Matrix elements  Double beta decay  Double beta decay

Recent results

There are three ways to look for double beta decay:
1) direct searches, in which one looks with a detector for the two electrons emitted by a source. The energy of the electrons is measured, so that the three decay modes can be distinguished.
2) geochemical experiments, in which one searches, in an ore containing double beta decay candidates, for an abnormal isotopic abundance of the daughter nuclei. The three modes cannot be distinguished.
3) radiochemical experiments, similar to the geochemical ones, but in which the daughter nuclei are unstable, and can be identified by their decay. We shall here focus on direct searches. Experiments have become more and more sensitive over the years, with larger target masses. Much progress has been achieved in selecting radiochemically clean components for the construction of detectors, leading to a substantial reduction of the background from natural activities. To minimise the background from direct cosmic hits, or from cosmogenic activations, detectors are usually operated in underground laboratories. Calorimeters with superior energy resolution are particularly well suited for the search of the $\beta\beta 0\nu$ decay peak. Tracking devices are also used. In these good event candidates can be selected from their topology, which leads to a further background reduction. Presently $\beta\beta 2\nu$ decay has been observed in several nuclei (⁴⁸Ca, ⁷⁶Ge, ⁸²Se, ¹⁰⁰Mo, ¹²⁸Te, ¹³⁰Te, ¹⁵⁰Nd). At the same time the limits on the other modes keep getting more constraining. We shall here mention only the most recent developments.
⁷⁶Ge. This nucleus has been studied for several years, since Ge detectors with large mass can be operated as calorimeters with excellent energy resolution. Presently the best results are those obtained by the Heidelberg-Moscow collaboration, which operates an array of 5 crystals made from Ge enriched to 86-88 % in ⁷⁶Ge, with a total mass of 11 kg, in the Gran Sasso underground lab. Data corresponding to 15 kg$\cdot$yr have been accumulated so far. $\beta\beta 2\nu$ was clearly observed, with a half life $T_{1/2}^{2\nu}=1.77^{+0.13}_{-0.11}\cdot 10^{21}$ yr. An upper limit of $T_{1/2}^{\chi^0} > 7.91\cdot 10^{21}$ yr for the half-life of the Majoron mode was derived. The energy resolution in the region of the transition energy E₀=2038.6 keV is of order 3 keV FWHM. There is no evidence of a peak there, and the limit $T_{1/2}^{0\nu}> 9.1\cdot 10^{24}$ yr (90% CL) was derived. A pulse shape discrimination system is being implemented ed. It allows to identify and reject multi-site events, for instance multi Compton scattering events. This reduces the background by a factor 5. A similar experiment, IGEX (South Carolina, Pacific Northwest, Zaragoza, ITEP, INR collaboration), has now also started producing data. It uses several Ge detectors enriched to 87.4 % in ⁷⁶Ge in Homestake, Canfranc and Baksan. The reported half-life for $\beta\beta 2\nu$ decay is in rough agreement with the Heidelberg-Moscow value. The limit $T_{1/2}^{0\nu}> 5.7\cdot 10^{24}$ yr (90% CL) was obtained. Pulse shape discrimination is also being implemented
¹³⁰Te. Another calorimeter, of a rather innovative type, a cryogenic bolometer, has been built by the Milano group to study ¹³⁰Te. The central component is a 334 g TeO₂ crystals mounted in a low background cryostat, and operated at 10 mK. Natural tellurium with 34.5 % ¹³⁰Te is used. Excellent energy resolution, around 10-15 keV FWHM, is achieved, close to that of Ge detectors. Data have been accumulated in the Gran Sasso laboratory during 10500 hours. No evidence of $\beta\beta 0\nu$ decay has been found, and the limit $T_{1/2}^{0\nu} > 2.1\cdot 10^{22}$ yr (90% CL) was derived. Now a 20 crystal detector is being built. Efforts are also made to grow crystals made from enriched ¹³⁰Te.
¹³⁶Xe. This nucleus is presently being investigated by the Caltech-Neuchâtel-PSI collaboration in the Gotthard underground lab. The detector is a tracking device, to be specific a gas time projection chamber filled with Xe enriched to 62.5 % in ¹³⁶Xe. The source mass is 3.3 kg of ¹³⁶Xe.

Table: Measured lower limits for the half-life of various $\beta\beta 0\nu$ transitions. Deduced upper limits on the neutrino mass $\langle m_{\nu} \rangle$ using various matrix elements.

 

	$T_{1/2}^{0\nu}$ [yr] (90 % CL)	$\langle m_{\nu} \rangle$ [eV] Caltech     Heidelberg      Tübingen      Stras.-Mad.
direct		<1.2-1.3  <5.2-5.6 <2.4-2.7	<0.51  <4.8  <2.2	< 0.48  < 5.1  < 1.8	< 1.4  < -  < 5.2
geochem. 128Te	> 2.6 - 7.7	<1.1-2.8	<1.0-1.7	<1.1-1.9	-

 

Data were taken during 10'000 hours. The limits yr (90% CL) and yr have been reported.
¹⁰⁰Mo. Another tracking device, NEMO II, is being used in the Fréjus lab. The source is a thin foil, stretched in the middle of the fiducial volume filled with He gas and instrumented with Geiger cells. Several nuclei can be investigated. The energy of the electrons is measured in two planes of scintillators. Using a 172 g source of Mo, enriched to 98.4 % in ¹⁰⁰Mo, $\beta\beta 2\nu$ decay was observed with the half-life yr. From the same data, a limit yr was derived for decay. The sensitivity to $\beta\beta 0\nu$ decay however is limited because of the small mass of the source. More recently, the NEMO group has also observed $\beta\beta 2\nu$ decay in ¹¹⁶Cd, as have the Osaka and Kiev groups.