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CERN | Geneva | Switzerland

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Getting our hands on dark matter

Second part in a series of four on Dark Matter

In a previous blog, I reviewed the many ways dark matter manifests itself through gravitational effects. But to this day, nobody has managed an unambiguous direct observation of dark matter.

This is not surprising given we are talking about a completely different and totally unknown type of matter, something not made of quarks and leptons like all visible matter (humans, planets, stars and galaxies).

Nevertheless, just as the quarks and leptons are the building blocks of visible matter, physicists expect dark matter is also made of fundamental particles, albeit dark particles. So we need to catch dark matter particles interacting in some way with particles of regular matter.

So far, all we know is that dark matter reacts to gravitation but not to electromagnetism since it does not emit any light. Maybe it interacts with ordinary matter through the weak nuclear force, the one responsible for radioactive decays. Dark matter would then be made of weakly interacting particles.

Weakly Interacting Massive Particles

One popular hypothesis is that dark matter particles would be WIMPs, which stands for Weakly Interacting Massive Particles. How often can WIMPs interact with matter? It should be less than 0.1 times per year per kilogram of sensitive material in the detector, depending on the WIMP mass.

The detection principle is simple: once in a while, a WIMP will strike a nucleus in one of the detector’s atoms, which will recoil and induce a small recordable vibration.

event-rate-vs-material

From Lauren Hsu’s review talk at ICHEP 2012.

The vertical axis shows the number of times a dark particle transfer a given amount of energy to a nucleus. The more massive the detector and the longer you operate it, the higher are the chances of recording a collision.

The detector material also matters as seen on the plot above: collisions are more energetic, hence easier to detect,  with Germanium (Ge) than with heavier nuclei like Xenon (Xe), but the total number of collisions is higher with the latter material.

These detectors are placed deep in mines or tunnels to block cosmic rays that would induce false signals in the detector. Eliminating all sources of background is the biggest challenge facing these experiments.

Dark matter wind

If the Universe is full of dark matter, we on Earth should feel a wind of dark particles as we travel around the Sun. This rate is evaluated to be of the order of a million particles per square centimetre per second for a WIMP ten times heavier than a proton.

And just like a cyclist riding on a circular track on a windy day, we should feel a head wind of dark matter particles in June and a tail wind in December since there is a greater concentration of dark matter in the centre of the galaxy.

 Wimp-wind

 

Imagine now a detector operating on Earth and sensitive to WIMPs. The variations in the wind intensity would be detected as an annual modulation in the number of dark matter particles hitting the detector throughout the year.

This is exactly what the DAMA/LIBRA experiment claims to observe for more than a decade now as shown on the plot below. Their signal is loud and clear (8.9 sigma) but unfortunately, refuted by several experiments.

DAMA-LIBRAThe number of events recorded by DAMA/LIBRA as a function of time (more than 10 years) shows a clear annual modulation.

Three other experiments have also reported signals: CoGent sees a faint modulation while both CRESST and CDMS observed a few events in excess of the expected background.

All would be great if these four experiments would all agree on the characteristics of the dark matter particle but that is unfortunately not the case.

Many theorists have deployed heroic efforts to devise new models to explain why some experiments see a signal while others do not, but no model is widely accepted yet. The situation remains terribly confusing as can be appreciated from the plot below.

CDEX

The vertical axis represents the possible rate at which a dark matter particle could interact with regular matter while the horizontal axis gives the mass of the hypothetical dark particle. The areas in solid colours delimit the possible values obtained by the four experiments having a signal. Only CoGent and CDMS agree.

The lines show the limits placed on the allowed dark matter interaction rate and mass by some of the experiments that reported no signal. All values above those lines are excluded, meaning the null experiments are in direct contradiction with the four groups that reported a signal.

As frustrating as this might seen, it is in fact not surprising given the complexity of these experiments. It could be due to experimental flaws or there might be a theoretical explanation.

Many experiments are collecting more data and new ones are being built. With theorists and experimentalists being hard at work, hopefully there will soon be a breakthrough.

First part in a Dark Matter series:       How do we know Dark Matter exists?

Second part in a Dark Matter series:  Getting our hands on dark matter

Third part in a Dark Matter series:     Cosmology and dark matter

Fourth part in a Dark Matter series:  Can the LHC solve the Dark Matter mystery?

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline
 or sign-up on this mailing list to receive and e-mail notification.

Further information:

Hangout with CERN: The Dark Side of the Universe

TED Ed clip: Dark matter: The matter we can’t see

TED talk by Pat Burchat: Shedding light on dark matter

 

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18 Responses to “Getting our hands on dark matter”

  1. Matthew says:

    Correction: “we should feel a head wind of dark matter particles during the summer months and a tail wind during the winter” … if we live in the northern hemisphere.

