What’s the Matter with Dark Matter, Matt?

It’s very rare that I disagree with Matt Strassler. That said, I can’t help but think that, when he criticizes the press for focusing their LHC stories on dark matter, he’s missing an important element.

From his perspective, when the media says that the goal of the new run of the LHC is to detect dark matter, they’re just being lazy. People have heard of dark matter. They might have read that it makes up 23% of the universe, more than regular matter at 4%. So when an LHC physicist wants to explain what they’re working on to a journalist, the easiest way is to talk about dark matter. And when the journalist wants to explain the LHC to the public, they do the same thing.

This explanation makes sense, but it’s a little glib. What Matt Strassler is missing is that, from the public’s perspective, dark matter really is a central part of the LHC’s justification.

Now, I’m not saying that the LHC’s main goal is to detect dark matter! Directly detecting dark matter is pretty low on the LHC’s list of priorities. Even if it detects a new particle with the right properties to be dark matter, it still wouldn’t be able to confirm that it really is dark matter without help from another experiment that actually observes some consequence of the new particle among the stars. I agree with Matt when he writes that the LHC’s priorities for the next run are

  1. studying the newly discovered Higgs particle in great detail, checking its properties very carefully against the predictions of the “Standard Model” (the equations that describe the known apparently-elementary particles and forces)  to see whether our current understanding of the Higgs field is complete and correct, and

  2. trying to find particles or other phenomena that might resolve the naturalness puzzle of the Standard Model, a puzzle which makes many particle physicists suspicious that we are missing an important part of the story, and

  3. seeking either dark matter particles or particles that may be shown someday to be “associated” with dark matter.

Here’s the thing, though:

From the public’s perspective, why do we need to study the properties of the Higgs? Because we think it might be different than the Standard Model predicts.

Why do we think it might be different than the Standard Model predicts? More generally, why do we expect the world to be different from the Standard Model at all? Well there are a few reasons, but they generally boil down to two things: the naturalness puzzle, and the fact that the Standard Model doesn’t have anything that could account for dark matter.

Naturalness is a powerful motivation, but it’s hard to sell to the general public. Does the universe appear fine-tuned? Then maybe it just is fine-tuned! Maybe someone fine-tuned it!

These arguments miss the real problem with fine-tuning, but they’re hard to correct in a short article. Getting the public worried about naturalness is tough, tough enough that I don’t think we can demand it of the average journalist, or accuse them of being lazy if they fail to do it.

That leaves dark matter. And for all that naturalness is philosophically murky, dark matter is remarkably clear. We don’t know what 96% of the universe is made of! That’s huge, and not just in a “gee-whiz-cool” way. It shows, directly and intuitively, that physics still has something it needs to solve, that we still have particles to find. Unless you are a fan of (increasingly dubious) modifications to gravity like MOND, dark matter is the strongest possible justification for machines like the LHC.

The LHC won’t confirm dark matter on its own. It might not directly detect it, that’s still quite up-in-the-air. And even if it finds deviations from the Standard Model, it’s not likely they’ll be directly caused by dark matter, at least not in a simple way.

But the reason that the press is describing the LHC’s mission in terms of dark matter isn’t just laziness. It’s because, from the public’s perspective, dark matter is the only vaguely plausible reason to spend billions of dollars searching for new particles, especially when we’ve already found the Higgs. We’re lucky it’s such a good reason.


6 thoughts on “What’s the Matter with Dark Matter, Matt?

  1. ohwilleke

    Dark matter theories are every bit as troubled as modified gravity theories right now. There are a lot of hard experimental constraints on dark matter properties, and they produce, very nearly, a null set. In particular, experimental constraints very strongly disfavor WIMP dark matter in the GeV and up mass range that SUSY and string theory would tend to favor.

