Neutrino physics gains new levels of popularity

The 21st International Conference on Neutrino Physics and Astrophysics was held on 14-19 June in the splendid Marguerite de Navarre auditorium at the Collège de France, in the heart of Paris. Organized by the CEA, the CNRS, the Collège de France and the University of Paris7-Denis Diderot, its aim was to review the latest developments in this rapidly evolving branch of physics. It attracted 520 participants – a record turnout for this series of meetings, and a clear sign of the renewed interest in neutrinos within the particle-physics community. All the advances made in neutrino physics were reviewed over the course of six days, and the most significant new results are summarized here.

Solar neutrinos formed the topic of the first session, beginning with a presentation of the latest measurements from the Sudbury Neutrino Observatory in Canada. This was followed by a report from Giorgio Gratta of Stanford on the results from the KamLAND experiment in Japan, which has provided new and definitive proof of neutrino oscillation in the energy range of a few million electron-volts.

Here, the neutrinos are not of extraterrestrial origin – instead they come from an artificial source, namely nuclear reactors. The detector, which has been built on the site of the old Kamiokande experiment, uses 1 kt of liquid scintillator as the target and seeks to observe the neutrino interactions of nearby reactors – mainly those at installations in Japan, but also some in South Korea. The average distance between the sources and the detector is 180 km, which has proven sufficient to confirm the deficit observed by the experiments designed to measure solar neutrinos.

After two years of taking data, KamLAND has reported 258 events, compared with an expected 365. Furthermore, the study of the energy distribution of these events indicates a spectral distortion in the low-energy range. This is a crucial result, because in addition to confirming oscillation, it allows a much more precise measurement than that made possible by solar neutrinos of the essential oscillation parameter Δm², the difference in the mass-squared of the two oscillating neutrinos. The result can be expressed as Δm² = 8.2 + 0.6 – 0.5 10^-5 eV². In a simple mass-hierarchy scenario, this determines the mass of the second neutrino ν_μ at 9 meV, which is about 100,000 million times lighter than the proton.

The results from KamLAND also open up a new line of research, namely the study of geoneutrinos, which was presented by Gianni Fiorentini of Ferrara and INFN. The Earth emits a tiny heat flux, and what scientists want to know is whether it comes exclusively from radioactivity. The uranium, thorium and potassium content could be determined by studying the neutrinos emitted, but the energy of these neutrinos is even lower than that of neutrinos from nuclear power plants, and KamLAND is close to the observation limits.

With neutrinos it is also possible to spy on what is going on inside nuclear reactors, and on the various fission products that produce neutrinos with different spectra, as John Learned from Hawaii described. The International Atomic Energy Agency, the watchdog organization for the non-proliferation of nuclear weapons, is beginning to be interested in this means of control.

Oscillation experiments

Following the presentations on low-energy neutrinos, it was the turn of atmospheric neutrinos and the results obtained with the new Super-Kamiokande detector. Edward Kearns from Boston presented the expected distribution of ν_μ interactions as a function of the L/E parameter, the ratio between the length of flight and the energy of the neutrinos detected, which agrees very well with the oscillation hypothesis. These results have been supported by those from the K2K experiment, in which neutrinos produced at the KEK laboratory are observed in the Super-Kamiokande detector 250 km away.

Overall, analyses from all the oscillation experiments are placing increasingly severe constraints on Δm² and on the mixing angles of the leptonic, or MNSP (Maki-Nakagawa-Sakata-Pontecorvo), mixing matrix. Srubabati Goswami of Allahabad summarized the state of progress. The third angle θ₁₃ is the least well known; we have only one limit, from the experiment at the Chooz reactor in France. The determination of this angle, together with CP violation in the field of neutrinos, is crucial, and various projects at different reactors were discussed. Intense activity surrounds the preparation of the longer-term future; pending the construction of neutrino factories, super beams and radioactive beams are under consideration, particularly at CERN.

Direct measurements of neutrino masses, limits on the magnetic moment, and searches for double beta-decay (with or without neutrinos) were also presented, in particular the first results from the Neutrino Ettore Majorana Observatory (NEMO3) experiment in Fréjus Underground Laboratory, France, and the Cuoricino project in the Gran Sasso Laboratory, Italy. The presentations on this subject covered evidence, indications and enigma. The latter category includes a signal for neutrinoless double-beta decay that comes from an analysis of the Heidelberg-Moscow germanium experiment, which will probably be the subject of discussion for several years to come. The field of unresolved enigma also includes the result from the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos, which should soon be clarified by the MiniBoone experiment at Fermilab.

The last part of the conference covered neutrino astrophysics. The neutrino sky map is still very incomplete; only the Sun and the brief signal from supernova SN1987A in February 1987 have been observed. Other sources are at work in the vast expanse of the sky, but their detection requires instruments 10,000 times larger than those that exist at present. Current projects, which tend towards a detector of 1 km³, were reviewed. In parallel with neutrinos, attention at the conference focused on research into astroparticles – high-energy photons, charged cosmic rays and gravitational waves. Michel Davier of Orsay demonstrated the potential richness of the “multi-messenger” approach for the future.

There is a fine line between astrophysics and cosmology, especially since constraints on neutrino mass are starting to emerge from experiments on the study of cosmic background radiation and
in-depth explorations of the universe. The constantly evolving results of this discipline were presented, and research into dark matter – both direct (new results from the Cryogenic Dark Matter Search, CDMS) and indirect – was discussed, along with dark energy.

Theory was not forgotten, and there was discussion of recent progress on neutrino mass models, tests of the various CP, T and CPT symmetries, flavour violations, and the implications of neutrinoless double-beta decay. The previous week, a two-day satellite conference involving almost 100 physicists had been held to commemorate the 25th anniversary of the discovery of the “seesaw mechanism”, currently the most promising explanation for the smallness of neutrino masses.

Remote visitors

Thanks to a successful webcast, some 800 Web users interested in neutrinos were able to follow the conference remotely, and anyone wishing to hear the presentations again will be able to do so during the next few months by logging onto the conference website at http://neutrino2004.in2p3.fr, where copies of the speakers’ transparencies are also available. The event was also a media success, thanks to a press conference organized the previous week involving around 15 journalists representing the main French newspapers and radio stations.

Neutrino physics has undergone dynamic changes in recent years. New ideas have been put forward in both the theoretical and the experimental fields, which should help this branch of physics to continue making major advances for many more years to come. The next step will be discussed during June 2006 in Santa Fe, at the next conference to be held in this series.