In a man-made cavern, deep beneath a mountain, scientists are hoping to shed light on one of the most mysterious substances in our Universe - dark matter.
The Gran Sasso National Laboratory seems more like a Bond villain's lair than a hub for world class physics.
It's buried under the highest peak of Italy's Gran Sasso mountain range; the entrance concealed behind a colossal steel door found halfway along a tunnel that cuts through the mountain.
Inside three vast halls, a raft of experiments are running - but with their latest addition, DarkSide50, scientists are setting their sights on dark matter.
Everything we know and can see in the Universe only makes up about 4% of the stuff that is out there.
The rest, scientists believe, comes in two enigmatic forms.
They predict that about 73% of the Universe is made up of dark energy - a pervasive energy field that acts as a sort of anti-gravity to stop the Universe from contracting back in on itself.
The other 23%, researchers believe, comes in the form of dark matter. The challenge is that until now nobody has seen it.
Layout of Gran Sasso National Laboratory in Italy's Gran Sasso mountain range houses the world's largest underground facility of its kind for conducting research on sub-atomic particles includes Project Xenon which is conducting research for detecting dark matter (Click Image To Enlarge)
What is dark matter?
- Normal matter gives out or absorbs light to make it visible, but matter doesn't have to interact with light this way
- Astrophysicists calculate that there isn't enough visible matter to explain the rotation of galaxies
- They proposed a type of matter that we can't detect in the normal way - dark matter
- You can't see dark matter directly with telescopes, but its gravitational effect can be seen on visible matter
- Dark matter should be all around us, so scientists are developing new ways to detect these mysterious particles
Dr Chamkaur Ghag, a particle physicist from University College London, explains:
"We think it is in the form of a particle. We have protons, neutrons and electrons and all these regular normal particles that you associate building things with. We think dark matter is a particle too, it's just an odd form of matter in the fact that we don't perceive it very readily. And that is because it doesn't feel the electromagnetic force - light doesn't bounce off it, we don't interact with it very strongly."
Physicists have called these dark matter contenders Weakly Interacting Massive Particles - or WIMPS.
They believe millions of them are passing through us every second without a trace.
But very occasionally one will bump into a piece of "regular" matter - and that is what they are hoping to detect with DarkSide50.
Inside a house-sized tank, a large metal sphere holds a particle detector called a scintillator.
This container is filled with 50kg of liquid argon and a thick layer of the element in its gas form.
Dr Ghag says.
"If a dark matter particle comes in and hits the argon, the recoiling atom gets a kick of energy and it quickly tries to get rid of it. In argon it gets rid of it by kicking out light; it sheds photons. But it also gives charge: some electrons that are liberated from the interaction site. And those electrons drift up into a gas layer, and when they hit the gas and you get another flash of light."
As dark matter particles steam through the detector, scientists hope that a few will collide with the argon atoms. This will generate two flashes of light - one in the liquid argon and another in the gas - which will be detected by the receptors. (Click Image To Enlarge)
In DarkSide, dark-matter particles are detected in a two-phase liquid-argon TPC surrounded by a neutron veto of liquid scintillator (spherical vessel), inside 1000 m3 of water to veto muons. Image credit: DarkSide collaboration. (Click Image To Enlarge)
Until now, the hunt for dark matter has proved elusive.
Some experiments claim to have seen signals of dark matter in the form of annual modulation.
This is the idea that the number of these particles we should be detecting changes as the seasons change.
That's because as the Earth moves around the Sun, it is moving into a stationary field of dark matter - and for half the year it will be moving against the tide of dark matter - just like driving into the rain. But for the other half it will be moving with this tide and less dark matter will hit.
But other researchers have questioned attributing these seasonal variations that have been detected to dark matter.
Other experiments have run for long periods of times without so much as a hint of the stuff.
One, called XENON100, which is also in Gran Sasso, ran for the course of a year, but only saw two "events" - not enough to rule out that this might have been some stray background radiation.
But with DarkSide50, there seems to be a new push to find some answers.
Alongside this experiment, another large detector - LUX - which is in a gold mine in South Dakota in the US will soon be coming online.
And in the next few years, scientists are planning even more ambitions detectors, such as XENON1T and LUX-Zeplin - they are hoping to find the first experimental evidence of these particles.
Aldo Ianni, from the DarkSide50 team, says:
"Dark matter is really a major scientific goal at the present time. It will help us understand a big fraction of our Universe that we don't understand at the present time. We know there is dark matter - but we have to understand what this dark matter is made of."Fruitless search?
Professor Stefano Ragazzi, director of the Gran Sasso National Laboratory, hopes that the first glimpse of dark matter will be in his research facility.
"There is competition amongst different experiments - so when you compete you prefer to win rather than coming second or third. The feeling is that dark matter could be just around the corner, so everybody is rushing to be the first to find it."
However, he admits that there is always the chance that these experiments may find nothing at all - and dark matter may not be in the form of WIMPs.
Professor Ragazzi says:
"We may find that we have the wrong hypothesis… [dark matter] may be something completely different. But it may be even more interesting not finding dark matter than finding it."
In the next few weeks DarkSide50 will be fully kitted out, the surrounding tank flooded with purified water, and then the scientists will have to watch and wait.
But Dr Ghag says despite the uncertainties, the potential reward of finding dark matter would be huge.
"If we did find dark matter, then we'd have done would be to solved one of nature's best kept secrets. And that would have been to have figured out what a quarter of the Universe is made of. That would be a revolutionary discovery - it would change our understanding of the Universe, the way it formed, and the way it will evolve."
