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 122.


  • rate this

    Comment number 121.

    101.Bovver Boy
    I agree with you, reasoned debate, and even thinking outside the box is not encoureged. Rather passive unquestiong acceptence of everything presented as fact.
    need I mention politics and advertising.

    you should of course include religion!
    But I disagree about the lack of questioning. Ours is avery questioning societyl we just don't promote the logic to go with it.

  • rate this

    Comment number 120.

    TerrD (98) Agree with the first part however you could replace the word Physicists with the words Religious people and your final 6 words with I am ignorant to the Trust because of my faith.
    Science says it doesn't know everything and it depends upon our knowlede as it is. Religions seems to say this is what we were told and despite proof saying otherwise, I still think the same.

  • rate this

    Comment number 119.

    It's always amusing how closed minded some believers in "science" are.

    Reminds me of an article the late Sir Patrick (RIP) wrote about 18 months back for The Sky at Night. One of the only ones I ever saw a negative reaction too.

    Why? Because he argued that even scientists pursuing unpopular theories should not be blacklisted, and given a platform to present their "evidence"

  • rate this

    Comment number 118.

    This seems like the emperor's new clothes. There's nothing there ! Really ! Trouble is too much money has been spent on it now and no scientist wants to be the first one to say they don't believe it and be classed as a fool. Life imitating art ?

  • rate this

    Comment number 117.

    112Joe blogs

    Oh yes...the Big Bang...is to scientists what the Garden of Eden is to theologists!
    I thought the big bang was when Mary got put up the duff.

  • rate this

    Comment number 116.

    Futurama quantum horse race joke for HYS regulars to help brighten your Wednesday =)
    Announcer: "It's a dead heat! They're checking the electron microscope..."

    Announcer: "...The winner is Number Three in a quantum finish"

    Disgrunted Punter "No fair! You've changed the outcome by measuring it"

  • rate this

    Comment number 115.

    I must admit to being a complete dummy regarding physics, it wasn't that long ago that I got myself all tied up with 'String Theory' now I'm trying to shed some light on 'Dark Matter'

    Imagine the EU being approached for a scientific research grant

    EU: Whats the money for?
    Boffin: Er to find Dark Matter, you do know what Dark Matter is don't you!
    EU: Oh yes obsolutely here take the money

  • rate this

    Comment number 114.

    Why is now any different to the last 2000 years?

  • rate this

    Comment number 113.

    I see various nutjobs have tried to hijack this to talk about religion (both pro- and anti- the holding of a religious belief) - when will you lot learn that science isn't the right tool to give you evidence either way?

    If there is a god, then science merely shows you how said deity did things.

    If there isn't any gods, well science is doing a fine job of explaining how the universe works.

  • rate this

    Comment number 112.

    Oh yes...the Big Bang...is to scientists what the Garden of Eden is to theologists!

  • rate this

    Comment number 111.

    Ah! but can it shed light on the inner world of the EU

  • Comment number 110.

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

  • rate this

    Comment number 109.

    Bovver Boy

    You're confusing not having your views taken seriously with not being allowed to hold or express those views at all. If you did not reach your views on the basis of reasoned argument (evidence, an understanding of the scientific method and accepted scientific principles) then no amount of reasoned argument will persuade you that you are wrong but don't expect to be taken seriously.

  • rate this

    Comment number 108.

    86.Bovver Boy
    Darwinism, ..the theory that we all come from monkeys. Whoops a theory, but presented as fact, though many scientists now question it.

    No work exists to question the workings of evolution. It would be easy to disprove if untrue, as even Darwin points out.
    Of course, as a theory, evolution is always open to question on the detail of its workings.

  • rate this

    Comment number 107.

    Was the bible written by people who were fed up with scientists?

  • rate this

    Comment number 106.

    Good to see things like this being done instead of poured down the welfare/NHS drain, but I hope DM/DE doesn't exist since the idea of a rapidly-accelerating universe is quite scary.
    In fact, if current models are correct, the universe will accelerate itself to destruction in time (a lot of time) which is depressing.
    Our medium/long term future is to live on large space ships - start work on that?

  • rate this

    Comment number 105.

    Dark energy sounds a bit dubious to me - a bit too much like something made up to cover our lack of knowledge of the dynamics of the big bang. But dark matter sound much more interesting. Does it only work at the level of the galazy and beyond, or also at the level of the solar system? Does it take into account the gravity of black holes?

  • rate this

    Comment number 104.

    It's expensive, looks cool and sounds complicated, so I'm all for it.

  • rate this

    Comment number 103.

    Infinite universes, definitely the worst theory ever invented. If true every word in every book ever written would be true in at least one universe..
    On the plus in at least one universe Terry Pratchetts Disk World will be real.
    On the negative in at least one really weird and unpleasant universe the Bible will be literally true!


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