Rare decay claim stirs controversy
Feb 11, 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 Princeton, told the publishers of Modern Physics Letters A that if the discovery were true, it would be a ‘real landmark…giving us an important window on physics beyond the Standard Model’.
Witten also points out that the German group’s estimate of neutrino mass – 0.39 eV – is much bigger than that suggested by the results of neutrino oscillation experiments. This estimate means that neutrinos could be a major source of dark matter in the universe.
But a group of 26 particle physicists from around the world has written to the journal, arguing that the data indicate a flat background with very little signal. They believe that Klapdor-Kleingrothaus and co-workers have analysed only selective data.
One of the co-authors of the letter, Frank Avignone of the University of South Carolina, told PhysicsWeb that such a discovery would automatically merit a Nobel prize. But he stresses that the level of uncertainty quoted by the German group is two to three sigma – this is equivalent to a confidence level of 95% or more, which would not be large enough to claim a discovery in most fields of particle physics.
About the author
Edwin Cartlidge is News Editor of Physics World