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CERN | Geneva | Switzerland

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Can the LHC solve the dark matter mystery?

Last part in a series of four on Dark Matter

After reviewing how dark matter reveals its presence through gravitational effects, the lack of direct evidence of interaction with regular matter and the cosmological evidence supporting its existence, here is what the Large Hadron Collider (LHC) at CERN can do.

We can find dark matter with the LHC but only if dark matter interacts with regular matter. Since we do not know how this may happen, we design traps suited for as many beasts as there are theories. Here are a few.

Supersymmetry

The current theory describing particle physics is the Standard Model. It has been extremely successful, explaining just about everything observed so far. Unfortunately, at higher energy, its equations start to break down.

This is why theorists developed Supersymmetry (or SUSY), building on the Standard Model and extending it further. What is truly remarkable is that this new theory invented to fix the flaws of the Standard Model predicts the existence of particles with the properties expected from dark matter, hence its great popularity.

All would be perfect except that no one has detected any of the many expected supersymmetric particles. This might simply mean that these particles are heavier than the current LHC reach. We will have more chances of discovering them once the LHC resumes in 2015 at much higher energy.

The lightest supersymmetric particle

In the LHC, protons collide, producing large amounts of energy. Since energy, E, and mass, m, are two forms of the same essence as stated by the famous E = mc2, energy can materialise into new particles.  Heavy particles are unstable and quickly decay into lighter ones.

Some variants of SUSY predict that all supersymmetric particles must decay into other supersymmetric particles. Under this assumption, the lightest SUSY particle cannot decay into anything else and remains stable, not interacting with anything else just like dark matter is expected to be.

sqark-cascade

A typical decay chain is shown above. A supersymmetric quark decays into another SUSY particle, χ2, and a normal quark, q. At the two subsequent stages, an electron or muon (denoted l+ and l-) and lighter SUSY particles are produced. The lightest one, in this case a particle called neutralino χ1, cannot decay into anything else and escapes the detector leaving no signal behind.

Seeing the invisible

An event is a snapshot capturing all lighter particles emitted when an unstable particle decays. And within each event, the energy needs to be balanced. So even when a particle flies across the detector leaving no signal, it can still be detected through the energy imbalance in the event. Invisible particles such as the lightest supersymmetric particles can be detected this way.

Both the CMS and ATLAS collaborations have been looking for events containing large amounts of unbalanced energy accompanying a single photon or a single jet (a jet is a bundle of particles made of quarks).

MET-photon-ATLAS

This figure displays an event from the ATLAS experiment containing a single photon (the energy deposit is shown in yellow around 4 o’clock on the left picture) and the missing energy represented by the pink dashed line around 10 o’clock.

This is exactly what an event containing the lightest supersymmetric particle and a photon would look like. But an event containing a Z boson and a photon would look just the same if the Z boson decayed into two neutrinos (other particles that do not interact with the detector).

Unfortunately, nothing has been observed in any of the channels studied so far that is in excess of what is expected from the background, i.e. other known types of events giving similar signatures.

Unlike the direct dark matter searches, the LHC analyses are sensitive to light dark matter particles. Remember the messy plot I showed about direct searches for dark matter? CMS and ATLAS can help clarify the situation, although their results depend on theoretical assumptions when the direct searches don’t.

Below are the CMS results for a search of events containing a single jet and missing energy.  The horizontal axis gives the mass of the dark matter candidate and the vertical axis, the allowed interaction rate with ordinary matter. Everything above the various lines is excluded. CMS (solid red line) exclude light dark matter particles for large interaction rates, a region inaccessible to XENON100, (solid blue curve) the most powerful experiment for direct dark matter searches.

