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 282.

    Science and our understanding of the universe is the most important thing for ensuring human kind lives on. Well done to the physicists undertaking this jaw-dropping research.

  • rate this

    Comment number 281.

    262"The strong nuclear force [ ]bosons"

    Just words, not an explanation.The Higgs Boson is intended to explain mass.The most massive of all particles and they can't find it.

    264 Those are just words, not an explanation

    265Some nutter once claimed gravity working because of the general theory of relativity

    How can you explain curvature of space when you haven't shown it has structure?

  • rate this

    Comment number 280.

    A small but crucial point: an object is visable when it emits or reflects light, not when it absorbs it.

    The statement on your page 'Normal matter gives out or absorbs light to make it visible, but matter doesn't have to interact with light this way.' should be amended.

  • rate this

    Comment number 279.

    275.Not So Bitter
    4 Minutes ago
    The hunt for dark matter happens because of one HUGE assumption, i.e. that all red shifts are due to recession, which is looking less and less likely - unless you are a Big Bang dogmatist.

    I think you mean dark energy.

    You can see the orbital consequences of dark matter in most galaxies, regardless of redshift.

  • rate this

    Comment number 278.

    One final appeal

    I suggest all interested in science read up on the ESO research led by Christian Moni-Bidin,

    Scott Tremaine, professor of physics at Princeton University's Institute for Advanced Study "If the authors' conclusions are correct, this is indeed a serious blow to dark matter"

    WIMPS are not the only option, if you are interested in science read up on some completed research!

  • rate this

    Comment number 277.

    "Drunken Hobo
    It's obviously good to have an appeal to the masses, but there doesn't appear much for the enthusiast anymore."

    Concurrent with SGL most universities, science learning centres and astronomical societies ran events for the public to turn up and find out more than they could in watching BC and DOB. All ages and genders were represented at these events.

  • rate this

    Comment number 276.

    #180 fuzzy

    The whole problem is that General Relativity only fails in the FTL context so it is almost impossible to test it or prove it directly either way. :( GR does fail in its logic because it predicts that the universe does not exist.
    UnF good refs for FTL research are in short supply. The best approach is to read widely through the sci literature, a good st point is Roger Penrose TENM.

  • rate this

    Comment number 275.

    The hunt for dark matter happens because of one HUGE assumption, i.e. that all red shifts are due to recession, which is looking less and less likely - unless you are a Big Bang dogmatist.

  • rate this

    Comment number 274.

    Cox has to be dumbed down just so that the lower end of the masses can get even the slightest grip on a huge complicated subject.

  • rate this

    Comment number 273.

    269 laughingdevil - I feel the same way about most of his programmes. Last year's Stargazing looked like it was for kids but I found this year's to be far better.
    It's obviously good to have an appeal to the masses, but there doesn't appear much for the enthusiast anymore. I long for a science programme taught by old men with long straggly beards and frumpy women with short hair and thick glasses.

  • rate this

    Comment number 272.

    I often think that the 'maps' of 'dark matter' look like tension lines in an elastic surface.

    It makes me think that the basic assumption of more mass is wrong.

    How about more force instead - a tension in the fabric of spacetime itself (Higgs field?) that resists the observed expansion of the universe.

  • rate this

    Comment number 271.

    Thought we were getting somewhere with becoming bipeds to use epicycles. Oh well back to the drawing board.

  • rate this

    Comment number 270.

    Let's hope this study bears good fruit. It's important to know the universe we live in and the more we know, the more we can explore and move further.

    Best of luck to you all.

  • rate this

    Comment number 269.

    Can't stand Cox, dumbed down isn't the word! As a keen amature astronomer I was so dissapointed with stargazing live, and his other things are worse. Dara O'Briens Science club was much more "grown up" than anything I've seen from Cox. His programs should be on CBBC!

  • rate this

    Comment number 268.

    EUREKA!!!!! I conducted an experiment recently and found dark matter!!!! SERIOUSLY, I DID! I drunk a whole bottle of vodka and looked up into the sky and I could see it. Millions of particles spinning around in the air. And then it went dark and I woke up a few hours later inspired by my discovery. The particles must've entered my head as well because they'd impacted every single brain cell I had!

  • rate this

    Comment number 267.

    People ask 'How will this benefit us?' It's an ignorant point, but there are two immediate rebuttals. 1) Does something have to have practical use? Knowledge has its own value. Like art, I guess. 2) Who knows what potential use something has until you find it? Antimatter is used routinely in medicine (PET scanners). Maxwell's waves were esoteric at first but are responsible for the modern world.

  • Comment number 266.

    This comment was removed because the moderators found it broke the house rules. Explain.

  • rate this

    Comment number 265.

    256. sieuarlu

    Some nutter once claimed gravity working because of the general theory of relativity. But of course I like your theory!

  • rate this

    Comment number 264.

    " sieuarlu
    they don't know how gravity works even though they can predict its effect.They don't know why like charges repel and opposites atract, don't know what magnetism really is, or why nucleii stay together

    Actually they do understand those things, though you may not understand the explanations. What they don't understand is how gravity AND those other forces operate in one unified theory.

  • rate this

    Comment number 263.

    I always love the comments on articles like this. "I don't really understand this, so I'll just claim its all nonsense!"


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