  2. SHREEKANT says:

    The location[condition] where we are searching ‘the high impact of DM’ is correct? Our atmosphere is now almost STABLE.
    At present condition, it is true that little more impact seen in Xe than Ge/Si [here interaction with Nucleus taken, not Atom].
    It is also true that DM is more near the GALAXY & …., but what about other places?
    DA,DM are not far away to feel & their interactions with Baryon is not too complicated to understand.Is only air comes out when we fill water in empty bottle?
    The role of DE energy is interesting. The role of 94-96% is very important in this universe. Our present theory is only based on the knowledge of 4-6%

    • CERN says:

      Hello,

      of course, it is hard to say we know how to look for dark matter. Nobody know yet if and how it may interact with matter. So we test many hypotheses.

      Dark matter is also seen to be concentrated in galaxy centres so indeed, it is a good place to look.

      Outside a galaxy centre, the density of dark matter is much reduced. For example, in our Solar system, far away from the galaxy center, the quantity of dark matter within the Solar system amounts to 0.0000000000001 times the mass of the sun. So yes, when you fill up a bottle, it is essentially just air coming out. There is very little dark matter here to start plus it permeates regular matter anyway.

      I hope this helps, Pauline

  3. SHREEKANT says:

    Tnx. for such a quick response.

    It means our immediate surrounding [invisible part] contain only air? Then what about SUPERSYMMETRY & 96%?

    • SHREEKANT says:

      Telecommunications expert suggests Earth may have dark matter disc
      Jan 03, 2014 by Bob Yirka
      http://phys.org/news/2014-01-telecommunications-expert-earth-dark-disc.html#nwlt

      It is true that dark atoms are present everywhere including near the earth. But the sizes of resulting dark matters are smaller near the earth with respect to that found near galactic cores or stars.
      It is also true that the size of dark matters near equator is little larger than at other places on the earth except inside the earth. Dark atoms are always balancing white atoms to maintain super-symmetry. If we put water in a bottle, both dark atoms & air (white atoms) comes out. White atoms are swimming in the ocean of dark atoms. Dark atoms & dark matters have been continuously adjusting them self in universe. Stars are not the source of dark atoms but they are the warehouse of dark atom.

      http://swarajgroups.blogspot.in/2013/06/out-of-box-thinking-is-essential-in.html

    • CERN says:

      Well.. while it is true that dark matter is more concentrated in the center of galaxies (and therefore there is less dark matter around us on Earth since the Earth is far from the galactic center), I doubt anyone can confirm that there is more around the equator than the poles. And you are talking about dark matter atoms: we do not know if dark matter forms composite objects like regular matter. And when you say: smaller, you mean in fact less concentrated or fewer dark matter. But I am afraid many of your assertions there are pure speculations. And you might well be right when you say that stars are not the source of dark matter (not atoms) but just warehouses for them. In fact, I think this statement is more tru for galaxies than for stars. Dark matter palyed a role in galaxy formation but not in star formation.

      Cheers, Pauline

    • SHREEKANT says:

      Kindly refer to your response dated January 8, 2014 at 7:29 am, I want to contradict on following points:

      1. Dark matter is also made up of dark atom.
      2. Dark matter is more concentrated in the centre of galaxies but it is not the warehouse of them, it is the PRODUCTION HOUSE.
      3. Dark matter do not play a major role in galaxy formation, it is a part of GALAXY.
      4. Earth has less dark matter but it is not due to very large distance from galactic core.
      5. When I say smaller dark atom it means size not its concentration.
      6. Dark matter do not plays a direct role in star formation but its alignment will help in star formation just away the galactic core in DISC not in HALO.

      Yes, my assertions are appearing as pure speculation but all my comments (very short) on black holes, dark matter, supernova, gravity etc.(ref-below links) are on the basis of dark matter & dark energy.

      I can explain the cause of lots of phenomenon on which our scientists are working. I can give the evidences of my speculation by explaining the causes of gravity, formation stars, formation of planets, formation of moon, formation of dark atom, formation of dark energy, how dark atom are continuously interacting with the environment, why dark matter is more near galactic core?, what is the destination of dark atom formed in galactic core?, formation of electron ring around the earth, why comet is not the source of life on earth, why neutrino found more near Antarctica, why a molded metal object remain in the same shape ….. these are only the few.

      I am delighted if you or your team member will guide me, where & how I can place my detailed work to cross check or amend.

      US experiment to vote on dark matter 16 Oct 2013 | 01:10 BST | Posted by Eugenie Samuel Reich | NATURE NEWS BLOG

      New Kind of Dark Matter Could Form ‘Dark Atoms’ by Charles Q. Choi, SPACE.com Contributor Date: 10 June 2013 Time: 05:32 PM ET

      “Fat gravity particle gives clues to dark energy” Force-carrying ‘gravitons’ with mass could help to explain Universe’s accelerating expansion. Zeeya Merali 10 September 2013

      Observations reveal carbon monoxide “snow line” in exosolar system Posted on July 22, 2013 by Physics Today

      Hidden mantle material may help explain Earth’s origins Posted on July 17, 2013 by Physics Today

      The Earth’s Gold –”A Neutron Star Collision Was the Source” -The Daily Galaxy via CfA, July 18, 2013

      “An Unknown Force of the Universe is Acting on Dark Matter” (4th of July Feature)-July 04, 2013

      White dwarf star throws light on possible variability of a constant of Nature [2 hours ago-in Phy.org]

      http://swarajgroups.blogspot.in/2013/06/out-of-box-thinking-is-essential-in.html?showComment=1372490547602#c3779664559132610836

