The beautiful interior of the San Vidal church in Venice was the setting for a workshop on neutrino telescopes, which looked at the many messages neutrinos carry.
The 10th International Workshop on Neutrino Telescopes, held in Venice on 11-14 March, brought together particle physicists, astrophysicists and cosmologists, all captivated by the fascinating properties of neutrinos. A total of 142 participants attended the meeting, which was organized by Milla Baldo Ceolin of Padova University and co-sponsored by the Istituto Nazionale di Fisica Nucleare (INFN) and the Istituto Veneto delle Scienze, Lettere ed Arti. In her opening address, Baldo Ceolin recalled the recent death of George Marx, a leading figure in Hungarian astroparticle physics. Further reminiscences followed, with warm and affectionate recollections of Bruno Pontecorvo by Luigi Radicati di Brozolo from Pisa, who talked about Pontecorvo’s deep human qualities and his invaluable scientific legacy, in particular regarding neutrinos.
The first two days of the workshop were devoted to neutrino oscillations, neutrino masses and mixing angles. John Bahcall of the Institute of Advanced Study, Princeton, began by reminding us of the tremendous challenge that the detection of solar neutrinos represented when it was first proposed. Like the Sun, which shone in Venice during most of the conference and dissolved the last of the winter fog, the joint effort of all experiments on solar neutrinos and solar physics has finally cast light on the long-standing solar neutrino problem. However, warned Bahcall, now that the solar standard model seems to work perfectly well, we should not stop testing it.
The elegance and completeness of the experiments at the Sudbury Neutrino Observatory (SNO) – with its outstanding feature of being sensitive to both charged and neutral current interactions on deuterium – emerged clearly from the talks by SNO project director Art McDonald of Queen’s University, Ontario, and by Richard Hahn from the Brookhaven National Laboratory. The experiment now has evidence at the 5.3σ level for neutrino flavour oscillation to active neutrinos. Yoichiro Suzuki of Tokyo described the contribution that Kamiokande and Super-Kamiokande have made in this field, including the first directional observation of solar neutrinos, the first measurement of the 8B neutrinos from the Sun, and the most precise detection of high-energy solar neutrinos through neutrino-electron scattering, as well as the detection of neutrinos emitted by the explosion of the supernova SN1987A. Kamiokande and Super-Kamiokande also produced the first clear evidence of neutrino oscillations through the distortion of the zenith distribution of atmospheric muon neutrinos. Support for this result came from the MACRO detector at the Gran Sasso Laboratory and Soudan2 in the US. Takaaki Kajita of Tokyo presented an exciting series of measurements in this field, together with the first confirmation of muon neutrino oscillations with a long baseline neutrino beam, which has been made by the K2K experiment. This experimental programme will continue with the long baseline experiment at the Japan Proton Accelerator Research Complex (J-PARC, formerly the Japan Hadron Facility).
Moving to the low-energy region, Vladimir Gavrin of the Institute for Nuclear Research (INR) of the Russian Academy of Sciences (RAS) reminded us of the contributions made by the Baksan Neutrino Observatory. Its most outstanding result was provided by SAGE, the radio-chemical experiment with metallic gallium that has been taking data for 13 years. Thanks to the low threshold of the neutrino capture reaction in the metal, gallium experiments are the only ones that are sensitive to all the components of the solar neutrino flux, in particular the pp neutrinos. On the same topic, Till Kirsten of the Max Planck Institute, Heidelberg, described the evolution over the years of the Gran Sasso Laboratory’s activities on solar neutrinos. The GALLEX experiment announced the first observation of solar pp neutrinos 10 years ago, and also made the first neutrino source calibration of any solar neutrino detector. Today, Gran Sasso’s involvement in solar neutrino physics continues with the Gallium Neutrino Observatory, which began running in 1998; the BOREXINO real-time, low-threshold solar neutrino detector, which is almost ready; and the LUNA experiment that aims to measure the cross section of the fusion reactions at the energy of the solar Gamow peak (that is, the optimum energy for the reactions).
The final brush-stroke to this picture of solar neutrino oscillations – after a quest that has lasted for 50 years – has come from the KamLAND experiment, which has shown the first strong evidence for the disappearance of reactor antineutrinos. As Atsuto Suzuki from Tohoku pointed out, the present KamLAND result is completely consistent with large mixing angle (LMA) solar neutrino oscillations. In addition, the experiment has such a low background that it can observe the antineutrinos from the decay of uranium and thorium in the Earth, and could in the near future provide a measurement of the beryllium-generated solar neutrinos. In this context, Gianno Fiorentini of Ferrara stressed that it is now time to use such geo-neutrinos to determine the radiogenic contribution to the energetics of the Earth. More generally, Gianluigi Fogli of Bari pointed out that we are entering the era of precision tests for several neutrino parameters. In the solar sector, the large angle solution at present includes two close, but separate regions in parameter space (see plots figure). Fogli showed that there is now statistical evidence for matter effects in the Sun, which were also nicely reviewed by Alexei Smirnov of INR/RAS and ICTP, one of the founding fathers of this field.
Artificial neutrino beams will continue to play a fundamental role in the precise determination of neutrino oscillation parameters. Bill Louis from Los Alamos presented the evidence for neutrino oscillation from the LSND experiment, through the appearance of electron antineutrinos in a muon antineutrino beam. He also described the status of MiniBooNe at Fermilab, which should unambiguously confirm or refute the LSND result, and which is now taking data with the results expected in early 2005. The final and complete mapping of the neutrino oscillation parameters will, however, require new facilities and new detectors. Deborah Harris from Fermilab presented future long baseline neutrino experiments, introducing new concepts in neutrino beams such as off-axis neutrino beams, “super beams” and “beta beams”. Ken Peach from the Rutherford Appleton Laboratory illustrated the road towards what appears to be a final neutrino beam facility – the Neutrino Factory – where neutrino beams of unprecedented intensity and purity will be produced by the decays of muons in flight.
