The Standard Model of particle physics looks like a perfect puzzle where all the pieces fit together, but there is one of them that resists: the nature of the neutrino!

Maybe this piece that doesn’t fit is telling us more, that our puzzle is incomplete, that whole sections are yet to be uncovered.

Neutrinos are the most elusive fundamental particles we know of and the only ones that can be its own antiparticle, the only one that can be completely neutral. The HEP experimental group of IGFAE participates in the international experiment NEXT,, installed in the Underground Laboratory of Canfranc,, under the mountain of Tobazo, in the Pyrenees. NEXT is one of the experiments currently competing to decipher the true nature of the neutrino, to understand if it is its own antiparticle.

This is not an easy task, neutrinos are so elusive, and so light, that what would be a trivial task for other particles, determining whether they are their antiparticle, is almost impossible in them. We can only do this by a process in which the neutrino behaves both as a particle and as an antiparticle. And this phenomenon could occur in a few, few, nuclei, where two of their neutrons become two protons exchanging between them a neutrino/anti-neutrino and releasing two electrons. This process is called neutrino-less double beta decay.

The NEXT detector is a cylindrical tank of gaseous Xenon, subjected to an intense electrical gradient and with its two covers instrumented with light sensors. The Xenon acts as a target, the nucleus where this hypothetical disintegration could occur, and as it is gaseous the electrons produced would leave a track, a trajectory of a few centimetres. Our detector then looks for an unequivocal signal: two electrons whose energy, by Einstein’s famous law, coincides with the lost mass of the parent nucleus, the Xenon. Our requirements are various and difficult to reach: a large amount of xenon (to have a large number of nuclei where decays can occur), an excellent reconstruction of the tracks (to identify the two electrons), a very precise measurement of the energy (to ensure that it corresponds to the lost mass of the parent nucleus) and finally an ultra-clean environment of radiation pollution (so that we do not have events that can confuse us, hence our laboratory is inside the mountain, protected from cosmic rays, and the detector elements are ultra-pure, avoiding radioactive contamination).

The first NEXT detector, called NEW, is installed in the Canfranc laboratory. The detector contains 10 kg of Xenon, with which we mainly aim to overcome all the technical and experimental difficulties involved in making this measurement with a new technique. The results are very promising, and for this reason we are already building a larger one, which may have a longer range, 100 kg of Xenon, in the coming years. And in a decade, reach one ton.

This is an experiment that requires a complex, precise and ultra-silent instrument; and to physicists: delicacy and patience; in return, if the neutrino is revealed as truly neutral, if it is its own antiparticle, we will have found a new doorway to a physics that could perhaps explain to us why the Universe and ourselves are matter.

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