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Dark Matter Searches at Colliders – part III May 6, 2008

Posted by dorigo in cosmology, physics, science.

Long overdue, here is the final part of a long post on the searches for new particles that may be the solution of a long-standing problem in astrophysics today: the missing mass in our Universe.

The large majority of cosmologists have become convinced, through the analysis of masses of data collected in the last two decades, that four-fifths of the matter in the Universe is non-baryonic. If we neglect particles which can only be created in high-energy collisions and decay in ridiculously small amounts of time, Baryons exists in just two forms: protons and neutrons. These make up the nuclei of atoms, and provide the fuel for stars to shine as they fuse into helium nuclei.

Non-baryonic matter does exist, and we know it well: we have electrons and neutrinos; but these are irrelevant. Electrons weigh less than a thousandth of a proton -and there are just as many electrons as protons around, to a very good approximation. As for neutrinos, despite our ignorance on their mass, they cannot make up the deficit of mass observed in the rotation speed of galaxies (exhibit one in support to Dark Matter: the speed of rotation does not decrease as much as it should if their mass was concentrated in stars) or in clusters of galaxies (exhibit two: gravitational effects we may detect visually do not match the observed distribution of galaxies in these agglomerates).

One intriguing solution to the problem lies in hypothesizing that a massive particle called neutralino wanders around in huge amounts, slow and unbothered by its close encounters with ordinary matter. Neutralinos would be electrically neutral, they would not interact strongly with matter, and they would be perfectly stable, lest they violate a very convenient quantum-mechanical conservation law. For more details on these hypotheses, see part II of this post.

So how can collider experiments detect this evanescent particle ? By producing pairs of higher-mass supersymmetric particles, which would chain-decay into non-supersymmetric ones plus a pair of those lightest supersymmetric particles, LSP. On the right you can see a decay chain whereby a gluino – a SUSY particle produced in large amounts in hadron collisions, due to its strongly interacting nature – emits a squark, the squark in turn emits another quark and decays into an excited neutralino, this emits a slepton, and the slepton ends up producing the lightest neutralino. All in all, from each of these chains (one per decay of each of the produced gluinos) one should observe two jets of hadrons from the quark hadronization, two leptons, and some missing energy. The missing transverse energy stolen by each neutralino would add as two vectors add in a plane: only rarely they would cancel each other out. In the graph below, for instance, two neutralinos leaving in different directions (the two dashed lines pointing towards the upper and lower left, in the transverse cut-away view of the ATLAS detector) would create a missing transverse energy vector pointing roughly mid-way between their exit directions.

The Tevatron experiments have searched for these events in their Run II data. The search in CDF considered the signature of two, three, or four hadronic jets plus a significant amount of missing energy from the neutralinos. This signature can be mimicked very effectively by the frequent, generic production of many jets by quantum chromodynamics interactions between quarks and gluons; the missing energy is thus required to be large and significant to suppress these processes.

The CDF experiment applied three different sets of selection cuts on their data to seek sensitivity to different regions of the parameter space of Supersymmetry. Indeed, as the mass of gluinos, squarks, and sleptons varies, so does the visible final state. For instance, if squarks and gluinos have a similar mass one is unlikely to detect a hadronic jet from the quark that is emitted in the transformation of the former into the latter. The signature pf pair-produced gluinos then more closely resembles one with only two jets and missing energy.

The figure on the right shows the final selection of the data in one of the three search regions. It is clear that known Standard Model processes provide a good modeling of the observed distribution of missing transverse energy in the data (black points with error bars), whereas a supersymmetric signal (the empty histogram in green, overlaid to SM contributions) would have instead stood out and created a disagreement.

From the distributions an upper limit can be extracted on the amount of signal contained in the data, and from the latter a limit is obtained in the cross section of gluino pair production: this translates into a mass exclusion range for squarks and gluinos. The final summarizing plot is shown below.

The plane is spanned by the mass of the two hypothetical particles. Colored areas have been excluded by different experiments; the CDF search extends the excluded region by the size of the red-painted area. We thus learn that gluinos cannot be lighter than 300 GeV, whatever the squark mass, otherwise CDF would have seen a bunch of anomalous events with large missing energy and jets.

