The 7,000-tonne titan that’s hunting for dark matter

by Peter Ray Allison

How do you find something that is smaller than an atom, barely interacts with ordinary matter, and may not even exist? The answer is you go big. Really big.

The 7,000-tonne titan that's hunting for dark matter

Like the Greek titan of the same name, ATLAS is an object of mammoth proportions. Its name stands for “A Toroidal LHC Apparatus” – one of the worst acronyms ever – but whereas its mythological counterpart was condemned to hold up the sky for eternity, this ATLAS is a subatomic particle detector that forms part of the Large Hadron Collider (LHC) at CERN.

Just as you need incrementally larger telescopes to see further (just look at the size of the Arecibo Observatory), so too do you need to build increasing larger detectors to view the subatomic level of the universe. In the case of ATLAS, which measures 25m in diameter and 44m in length (large enough to comfortably accommodate 27 medium-sized terrace houses), it is for detecting and confirming the presence of the theoretical dark matter.

Not that ATLAS gets to rest on its gargantuan laurels. The recently upgraded LHC, which has improved stability and reliability, is now producing a billion proton-proton collisions every second. As such, the ATLAS experiment has collected more than a quadrillion particle collisions, which is twice the amount of data they were expecting. This increased rate of data collection means that ATLAS will need to be upgraded next year.

So why are we interested in dark matter?

The reason we are so interested in dark matter is that the visible universe – that which we can see and physically touch – only makes up a small proportion of what is actually there. It has been approximated that ordinary (baryonic) matter, which is visible to us, comprises only 4% of the universe. Dark matter is thought to compromise 23% and the remainder is made up of dark energy.

Dark matter was first hypothesised nearly 100 years ago, as a result of the observations of galactic rotation by Dutch astronomer Jacobus Kapteyn. Studies of galaxies revealed that there was a significant amount of matter missing from the universe. Essentially, the universe would not be structured as it were, unless there was a lot more matter than there was initially believed to be. Either that or the equations were wrong…claudia_marcelloni_2

Dark matter and dark energy are so called, not because they are literally dark or black, but because we cannot see them. Dark matter is believed to consist of subatomic particles that interact with the visible universe in only limited ways, but nonetheless have a profound effect on the universe as a whole.

It has incredible implications for our understanding of the universe.

Given that the hypothesis for dark matter has been corroborated many times, the only plausible explanation is that the matter has not been detected yet. Even though dark matter is incredibly small, and has a minute gravitational pull, it is so abundant compared with visible matter that it has incredible implications for our understanding of the universe.

Were it not for the presence of dark matter, galaxies would not form as they do, in terms of their angle of rotation and radial velocity (how fast they turn). The recently discovered Dragonfly 44 (awesome name for a sci-fi show) is the Milky Way’s dark twin. This “ghost” galaxy has approximately the same amount of mass as our own, but is its opposite in terms of number of stars and structure.maximilien_brice

Due to the cumulative effect of dark matter’s gravitational pull, it can deflect light – an effect known as gravitational lensing. By observing the gravitational lensing effect on the light from distant galaxies, scientists and astronomers can gain an understanding of the distribution of dark matter within the universe.

“The thing with dark matter is that even though we can’t see it, it still has a gravitational effect,” explains Professor Davide Costanzo of the University of Sheffield. “So if you had a lump of it, you would still be attracted to it [by gravity].”

Bringing ATLAS into the equation

At the moment, we are unable to directly detect and measure dark matter, but what we can detect is the interaction that dark matter has with visible matter. Through these observations, and by measuring the entropy of the cosmic background, we can subsequently infer how much dark matter is present.

We can also detect the way in which dark matter interacts with other subatomic particles. Through these interactions, we can determine the existence of dark matter. One such example of how this interaction is detected is through the ATLAS experiment.claudia_marcelloni_3

It operates by monitoring the point at which the LHC smashes two particles together while they’re travelling at speeds of 0.999999990c, which is approximately 11km/h (nearly 7mph) less than the speed of light, or slightly slower than Star Trek’s warp factor 1. The resulting collision generates up to 13TeV (1.3×1013V, or 13 trillion electron volts).

The resulting collision may create one subatomic particle that fires off in one direction, and a second particle of dark matter that fires off in the opposite direction.

To detect the particles that are created as a result of the collision, there are several layers of the ATLAS detector surrounding the collider. Each of these layers are intended to capture and/or detect the types of different particles created from the collision.

ATLAS uses lead and iron. Gold would have been nice, as it is far denser than lead, but it can be a tad expensive.

The innermost layer of the ATLAS experiment is comprised of silicon detectors, which are the same as those used in a digital camera, except on a much larger scale. The outer layer of ATLAS is intended to capture high-energy particles, which requires the use of large amounts of dense matter. In this case ATLAS uses lead and iron. Gold would have been nice, as it is far denser than lead, but it can be a tad expensive.

“What we detect at the LHC is not dark matter from the Big Bang, but new dark matter we produce as a result of the collision,” explains Professor Costanzo. “The problem is that after we have produced them, we cannot detect them. When we produce them we produce other particles, and we infer we have produced [dark matter] because something is missing.”


The reason that ATLAS is so large is that it performs incredibly precise measurements, which necessitates complex engineering. In addition to this, the ATLAS casing also shields the experiment from cosmic rays, particularly muons, to prevent them from interfering with the sensors. Boulby Mine in Yorkshire also conducts dark matter research a kilometre underground, protecting it from interference of cosmic rays.

Through research into dark matter, CERN has developed new sensors and means by which we can detect incrementally smaller particles. “We have added a whole lot of progress in many fields, and you never know what new discoveries can bring you,” says Professor Costanzo. “When the electron was discovered it was fine, but now everything is based on electricity.”

As there are several different experiments coming from different angles, it should only be a matter of time before we detect dark matter. The largest of the next generation in dark matter experiments will be the LUX-Zeppelin (Large Underground Xenon and ZonEd Proportional scintillation in LIquid Noble gases), which will be conducted deep underground in South Dakota.

“Like the Higgs boson, we knew it [had] to exist and then took a while to confirm its existence,” concludes Professor Costanzo. “With dark matter, we’re closing in from several corners, and hopefully in the next ten years that will happen.”

Photographs: CERN (Claudia Marcelloni and Maximilien Brice)

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