This gleaming experiment may solve the cosmic mystery of antimatter

Two views of LEGEND’s scintillation light detector fibre modules (above and below) with light-capturing fibres (green), part of the equipment needed to try to spot antineutrinos annihilating each other

Enrico Sacchetti

Photographer Enrico Sacchetti

THESE gleaming images, taken by photographer Enrico Sacchetti, show key components of an experiment that could finally shed light on one of the biggest mysteries in modern physics.

LEGEND is an international project that aims to explain why there is so much more matter than antimatter in the universe. Starting this year, its first stage, called LEGEND-200, will use highly sensitive germanium detectors to collect data for the next five years at the Gran Sasso National Laboratory in Italy.

Side view of the inner fiber barrel and the open lock system. It serves to detect the scintillation light from the liquid argon to discriminate neutrinoless double beta decay signal events from background events.

Enrico Sacchett

Antimatter is composed of antiparticles that have the same mass as “standard” particles, but with other properties, like charge, opposite to them. LEGEND-200 is examining the hypothesis that minuscule, light and uncharged subatomic particles called neutrinos – themselves a bit of a mystery – are their own antiparticles. In other words, neutrinos and antineutrinos may be one and the same, and could therefore annihilate a like particle.

Assembly of a light sensing module. This is a part of the inner fiber barrel shown below. The light sensing module consists of silicon photomultipliers which are coupled to wavelength-shifting fibers (green).

The assembly of one fibre module

Enrico Sacchetti

LEGEND-200 is probing this possibility by searching for evidence of a rare, theoretical process called neutrinoless double beta decay. This is when two neutrons spontaneously change into two protons, emitting two electrons and two antineutrinos.

Bottom view of the LEGEND-200 liquid argon cryostat and the enclosing water tank. The inner walls of the water tank and the outer wall of the cryostat are covered with a wavelength-shifting mirror film. The purpose is to shift and reflect the light produced by muons via the Cherenkov effect in the water. The photons are detected by photosensors. The purpose of this detector component is to detect and identify cosmic ray muons, some of which still reach the experiment despite being located deep underground the Gran Sasso mountain. (fyi: The muon flux is reduced by a factor 106 (one million) by the rock). If not identified, the muon induced events could mimic the searched signal events.

Underside of the cryostat that will be filled with liquid argon and hold the fibre modules

Enrico Sacchetti

A pair of antineutrinos emitted by a doubly decaying germanium nucleus will, in theory, sometimes annihilate each other, leaving only the emission of electrons – proof of an event that selectively destroys antimatter. If this is observed, we will have seen for the first time a process that favours the existence of matter over antimatter, possibly explaining the matter-antimatter imbalance in the universe.

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