Limits on the Majorana Neutrino Mass in the 0.1 eV Range

Abstract

The Heidelberg-Moscow experiment gives the most stringent limit on the Majorana neutrino mass. After 24 kg yr of data with pulse shape measurements, we set a lower limit on the half-life of the 0νββ-decay in 76 Ge of T 0ν 1/2 ≥ 5.7 × 10 25 yr at 90% C.L., thus excluding an effective Majorana neutrino mass greater than 0.2 eV. This allows to set strong constraints on degenerate neutrino mass models.Neutrinoless double beta (0νββ) decay is an extremely sensitive tool to probe theories beyond the standard model (see [1]). While the standard model exactly conserves B-L, 0νββ-decay violates lepton number, and B-L, by two units. The simplest mechanism which can induce 0νββ-decay is the exchange of a Majorana neutrino between the decaying neutrons. Alternatively, any theory that contains lepton number violating interactions can lead to the process. Independently of the underlying mechanism, an observation of the 0νββ-decay would be an evidence for a nonzero Majorana neutrino mass [2]. There are several indications for nonzero neutrino masses, the most stringent ones come from solar and atmospheric neutrino experiments. In particular, the confirmation by Super Kamiokande of the atmospheric neutrino deficit [3], provides strong evidence for neutrino oscillations, although also other solutions are possible [4]. If a neutrino as a hot dark matter (HDM) component is taken into account, then fitting the atmospheric, solar and HDM scales with three neutrinos is only possible in the degenerate mass scenario, where all neutrinos have nearly the same mass, in the order of O(eV) [5]. This would lead to an amplitude for 0νββ-decay mediated by the neutrino mass which is accessible by the present sensitivity of the Heidelberg-Moscow experiment.The Heidelberg-Moscow experiment operates five p-type HPGe detectors in the Gran Sasso Underground Laboratory. The Ge crystals were grown out of 19.2 kg of 86% enriched 76 Ge material. The total active mass of the detectors is 10.96 kg, corresponding to 125.5 mol of 76 Ge, the presently largest source strength of all double beta experiments. Four detectors are placed in a common 30 cm thick lead shielding in a radon free nitrogen atmosphere, surrounded by 10 cm of boron-loaded polyethylene and with two layers of 1 cm thick scintillators on top. The remaining detector is situated in a separate box with 27 cm electrolytical copper and 20 cm lead shielding, flushed with gaseous nitrogen and with 10 cm of boron-loaded polyethylene below the box. A detailed description of the experiment and its background is given in [6]. For a further reduction of the already very low background of the experiment, a pulse shape analysis (PSA) method was developed [7]. The analysis distinguishes between multiple scattered interaction in the Ge crystal, so called multiple site events (MSE) and pointlike interactions, i.e. single site events (SSE). Since double beta decay events belong to the SSE category, the method allows to effectively reduce the background of multiple Compton scattered photons. The probability of...

Cancellations in neutrinoless double beta decay and the neutrino mass matrix

“…[4] and references therein. The most stringent limit however comes from the first process, investigating the effective Majorana mass of the electron neutrino, with a current bound of [5] m < ∼ 0.2 eV.…”

Double Beta Decay: Theory, Experiment and Implications

“…Again, only the most restrictive limits for the given transition are shown. The longest half-life limit, reported for 76 Ge by the Heidelberg-Moscow collaboration [33], is based on 24.16 kg yr of exposure and uses pulse shape discrimination to suppress the background (in the relevant energy region the background is a mere (0.06±0.02) events/(kg·yr·keV)). In that experiment 7 events were observed in the 3σ region around the 0ν decay Q value, while from the background extrapolation one expects 13 events.…”

“…On the other hand, if one would accept the more stringent limit of 5.7 × 10 25 [33], even the more pessimistic matrix elements restrict m ν < 0.5 eV. Needless to say, a more objective measure of the theoretical uncertainty would be highly desirable.…”

Phenomenology of neutrino oscillations

“…However the latter has not been seen yet and the experimental limits on it are getting stronger. The strongest upper limit so far comes from the Germanium experiment and it is [27] …”