SpinIndependent_woScalar-CMS

The Higgs boson and dark matter

Another approach to find dark matter relies on some theories that predict that the Higgs boson could decay into dark matter particles. Higgs bosons can be produced with another boson, such as with a Z boson. If the Higgs boson decays to any type of dark matter, we would only see the decay products of the Z and missing energy for the Higgs boson. Searches for such decays have so far not revealed anything above the expected background level. inv-Higgs

A dark parallel world

A group of theorists developed an amazing Theory of Dark Matter incorporating ideas of a Hidden Valley where two worlds would evolve in parallel: our world with Standard Model and the yet undiscovered supersymmetric particles, and a dark world populated with dark particles as depicted below, where each horizontal line represents a particle of a given mass.

HiddenValley

The idea is that the LHC could produce heavy supersymmetric particles. These particles would decay in a cascade into lighter ones down to the lightest SUSY one. That particle would be a “messenger” capable of crossing over the Hidden Valley, escaping into the dark sector and becoming invisible to us.

In the dark sector, this particle could decay in a cascade into lighter dark particles until it reaches the lighest supersymmetric dark particle, another messenger capable of tunnelling back to our world where it would reappear into many pairs of electrons or muons.

This may sound like pure science fiction but it is all rooted in sound, but still unproven, physics as a quick check with the original papers cited above will demonstrate.

I was until recently one of the experimentalists looking for signs of this Hidden Valley, selecting events containing regrouped pairs of electrons and muons but so far, nothing has been found.

Experimentalists are still looking, there and in many other places, constantly refining their searches and trying new strategies. If dark matter interacts with matter, we ought to find it.

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

Pauline Gagnon

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27 Responses to “Can the LHC solve the dark matter mystery?”

  1. Ivan Kirkpatrick says:

    I find this Dark Matter subject to of extraordinary interest…..

    Back in my college days I remember discussions with my friends regarding the missing mass of the universe and how Isaac Asimov suggested it was neutrinos that would be found to make this missing fraction.

    I really appreciate your writing. Are you working on a book? Maybe even SciFi?

    • CERN says:

      Dear Ivan,

      thanks for letting me know you appreciate these blogs. I might consider writing a book, why not?

      Neutrinos have been considered for a long time as dark matter candidate. Now we know they have a mass although very small so that could do if there are lots of them. But they would belong to a broad category of dark matter candidates called hot dark matter. These are for particles moving near the speed of light, contrary to slow moving WIMPs of Cold Dark Matter theory, the theory that is most popular and now supported by the Subaru observation. Hot dark matter is not favored for many reasons but I am not an expert on this and cannot say much more on this except that it was seriously considered.

      Cheers, Pauline

  2. Shane Huntet says:

    What about looking for dark matter repeling matter instead of interacting with matter? You could look for unexpected movements of particles moving in a straight line. I am not a physicists and I certainly could not do the math to prove it out but it seems to me that if matter and dark mattet repealed each other it would help explain some of the phenomenon that baffles us now,o like the galaxies speeding up heading to the edges of the known universe an why galaxies don’t fly apart. It maybe dark matter only works on a macro scale and that is why the expected traces of it on such a small scale cannot be found.

    • CERN says:

      Hello Shane,
      as far as I can see, the direct searches for dark matter (looking for a dark matter particle recoiling against a regular matter nucleus) would be sensitive to attractive and repulsive forces. But what has been observed so far in the computer simulations re-enacting the evolution of the Universe since the Big Bang and the formation of galaxies, is that dark matter first formed small structures, then attracted regular matter. So one of the way it interacts with matter gives an attractive force (gravitational force here). But I agree with you that it will be nice to get a clearer picture.

      Cheers, pauline

  3. John says:

    Why are scientists convinced that the answer to the dark matter mystery will be would in particle physics?

    • CERN says:

      Sorry for the late reply. This is a good point. We think so because dark matter has to be made of something. This something may or may not be made of particles, just like regular matter. If so, particle physicists are the most appropriate people to look for it.

      But of course, cosmologists and astronomers can also contribute information and clues. This search is becoming very multiidisciplinary already.

