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L. Camilleri
8.4.1 Beta Beams
Beta beams [122, 123] are beams of neutrinos based on the production, storage and
β-decay of radioactive ions. A possible European solution was studied in the context
of the Eurisol project [124]. 6 He ions which, decaying via β − , produce ν e and
18 Ne ions, which decaying via β + , yield ¯
ν e would be stored. These ions would be
accelerated to the energy required to produce decay neutrinos of the required energy
and then stored in a race-track shaped storage ring. The Lorentz boost produces a
well focussed forward beam which would illuminate one or more detectors in line
with the straight sections of the storage ring. The search for CP violation would
proceed by observing ν e → ν μ and ¯
ν e → ¯
ν μ oscillations. These oscillations would
result in the observation of muons produced via the charged current interaction of
ν μ ’s and ¯
ν μ ’s. These beams are particularly advantageous for the study of these
oscillations as they do not, unlike present accelerator neutrino beams, have an
intrinsic oscillated flavour component. Thus an important background is eliminated.
The detector envisaged for this project was MEMPHYS, a water Cerenkov counter
described in Sect. 8.3.2, located 130 km away from a potential beta beam source
at CERN. This distance would require neutrino energies of a few hundred MeV to
be at oscillation maximum. These low energies and distances would preclude any
resolution of the mass hierarchy. As usual, an additional detector near the storage
ring would be needed to study the beam before oscillations can occur. It is estimated
that 2.9 × 10 18 6 He ions and 1.2 × 10 18 18 Ne ions decaying per year in the straight
sections would be needed to meet the physics requirements. Whereas this seems
achievable for 6 He, new production methods [125] would be needed for 18 Ne.
8.4.2 Neutrino Factory
A neutrino factory [126, 127] uses the decay of muons to produce neutrinos. The
first step is to produce pions using a very high intensity proton beam impinging on
a target. The decay of these pions then produce muons. Before they are injected into
a storage ring their momentum and angular spread must be reduced to maximize
their capture efficiency. This is done by phase rotation and ionization cooling.
Longitudinal momentum spread would be reduced by phase rotation using the
Neuffer scheme. This entails capturing multi bunches of muons with a very high
Radio Frequency (RF) and rotating their phase with decreasing RF along the
cooling channel. Angular spread (transverse momentum) would be reduced through
ionization and subsequent longitudinal acceleration using RF cavities. The storage
ring includes straight sections pointing to one or more detectors [129]. In the scheme
studied in the context of the International Scoping Study [128] muons of both
signs of about 20 GeV/c can be captured and stored simultaneously. Storage ring
geometries have been identified that can deliver both neutrinos and antineutrinos to
one or more detectors. In a race track geometry neutrinos and antineutrinos would
L. Camilleri
8.4.1 Beta Beams
Beta beams [122, 123] are beams of neutrinos based on the production, storage and
β-decay of radioactive ions. A possible European solution was studied in the context
of the Eurisol project [124]. 6 He ions which, decaying via β − , produce ν e and
18 Ne ions, which decaying via β + , yield ¯
ν e would be stored. These ions would be
accelerated to the energy required to produce decay neutrinos of the required energy
and then stored in a race-track shaped storage ring. The Lorentz boost produces a
well focussed forward beam which would illuminate one or more detectors in line
with the straight sections of the storage ring. The search for CP violation would
proceed by observing ν e → ν μ and ¯
ν e → ¯
ν μ oscillations. These oscillations would
result in the observation of muons produced via the charged current interaction of
ν μ ’s and ¯
ν μ ’s. These beams are particularly advantageous for the study of these
oscillations as they do not, unlike present accelerator neutrino beams, have an
intrinsic oscillated flavour component. Thus an important background is eliminated.
The detector envisaged for this project was MEMPHYS, a water Cerenkov counter
described in Sect. 8.3.2, located 130 km away from a potential beta beam source
at CERN. This distance would require neutrino energies of a few hundred MeV to
be at oscillation maximum. These low energies and distances would preclude any
resolution of the mass hierarchy. As usual, an additional detector near the storage
ring would be needed to study the beam before oscillations can occur. It is estimated
that 2.9 × 10 18 6 He ions and 1.2 × 10 18 18 Ne ions decaying per year in the straight
sections would be needed to meet the physics requirements. Whereas this seems
achievable for 6 He, new production methods [125] would be needed for 18 Ne.
8.4.2 Neutrino Factory
A neutrino factory [126, 127] uses the decay of muons to produce neutrinos. The
first step is to produce pions using a very high intensity proton beam impinging on
a target. The decay of these pions then produce muons. Before they are injected into
a storage ring their momentum and angular spread must be reduced to maximize
their capture efficiency. This is done by phase rotation and ionization cooling.
Longitudinal momentum spread would be reduced by phase rotation using the
Neuffer scheme. This entails capturing multi bunches of muons with a very high
Radio Frequency (RF) and rotating their phase with decreasing RF along the
cooling channel. Angular spread (transverse momentum) would be reduced through
ionization and subsequent longitudinal acceleration using RF cavities. The storage
ring includes straight sections pointing to one or more detectors [129]. In the scheme
studied in the context of the International Scoping Study [128] muons of both
signs of about 20 GeV/c can be captured and stored simultaneously. Storage ring
geometries have been identified that can deliver both neutrinos and antineutrinos to
one or more detectors. In a race track geometry neutrinos and antineutrinos would