The Tevatron protons and antiprotons do not have enough energy to investigate supersymmetric particles of mass much larger than the limit discussed above: so if Supersymmetry is the right theory of Nature, it may turn out to be the job of the Large Hadron Collider to discover it. With its 7-fold increase in energy and hundred-fold increase in interaction rates, the LHC is expected to provide a clear-cut answer: discover supersymmetry, or rule it out for good. As you can see in the plot below (where the plane is spanned by two convenient parameters among the multitude of choices: M_0 and M_{1/2}), the discovery reach of the CMS experiment extends to mass values in excess of a TeV – where supersymmetric particles would be close to useless, because they would not have a chance to solve the problems of electroweak symmetry breaking for which they were once invented.

The graph is complicated and it requires some more explanation: the blue areas are excluded by theoretical constraints and experimental searches, and the green area is also excluded. The colored wavy lines show instead the limits that CMS will be able to set in the plane -intending it will exclude anything to the left of the curves – with different searches, labeled by their respective “smoking guns”. The red curve is labeled E_T^{miss} for missing transverse energy, and it is one of the most performant in excluding the parameter space.

So, indeed, CMS and ATLAS will have an easy way to find signals of supersymmetry across the table -the wide space of parameters: they just need to study their distribution of missing transverse energy, just as we saw CDF do in the analysis mentioned above. The fanthom signal of a neutralino, which cannot interact with the detector and leaves unseen, turns out to be more striking at the end of the day than the multitude of jets and charged leptons the pyroclastic Supersymmetric production events would give rise to. Seeing events with a large amount of missing transverse energy would not allow us to determine which form of supersymmetry we are dealing with – whether a minimal supersymmetric extension of the Standard Model with two higgs boson doublets, or more complicated schemes. However, it would still allow us to claim that we have evidence for THE candidate particle which constitutes 80% of the stuff the Universe is made of.

I need to warn the reader here: of course, ATLAS and CMS have already studied dozens of methods, some of which are quite complicated, to extract very detailed information on Supersymmetry and very clean signatures of its presence from LHC data. These analyses focus on kinematical properties of the supersymmetric decays which are very model-dependent, and very complicated to explain. Although I reported about these methods in my seminar, I take the liberty here of jumping ahead a little…

So what instead if SUSY is not, after all, the right idea ?

Despite the general enthusiasm of theorists, phenomenologists, and other assorted believers, in fact, we have to keep a cool mind. Let’s review the cost of the purchase we have to make if we are to marry Supersymmetry:

  • twenty brand-new particles, never before seen
  • at least 104 new parameters, whose value is unknown and to be determined by improbable experiments
  • a strict conservation of R-parity, the number you get by adding together spin, baryon, and lepton number in a suitable combination – the combination allows the proton and the lightest neutralino to remain stable
  • We also have to agree that despite the fact that in principle the Tevatron and LEP colliders could have well stumbled into Supersymmetry, they haven’t – new physics chose to hide in the far away corner, just like the small coin that you dropped from your pocket.

Some of us think the above is too much to buy, for a theory which “solves” the mystery of a unnaturally small mass of the Higgs boson (provided the Higgs exists and is light as every evidence still suggests) and which collapses two crossings between running coupling constants into one single point. Ockham’s razor comes a-slashing: “entia non sunt multiplicanda praeter necessitatem“, one must not multiply entities. The most economical explanation is the best one… The razor cuts unnecessary entities.

One should mention, at the end of this long post which focused on the searches for just one candidate for dark matter – the one which hadron colliders may have a chance to find, the neutralino – that there is a long list of alternatives, of many flavors: kaluza-klein gravitons, sneutrinos, gravitinos, little higgses, axions, primordial black holes, charged massive particles, heavy neutrinos, sterile neutrinos, you name them.

It is for this very reason that in the end, LHC searches will require to follow the very important two-step procedure outlined by M.Mangano in a recent paper: first establish that an anomaly exists in the data, and only after it has been demonstrated to be utterly unexplainable by known phenomena, proceed with an exotic explanation.

To conclude, dark matter candidates have been searched at past and present collider experiments with no success. LHC appears to have the right energy and the potential to finally discover the source of this astounding enigma. In any case, we will know in a few years whether Supersymmetry is real or just a crazy concoction. If SUSY exists, new accelerators will be needed to investigate it in detail, but if it doesn’t, particle physics may be at a dead end. Despite this threatening possibility, we have extremely exciting years ahead of us!


1. forrest noble - May 6, 2008

Hi Tomasso,

Hope you’re still not typing with one hand. Not cool for someone who needs to be on the computer as often as you do. Nice graphics above. I tried to follow some of it the best that I could.

