Neutrinoless double beta decay

Neutrinoless double beta decay (0νββ) is a commonly proposed and experimentally pursued theoretical radioactive decay process that would prove a Majorana nature of the neutrino particle. To this day, it has not been found. The discovery of neutrinoless double beta decay could shed light on the absolute neutrino masses and on their mass hierarchy (Neutrino mass). It would mean the first ever signal of the violation of total lepton number conservation. A Majorana nature of neutrinos would confirm that the neutrino is its own antiparticle. To search for neutrinoless double beta decay, there are currently a number of experiments underway, with several future experiments for increased sensitivity proposed as well.

History

In 1935 Maria Goeppert Mayer proposed the idea of double beta decay. Two years later, in 1937, the Italian physicist Ettore Majorana first introduced the concept of a particle being its own antiparticle, explicitly mentioning his theory's possible application to neutrinos. Particles of this nature were subsequently named after him as Majorana particles. In 1939, Wendell H. Furry realized that Majorana's theory allowed a new decay channel. Furry stated the transition probability to even be higher for double beta decay in Majorana's symmetrical theory of the neutrino when compared with the original Dirac-Fermi theory. The privative label "neutrino-less" appeared in 1953 and largely replaced the original terminology. This theoretical decay was the first idea proposed which could be used to search for the violation of lepton number conservation. It has, since then, drawn attention for being useful to study the nature of neutrinos (see quote).

Physical relevance

Conventional double beta decay Neutrinos are conventionally produced in weak decays. Weak beta decays normally produce one electron (or positron), emit an antineutrino (or neutrino) and increase (or decrease) the nucleus' proton number, Z , {\displaystyle \ Z\ ,} by one, while leaving its atomic weight, A , {\displaystyle \ A\ ,} unchanged:

( A , Z ) ⟶ ( A , Z + 1 ) + e − + ν ¯ e {\displaystyle \ (A,Z)\ \longrightarrow \ (A,Z+1)\ +\ e^{-}\ +\ {\bar {\nu }}_{e}\quad } and

( A , Z ) ⟶ ( A , Z − 1 ) + e + + ν e . {\displaystyle \ (A,Z)\ \longrightarrow \ (A,Z-1)\ +\ e^{+}\ +\ \nu _{e}~.}

The nucleus' mass (i.e. binding energy) is then lower and thus more favorable. There exist a number of elements that can decay into a nucleus of lower mass, but they cannot emit one electron only because the resulting nucleus is kinematically (that is, in terms of energy) not favorable (its energy would be higher). These nuclei can only decay by emitting two electrons (that is, via double beta decay). There are about a dozen nuclei that have been confirmed to decay only via a double beta decay. The corresponding decay equation is:

( A , Z ) ⟶ ( A , Z + 2 ) + 2 e − + 2 ν ¯ e . {\displaystyle \ (A,Z)\ \longrightarrow \ (A,Z+2)\ +\ 2\ e^{-}\ +\ 2\ {\bar {\nu }}_{e}~.}

It is a weak process of second order. A simultaneous decay of two nucleons in the same nucleus is extremely unlikely. Thus, the experimentally observed lifetimes of such decay processes are on the order of 1018 ~ 1021 years. A number of isotopes have been observed already to show this two-neutrino double beta decay. This conventional double beta decay is allowed in the Standard Model of particle physics. It has thus both a theoretical and an experimental foundation.

Overview

If the nature of the neutrinos is Majorana, then they can be emitted and absorbed in the same process without showing up in the corresponding final state. As Dirac particles, both the neutrinos produced by the decay of the W bosons would be emitted, and not absorbed after. Neutrinoless double beta decay can only occur if both

the neutrino particle is Majorana, and there exists a right-handed component of the weak leptonic current or the neutrino can change its handedness between emission and absorption (between the two W vertices), which is possible for a non-zero neutrino mass (for at least one of the neutrino species). The simplest decay process is known as the light neutrino exchange. It features one neutrino emitted by one nucleon and absorbed by another nucleon (see figure to the right). In the final state, the only remaining parts are the nucleus (with its changed proton number Z {\displaystyle \ Z\ } ) and two electrons:

( A , Z ) ⟶ ( A , Z + 2 ) + 2 e − . {\displaystyle \ (A,Z)\longrightarrow (A,Z+2)+2e^{-}~.}

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