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Introduction

The Heidelberg-Moscow experiment is already for nine years now the most sensitive neutrinoless double beta decay experiment worldwide (see Fig. 1). It has contributed in an extraordinary way to the research in neutrino physics and more general beyond standard model physics, and limits for the latter are competing with those from the largest high-energy accelerators. It will keep its outstanding position in non-accelerator particle physics for several further years to come, before it may be succeeded by future large projects. Its unique position may justify that we give some historical keypoints of the development of this experiment and its background on the next pages.

The realization of the experiment goes back to first discussions in 1986 about a cooperation between Max Planck Institut für Kernphysik, Heidelberg, Leningrad Institute of Nuclear Physics, Gatchina, and Kurchatov Institute, Moscow. The first public announcement of the idea was made at the first WEIN conference in Heidelberg 1986, held on the occasion of the {\bf 600th} anniversary of the University of Heidelberg. Later conferences of this series at Montreal, Dubna, Osaka and Santa Fe, WEIN98, USA, provided centers of active discussions of among others double beta decay and related particle physics. A proposal of the experiment was presented at the MPI Heidelberg in 1987 (H.V. Klapdor, MPI- Report 1987-V 17). In the early time we also tried to include French colleagues from Bordeaux into a collaboration. The search for suitable sites for the experiment included at that time Baksan (Kaukasus), Solotvina (Ukraine), Gran Sasso (Italy) and Frejus (France). On the basis of exploratory research of the different parties on the optimal background, the decision was finally made to use the shielding suggested and built by the Heidelberg group, and to locate the experiment in the Gran Sasso Underground Laboratory (LNGS) , and the collaboration between MPI Heidelberg and Kurchatov Institute, Moscow was formed. The president of the Istituto Nazionale di Fisica Nucleare (INFN), at that time Prof. N. Cabibbo , and the director of the Laboratori Nazionali di Gran Sasso, Prof. E. Bellotti, generously supported the installation of the experiment. With the support of the LNGS the experimental building of the experiment was built between Halls A and B in Gran Sasso, into which the first enriched 76Ge detector (the first high-purity enriched 76Ge detector worldwide) was installed in July 1990 . First preparational work had been done since 1989 in a provisional tent in Hall C.

Into the same year of 1990 fell the edition of the English version of our book, with K. Grotz, on "The Weak Interaction in Nuclear, Particle and Astrophysics", followed by the Russian edition in 1992 and the Chinese version in 1996.

In 1992, as a by-product of this experiment we provided together with our Russian colleagues two enriched 70Ge detectors, which were launched in the same year in a joint experiment with NASA in the GRIS and HEXAGONE ballon experiments in Alice Springs Australia, to do research into gamma-rays for the center of the galaxy.

The full amount of five enriched 76Ge detectors of in total 11 kg was finally installed in 1995 and were operated since 1996 with a newly developped pulse shape discrimination method (J. Hellmig and H.V. Klapdor-Kleingrothaus, Nucl. Instrum. Meth. A 455 (2000) 638 - 644. J. Hellmig, F. Petry and H.V. Klapdor-Kleingrothaus, Patent DE19721323A; B. Majorovits and H.V. Klapdor-Kleingrothaus, Eur. Phys. J. A 6 (1999) 463.) Since this time the full final experiment is delivering data.

1995 was also the year of the edition of the German and English editions of our book, with A. Staudt, on "Non-Accelerator Particle Physics", followed by a Russian edition in 1997, the year in which also the German and English editions of our book, with K. Zuber, on "Particle Astrophysics" were published. While during the eighties and the early nineties the Heidelberg group made important contributions to the investigation of nuclear matrix elements for double beta decay, it contributed in the last seven years extensively also to the theoretical exploration of the wide potential of double beta decay to study branches of beyond standard model particle physics other then neutrino physics. Since 1992/93 the experiment yields the sharpest limits on neutrinoless double beta decay. For some years it also provided the most sensitive raw data in the search for cold dark matter (WIMPs) in the universe.
The present values - derived in the year 2001, after 10 years since operation of the first detector of this experiment in Gran Sasso - for the half-life for neutrinoless double beta decay and the Majorana neutrino mass are 1.5 x 1025 years and 0.39 eV , respectively.

H.V. Klapdor-Kleingrothaus et al., hep-ph/0201231 and Mod. Phys. Lett. A 16 (2001) 2409;
H.V. Klapdor-Kleingrothaus et al.
Part. and Nucl. 110 (2002) 57-79 and
Foundation of Physics 31 (2002) 1181-1223.

The sensitivity thus entered into a range where the results decisively influence neutrino mass scenarios and cosmological parameters presently considered on the basis of the most recent neutrino oscillation experiments such as Superkamiokande etc. and Cosmic Microwave Background observations. The result of the HEIDELBERG-MOSCOW experiment allows only for degenerate neutrino masses, or for an inverse hierarchy of three or four neutrinos (see Fig. 2 )
(H.V. Klapdor-Kleingrothaus and U. Sarkar, Mod. Phys. Lett. A 16 (2001) 2409).


Fig.1



Fig.2


Fig.3

The data of the Heidelberg-Moscow double beta decay experiment for the measuring period August 1990 - May 2000 (54.9813 kg y or 723.44 molyears).

First evidence for neutrinoless double beta decay is observed giving first evidence for lepton number violation.

Yield Analysis evidence for the neutrinoless decay mode at 99.8% c.l. (3.1 Sigma) with the Feldman-Cousins method, and 97% (2.2 Sigma) with the Bayesian method .

The half-life of the process is found to be

T1/20v = (0.8 - 18.3) x 1025 y (95% c.l.) with a best value of 1.5 x 1025 y,

H.V. Klapdor-Kleingrothaus et al., hep-ph/0201231 and Mod. Phys. Lett. A 16 (2001) 2409-2420;
H.V. Klapdor-Kleingrothaus et al.
Part. and Nucl. 110 (2002) 57-79 and
Foundation of Physics 31 (2002) 1181-1223.

 

The deduced value of the effective neutrino mass is, with the nuclear matrix elements from A. Staudt, K. Muto and H.V. Klapdor-Kleingrothaus, Eur. Lett. 13 (1990) 31,
< m >= (0.11 - 0.56) eV (95% c.l.), with a best value of 0.39 eV .
Uncertainties in the nuclear matrix elements may widen the range given for the effective neutrino mass by at most a factor 2.

Our observation at the same time means evidence that the neutrino is a Majorana particle.

For more details, have a look to:
"SIXTY YEARS OF DOUBLE BETA DECAY"
From Nuclear Physics to Beyond Standard Model Particle Physics,
H.V. Klapdor-Kleingrothaus, World Scientific (2001) 1316 pp.

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