Neutrinos lead beyond the desert

1 March 2003

The Finnish town of Oulu, on the Gulf of Bothnia, almost at the Arctic Circle, provided a pleasant atmosphere for a conference on particle physics beyond the Standard Model.


The Beyond the Desert 02 – Accelerator, Non-accelerator and Space Approaches conference was held on 2-7 June 2002. It was the third in the series of “Beyond conferences” that began in 1997. Traditionally the scientific programme has covered almost all of modern particle physics, and this meeting was no exception, ranging from SUSY and extra dimensions to dark matter and neutrino mass.

The conference began with sessions on new theoretical developments in extending the Standard Model by means of grand unified and SUSY theories, followed by new results on the search for Higgses, SUSY particles, R-parity violation, leptoquarks and excited fermions at the LEP and HERA colliders. The revival of a g-2 signal for the muon deviating from the Standard Model, and its consequences for SUGRA models, were addressed by Pran Nath of Boston who, together with Dick Arnowitt of Texas, first introduced SUGRA 20 years ago. Later, extra dimensions, M-theory and fundamental symmetries were also presented. Ignatios Antoniadis of CERN, while talking about string and D-brane physics at low energies, pointed out that although no-one has ever observed strings or the space of extra dimensions where they live, the “hidden” dimensions of string theory may be much larger than we thought in the past, and may come within experimental reach in the near future.

The long-standing and very intriguing problem of dark matter in the universe, with its connection to new physics and new phenomena, was another important topic. Results and perspectives for direct dark matter experiments with scintillators (DAMA and LIBRA) and germanium detectors with big target mass (GENIUS and GENIUS-TF) were presented by Rita Bernabei of Roma and Irina V Krivosheina of Heidelberg and Nizhnij Novgorod. These are currently the only two experiments that can in principle use seasonal modulation to see (and indeed DAMA has seen) a positive signal from the interactions of dark matter particles by direct detection. Other experiments, for example with sophisticated cryogenic detectors exploiting ionization (or scintillation)-to-heat discrimination, are currently unable to register such a modulation in WIMP interactions because of their very small detecting mass.

Astrophysical data are becoming increasingly important for modern particle physics. For example, the excellent talk by Naoshi Sugiyama of Tokyo – “Cosmic Microwave Background: a new tool for cosmology and fundamental physics” – made it evident that an unexpectedly huge amount of fundamental information can be extracted from current research into the cosmic microwave background. Astrophysical investigations are also intimately connected with the exciting question of neutrino properties. Cosmic high-energy neutrinos can interact with relic neutrinos, producing Z-bursts which could explain the mysterious origin of extremely-high-energy cosmic rays, as Sandor Katz of Eotvos, Hungary, explained. This mechanism requires the neutrino mass to be in the 0.02-2.2 eV range, which intriguingly fits with recent results obtained from neutrinoless beta decay of germanium in the Heidelberg-Moscow experiment. Neutrinos from supernovae also figure in current theoretical investigations, as Alexei Yu Smirnov of Trieste and Moscow described.

Neutrino physics was undoubtedly the central topic of the conference. Rabindra Mohapatra of Maryland presented the modern understanding and a general view of neutrino masses and mixings. This was followed by several presentations on solar neutrinos, with Oliver K Manuel of Missouri describing the Standard Solar Model and modern experimental hints for an elemental composition of the Sun that is radically different from the usual current assumptions. Extended discussion of the experimental achievements in solar and atmospheric neutrino oscillation experiments included the Sudbury Neutrino Observatory (SNO) and its results from the recent analysis with a pure heavy water target, presented by Mike Dragowsky of Los Alamos. The consequences of the neutral current rate measured in SNO for resolving the solar neutrino puzzle were discussed by Sandhya Choubey of Southampton. SNO performed the first measurements of the total active neutrino flux, and claims evidence for neutrino flavour transformation at a 5.3 sigma level.

Global MSW analysis of the neutrino oscillation experiments favours the large mixing angle (LMA) region, and can be tested in new experiments. At the conference, the running status and prospects for the new and near-future neutrino oscillation experiments KamLAND, K2K and Superkamiokande, and new facilities such as neutrino factories and the JHF-SK project, were presented and discussed. For example, KamLAND (presented by Fumihiko Suekane of Tohoku), is a very long baseline reactor neutrino oscillation experiment with a 1000 tonne liquid scintillator detector. It can directly test the MSW-LMA solution with only six months of data, and will determine the oscillation parameters with very high accuracy if the LMA case is true. The experiment started data-taking in 2002, and the first results have been announced (KamLAND experiment discovers that reactor antineutrinos ‘disappear’). Rebuilding of the Superkamiokande detector began in 2002, and full reconstruction is expected by 2007, as Takaaki Kajita of Tokyo described. The physics potential and status of the second-generation proton decay and neutrino experiment ICARUS (Imaging Cosmic And Rare Underground Signals) in the Gran Sasso Laboratory were also discussed by Fulvio Mauri of Pavia and Ines Gil-Botella of Zurich.

The exact nature of neutrinos remains an exciting problem. Are these most mysterious objects Dirac or Majorana particles, and what are their masses? One of the best tools to find the answer is neutrinoless double beta decay. The evidence for observation of neutrinoless double beta decay of the isotope 76Ge claimed by the Heidelberg-Moscow collaboration took a central part in the discussions. Alexander Dietz of MPI, Heidelberg, described the mathematical approach to the accurate treatment of statistics of rare events used by this collaboration. The very accurate data on the Q-value of the 76Ge double beta-decay – which are crucial to the analysis and are determined from accurate mass measurements in a Penning trap – were presented by Ingmar Bergstrom of Stockholm.


Hans Volker Klapdor-Kleingrothaus of Heidelberg then outlined the present evidence for neutrinoless beta decay, as well as the general future for double beta decay experiments. The Heidelberg-Moscow collaboration fixes the effective neutrino mass in the region of 0.05-0.84 eV (95% confidence level). The important question of nuclear matrix elements for double beta decay was described thoroughly by Fedor Simkovic of Bratislava, who showed that transitions to different excited daughter states could help to distinguish between different mechanisms triggering the neutrinoless beta decay process. Important new constraints on neutrino mixing parameters following from the results of the Heidelberg-Moscow collaboration were also discussed by Hiroaki Sugiyama of Tokyo.

Still on the question of the nature of the neutrino, Dharamvir V Ahluwalia of Zacatecas, Mexico, reported on a new theoretical concept concerning massive Majorana particles and outlined the consequences for the structure of space-time. He showed that the Majorana nature of the neutrino tells us that space-time has realized a construct that is central to the formulation of supersymmetric theories. These various discussions showed that neutrinos at extremely low energies, as well as at extremely high energies, are particles that can supply us with exciting discoveries in the future. Together with the other topics, they made the conference a valuable contribution to the fruitful exchange of ideas between physicists working in particle physics, nuclear physics and cosmology.

Proceedings will be published by Institute of Physics Publishing, Bristol, UK.

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