Dark Matter: Experiment to shed light on dark particles


BBC World Service science reporter Rebecca Morelle has been inside the Darkside50 experiment

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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.

Start Quote

The feeling is that dark matter could be just around the corner, so everybody is rushing to be the first to find it”

End Quote Stefano Ragazzi Director, Gran Sasso, National Lab

But there's a good reason for its subterranean location. The 1,400m of rock above means that it is shielded from the cosmic rays that constantly bombard the surface of our planet.

It provides scientists with the "silence" they need to understand some of the strangest phenomena known to physics.

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.

What is dark matter?

dark matter superstructure
  • 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

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.

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.

Gran Sasso Scientists hope to detect that rare event when a particle of dark "stuff" bumps into regular matter

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.

"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," says Dr Ghag.

"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."

Dark matter detector 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.

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.

Cavern The experiments are housed in a cavernous room underground

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," he explains.

"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.

Gran Sasso sign The director of Gran Sasso hopes the first glimpse of dark matter will be made at his facility

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," he explains.

"That would be a revolutionary discovery - it would change our understanding of the Universe, the way it formed, and the way it will evolve."


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  • rate this

    Comment number 162.

    Here's a mad idea. Has anyone thought of 'painting it white'; might make it easier to see?

  • rate this

    Comment number 161.

    God > Science = Delusional
    Science > God = Blinkered
    God + Science = Radical
    God - Science = Creationist
    Science - God = Rational

  • rate this

    Comment number 160.

    it is an example of a change in the population in response to the environment"

    In the same way bipedalism developed in our human ancestors in response to environmental changes making grasslands a more fruitful hunting ground than the jungle.

  • rate this

    Comment number 159.

    @108: alexicon
    " Darwinism doesn't state that we are descended from monkeys. "
    I never said it did. Try reading!

    "Also you simply have no idea of how many scientists are working on the evolutionary process." Dont I? You know that? Of course not! But i do know there is no actual science that challenges the theory.

  • rate this

    Comment number 158.

    laughingdevil is one nippy wee sweety

  • rate this

    Comment number 157.

    Ah the old "can't disprove God" line - Russell's teapot might be an interesting one to look up on that one.

    I am sure that most religious folk are equally excited by this kind of stuff, just a shame the hardcore, defensive nutjobs get all of the air time.

  • rate this

    Comment number 156.

    153. alexicon

    Isn't this how evolution works? A genetic mutation (no tusks) is favoured by environmental conditions (ivory trade). The mutated individuals survive and reproduce while the base population are less successful, changing the nature of the population. Where the environmental pressure did not exist (reserves) the tusked elephants would continue to dominate and the species branches.

  • rate this

    Comment number 155.

    @108: alexicon
    You're wrong in so many ways. Darwinism doesn't state that we are descended from monkeys. Neither it is a theory, it's a scientific theory which is an entirely different thing.
    Also you simply have no idea of how many scientists are working on the evolutionary process.

  • rate this

    Comment number 154.

    Bovver Boy "we all come from monkeys. Whoops a theory, but presented as fact."

    In science (unlike religion) absolute truth is not something we allow, ideas can be revised in the light of contradictary evidence, so we call them theories. Since Darwin proposed his theories no single piece of peer reviewed evidence has cast doubt on his basic premise. In fact even modern DNA analysis supports Darwin

  • rate this

    Comment number 153.

    . We see evolution happening now with Elephants without tusks being born due to fact they are not being poached

    Take care here.. you mean that elephants without tusks are more likely to survive because poachers don't want them, but ,it is an example of a change in the population in response to the environment.

  • rate this

    Comment number 152.

    138Robert Lucien

    The speed of light while very fast to us is almost impossibly slow on the scale of the universe, FTL theory tries to deal with the 99.999..% of the universe inside the FTL barrier.
    J Gribbin in his book Schrodinger's Kittens, gives a neat 'explanation' how the double slit experiment might work using light that travels back in time. Does FLT involve such concepts?

  • rate this

    Comment number 151.

    135. laughingdevil

    Climate scientists have no say in what telescope time solar astronomers get. Indeed solar astronomers would be very happy to get more grant money by challenging the consensus on climate science but regrettably for them (and us) the evidence from telescopes and satellites shows changes in solar output are not responsible for the recent warming trend. Name this astronomer.

  • rate this

    Comment number 150.

    129. Howesyourview
    How to lightly troll on a science based article. Say the word god. If how ever you do truely believe physics is a waste of time, then i challenge you to express your views on HYS without it. Good luck
    Man has invented and spoilt both.

  • rate this

    Comment number 149.

    We can split the atom and find and name all the particles or an atom.....and yet these scientists claim that 73% of everything is made of something that we know nothing about!
    Really makes me laugh - and I have a strong scientific background.
    Undeniably there is something that affects the universe's gravitational attraction but searches like this are just so much phrenology!

  • rate this

    Comment number 148.

    70. paulmerhaba: "Call me an old fashioned romantic but a choice between adam and eve copulating and a newt like organism pulling itself from the ocean in search of guinness is a no brainer"

    Adam and Eve -> Kane and Abel. Then what? Incest and inbreeding? I'll go with the newt thanks!

  • rate this

    Comment number 147.

    Physics is a pretty interesting science because the vast majority of theories proposed couldn't be formally tested until more recently. They make educated guesses in the meantime and come to some consensus over time. Will be really interesting to see if this particular theory turns out to be true.

  • rate this

    Comment number 146.

    My point through all my posts is that there are other theories other than WIMPs, this is consistantly marked down. These theories are not made up, they are in respected scientific journals.

    I've never said I'm an expert, I'm simply an advocate of people educating themselves on alternative theories and the science behind them.

    Please explain what is wrong with saying this?

  • rate this

    Comment number 145.

    It would be ironic if @12 Jack Napier (highest rated) was being sarcastic and believed that electricity, CAT scanners and X-ray machines really were a waste of time and money

  • rate this

    Comment number 144.

    96.Jack Napier
    "some of the ever-optimistic users of this comments page seem to think that they can put put forwards .. arguments against the current theory ."

    You are right to point this out but perhaps thats because the idea of dark matter does have a hint of "desperation" about it. Anyway I would see armchair physics here than religious propaganda.

  • rate this

    Comment number 143.

    It's a scientific fact that we do not see, hear, or otherwise sense even a small fraction of "stuff" exists in the universe. Dark matter, God's particle - how can we study "this stuff" that we cannot even sense...& if we did discover/sense such "stuff" we humans would only use the discovery to create more weapons, kill more people, make more war...beat our chests!


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