As you know, I believe that the standard model of particle physics has got it wrong. One of the reasons that I believe this is true is also because of Ockham’s razor “entia non sunt multiplicanda praeter necessitatem“, one must not multiply entities. The most economical explanation (all else being equal) is (most often) the best one… The razor should cut (clear the board of) unnecessary entities.

Primarily for this reason I do believe there is just one fundamental particle (and no more) which for, which has been called the god particle by Leon Lederman, for one. We do have a lot of knowledge concerning stable particles, fermions which are the constituents of all matter. Although we have Murray Gell-Mann’s mathematical system involving quarks and gluons, I believe this system is completely unnecessary excepting for analytic purposes.

To explain all of reality according to Ockham’s razor, it seems to me, all one needs is a basic particle to perpetuate time, substance and extension, which is change. I would assert the none of the so-called fundamental forces are needed to explain anything, since they can all be explained by physical engagements or the field motion of particles such as gravity and magnetism.

The only primary characteristics, as the building block of all matter, that this particle would need would be the cause of particle spin and has the physical form that allows for self, and other particle engagements. All the other theoretical particles such as quarks and gluons are not needed as other string theorists assert.

All the other real but short lived particles are ultimately not needed to explain reality. They accordingly would be the effect of certain reactions and the primary cause of none.

Mass, and therefore gravity, can be simply explained as to its cause as the result of pushing gravity. From this elementary particle only the creation of two fermions need be explained, the proton and the electron. All the rest of the elements in the universe can be explained by both fusion and fission processes within stars which also create neutrons. Positrons have been shown to be stable particles and need to be explained, with only spin difference between them and electrons. Anti-protons are not stable particles unless their spin is magnetically reinforced. There is evidence to support this assertion.

This, I believe, is not only the ultimate Ockham’s razor and the simplest possible explanation, there is also considerable evidence to support it, such as the fact that quarks and gluons have never been observed as separate entities. The simplest of theories, I believe, will never be disproved because it is the simplest of conceivable possibilities and therefore a more likely truth.

your friend forrest

cheers in Italy is primarily vino, right?

2. jtankers - May 7, 2008

LHCConcerns.com will pay $500.00 US to the best proposal that can reasonably prove 5% or less Risk of Planetary destruction from Micro Black Holes.

The contest will conclude in a vote by site visitors on all reasonable proofs received, all proofs will be published and the contest will end not sooner than May 20th. (LHCConcerns will make the final call on best proposal that reasonably proves 5% or lower risk from micro black holes being created by the Large Hadron Collider).

You may prove that ANY ONE of the following or provide any other reasonable Proof or method to prevent Micro Black Holes from being created by the Large Hadron Collider or prove that they are harmless!

1. The Large Hadron Collider will not make micro black holes.
2. Micro black holes created will be sent safely into space.
3. Micro black holes will evaporate.
4. Micro black holes will take more than 2 billion years to accrete the Earth. (If you can only prove a lesser time frame, then the prize will be reduced proportionately…)
5. Any form of cosmic ray argument that proves 5% risk or lower.
6. Find a way to make the Large Hadron Collider safe from creating micro black holes (we already requested different speed collissions or different mass collisions, LSAG told us it was not possible, they already thought of it).

It is harder than it looks, the LHC Safety Assessment Group (LSAG) could not produce a safety report… (CERN and LSAG are still using the 1999 RHIC safety report that does not even address what might happen if micro black holes were created, because they did not know that it was possible at that time. We are also being generous on the 2 billion years, we want to be reasonable)


3. jtankers - May 7, 2008

fyi: “forrest” is a disinformation campaign… if you can’t win on science, then try confusing the issues…

4. island - May 7, 2008

Great post, “Hunt-n-Peck”.

Tommaso said:
Some of us think the above is too much to buy,

Well, I should hope so, good grief!

If there isn’t a big push to again look back into fundamental physics for something that got missed, then they get what they deserve, and I hope that funding for this load of crap rapidly sinks like a rock to the bottom of the sea when it doesn’t work.

Dirac’s inability to unify GR and QM as he had done with SR and QM should have clued them that rationalizing the negative energy states was not the way to go.

It doesn’t take a rocket scientist to see this.

If these theoretically righteous morons go off on a tangent trying to PUSH theory, then I’m gonna freaking flip. Just how complex can you make something before it finally hits you in the face that nature ain’t that complicated, dudes!!!