  4. veeramohan says:

    Without a geometry (reference frame) there was no closed system of baryons and leptons – in the early universe – otherwise it was became undetectable (in reference with c) like dark matter.
    More momentum more mass (lot*)- so it is relativistic. #2
    Because dark matter doesn’t collide in the traditional sense, it has no way to shed its momentum and angular momentum (in the form of emitting photons) the way normal matter does.
    Focusing on the evolution of angular momentum – not only is individual particle angular momentum not conserved, but the angular momentum of radial shells also varies over the age of the Universe by up to factors of a few. It was found that torques from external structure are the most likely cause for this distribution shift.

    Frictional forces among the baryons have the general effect of removing angular momentum from baryons that have little and transferring it to baryons that have a “lot”*. Dynamical friction of dark matter on clumps of baryonic matter has the general effect of transferring angular momentum from the baryons to the dark matter.

    So the relativistic mass could be changed by the dark matter ?

  5. Suresh Kumar says:

    What does the yellow arrows indicate here. If it represents transition from one to another, then what is the role of valley does it enhances the transition rate or act like barrier. And if valley has a finite range in mass dimension, so what one would expect beyond this range. It would be nice to put the interpretation of diagram as much thought must have gone in this depiction.

    • CERN says:

      Hello Suresh,

      the yellow arrows symbolise the possibility that the lightest supersymmetric partciles (both in the visible world and in the dark sector) are messengers that can go through the barrier of the Hidden Valley and access the other sector. So the Hidden Valley is like a huge barrier that prevents particles from the visible sector to go to the dark sector, and vice versa. Only these two messengers, the lightest SUSY particles from each sector, can travel through the barrier and access the other side.

      The way I understood the Hidden Valley concept is that is is an infinitely high barrier and nothing can cross it except these special messengers.

      This particular picture is based on what some of the theorists who developed this theory drew themselves and partly my own visualisation of it. But until we find a proof of its existence, all this is just pure speculation.

      I hope this helps, Pauline

  6. maurice folliot says:

    if math is not a sufficient means to explore the truth should thought in pure consciousness be developed to explore the physical and any other forms of that exist in the universe or infinite multiverse?

    • CERN says:

      Hello Maurice,

      as I wrote in the blog, theorists are using mathematical tools to try to tackle these puzzling issues. But even though it may sound as science fiction, it is still dealt with using scientific tools such as mathematics to describe the reality and experiments to confirm it. You are asking your question to the wrong person. What you propose is not in the line of thinking of a physicist. So you must suggest it elsewhere…

      Cheers, Pauline

  7. Torbjörn Larsson, OM says:

    Maybe I missed it, but I didn’t see the observed cosmological bound on WIMPS of > 40 GeV (IIRC). That, and XENON100, puts a decided cut into the “WIMP miracle”.

    • CERN says:

      Indeed, I did not put that in. But the Xenon100 bounds do not rule out everything. It strongly limits the possibilities tough as you point out.

      Cheers, Pauline

  8. Hi Pauline,

    I am just wondering why this model is preferred to the model where SUSY particles actually are the dark matter?

    • CERN says:

      Hello David,

      there are lots of different models proposed by theorists. Everybody is trying to provide something coherent and that would fill the bill. SUSY is one of the most popular model but it comes in several different forms. The model I described here is just one of many. And in SUSY, not all particles are dark matter, only the lightest supersymmmetric one, the one that cannot decay into anything else, is the best candidate to be a dark matter particle.

      The model I described here contains supersymmetry but attempts to give a more general picture.

      Cheers, Pauline

  9. JL. Picard says:

    hi!

    just a question from a (relative) layman:

    the consensus of the scientific community is – apparently – that dark matter (I mean the mystery one, that explains why stars at the edge of galaxies have same linear speed as those at the centre – I believe you guys call it “cold dark matter”) is made of WIMPS hence those underground experiments trying to capture a few of those particles

    but since essentially nothing is known of dark matter beyond its gravitational effects, how do you know they are WIMPS? ie. who said they have to interact with the weak force at all?

    and related question: IF dark matter only interacts with gravity, does this mean the search is doomed?
    ’cause that would be essentially akin to trying to catch ghosts wouldn’t it? since such particles would be completely impossible to detect (much less contain)

    • CERN says:

      Hello,

      indeed, there are numerous proofs today pointing towards cold dark matter, that is dark matter that does not travel at the speed of light or near it like neutrinos. WIMPs are still just one hypothesis. It has neither been proven right or been ruled out. The underground experiments are precisely trying to establish this fact. If they are weakly interacting particles, underground experiments or the LHC experiments should see it. So this is all work in progress.

