Well then, how can we possibly make it visible? Here at CERN, my colleagues are trying to produce such new particles by creating conditions as they existed shortly after the Big Bang. They look for WIMPs emerging in collisions of very-high-energy protons. We, on the other hand, look for collisions of dark matter particles — those that are here and out there forming the halo of our Milky Way — with atoms in a terrestrial detector. In this rare process, which you can picture like an elastic collision between billiard balls, a tiny amount of energy is transferred onto the atom, which then gets excited and emits a faint flash of light.
We have calculated that about [100 million] dark matter particles pass through your hand every second. So, hold out your hand and just imagine this: can you feel anything? No. And you also don’t feel the billions of neutrinos and all the other cosmic ray particles that pass through you and your body in very high numbers. These cosmic rays would create a huge noise in our detectors preventing us from listening to the rare and faint dark matter interactions.
So, where is the best place to look for something that’s invisible? At the bottom of a dark, deep mine. To have even the slightest chance of detecting WIMPs, we have to go deep underground. We use mountains as shields against this noise caused by cosmic radiation. And I work in one of the largest underground laboratories in the world, the Gran Sasso Laboratory in Italy. It is located beneath the Cardinal Grand Massif of the Abruzzi mountains and offers about 1, 400 meters of shield against noise, both real and cosmic. In this quiet location, we recently placed a large detector filled with three and a half tons of liquid xenon kept cold at a temperature of minus 100 degrees Celsius.
The liquid xenon emits a tiny flash of light when a particle collides with a xenon atom. And we can detect this light, and we can also measure the few electrons that are produced in such a collision. The trick is then to make sure that no other particle ever makes it to the center of your detector, and that only dark matter particles can leave the faint trace that we are looking for.
So we have to surround the xenon by various shields. It is contained in an inner detector itself, in a large stainless steel cryostat which then, is surrounded by ten meters of water. The water protects it against those cosmic rays that can still penetrate deep underground. The core of this detector, its inner part, is called time projection chamber. It delivers a precise, three-dimensional image of any interaction that happens in the liquid by means of 248 photo-sensors, or eyes, that are constantly monitoring the cold liquid.
So, considering now the flux of the dark matter particles, the ten millions through your hand per second, and the fact that we haven’t seen any yet with the previous generation of detectors, we can estimate that only a handful will scatter in our new detector in one full year of data, maybe, even just one.
We are looking for very rare events indeed, but there was another challenge that we had to overcome. It is not sufficient to bring your detector underground and to surround it by water, we also had to ensure that every material that we employed in its construction had incredible low levels of natural radioactivity.
Natural radioactivity is everywhere; it is around us, inside us, it’s extremely difficult to shield. For example, every single one of you would be way too radioactive to be standing next to our unshielded detector. This is why we are constantly searching for new materials that have only minor traces of natural radioactivity.
So, after we designed our time projection chamber, and we built it out of the cleanest materials that we could find, we first assembled everything, and tested it at the Earth’s surface. We then cleaned it and reassembled the entire detector in a clean room at the Gran Sasso Laboratory, after which we finally brought the detector underground.
Here, you can see the entire experiment in the underground laboratory: the ten-meter tall service building, and next to it, the large water shield with a cryostat and the time projection chamber inside. It took two years to build, after many years of R&D and design work.
So, after this long journey, we have created the largest and the most sensitive dark matter detector in operation. And now we took the last month to characterize it, to understand its behavior, and we are finally ready to start searching for dark matter particles. We are hopeful that soon, we can bring some light into the dark.
So, in the future, when you gaze into the stars, you might not see any obvious changes in this beautiful image, but we might have all come a little closer in our quest to understand how galaxies in the universe, including our own Milky Way, formed.