5. dorigo - May 7, 2008


antiprotons are perfectly stable, and whatever “evidence” you may bring to prove otherwise is a wrong, since it is falsified by a mountain of facts.


6. dorigo - May 7, 2008


your endeavour is despicable. The LHC will not create black holes, and you will soon have to invent another doomsday scenario if you like what you are doing.


7. dorigo - May 7, 2008

Hi Island,

despite my hostility with SUSY, I feel I have to tone down the criticism a bit here: the effort of SUSY theorists is not one to be too critical about. It is a reasonable theory that solves a very awkward problem with electroweak symmetry breaking. That is insufficient justification to me and to you, but it is a valid try.


8. dorigo - May 7, 2008

And forrest, in Italy we toast with wine, sure, but we do it just as well with other alcoholic beverages.


9. Guess Who - May 7, 2008

Hi TD, last night I tried to remind you of


but no luck, apparently the comment was blocked (spam filter acting on text/link ratio?). Trying again; it’s not a scenario I like but it’s logically possible, so even if no SUSY is found at the LHC, you know it won’t really be ruled out for good.😦

10. dorigo - May 7, 2008

Thank you GW, indeed this post was in the spam filter too, but I recovered it… Yes, it must be the t/l ratio indeed.

yes, split susy… I know we cannot really rule anything out. We content ourselves with 95% C.L. limits for a reason, after all😉

In any case it is a good point. I tried to be rather clear-cut in the post, and ignored this possibility. I honestly think split susy is not going to be very popular after we find no regular SUSY at the LHC… I want to see just how much more unsupported theories are physicists willing to buy in order to avoid their research field be wiped out of the board…


11. arcadianfunctor - May 7, 2008

correction: many more…. and then switch the order of are physicists. Finally, it’s wiped OFF the board. Cheers.

12. forrest noble - May 8, 2008

Most of my postings and theories have been prefaced with Non-conventional or alternative theory (NCT) or similar wording like I believe, I think, in my opinion, etc. including this one of course and all of my assertions below which may apply. I try to always qualify my statements (I am forgetful) but since most of my days are spent writing technical papers and theories, these qualification statements are assumed by the reader. For non-personal assertions URL’s are presented.


We know each other on-line from your site.
I’m not trying for the prize but the answer to your question, is #1
The Large Hadron Collider will not make micro black holes; going further than that, I would say the a collider could never create a black hole. The energy of the compression needed must be equal on all sides in a spherical compression. Accordingly, a black hole has a physical diameter and is entirely composed of dark matter. If one starts with baryonic matter it first would be crushed into coiled strings of dark matter particles that must be entangled solely based upon perfectly uniform surrounding pressures. It could have no more mass or gravitational influence than the beginning baryons had, nor could it grow in size anymore than the few protons which might be its mass equivalent. If one were ever created in a lab its density would reduce its size to maybe a thousandth the diameter of an electron and it would fall to the earth maybe at 32ft. per second, as does all larger matter. Once it hit the ground, it would very slowly sink through it because it still would have little mass and still could be pushed around by greater forces. Some day it might end up in the center of the Earth where it would settle. It might last as long as a few billion years before it would come apart to become an ordinary part of the surrounding dark matter field which surrounds all matter in a vortex.

This is your question, so I can’t be confusing the issue. Alternative theory is alternative theory, nothing to be concerned with unless you find value in it. Then you could contact me. You already have my e-mail address. Interesting proposal and question Jim.


The only evidence that I know of concerning anti-protons other than theory is that the best containment to date requires a cyclotron which would inadvertently reinforce an anti-proton’s spin rate. To my knowledge, the greatest half life yet achieved is 18 days. There is no proof that this particle loss rate is solely due to interaction with ordinary matter. This is only a theory. The alternative theory is that they are not stable particles.


If you have more current info please advise me, thanks

As to the wine, I’m going down to the bar to get mine right now.
and yes it’s not the first time ——- or the last, I hope.

your friend forrest


13. dorigo - May 8, 2008

🙂 Kea, thank you for the corrections. I was writing a bit too fast and did not stop and think!


14. dorigo - May 9, 2008

Hi Forrest,

I am sorry I could not find time to get some references for you about antiproton traps. And the pdg site is down – I was hoping to get some information for you there. Maybe tomorrow…


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[…] in jets and missing transverse energy. I also discussed these results recently in this blog, in a recent series of posts about dark matter searches at […]

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