General presentation and scientific context of the conference

General Presentation
Scientific Context

General presentation

The evidence for solar (Homestake, SuperKamioKa, Gallex, Sage) and atmospheric oscillations (SuperKamioKa, Macro, Soudan2) has recently reached a maturity suggesting that we are for the first time after 50 years of neutrino studies witnessing the effects of neutrino masses and mixing.

The main goal of this euroconference is the interdisciplinary study of the evidence for neutrino masses and mixing and the prospects of its further definition.

It intends to gather representatives from the community of accelerator physicists, astroparticle physicists, solar physicists, phenomenologists, formal theorists and cosmologists in order to evaluate a) the evidence for neutrino mass and oscillation, b) the veracity and completeness of solar and atmospheric data and modeling used in the extraction of the signals c) the generality of the proposed phenomenological analyses d) the relative theoretical importance of the different parameters of the mass and mixing matrix. It will further prospect which measurements will help disentangle competing analyses and theoretical models, review the existing long baseline program and set the axes of study for the future upgrades in the context of a neutrino factory. It will also study alternative scenarii and signals seen in medium baseline experiments and the changes they might induce, if verified, in the above program. It will finally study the cosmological constraints and the possibilities of detection of neutrinos coming from distant sources, e.g supernovae, sources of gamma-bursts and active galactic nuclei, from the point of view of the the determination of the mass and mixing parameters.

The Euroconference will also serve as a training ground for a small number of young researchers. It will offer them the unique opportunity to study problems usually spread about different fields and to interact with leading scientists, usually attending other conferences or meetings. We hope that the intense discussion sessions in the evening and the availability of good computing facilities at Les Houches, will be the starting point for many new contributions and collaborative efforts in the field, which though quite old still has a lot of theoretical space available for new intuitions organizing its future.

Scientific Context

The second half of the twentieth century has been marked by the progressive revealing and deeper understanding of the gauge structure of the fundamental interactions; what is presently called the Standard Model (SM).

This discovery, in a continuous interplay of theory and experiment, lead to:

a first unification of the electro-magnetic with the weak interaction: the electroweak theory that withstood with success the per-mil precision measurements at the LEP collider and elsewhere,
hints of further unification with the strong interactions at a high mass scale, the Grand Unified Theory (GUT) scale, provided the supersymmetric extension of the SM is considered.

Neutrino physics played a major role in this evolution: from the discovery of parity violation, through the discovery of neutral current interactions to the establishment of the scaling violations in hadronic interactions and the determination of the number of 3 "active" neutrinos ( $\nu _{e,}\nu _{\mu },\nu _{\tau }$ ) by the Z width and consequently of the number of fermion families.

This highly successful program awaits only one experimental question to be solved before it is considered complete: the existence of Higgs, the scalar particle responsible for the masses of gauge particles and fermions within the SM. The direct searches at LEP/CERN put a lower limit on the Higgs mass $\sim 100$ GeV/c², and the radiative corrections to the LEP/SLC and FNAL observables put an upper limit of $\sim 200$ GeV/c². This sets the stage for the discovery of the Higgs at the Tevatron at Fermilab and the LHC/CERN. The answer to the Higgs puzzle will be known before 2010. Many workshops and conferences have addressed already this topic, and it will be only indirectly treated in our workshop.

Nevertheless, even if the Higgs is discovered, the patterns of masses and their electroweak mixing remain unexplained in the context of the SM, or equivalently the couplings of the Higgs to the fermions remain arbitrary and fixed only by experiment. Furthermore, the progressive understanding of the quark sector from the strange (V particles $\sim 1948$ ) to the top quark (1995) and the determination of their mixing matrix, the so called Cabibbo-Kobayashi-Maskawa (CKM) matrix, while successful and in full development ( see e.g the Babar experiment at SLAC) gives measurements which are difficult to use theoretically, since the strong interaction is blind to the mixing and introduces large uncomputable uncertainties.

Luckily, the last quarter of the 20th century has also witnessed experimental indications that neutrinos may have a mass and therefore non-zero mixing. These indications became much stronger during the last years. The leptonic mixing matrix is the perfect laboratory for testing theories generating masses for fermions, since it is not affected by strong interaction corrections.

There has been 3 types of solar neutrino experiments (using Cl,Ga,H₂O as target) measuring a deficit of $\nu _{e}$ interactions, giving thus indications of an oscillation of $\nu _{e}$ to some other neutrino type. This by itself is a strong sign that neutrinos have mass. Even if one of these experiments is wrong, the data are not compatible with solar physics. Present observables are not yet sufficient to fix one solution for the oscillation formula and so we are currently left with 3 possible solutions: two at $\delta m^{2}\sim 10^{-4}eV^{2}$ and large or small mixing and one at much lower mass difference ( $\Delta m^{2}\sim 10^{-11}eV^{2}$ ) and large mass mixing¹. The situation will certainly become clearer by the time of the proposed conference, since the Sudbury Neutrino Observatory (SNO) experiment in Canada will report on its first results during 2000 and the SuperKamioka (SK) water detector in Japan will increase its statistics. An ambitious future program (Kamland, Borexino, LENS, etc...) will further diminish the uncertainties and fix a unique solution for the oscillation pattern.