      Astronomers discover pulsations from crystalized dying star This discovery will allow scientists to see below the white dwarf’s atmosphere and into its interior. By McDonald Observatory at University of Texas, Austin — Published: June 24, 2013

      Spectacular Sun Storm Sheds Light on Star Formation by Mike Wall, SPACE.com Senior WriterDate: 20 June 2013 Time: 02:01 PM ET

      Giant Black Hole’s Dust Oddity Surprises Scientists by Clara Moskowitz, SPACE.com Assistant Managing EditorDate: 20 June 2013 Time: 06:00 AM ET

      Study explains decades of black hole observations 19 hours ago by Susan Gawlowicz

      Alien Life Unlikely Around White and Brown Dwarfs, Study Finds Charles Q. Choi, Astrobiology
      Magazine Contributor Date: 05 June 2013 Time: 07:00 AM ET

      A Rare Opportunity to Watch a Blue Straggler Forming by SHANNON HALL on JUNE 11, 2013
      Theorists weigh in on where to hunt dark matter May 22, 2013 by Lori Ann White
      http://swarajgroups.blogspot.in/2013/05/grains-of-sandfrom-ancient-supernova.html#comment-form

    • CERN says:

      Hello again,

      it’s great to hear that you have the answers or assertions to all these questions.But what matters most in science is tangible proofs. A theory, as elegant as it might be, remains just hypothetical until it is proven by hard facts. This is exactly what happened with the Higgs boson and the Brout-Englert-Higgs field. It all remained pure speculation until the LHC experiments discovered a type of Higgs boson in 2012. I agree with you: it seems logical to suppose that dark matter is also made of dark atoms, themselves being built of dark particles. This is a widely accepted idea in the physics community but no one can say it is so until he or she has proven it.

      The best place to expose your ideas is to submit your paper to the Physics archives: http://arxiv.org/ The whole physics community will then be able to see your ideas. It is certainly a more suited place than this blog. It is indeed the policy of the Quantum Diaries site to only accept short comments on the topics addressed in blogs.

      Cheers, Pauline

  4. veeramohan says:

    Why we are interested about Darkmatter, if it is only 0.0000000000001 times the mass of the sun, in our solar system – anyway without darkmatter, we will be safe ?
    Any Quantum entanglement from the centre of the galaxy ?

    • CERN says:

      Hello,
      in our solar system, far away from the galactic center, there is little dark matter. But overall in the whole Universe, it accounts for 27% of all there is out there. That’s why we care.

      I hope this helps, Pauline

  5. The above document claims: “we are talking about a completely different type of matter, something not made of quarks and leptons like all visible matter (humans, planets, stars and galaxies).”
    The problem is there exists not something like visible-matter. We have baryonic and non baryonic matter and we have baryonic matter that is visible (stars) and that is not visible (planets). The problem is when you do a simulation(calculation) using all matter that is visible the calculated galaxy rotation curve versus the observed galaxy rotation curve do not match. Performing the same calculation but adding extra matter that is not visible the two can match. This added extra matter has a certain density. When this density is below the Oort Cloud density then this extra matter becomes trully invisible at the same time making the need to include WIMP’s vanishing to zero.

    The article also claims: “If the Universe is full of dark matter, we on Earth should feel a wind of dark particles as we travel around the Sun. ” There are three conflicting issues: The amount of matter in the Universe, the amount of matter in any(our) Galaxy and the amount of matter in the Solar system. In each of this cases; matter is the sum of visible baryonic, invisible baryonic and nonbaryonic matter. For the solar system the amount of nonbaryonic matter IMO is very small otherwise it will influence the movement of the planets. Anyway if darkmatter is detected it will not solve the darkmatter problem.

    http://users.pandora.be/nicvroom/

    • CERN says:

      Hello Nicolaas,

      thank you for sharing your thoughts on this. Let me first clarify that I used the term visible and ordinary matter interchangeably. I mean there all matter described by the Standard Model. You, I and all the planets are made of visible matter. We reflect light and we also emit electromagnetic radiation (mostly in the infrared range as heat).

      Yes, the Universe is full of dark matter but you are right to point out that its density varies greatly. It is very low in the solar system (hence it does not affect the motion of planets and Kepler’s third law for the rotation speed of planet is still valid) but very high in the galactic center.

      Cheers, Pauline

  6. Amir Livne Bar-on says:

    How is dark matter distributed in the space around a galaxy? From the responses in in this post I gather that it’s much denser near the galactic center, But from the rotation-speed graph in the previous post my intuition about rotation tells me it should have a uniform density in a volume much larger than the galaxy.

    Is my intuition wrong? How can an addition of mass to the center cause the angular velocity to be the same for stars in different orbits around the center?

    Is there any other kind of evidence for the distribution of dark matter in some galaxies, or in out own galaxy? Do we know if it come in large clumps like normal matter is made mostly of big stars that are wide apart?

  7. CERN says:

    sorry, could you be more explicit (i.e. give me the link of interest) I am afraid I do not get what you want me to look at.

    Thanks, Pauline

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