Perspectives of Nuclear Physics in Europe - European Science ...

4.5 Fundamental Interactions
The next generation of experiments (cf. Table 1) will
test this claim and will explore part of the mass range
predicted by oscillation experiments for the inverted hierarchy
( Δm 2 23< 0). These experiments will also serve as
bench tests for the following one ton scale experiments
which are required to explore completely the inverted
mass hierarchy. The meV mass range, predicted in the
case of the normal hierarchy ( Δm
2
23
> 0), is beyond the
reach of current technologies.
Two main experimental approaches are pursued:
calorimeters measure the sum energy of the two electrons
while tracking calorimeters or TPCs record the
kinematics of the single electrons. In one experimental
approach, it is planned to identify the daughter nucleus
as an additional method to discriminate background
events. Different experimental approaches are required
in order to reduce possible systematic uncertainties as
well as experiment-specific backgrounds.
The enrichment of isotopes is crucial for double β
decay experiments. Presently, all isotopes have been
enriched by the centrifugation method in Russia.
However, some isotopes, in particular 48 Ca and 150 Nd
which are favourable from the point of view of transition
energy and phase space factor, cannot be enriched by
centrifugation. A European facility to provide double β
decay isotopes at the hundred kilogram scale based on
an ion cyclotron resonance separation method would be
desirable. Such a facility would be able to enrich almost
all interesting double β decay emitters. Related to the
enrichment, physical or chemical purification methods
are also needed.
Important progress has been made for the Nuclear
Matrix Element calculations over recent years.
Nonetheless, the development of these calculations
should be strongly supported in the future in order to
Figure 1. The effective neutrino mass (m ee ) as a function
of the lightest neutrino mass shown for the normal (i.e. Δm 2 23< 0),
inverted ( Δm 2 23 < 0) and quasi degenerate neutrino mass scheme
as predicted by neutrino oscillation experiments. The different lines
and colours correspond to various assumptions on CP violating
phases. Limits from neutrinoless double β decay experiments
and constraints from cosmology are displayed.
provide guidance in the choice of double β decay isotopes
and to extract the effective neutrino mass from
the measured lifetime. Auxiliary experiments in support
of the nuclear structure calculations as charge exchange
reactions or muon capture should be carried out.
It is noteworthy that some of the detector materials
used in neutrinoless double β decay, e.g. Ge and Xe,
Table 1. Overview of funded next-generation double β decay experiments and projects with substantial R&D grants. *Mass of double β
decay isotopes not including detection and analysis efficiencies which range from about 30% (e.g SuperNEMO) to about 90% (e.g. GERDA,
CUORE). The numbers given for ‘expected final sensitivity’ depend on the assumed background level, the expected duration of data taking
and choice of nuclear matrix elements and are therefore only indicative values. TPC, time projection chamber; LNGS, Gran Sasso National
Laboratory, Italy; WIPP, Waste Isolation Pilot Plant, USA; DUSL, Deep Underground Science Laboratory, USA; LSC, Laboratorio Subterraneo
de Canfranc, Spain; SNOlab, Sudbury Neutrino Observatory, Canada; LSM, Laboratoire Souterrain de Modane, France. u.e., under
evaluation. Not listed is the KamLAND experiment in Japan, which plans to load 136 Xe in the liquid scintillator detector.
156 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010