15 January 2022 | New Scientist | 31Stuff of legend
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.
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.
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.
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. ❚Gege LiPhotographer Enrico SacchettiLeft and bottom right: Two views of LEGEND’s
scintillation light detector fibre modules with
light-capturing fibres (green), part of the
equipment needed to try to spot antineutrinos
annihilating each other (see main text, right);
Above: The assembly of one fibre module;
Top: Underside of the cryostat that will be filled
with liquid argon and hold the fibre modules