    For example:
    1. Cold dark matter theories imply a NFW halo shape which is not what is observed. Real halos are isothermal and rugby ball shaped, and most DM theorists hope that this is due to dark sector self-interaction or interactions with baryons, but haven’t come up with a universal set of parameters that can reproduce this result in simulations.
    2. Cold dark matter theories predict too much small scale structure.
    3. The observed remarkably close link between baryonic matter distributions and dark matter phenomena is a natural feature of gravity modifications, while it is a mere approximate average relationship in dark matter models which should feature considerable random halo variation/scatter even for otherwise identical baryonic matter distributions.
    4. Dark dark matter detection experiments rule out dark matter with meaningful interactions with ordinary matter over a range of roughly 5 GeV to 600 GeV, and heavier dark matter has all of the problems of CDM theories. These include LUX, CMS, and CDMS Lite. Results that rule out parameter space have for the most part been confirmed multiple times, while no direct search detection has been replicated.
    5. WDM models ( with keV mass thermal relic dark matter) is tightly constrained from both below and above, with bounds that seem to overlap at times.
    6. Planck data largely rules out DM models with particles that are not completely stable over time (10^17 seconds is the minimum mean lifetime).
    7. A variety of data strongly disfavor DM that interacts with ordinary matter by any means other than gravity. For example, any kind of DM under 45 GeV that could be produced in W or Z boson decay has been ruled out. A fractional weak force coupling would have to be less than, or on the order of 0.2% of the strength of a neutrino coupling (i.e. one five hundredth) to fit the precision electroweak data and there is absolute no precedent in Standard Model physics for any such tiny fractional weak force coupling. W and Z boson decays are “democratic” and come in neat integer proportions in every case, at tree-level.
    8. DM simulations strongly favor single component or singlet with a dark sector force carrier models, over more complex dark sectors; the fact that modified gravity equations can describe the same phenomena with just a couple of parameters or less over many orders of magnitude also argues for that conclusion. If this were not the case, it would be possible, as it in fact is possible, to model almost every galactic scale system with a single parameter for surprising accuracy.
    9. The Bullet cluster is not a great fit to standard lamdaCDM.
    10 Lyman alpha forest data from Planck seem to rule out WDM with particles less than 3.3 keV in mass which are seemingly too heavy to solve other CDM problems, which WDM masses of ca. 2.2 KeV are favored.
    11. About 70% of disk galaxies are “bulgeless” and modified gravity theories can easily reproduce these dynamics in the face of various galaxy formation scenarios. But, WDM and CDM models create far, far too few bulgeless galaxies, because those models tend to create serious bulges whenever DM halos or galaxies merge, which happens a lot. Some WDM/CDM simulations cheat to prevent relatively equal galaxy collisions like the Bullet cluster and get results more like real life, but there is no physical mechanism to cause that to happen and we know that big collisions are far more common than the “tame” simulations permit.
    12. DM theories have made “post-dictations” but not any meaningful predictions, because they have too many free parameters and the galaxy formation process is not well understood.
    13. No mechanism for DM formation consistent with the SM has been developed.
    14. DM that acquired mass via the Higgs mechanism would throw a real wrench in the decays of Higgs bosons relative to the SM expectation, unless they are very light (low single digit GeV or less) but this is not observed.
    15. Generically, experimental bounds on dark matter candidates from SUSY theories are in the hundreds of GeV and up mass range which implies heavish CDM which is experimentally ruled out.

    Nobody doubts that MOND is not an accurate description of the universe; it is a toy model and not even relativistic.

    But, some proposals to modify gravity from Moffat (e.g. http://arxiv.org/abs/gr-qc/0506021 aka SVTG) and Deur (who treats the self-interaction of the graviton differently from the treatment implied by the standard GR equations in a manner analogous to the self-interactions of gluons in QCD), are looking quite promising at the moment across a broader range of scales from solar system to cluster scale, and the proposition that only dark matter can explain the bullet cluster has been disproven.

    One of the better experimental arguments for Deur’s approach is its accurate description of the relationship between the mass to light ratio of an elliptical galaxy and the extent to which the galaxy is not perfectly spherical. Deur’s approach also naturally explains why dark matter, dark energy and baryonic matter in the LamdaCDM standard model of cosmology have the same order of magnitude. While RAVE star motion in the Milky Way is a poor fit to some of the less successful gravity modification theories, this hasn’t been examined for these more promising approaches.


    1. 4gravitonsandagradstudent Post author

      Ok, so what do the UV completions of modified gravity proposals look like, and what sorts of LHC signatures might they produce? 😉

      More seriously, as a proponent of modified gravity, do you think that there’s likely to be BSM physics for run two of the LHC to find, in general? And if so, can you think of a reason for it to be there that’s more persuasive (to the broader public) than naturalness?


      1. ohwilleke

        While we could find new particles or force at the LHC in principle, my Baysean prior is that this is very unlikely. I also think that naturalness has been positively counterproductive as a hypothesis generator.

        Maybe the strongest possibility for new physics that we might see at the LHC would be to discover systemic violation of lepton universality, or maybe even to discover a boson that mediates lepton flavor oscillation but is massively suppressed in charged leptons, but isn’t in neutrinos for some reason. On the other hand, I’d put the odds of B number violation or L number violation at the vanishing small level – I personally think that we’ve had matter-antimatter asymmetry to the degree we do today since at least the end of the inflationary era or so. I also wouldn’t be too surprised if we discovered that CP violation was a separate phenomena entirely from the CKM matrix and only presents itself a CP violating phase in that matrix because flavor changing W boson interactions and CP violating interactions almost always present together by “coincidence”. Another area worth looking at if we have computing power to burn would be to develop a program to look more rigorously for non-local correlations in decays that might provide some new insights or measure things that can’t be observed directly.

