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experiment that is currently being set up at the Karlsruhe
Institute of Technology is a further development of these
experiments. The concept is based on a very strong
windowless gaseous tritium source to reduce the systematic
uncertainties and on a high-luminosity and
high-resolution electrostatic spectrometer of MAC-E-
Filter type resulting in a sensitivity on the neutrino mass
of 200 meV/c 2 . This aimed improvement of two orders
of magnitude in the m(ν e ) 2 is connected to the requirement
of new developments and severe challenges of
controlling the tritium source pressure and temperature
at the per mille level, of providing extreme vacuum conditions
of about 10 -11 mbar, of developing new background
reducing methods and of controlling the retarding high
voltage at the ppm level. The β spectrum of 10 11 decays
per second has to be measured with an energy resolution
of less than 1 eV at a background level of 0.01 events
per second. This also requires the 3 He/ 3 H mass ratio to
be determined with high precision using Penning trap
based mass spectrometry. The KATRIN experiment will
start taking data in 2012 and requires three years of full
data taking within 2012–2018 to reach its design sensitivity.
As to the second isotope, i.e. 187 Re, the advantage
of the lower endpoint energy of 2.47 keV can only be
exploited if the entire released energy, except that of
the neutrino, is measured in view of its long half-life of
4.3 x 10 10 y and the complicated electronic structure.
This can be realised by using a cryogenic bolometer as
the β spectrometer, which at the same time contains
the β emitter. This was pioneered at Milan and Genoa
with proof of principle experiments that yielded limits on
the neutrino mass of 15 eV/c 2 and 26 eV/c 2 . To further
increase significantly the sensitivity three improvements
have to be achieved: much better energy resolution in the
eV range, time constants in the µs range and large arrays
of many thousands of detectors. The former Milan and
MANU groups are working together with new groups,
forming the MARE collaboration, to improve the sensitivity
in two steps, down to a few eV/c 2 and to a few
hundred meV/c 2 .
The results from neutrino oscillation experiments
show that the “average electron neutrino mass” can
be as small as 50 meV/c 2 (inverted hierarchy) or even
as small as about 10 meV/c 2 (normal hierarchy). For the
cryo-bolometer technique there is no principle limitation
in size to reach even higher sensitivities, in contrast
to the KATRIN technique with its 70 m overall length
and 10 m diameter main spectrometer. On the other
hand, a sensitivity on the neutrino mass of 200 meV/c 2
requires an improvement of four orders of magnitude in
the observable m(ν e ) 2 , which is by far a non-trivial challenge.
Therefore, other possibilities, also with tritium,
have to be considered to achieve a sensitivity in the 10
meV/c 2 range. Some initial ideas of alternative ways to
directly measure the neutrino mass are being discussed
in the community.
Neutrinoless double β decay
Establishing whether neutrinos are Dirac fermions (different
from their antiparticle) or Majorana fermions (spin 1/2
particles identical to their antiparticles) is of paramount
importance for understanding the underlying symmetries
of particle interactions and the origin of neutrino masses.
The only practical way to test whether neutrinos are
Majorana particles is to search for neutrinoless double β
decay (0νββ).
The neutrinoless double β decay of a nucleus consists
of the simultaneous transition of two neutrons into two
protons with the emission of two electrons. This process
is forbidden in the Standard Model. The experimental
signature of this decay is a peak in the distribution of the
electron sum energy at the transition energy. Precision
measurements of the Q-values for double beta decay
isotopes with Penning trap setups allow an accurate fix
of the peak position.
There are several possible mechanisms leading to the
0νββ process: exchange of a light neutrino, right-handed
weak currents, exchange of super-symmetric particles,
and other non-standard interactions. Independent of the
leading term, the observation of 0νββ decay would unambiguously
establish the Majorana nature of neutrinos.
Once the 0νββ decay has been observed experimentally,
the nature of the leading term could be studied by
measuring the energy and angular distribution of the
single electrons, by measuring the branching ratios of
0νββ decays to excited levels, and by comparison of the
decay rates of different nuclei.
In the case of light neutrino exchange, the half-life
of the 0νββ decay depends on the effective Majorana
neutrino mass (m ee ) as ,
where is a calculable phase space factor,
is the nuclear matrix element of the process, and
is the effective
Majorana neutrino mass, with m 1 , m 2 , m 3 the masses
related to the three neutrino mass eigenstates, U e1 , U e2 ,
U e3 the elements of the first row of the neutrino mixing
matrix and φ 1 , φ 2 the Majorana CP phases (±1 if CP is
conserved). Figure 2 shows the range of as predicted
by neutrino oscillation experiments for a normal,
inverted and quasi degenerate neutrino mass scheme.
The present experimental sensitivity corresponds to
the quasi degenerate mass scheme (m 1 ≅ m 2 ≅ m 3 ) and
one experiment claims the observation of 0νββ decays
of 76 Ge.
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