Particle physics

Radioactive disputes
Feb 14th 2002
From The Economist print edition


A potentially Nobel-prize winning discovery. Or maybe not

IF IT proves true, remember that you read it here first. Hans
Klapdor-Kleingrothaus and his colleagues at the Max Planck
Institute for Nuclear Physics in Heidelberg have just reported the
first recorded instance of neutrinoless double beta decay.

That might not sound like news worth holding the front page for,
but to those interested in fundamental physics, it is. If it turns out
to be correct, it will require a substantial rewriting of the Standard
Model, the current repository of all knowledge and wisdom about
particle physics.

In normal beta decay, one of the neutrons in an unstable atomic
nucleus turns into a proton, prompting the emission of two
particles: an electron and an anti-neutrino. This fits with the
Standard Model, which says that a fundamental quantity of the
universe, called lepton number, must be conserved. Electrons and
neutrinos have a lepton number of +1; their anti-matter
counterparts, positrons and anti-neutrinos have a lepton number
of -1. The result is that the net change of lepton number in beta
decay is +1-1, in other words, zero.

There is a very small chance, however, that two neutrons will
decay at the same time, resulting in the simultaneous emission of
two electrons. That, according to a heretical theory, might occur
without the emission of any anti-neutrinos at all, which would
violate the conservation of lepton number and put the Standard
Model in trouble. Such a neutrinoless double beta decay could,
according to the theory, be detected by monitoring the energy of
the electrons given off.

Dr Klapdor-Kleingrothaus and his colleagues looked for the tell-tale
signal in ten years' worth of data collected from a
radioactive-decay experiment being carried out at a laboratory
inside Gran Sasso, a mountain in central Italy. In a paper in
Modern Physics A, they say they have found it.

If they have, the result would not only violate the conservation of
lepton number, it would also mean that anti-neutrinos and
neutrinos are actually the same thing; in other words a neutrino is
its own antiparticle. It would also have cosmological
consequences, since it would make neutrinos into objects a lot
more massive than is currently believed. Since neutrinos are
extremely abundant, that would go some way towards explaining
the so-called exotic dark matter in the universe, which observation
shows is there, but is not made of ordinary atoms.

Not surprisingly, the announcement has provoked a backlash. An
international group of researchers has written a letter to Modern
Physics A, arguing that Dr Klapdor-Kleingrothaus has been
selective in his analysis, and that the data do not show the result
he claims. Whether that result is indeed a lemon, or the letter
proves to be sour grapes, remains to be seen.

Quelle: Economist/Science Section

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