        While superficially boring, one of the really important things that the LHC can do is to narrow the margin of error on the top quark and Higgs boson mass measurements. A conjecture that is strongly favored by current data and could be much more tightly constrained with greater precision in these measurements is that the sum of the square of the masses of the fundamental bosons is exactly equal to the sum of the square of the masses of the fundamental fermions is exactly equal to half of the Higgs vev squared. This would imply a Higgs mass of about 124.65 GeV, and a top quark mass of about 174.03 GeV, and more importantly would be a window into the deeper relationships of the Standard Model constants with each other. It would be supersymmetry of a sort, without the extra particles.

        There are also some very interesting suggestions of deeper structure in the CKM matrix that we lack the precision to confirm or reject convincingly (it seems very plausible that you don’t need all four of the theoretically maximal set of four parameters to parameterize the CKM matrix with perfect accuracy – one real parameter and one complex parameter (which really counts as to parameters), might be enough).

        Another thing that the LHC can do which probes energy scales much, much higher than what it can directly generate is to test the running of the three Standard Model coupling constants at the highest possible energy scales, something that only the LHC can do. This is very sensitive to BSM physics at the UV scale, and is likely to vanquish SUSY, but possibly also point to gauge unification possibilities within the Standard Model – it only takes a 1-2% tweak here and there over a dozen orders of magnitude in one or two of the coupling constants for that to happen, which quantum gravity could easily bring about. For that matter, greater precision in the strong force coupling constant measurement all by itself by just an order of magnitude or so, combined with increasingly muscular computing power could dramatically increase the practical engineering and applied uses of QCD. You can do a lot with 0.1% precision that you can’t with 1% precision in your calculations.

        I also think that there is a lot of potential for new discoveries in the area of composite particles. Its messy, but we have gobs and gobs of scalar meson and axial vector meson resonances we don’t understand well at all, and while we’ve seen some dimeson molecules, we still haven’t had any confirmed sightings of basic QCD predictions like glueballs, genuine tetraquarks and pentaquarks. QCD is the only area where experiment does not confirm a priori Standard Model predictions with some regularity and there is a lot of room for new discovery there. More mundanely, we are closing in on an analytical generalized parton distribution function which could conceivably hold surprises. My intuition is that we may be missing one or two fundamental laws of nature in QCD (or at least profoundly improved ways to calculated QCD phenomena in practical contexts comparable to the OZI rule) that are necessary to really understand its more confounding aspects of QCD. Exploring the boundary between perturbative and non-perturbative QCD regimes would be very worthwhile.

        I’m not sure the LHC is powerful enough for it, but top quark hadrons should in principle be very rare, but not actually impossible, just exceedingly rare, even at very high energies. But, the decay of a top quark meson ought to be a really fabulous thing with a very distinctive signature. If one happened, I don’t think we’d miss it.

        Honestly, though, I don’t think that we should build another collider right away after the LHC run is over. In the medium term, our money is better spent on neutrino physics, infrared QCD, space based telescopes and inferometers, neutrinoless beta decay experiments, more powerful lattice QCD, and funding theoretical physicists who are zigging when everyone else zags. The physics community has a serious too many eggs in one basket problem right now. Letting ideas percolate and other areas develop for a decade or so after the LHC run end so we can take time to get a better idea about what we are looking for and how to look for it would be very prudent.


        1. 4gravitonsandagradstudent Post author

          So to translate into “how can we get the public to care about this” space (i.e. the topic of the post 😉 ), we’ve got matter-antimatter asymmetry (definitely something we can get people excited about), various weird relations between masses (probably not worth telling the public about without deeper conjectures as to why, since the numerological crackpots out there would see it as endorsement of their “methods”), gauge unification (tends to be a crowd-pleaser, though saying we’ll get the extra 1-2% from “quantum gravity” is pretty nebulous), and getting QCD right (not part of the current HEP popularization paradigm, but honestly probably should be…still, would take some dedicated exposure before it becomes the sort of thing one can casually mention in a news article).

          From your perspective, it seems like there should also be the motivation of “kill low-energy SUSY/dark matter once and for all”, which would mean you ought to approve of the press’s current treatment on some level, unless you think the case is already completely closed. 😉


          1. ohwilleke

            The LHC will be hard pressed to entirely kill SUSY, at least, by itself. The parameter space leaves too much room for retreating and regrouping. I’d like to see SUSY killed once and for all, but I don’t believe that it will happen.

            More importantly, I’m not really so much against exploring SUSY theories as one possible solution, as I am deeply concerned about the fact that SUSY is sucking up the lion’s share of HEP-experimental resources and theoretical physicist attention, to the detriment of more promising lines of inquiry.



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