      And you are right: if dark matter only interacts with gravity, the search for catching dark matter particles would be doomed.

      I hope this helps, Pauline

  10. Michael says:

    How come we can’t Catch dark matters using zero gravity
    I watched a documentary where it said dark matter was flowing through us and being constantly pulled though us with gravity.. Well if there is no gravity would it be easier to attain

  11. Tom says:

    Thank you for a very informative series of articles. This may seem a very naive question but if dark matter only interacts via gravity shoudn’t one expect it to clump together to form larger structures ie like “dark stars”, or sink to the centre ordinary stars and interact with black holes. Given that there is supposed to be so much of it out there is seems very passive!

    • CERN says:

      Sorry for the very delayed response. I was trying to get a more exact answer form an expert in the filed but have not heard back yet from my colleague. Dark matter is known to cluster in the center of galaxies but I have not heard specifically about stars made of dark matter. It is know that dark matter played a catalyst role in galaxy formation but it is not quite the same for stars. So then I would think this means dark matter clusters more on galactic scales than star scales.

      Sorry I cannot be more definitive on this. I will try to answer more precisely later on. Cheers, Pauline.

  12. Mr Roy mullings says:

    I have read your blogs and enjoyed them. I have no qualifications but have a theory that the weight of the heavy dark matter crushes the light dark matter which distorts the fabric of the matter which creates more dark energy hence the rapid expansion of the universe. As I said I have no qualifications but hope this is relative to your blog which I found interesting . Read the 4 parts

    • Mr Roy mullings says:

      Also this would in my opinion explain the creation of black holes! Ok I don’t know but think that add the gravitational pull of any mass dark or visible and the repulsion in polarity of all mass. Sorry if it makes no sense and if it has wasted your time

    • CERN says:

      And the creation of black holes is very well understood. It comes from a star that collapses unto itself under its own gravity. Nothing to do with dark matter… You’ll find all details on wikipedia: http://en.wikipedia.org/wiki/Black_hole

      Cheers, Pauline.

    • CERN says:

      Glad you enjoyed these blogs. Unfortunately, one cannot just make theories like that. They have to be grounded on what we already know from experimental facts. We cannot just throw ideas up in the air like that. At least, not in science. So for your theory to have any merit, we would have first to establish if there is heavy and light dark matter. There is no scientific proof that such things exist, we just know there is dark matter. There is no evidence that it comes in two types, light and heavy. Then distorting the fabric of the Universe is not known as the source of dark energy. So I am afraid I cannot say more than this… But of course, theorists do it all the time: they propose ideas they think could explain what we see. But these ideas have to respect all the constraints of what is known so far, and that’s the huge challenge. As a general rule, if it’s that easy, someone else who spends all his or her time on it should have thought of it…

      Cheers, Pauline

  13. Stalin Beltran says:

    Hi Pauline,

    I have a theory that could explain pretty much all I have read in the Internet about Dark Matter, but it goes just in the opposite direction of today’s efforts, so I prefer get some physical support before releasing it. I am a software engineer and I lack the scientific background to solve it by myself.

    Because of that, my only option could be find support by computer processing. What I need is a software to find numeric solutions to Einstein’s equations (one able to run on domestics computers -I know I’m asking too much). Could you help me with this, please? Any help is welcome.

    Thanks in advance

    PD: I am not good in English, I apologize for any mistake above.

  14. Jacob Owens says:

    Hello, fantastic information and an exciting article post, it’s going to be exciting if this is still the case in a few years time!

  15. Layla Campbell says:

    I seldom comment on these items, but I thought this on deserved a well done

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