Double Beta Decay

Double beta decay is an exceptionally rare nuclear process in which two neutrons become protons at the same time. It occurs in nature — and in a second, never observed form it could answer one of the oldest open questions in physics.

The ordinary form

In some atomic nuclei simple beta decay is energetically forbidden while the double one is allowed. Two neutrons then convert at the same time, and two electrons and two antineutrinos are produced.

Because two processes have to take place simultaneously, this is enormously improbable:

That is a hundred million to a hundred billion times longer than the age of the universe. It becomes measurable only through the sheer number of atoms: a kilogramme contains so many nuclei that a few decays occur per year.

The form being sought

More interesting is the variant without neutrinos. In it only the two electrons would be produced.

That is possible only if the neutrino is its own antiparticle — a Majorana particle in the sense of Ettore Majorana. The antineutrino emitted by one neutron could then be taken up again immediately by the second neutron as a neutrino. Nothing escapes to the outside.

In this process lepton number would be violated by two. A conserved quantity that has held in every experiment so far would thereby fall.

How it would be recognised

One measures the sum of the energies of both electrons.

In ordinary double beta decay the neutrinos carry away an indeterminate share; the sum yields a broad spectrum. Without neutrinos the electrons would have to carry the entire energy — the result would be a sharp peak at the upper end.

What is sought is thus a narrow line above a broad background. That demands outstanding energy resolution and extremely pure materials.

The competitors

Several experiments work with different nuclides and techniques — which is deliberate, for a find in only one nuclide would be hard to believe.

GERDA and LEGEND use germanium-76 as a semiconductor detector. The trick: the nuclide is at once source and detector, which makes the energy resolution extraordinarily good.

KamLAND-Zen dissolves xenon-136 in the scintillator of KamLAND and so makes use of the large volume already there.

CUORE cools crystals of tellurium-130 to a hundredth of a degree above absolute zero and measures the minute rise in temperature caused by a single decay.

None of them has found it. The lower limits stand at more than 10²⁶ years.

Why a non-find is progress

Every limit pushed further constrains the possible effective Majorana mass — at present to a few hundredths to tenths of an electronvolt.

The search is thereby approaching the range that the neutrino mass would lead one to expect for inverted mass ordering. If the next generation finds nothing, this arrangement is largely excluded — which would also be a result.

One qualification belongs here: the route from half-life to mass leads via nuclear matrix elements, which can only be calculated. Different methods differ by a factor of two to three. This uncertainty accompanies every figure in this field.

What is at stake

A detection would have three consequences at one stroke. It would show that the neutrino is its own antiparticle. It would refute the conservation of lepton number. And it would support the seesaw mechanism — and with it one possible way of explaining the preponderance of matter in the universe.

Related

• Ettore Majorana — the idea of 1937
• Helicity — why the question arises
• Seesaw mechanism
• Neutrino mass

Sources

• GERDA Collaboration: Final Results of GERDA, Physical Review Letters 125, 252502 (2020).
• KamLAND-Zen Collaboration: Search for Majorana Neutrinos with the Complete KamLAND-Zen Dataset, Physical Review Letters 130, 051801 (2023).