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.