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highest experimental sensitivities were reached in dedicated
µ and K experiments, τ decay starts to become
competitive.
L 1 ↔ L 2
Whereas most models favour µ + → e + γ the experimental
sensitivity is limited by accidental e + γ coincidences
so available beam intensities cannot be fully exploited.
Searches for µ−e conversion, on the other hand, are
limited by the beam intensities and large improvements
in sensitivity may still be achieved.
The present µ→ e + γ upper limit of 1.2 × 10 -11 was
established by MEGA at LAMPF. The new MEG experiment
at PSI, which uses a novel liquid Xe scintillation
calorimeter viewed by photomultipliers from all sides,
aims at a single-event sensitivity of ∼10 -13 . Ten times
larger surface muon rates than used by MEG can be
achieved at PSI today already but the background suppression
would have to be improved by two orders of
magnitude.
The present best limits on µ − e conversion have all
been measured with the SINDRUM II spectrometer at PSI.
New µ − e conversion experiments using pulsed proton
beams are currently being considered both at Fermilab
in the USA and at J-PARC in Japan (see Table 2). Key
improvements concern the muon momentum transmission
and reduced pion induced background.
L 1,2 ↔ L 3
B-factories operating around the Υ(4S) resonance also
serve as τ factories. The decay products of the tau pair
produced are well separated in space thus offering the
possibility to tag one of them by selecting a dominant
decay mode for the other. Upper limits of O(10 -8 ) have
been reached for the branching ratios of the various
channels. Similar upper limits will be achieved for the
τ → 3µ branching ratio at LHC during the low luminosity
phase. Lepton flavour violating τ decays are predicted in
many extensions of the Standard Model with branching
ratios partially next to the current experimental upper
limit. Therefore, lepton flavour violating τ decays are
an interesting option in the search for new physics.
Improved searches, with upper limits of O(10 -10 ), received
high priority at the Super B-factories proposed in Italy
and Japan.
Note, finally, that the decays of the lowest-lying pseudoscalar
mesons also offer a wide variety of symmetry
tests. Such tests are being pursued at the BEPC, COSY,
ELSA, DAPHNE and MAMI facilities. It is expected that
the number of π 0 , η, η’ and J/φ decays detected will
be multiplied in the upcoming years, leading to much
improved limits or discoveries.
New (time reversal invariant)
interactions in nuclear and neutron
β decays
The Vector-Axial vector character of the weak interaction,
discovered in experiments in nuclear β decay, seems well
established. However, even though all experimental data
agree with the V-A theory, other interactions could still
participate at about the 5 to 10% level. In β decay new
interactions can be probed by precision experiments
which measure several types of correlation between
the spins and momenta of the particles involved in the
decay. Thus, the presence of exotic interactions (e.g.
scalar S and tensor T) can be investigated, as well as
the masses and couplings of the corresponding bosons
that are related to such new interactions. Both neutron
and nuclear decays are being studied. In the first case
the precision is not affected by nuclear structure corrections.
However, in nuclear β decay nature provides a
large amount of nuclear states with different properties
so that transitions can be selected to yield sensitivity
to particular physics beyond the SM and at the same
time ensure that nuclear structure related corrections
are small or well under control. Note that reaching the
required precision requires long beam times to collect
the necessary statistics as well as to get good control
of systematic errors. The experiments would therefore
benefit significantly from dedicated facilities providing
sufficient beam time, such as the ISOL@MYRRHA facility
that is planned in Belgium.
The pseudoscalar contribution to β decay vanishes
in the non-relativistic approximation for nuclei. A very
stringent constraint (∼10 -4 level) was obtained from the
pion-decay branching ratio Γ(π → eν)/Γ(π → µν). The
new interactions can have time reversal (T) invariant
and time reversal violating components. The latter will
be discussed in section 4.5.3.
New time reversal invariant vector and
axial-vector interactions
Precision measurements of observables that are sensitive
to right handed (V+A) interactions provide powerful
means to probe specific scenarios of new physics beyond
the SM in which the parity symmetry is restored at some
level due to the exchange of non-standard new bosons.
The most popular ones are the so-called left-right symmetric
models, which allow for the presence of a W
gauge boson that couples to right-handed particles.
Until recently, relative measurements in nuclear
decays comparing the longitudinal polarisation of positrons
emitted along two opposite directions with respect
to the nuclear spin, provided the most stringent tests
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