The general implications of the recent neutrino results on physics beyond the Standard Model were discussed by Guido Altarelli from CERN. He remarked that the neutrino properties fit nicely, and actually support the framework of grand unification in its supersymmetric version, where dark matter and baryogenesis can be included naturally. In such a context, the dominant source of neutrino masses could be the “see-saw” mechanism, which was reviewed by Steve King of Southampton, Rabindra Mohapatra of Maryland and Ferruccio Feruglio of Padova. Neutrino masses are also considered in models with extra dimensions, as summarized by Qaisar Shafi from the Bartol Research Institute. The challenging and fundamental direct determination of neutrino mass, both with integral spectrometers and cryogenic detectors, was discussed by Christian Weinheimer of Bonn and Angelo Nucciotti of Milano-Bicocca. The relevance of a positive signal in neutrinoless double beta decay for understanding the neutrino spectrum was emphasized by Serguey Petcov of SISSA/INFN and INRNE, Sofia, while Alessandro Strumia of Pisa reminded us about the existing unconfirmed neutrino “anomalies”.
On the third day of the workshop, the discussion moved to neutrino astrophysics. Petr Vogel of Caltech explained that supernova neutrinos are essential for improving our knowledge about the emission models in gravitational collapses. Neutrinos with energies in the MeV range could also shed light on other types of gravitational collapse, such as the one leading to a strange-quark star starting from a neutron star, as described by Arnon Dar of Technion and CERN.
The detection of high-energy cosmic neutrinos represents one of the most exciting future prospects in astrophysics – indeed, in 1988 the first Neutrino Telescope Workshop held in Venice promoted the birth of this new field. Dar, Daniele Fargion of Rome and Francis Halzen of Wisconsin reviewed the theoretical motivations for studying high-energy cosmic neutrinos. Such studies are expected to play an important role in unravelling the mysteries associated with major cosmic accelerators, such as active galactic nuclei and gamma-ray bursters. There are several existing models, and competition between them is fierce, as noted by Alvaro de Rujula of CERN, so observations will be crucial. Sandip Pakvasa of Hawaii pointed out that neutrino decay might affect the flavour composition of astrophysical neutrinos.
Impressive progress has been made in the construction of neutrino telescopes since the first workshop in 1988. As Stephan Hundertmark of Stockholm reported, AMANDA, the muon and neutrino detector array at the South Pole, is now a successfully operating telescope. It currently has the best limit on a neutrino source above the TeV region, with a sensitivity of about 0.1 event per km2 per year. The future of AMANDA will be IceCube, a kilometre-scale neutrino observatory designed to detect neutrinos of all flavours at energies from 107 eV to 1020 eV. The first of its 80 strings will be deployed in 2004, and the detector will be completed in 2009. Closer to Europe, Jürgen Brunner of Marseille reported on ANTARES, the neutrino telescope under construction off the Toulon coast, which will be ready to take data in less than three years. A strong programme has also already begun on the Neutrino Mediterranean Observatory (NEMO), a km3 deep-sea neutrino telescope. The wish of all the participants is that the first “light” through this new window on the universe might be announced in Venice at a future Neutrino Telescope Workshop.
Neutrinos in cosmology were the subject of the fourth and last day of the workshop. Few things in recent years have had the same impact on our view of particle interactions as the recent impressive experimental achievements in cosmology. The data from WMAP, for example, confirm that we are now performing precision tests of cosmological models. Evidence for dark matter was reviewed in great detail by Marco Roncadelli of INFN Pavia, while Rita Bernabei of Rome reported the status of LIBRA. This is the upgraded version of DAMA, the experiment at Gran Sasso that is reporting a signal from dark-matter particles. We know that neutrinos can be, at most, a sub-dominant component of dark matter, and as Sergio Pastor of Valencia recalled, we can infer from the power spectrum of density fluctuations, an upper bound on the sum of neutrino masses of about 1 eV. The canonical dark-matter candidate remains the lightest particle – possibly a neutralino – in a supersymmetric extension of our world. Neutrinos could, however, provide an efficient way of revealing neutralinos captured by the Sun, after annihilation, as suggested by Antonio Masiero of Padova. On a related topic, the observed baryon density could have been produced by leptogenesis, through the CP-violating, non-equilibrium decay of a right-handed neutrino. Franco Buccella of Napoli showed how leptogenesis can be elegantly incorporated into grand unified theories such as SO(10). It is remarkable that the range of neutrino masses required for successful leptogenesis is essentially the same as the one obtained from neutrino oscillations, as was discussed by Wilfried Buchmüller of DESY.
Dark energy is currently one of the most intriguing mysteries of our universe, and was described for the conference by Masiero and Sabino Matarrese of Padova. Dark matter and dark energy are also expected to affect the time variation of fundamental constants in specific frameworks such as string theory, as Thibault Damour of IHES, Bures-sur-Yvette, explained. Neutrinos are apparently not involved here – unless, as Guido Altarelli observed, the equality between the dark-energy scale and the scale of neutrino masses is not a numerical coincidence but is instead an indication of some deep and yet unidentified relationship.
Scrutinized at any length scale, from the microscopic to the cosmological, our world is fascinating. This workshop illustrated that neutrinos are capable of carrying information about our universe from the smallest scales to the largest distances, and this, in the words of Sheldon Glashow of Boston who closed the meeting, is the kind of unification that we should really be looking for.
