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.
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.
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.
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.
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.
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.
Theory
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.
| FOR |
AGAINST |
- 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."
|
Conclusions
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:
http://nu2000.sno.laurentian.ca/.
Disclaimers
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
(Juha.Peltoniemi@oulu.fi)