Did you know?

Many concepts in the world come with their opposite: good and bad, black and white, up and down, etc. This is true for a lot of things but not for the totality. For example, the majority of particles also have a corresponding antiparticle, but for some of them, the antiparticle is the particle itself. These are called intrinsically neutral particles.

The fermionic (i.e. spin 1/2) sector is the only one in the Standard Model for which we don’t yet know if there exist intrinsically neutral fundamental particles. In general, an intrinsically neutral fermion is called a Majorana particle. Otherwise, it is called a Dirac particle. We know, for example, that Majorana fermions can emerge in superconducting materials as quasi-particle bound states, but we don’t know if fundamental Majorana fermions exist. Currently, the only known candidates for this role are the neutrinos.

The situation is more clear for the other sectors in the Standard Model. For example, if the analysis at LHC would confirm that the observed Higgs particle is indeed the Standard Model Higgs boson, it would be the first example of a scalar (i.e. spin 0) intrinsically neutral fundamental particle. Before the LHC discovery we only knew examples of composite intrinsically neutral scalar particles, as for instance the neutral pion, which is a meson formed by a bounded quark-antiquark pair.

In the vector (i.e. spin 1) sector there are two examples of intrinsically neutral fundamental particles: the photon, that is the carrier of electromagnetic interactions, and the Z boson, that mediates neutral weak interactions.

Also the graviton, the unique spin 2 fundamental particle which is at the moment only hypothetical, should be an intrinsically neutral particle.

If neutrinos are Majorana particles, they could generate a peculiar process that consists in a nuclear decay without any neutrino emission. This process is called neutrinoless double beta decay and is forbidden if neutrinos are Dirac particles.

A possible future observation of a neutrinoless double beta decay would enable us to conclude that neutrinos are Majorana particles, fitting an intrinsically neutral fundamental particle also in the fermionic sector.

Text by Michele Lucente.