Why Scientists Believe Dark Matter Exists
The following video from Canada's Perimeter Institute For Theoretical Physics explains why scientists believe there is "missing or unseen mass" that does not emit, reflect or absorb any type of electromagnetic radiation or light and is necessary in order to keep galaxies from spinning out of control and breaking apart as they spin. This missing mass is now referred to as dark matter, or matter that cannot be seen.
The Technology Behind DarkSide50
DarkSide50 is a new experiment that uses novel techniques to suppress background sources as much as possible, while also understanding them well. The programme centres on a series of detectors of increasing mass, each making possible a convincing claim for the detection of dark matter based on the observation of a few well characterized nuclear-recoil events in an exposure of several years. The design concept involves a two-phase, liquid-argon time-projection chamber (LAr-TPC) in which the energy released in WIMP-induced nuclear recoils can produce both scintillation and ionization. Arrays of photomultiplier tubes at the bottom and top of the cylindrical active volume detect the scintillation light. A pair of novel transparent high-voltage electrodes and a field cage provide a uniform drift field of about 1 kV/cm to extract the ionization produced. A reflective, wavelength-shifting lining renders the scintillation light from the argon (wavelength 128 nm) visible to the photomultipliers.
In a two-phase argon TPC, rejection of background comes from three independent discrimination parameters: pulse-shape analysis of the direct liquid-argon scintillation signal (S1); the ratio of ionization produced in an event to scintillation, where the former is read out by extracting ionization electrons from the liquid into the gaseous argon phase, where they are accelerated and emit light through electroluminescence (S2); and reconstruction of the event’s location in 3D using the TPC. The z co-ordinates for the event are determined by the time delay between S2 and S1, while the transverse co-ordinates are determined through the distribution of the S2 light across the layer of photomultiplier tubes.
In a typical target, there are three main sources of background at energies up to tens of kilo-electron-volts: natural β and γ radioactivity, which induces electron recoils; α decays on the surface of the target in which the daughter nucleus recoils into the target and the α particle remains undetected; and nuclear recoils produced by the elastic scattering of background neutrons. This latter process is nearly indistinguishable from the signals expected for WIMPs and requires an efficient neutron veto in the apparatus.
As in other experiments searching for rare events, DarkSide’s detectors will be constructed using materials with low intrinsic radioactivity. In particular, the experiment uses underground argon with extremely low quantities of 39Ar, which is present in atmospheric argon at levels of about 1Bq/kg as a result of the interaction of cosmic rays, primarily with 40Ar. The DarkSide collaboration has developed processes to extract argon from underground gas wells, where the proportion of 39Ar is low. A particularly good source of underground argon is in the Kinder Morgan Doe Canyon Complex in Colorado. The CO2 natural gas extracted there contains about 600 ppm of argon. The DarkSide collaboration has operated an extraction facility at the Kinder Morgan site since February 2010; it has to date extracted some 90 kg of underground depleted argon and subsequently distilled 23 kg to about 99.99% purity. (The throughput is about 1 kg/day, with 99% efficiency.) Studies of the residual 39Ar content of the distilled gas with a low-background detector at the Kimballton Underground Research Facility, Virginia, give an upper limit for the 39Ar content equivalent to 0.6% of the 39Ar in atmospheric argon.
It is not only the argon that has to have low intrinsic radioactivity. Nuclear recoils produced by energetic neutrons that scatter only once in the active volume form a background that is, on an event-by-event basis, indistinguishable from dark-matter interactions. Neutrons capable of producing these recoil backgrounds are created by radiogenic processes in the detector material. In detectors made from clean materials, the dominant source of the radiogenic neutrons is typically the photodetectors, so ultralow background photodetectors are another important goal for DarkSide. A long-term collaboration with the Hamamatsu Corporation has resulted in the commercialization of 3-inch photomultiplier tubes with a total γ activity of around only 60 mBq per tube, with a further 10-fold reduction foreseen in the near future.
To measure and exclude neutron background produced by cosmic-ray muons, the DarkSide TPC will be deployed within an active neutron veto based on liquid scintillator, which will in turn be deployed within 1000 m3 of water in a tank 10 m high and 11 m in diameter, which was previously used in the Borexino Counting Test Facility at Gran Sasso. The liquid-scintillator neutron veto is a unique feature of the DarkSide design and is filled with ultrapure, boron-loaded organic scintillator, which has been distilled using the purification system of the Borexino experiment. The water serves as a Cherenkov detector to veto muons. Monte Carlo simulations suggest that with this combined veto system, the number of neutron events generated by cosmic-rays at the depth of the Gran Sasso Laboratory should be negligible, even for exposures of the order of tonne-years.
The DarkSide programme will follow a staged approach. The collaboration has been operating DarkSide 10, a prototype detector with a 10 kg active mass, in the underground laboratory at Gran Sasso since September 2011. This has been a valuable test bed during the construction of the veto system. It has allowed the light-collection, high-voltage and TPC field structures – and the data-acquisition and particle-discrimination analysis systems – to be optimized using γ and americium-beryllium sources. The first physics detector in the programme, DarkSide 50, should be deployed inside the completed veto system in the Gran Sasso Laboratory by the end of 2012. Looking forward to the second generation, upgrades to the underground argon plants are planned, and the nearly completed veto system has been designed to accommodate a DarkSide-G2 detector, which will have a fiducial mass of 3.5 tonnes.