical.

The most interesting question around supernovae seems to be the synthesis of the heaviest elements by r-process, which may likely take place in the hot bubble of the exploding supernova. Simulating the rapid nuclear reactions in the exploding star is a very challenging task, and it is essentially harder than simulating the stellar energy production or big bang nucleosynthesis. The present simulations do not yet fit well enough to the data.

Neutrinos play a substantial role for making the appropriate conditions for the r-process. Namely, a successcul nucleosynthesis requires the matter to be neutron rich, and the proton to neutron ratio is controlled by the neutrino radiation. Hence, changes in the neutrino flux, e.g. by MSW conversion, may directly affect the production of heavy nuclei. Particularly, the conversions to sterile neutrinos may help the nucleosynthesis (Peltoniemi 95), and David Caldwell in his talk stated that "the model with sterile neutrinos is the only robust solution".

Several new experiments will be able to measure a future supernova. Some of these were discussed during the meeting, both in form of an oral presentation and a poster.

Neutrino oscillation experiments: LSND vs Karmen

The LSND experiment still stands behind their claims of neutrino oscillation. Prof White reported a total excess of 83 +- 24 electron neutrinos, compared to background, which is consistent with an oscillation probability 0.31 +- 0.09 +- 0.05. They have used two kinds of targets, and have studied both decay at rest and decay in flight, for neutrino and antineutrino channels.

On the contrary, the upgraded Karmen experiment sees no signal of neutrino oscillation. As reported by Zeitnitz, they are about to approach a conflict with LSND masurements, but there is still a small range of parameters allowed by both experiments (and all we need is a point). Within a couple of years they may be able to reach the sufficient statistics to make stronger comparisions.

It is worthwile to repeat the comment by David Caldwell that one should not compare apples to oranges. The plots with the allowed regions for the oscillation parameters displayed by these two groups apply two different statistical procedures, one uses confidence levels and the other likelihood contours. Alhtough they sound quite similar, they are different enough not to be mixed in the same figure.

The Karmen experiment keeps seeing the time anomalies. Totally they see an excess of 99 events above the typical decay curve, which is 3.8 sigmas off. This looks like an instrumental effect, but they have not been able to find any reason for it, although studying very carefully all the possible systematics, including the most obvious causes like afterpulses. If it were something real, it could be a weakly interacting particle of 33.9 MeV. (A sterile neutrino mixing with electron and muon neutrinos may do). Nevertheless, it sound very weird that there could be a particle whose mass is almost exactly the mass difference of pions and muons! The origin of the time anomalies should be clarified before Karmen can claim to exclude LSND results.

Neutrinos and cosmology

There were some interesting contributions to neutrino cosmology. However, they did not enhance essentially our understanding of neutrinos. There seems to be some kind of conflict within cosmology: observations of distant objects favor a large cosmological constant with no hot dark matter while the simulations of structure formations fit the observations only when one has some 20 % of hot dark matter, the best fit being two neutrinos with a total mass of 4.7 eV.

The quest of sterile neutrinos

Several speakers, including Smirnov, Langacker and Caldwell, speculated on the existence of sterile neutrinos. 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.

  • A conversion to sterile neutrinos fits the solar neutrino data slightly better than conversions to muon neutrinos.
  • 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 or to have neutrinos as hot dark matter 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].
  • Right-handed neutrinos may solve the time anomalies of Karmen.
  • Recent simulations of structure formation of the universe need hot dark matter. (Gawiser and Silk 98)
  • 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).
  • Right-handed neutrinos are the most evident extension of the standard model. Some sophisticated superstring-inspired models can accommodate light sterile neutrinos (BenakliSmirnov97).
  • The limits for sterile neutrinos from early universe are too unreliable to be taken at face value, and can even be avoided by ingenius mechanisms (FootVolkas98). Sterile neutrinos may even help to generate lepton and baryon asymmteries.
  • The observations of Karmen exclude most of the parameter range needed for LSND solution.
  • The most evident solution to the Karmen time anomalies, a sterile neutrino of 33.9 MeV is too weird to be taken seriously. How an earth a particle can have a mass equal to the difference of masses of two completely different particles, one of which is even a composite meson?
  • Recent observations (esp distant supernovae) for cosmological parameters suggest a large cosmological constant and no hot dark matter.
  • The nuclear, subnuclear and macroscopic phenomena in supernovae are too poorly known to make any reliable conclusions by requiring successful nucleosynthesis.
  • 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."


Something is happening in neutrino physics right now. The results of SuperKamiokande, combined with the results of many other experiments, strongly suggest neutrinos having non-zero masses. However, all these are indirect observations, mostly observing a deficit of neutrinos, so they cannot be taken as a definite proof for neutrino masses. To actually measure a mass for a neutrino may take a lot of time and money. In any case, there is very little reason to believe that neutrinos are massless.

More information

Transparencies of the talks are available for download.

SuperKamiokande press release

Neutrino news page at Hawaii


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 15th July 1998 Juha Peltoniemi (