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

4.5 Fundamental Interactions
The CP violation search will require neutrino beams
with the highest possible intensities, tiny intrinsic backgrounds
and very good control of systematic errors. The
goal of long term oscillation experiments is to address
leptonic CP violation, the neutrino hierarchy and small
values of the third neutrino mixing angle. Three possibilities
can be envisaged: super-beams, beta-beams and
neutrino factories. Super- beams are neutrino beams
coming from conventional sources (pion and muon
decays) pushed to their ultimate intensity. Neutrino factories
are based on the production, acceleration and
storage of muons to obtain intense muon and electron
(anti)neutrino beams.
The beta-beam concept, proposed by Zucchelli, is a
new method for the production of intense neutrino beams
based on the β decay of boosted radioactive ions. Such
beams are pure in flavour (only ν e or ⎺νe depending on the β
decaying ion) and have well known fluxes. In the original
scenario the beta-beam baseline is hosted at CERN.
The ions are produced through the ISOL technique and
stored in a storage ring. When they decay, they produce
a neutrino beam pointed to a far large-size detector
located in an enlarged Frejus Underground Laboratory
to compare ν e → ν µ with ⎺νe → ⎺ νµ . The megaton size
Cherenkov detector (about 20 times Super-Kamiokande)
would be a multipurpose detector for the search of CP
violation and proton decay, as well as the observation of
(relic) supernova neutrinos. The (3σ) discovery limit for
sin 2 2θ 13 > 3 × 10 -4 covers a large fraction of all δ values
making the beta-beam option a very competitive strategy.
As far as the hierarchy is concerned, the neutrino
factory can identify the mass hierarchy through matter
effects, while the beta-beam can use a combination
with atmospheric data in the same detector if the third
neutrino angle is large.
Note that a low energy beta-beam facility has also
been proposed with the purpose of performing neutrino
interaction measurements of interest for nuclear physics,
for the study of fundamental interactions and for corecollapse
supernova physics. A feasibility study of the
original beta-beam baseline has been performed within
the EURISOL Design Study (FP6, 2005-2009). Clearly a
key issue is to reach the required ion intensities. Further
investigation on high ion production techniques for the
isotopes of interest is crucial.
As far as a comparison among the facilities is
concerned, if sin 2 2θ 13 > 0.02 (large) the superbeam, betabeam
and neutrino factory have similar sensitivity to the
Dirac phase. For values of 5 × 10 -4 < sin 2 2θ 13 < 0.02 the
superbeams are outperformed by the beta-beam and
the neutrino factory. Only the optimised neutrino factory
can reach values of sin 2 2θ 13 smaller than 5 × 10 -4 , while
the optimised beta-beam and the conservative neutrino
factory option have a comparable performance.
Neutrino masses
Neutrino oscillation experiments have provided clear evidence
for the neutrinos to be massive particles. However,
the fact that they turn out to be lighter by at least 6
orders of magnitude than any charged fermion is difficult
to ascribe simply to much smaller Yukawa couplings
to the Higgs. It is therefore much more reasonable to
assume that neutrino masses are based on so-called
Majorana mass terms, which are only allowed for the
neutral neutrinos. The seesaw mechanism would be a
natural explanation for the smallness of neutrino masses,
but it would require new physics beyond the Standard
Model. Determining the neutrino masses would allow a
distinction to be made between different theories.
Unfortunately, neutrino oscillation experiments provide
us with the differences of the squared neutrino masses
Δm 2 ij and cannot determine the sign of Δm 2 ij nor the absolute
neutrino masses. Still, once one neutrino mass has
been determined by different means, the other neutrino
masses could be reconstructed using the values from
neutrino oscillation experiments. The absolute neutrino
mass has strong consequences for astrophysics and
cosmology as well as for nuclear and particle physics.
Therefore, an absolute determination of one neutrino
mass is one of the most important next steps in neutrino
physics. In cosmology an upper limit of Σm i < 0.61 eV/c 2
on the sum of the three neutrino masses has been
obtained from the size of fluctuations in the microwave
background at small scales. This is, however, to some
extent model- and analysis-dependent. It is expected
that the data from the recently launched PLANCK satellite
will allow improvement to the sensitivity of the neutrino
mass to the 100 meV/c 2 range within the next decade.
Other approaches are direct mass measurements and
searches for neutrinoless double β decay (0νββ).
Direct mass measurements
Direct neutrino mass determination is based on the
investigation of the kinematics of weak decays, the signature
of a non-zero neutrino mass being a tiny
modification of the spectrum of the β electrons near its
endpoint. This allows determination of the “average
electron neutrino mass”
with
this incoherent sum not being sensitive to phases of the
neutrino mixing matrix. For optimal sensitivity β emitters
with low endpoint energy are favoured. Two different
isotopes, tritium ( 3 H) and 187 Re, are suited for these
experiments. Tritium (with an endpoint energy of 18.6
keV and a half-life of 12.3 y) β decay experiments have
been performed in the search for the neutrino mass for
more than 50 years, yielding a sensitivity of 2 eV/c 2 by
the experiments at Mainz and Troitsk. The KATRIN
154 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010