Five experiments (SuperKamioka, Kamioka, IMB,Macro, Soudan2) have seen an anomalously low rate of $\nu _{\mu }$ coming from the interaction of cosmic rays with the atmosphere. Among these, SuperKamioka having by far the largest statistics and sensitivity, has given an array of evidence that the anomaly is indeed consistent with $\nu _{\mu }\rightarrow \nu _{\tau }$ oscillations, with maximal mixing and $\Delta m^{2}\sim 3\times 10^{-3}eV^{2}$ .

The uncertainties of the atmospheric modeling and data, while they cannot put in doubt the existence of the above anomaly, need to be reduced in order to obtain a precise determination of the oscillation parameters ( $\Delta m^{2},\theta$ ). That is why an intensive program has started, measuring the experimental inputs to the modelisation

with balloon and satellite experiments (e.g MASS2, AMS) testing the primary proton spectrum and
accelerator experiments at CERN (P214) testing the model of hadronic interactions.

In parallel, upgrades in the modelisation (3D vs 1D) are also actively pursued. The discussion around these matters will also be a theme of the conference.

Surprisingly enough these two sets of measurements (atmospheric and solar), due to the simplification that the large mass hierarchy imposes, are sufficient to fix the gross features of the leptonic mixing matrix. There is currently a wealth of phenomenological analyses fitting the data under the assumption of 3 neutrino flavors or permitting a fourth "sterile" (without electroweak interactions) one. These analyses need to be refined and augmented with theoretical input (consistency with radiative corrections etc) in order to set the guidelines for the future neutrino experimental program.

The next few years will be very exciting. The first long baseline experiment is already in operation between KEK and the SuperKamioka site (250 Km) and the year 2000 will be the year that its first results will be presented to the scientific community. The conference, will evaluate their impact on ongoing and planned experimental and theoretical studies. The above data from K2K will be for instance crucial for setting the final experimental goals, and define the strategy of the long baseline program in the US (MINOS) but most importantly for the two European efforts (with international collaboration) OPERA and ICARUS now at the proposal stage. They will be operated in the recently approved long baseline neutrino beam from CERN to Gran Sasso (750 Km). The complementarity of the American program where the signal is the pattern of neutrino oscillation through disappearance and the European program where the signal is direct detection of the $\tau$ signal coming from $\nu _{\mu }\rightarrow \nu _{\tau }$ appearance guarantees a no-loose situation. Definitive answers are expected by 2007.

The present neutrino program ending in 2010 is not a dead-end. Neutrinos coming from muon colliders, the so-called neutrino-factories will be intensive sources of neutrinos probing with unprecedented precision the leptonic mixing matrix and eventually even measure the CP violation effects in the leptonic sector.

Neutrino masses cannot be introduced in the standard model without assuming new interactions at the GUT scale. In some sense the smallness of the $\nu$ mass is a hint of the large suppression induced by symmetry breaking at a high mass scale. The mass of the neutrino becomes therefore a new probe, after the proton lifetime, of the physics at the Grand Unified Scale. Neutrino masses make Grand Unified Theories and predictions experimentally testable once more, in parallel with the unification hints coming from LEP precision measurements. The present data already exclude, due to the relatively straightforward Renormalisation Group running of the leptonic evolution, many proposed theories.

The above theoretical implications, mass models, textures and their implications for cosmology will be examined.

Other topics with relevance to the progress explicited above are the results of the medium baseline experiment at Los Alamos LSND observing an anomaly that might also be interpreted as $\nu _{e}$ oscillation, which could be only accommodated in the above picture if there was a fourth "sterile" neutrino. This anomaly is not confirmed by KARMEN an experiment of lower sensitivity and will be tested by the Boone experiment at Fermilab. Though no new developments are expected till 2001, the review of these medium baseline results is planned so that their relevance and implications for the rest of the program is examined.

Finally, there is an ongoing discussion for the construction of a large underwater telescope, possibly in the Mediterranean, using neutrinos as a probe to map violent effects in the universe (active galactic nuclei, gamma-bursts etc). The Mega-Science forum in Sicily in 1997 already set an international committee, and a European branch to evaluate the feasibility, and the co-ordination of current projects (AMANDA, ANTARES (France), BAIKAL, NEMO (Italy), NESTOR (Greece) etc). The EuroConference will study the potential of these detectors under the point of view of neutrino masses and mixing, as well as the use of existing detectors for supernova detection. The consequences of neutrino masses on the formation of the universe, the baryon asymmetry, and the cosmological history will also be largely debated.

The European experimentalists and theorists already play a leading role in the above context, we hope that the above workshop will strengthen their collaborative ties and create new opportunities for fresh ideas and interaction with their younger colleagues.

1. The probability of oscillation in the 2 flavor oscillation case is: $P(\nu _{a}\rightarrow \nu _{b})=\sin ^{2}2\theta \sin ^{2}(\Delta m^{2}L/4E)$ where $\theta$ is the mixing angle, $\Delta m^{2}$ is the mass difference, L is the observation baseline and E the neutrino energy