11 February 2002
Physicists from the Max Planck Institute for Nuclear Physics in
Heidelberg claim to have observed the rarest known nuclear
decay for the first time. Hans Klapdor-Kleingrothaus and
colleagues have analysed data from an experiment at the Gran
Sasso laboratory in Italy and say they have strong evidence for
so-called neutrinoless double-beta decay. Such a discovery
would be one of the biggest breakthroughs in particle physics
for many years, but other researchers in the field believe the
claim is based on a flawed analysis of the data (Mod. Phys.
Lett. A 16 2409; xxx.lanl.gov/abs/hep-ph/0201231).
In beta decay, one of the neutrons in an unstable nucleus turns
into a proton, prompting the emission of an electron and an
electron antineutrino. There is a tiny chance, however, that two
neutrons will be converted simultaneously, resulting in the
emission of two electrons with precisely defined energies.
Such a decay could occur without the emission of any neutrinos,
but this would violate one of the fundamental rules in the
Standard Model of particle physics, the conservation of ‘lepton
number’: electrons and electron neutrinos have a lepton number
of 1, and positrons and electron antineutrinos have a lepton
number of –1. It would also mean that the neutrino is a so-called
Majorana particle – its own anti-particle – and would provide a
value for the absolute mass of the neutrino, which can be
calculated from the decay half-life.
In a paper published in Modern Physics Letters A, the German
group claims to have found the tell-tale peak in the spectrum of
electron energies produced by the Heidelberg-Moscow
experiment, which looks for nuclear decays in 11.5 kilograms of
germanium-76. Ed Witten, a theoretical physicist at the Institute
of Advanced Study in…
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