Nowadays we understand the nature
of the Universe and its components
to an unprecedented level of detail.
Scientific evidence tells us
 that if we could gather
all the ordinary matter
content in the Universe
 - gas, planets, stars, galaxies -
it would account for only
about 15% of the total,
while 85% is believed to be
of a mysterious nature,
and is referred to as "Dark Matter".
The first indication
for the existence of Dark Matter
came in 1933.
Astronomer Fritz Zwicky
coined the term to describe
an invisible matter
whose presence he deemed necessary
to explain galaxy motions
in a large galaxy cluster.
Clearer proof came decades later
from several sets
of astronomical observations.
For instance,
the rotational behaviour
of spiral galaxies
such as our Milky Way,
is proof of the existence of Dark Matter.
If a large spiral galaxy
consisted only of atoms,
the ordinary visible matter,
the inner stars would rotate
at a higher speed
than the outer ones.
This would be seen clearly
in the rotational velocity plot.
However, from rotation velocity
astronomical measurements
we see a different behaviour:
the outer stars rotate
around the galactic center
as fast, if not faster, than the inner ones.
This can only be physically possible
if the galaxy itself is immersed
in a much bigger and massive "halo"
of a Dark Matter we can't see,
with a total mass about 6 times larger
than the ordinary visible matter.
Scientists have been producing theories
on possible Dark Matter
candidates for decades.
Among the leading candidates,
motivated by the so-called
supersymmetry theory,
are WIMPs
- Weakly Interacting Massive Particles -
that only interact with atoms through gravity
or through very rare collisions.
We could, in principle,
observe these collisions
in detectors with very large
 target masses (1-100 tons),
located in deep underground sites
to eliminate particle background.
The DarkSide collaboration
is an international affiliation
of over 60 universities
and research institutes
 from around the world,
that aims at the direct detection of WIMPs
at the Gran Sasso National Laboratory
 in Assergi, Italy.
The target material in DarkSide-20k
is liquid Argon,
a cryogenic material
with optimal properties for
a Dark Matter experiment.
The collision between a WIMP
and an Argon nucleus
would produce a trail of free
 electrons and photon emissions
that DarkSide-20k would be able to detect.
Argon is a gas present in the atmosphere.
however, not all Argon is the same.
Argon 40
(with 40 protons and neutrons in its nucleus)
is stable and optimal
for a Dark Matter experiment,
unlike Argon 39,
produced in small quantities
in the atmosphere
by interaction with cosmic rays
coming from space.
This is why the DarkSide-20k target Argon
is extracted at the URANIA plant
from the gas coming from
 a deep underground well in Colorado.
Located there for thousands of years,
the stable Argon 40
is shielded from the cosmic rays,
while the Argon 39
has had the time to vanish
 almost entirely by decaying.
The underground Argon
is then sent for further purification
to the ARIA laboratory
at the Seruci mines in Sardinia,
before its use in the experiment.
The production of the extremely
 sophisticated instrumentation
for DarkSide-20k takes place
in several labs around the world.
Researchers from the Cryogenics
 Laboratory of INFN in Naples, Italy,
test the delicate photons detectors,
the so-called Silicon photomultipliers,
or SiPM.
SiPM are one of the key
enabling technologies
for large scale liquid Argon based
Dark Matter experiments
and are produced and assembled at NOA,
Nuova Officina Assergi, in Abruzzo.
The local governments of the Italian
regions of Sardinia and Abruzzo
gave a crucial financial
 and political contribution
to the development
of the research facilities
in Seruci and L'Aquila,
with a positive
economic and social return
for those territories.
Ultimately all the highly
sophisticated components
will converge at the underground
Gran Sasso laboratory,
where DarkSide-20k will be installed
and operated starting from 2024.
A huge challenge for the experiment
is that all components and materials
must have the lowest possible level
of natural radioactivity,
to reduce neutron background.
The outer structure of DarkSide-20k
is a large cubic cryostat chamber
that contains about
700 tons of liquid Argon
and acts as a shield from
outcoming background particles.
Within the cryostat is the "Veto chamber",
which has the crucial task of separating
the effects of WIMP-Argon collisions
from background events,
such as Argon collisions with neutrons
produced by cosmic rays
and by natural radioactivity,
which, although unavoidable,
is minimised in all components
and materials of the experiment.
The inner chamber and the core
of the DarkSide-20k experiment,
is the so-called
Time Projection Chamber, or TPC.
This chamber contains 20 tons
of target underground liquid Argon,
which is viewed by arrays of SiPM.
To observe the signal
of a WIMP-Argon collision,
the experiment needs to trace
with extreme accuracy
both the photons and the electrons
produced by the impact.
Silicon-PMs detect with great accuracy
the primary scintillation photons
produced by WIMP-Argon collision
thanks to their high photon detection
efficiency and single-photon resolution.
A WIMP collision also extracts
some of the electrons
orbiting the Argon nucleus.
These are detected thanks to the
TPC's "two-phase configuration",
where a small region of gaseous Argon
lies above the larger volume
of liquid underground Argon.
A uniform electric field
is applied to drift
the free electrons upward
to the surface of the liquid Argon.
There, another electric field is applied
to pull the free electrons
 into the gaseous Argon,
where they produce
secondary scintillations photons
by a process called "electroluminescence".
When a WIMP collides
with an Argon nucleus,
it produces primary
scintillations photons
which are instantly detected
by the Silicon-PMs inside the TPC:
this first signal is called S1.
The free electrons produced in the collision
drift upwards to the surface
 of the liquid Argon,
and are pulled into the gaseous Argon
where they produce
secondary scintillation photons,
also viewed by the Silicon-PM arrays.
This signal, S2,
happens at a later time
with respect to the primary
scintillation signal S1.
This configuration allows
for the WIMP-Argon events
to be accurately localised
 in three dimensions:
the delay time between
 the S1 and S2 signals
accurately defines
the vertical position of each event,
with millimeter precision.
The distribution of light
over the top photo detector array
gives the horizontal position,
 with centimeter like precision.
The veto chamber is then used
to discriminate real
Dark Matter events
from background signals.
Thanks to DarkSide-20k
we will be able to either detect
Dark Matter for the first time,
or strongly constrain
its nature and its properties.
The quest for the discovery
of this mysterious
component of the Universe
is on!
