Prof.Dr.H.V.Klapdor-Kleingrothaus

Why is the Conclusion of the GERDA Experiment not Justified ?

Authors: Hans Volker Klapdor-Kleingrothaus (Heidelberg, Germany), Irina V. Krivosheina (Heidelberg, Germany and Nishnij Novgorod, Russia) (Submitted on 12 Aug 2013 (v1), last revised 29 Aug 2013 (this version, v4))

The first results of the GERDA double beta experiment in Gran Sasso were recently presented. They are fully consistent with the Heidelberg-Moscow experiment, but because of its low statistics cannot proof anything at this moment. It is no surprise that the statistics is still far from being able to test the signal claimed by the HEIDELBERG-MOSCOW experiment. The energy resolution of the coaxial detectors is a factor of 1.5 worse than in the HEIDELBERG-MOSCOW experiment. The original goal of background reduction to 10^{-2}counts/kgykeV, or by an order of magnitude compared to the HEIDELBERG-MOSCOW experiment, has not been reached. The background is only a factor 2.3 lower if we refer it to the experimental line width, i.e. in units counts/kgy energy resolution.
With pulse shape analysis (PSA) the background in the HEIDELBERG-MOSCOW experiment around Q_ββ is 4x10^-3 counts/kgykeV, which is a factor of 4 (5 referring to the line width) lower than that of GERDA with pulse shape analysis.
The amount of enriched material used in the GERDA measurement is 14.6kg, only a factor of 1.34 larger than that used in the HM experiment. The background model is oversimplified and not yet adequate. It is not shown that the lines of their background can be identified. GERDA has to continue the measurement further ~5years, until they can responsibly present an understood background. The present half life limit presented by GERDA of T_1/2^(0ν) > 2.1 x 10²⁵ y (90% confidence level, i.e. 1.6σ)) is still lower than the half-life of T_1/2^(0ν) = 2.23{{+0.44}_{-0.31}} x 10²⁵ y determined in the HEIDELBERG-MOSCOW experiment.
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