I tell everyone I meet that we are at the dawn of the Dark Matter Decade. Usually they slowly back away, but I’m pretty persistent. Our technology has reached the point that we have an excellent chance of actually detecting most of the matter in the universe for the first time.
We’re very happy to have a guest post from Neal Weiner, one of the leading theorists working in the fast-moving area. (Don’t forget our previous guest post from one of the leading experimentalists.) Neal is responsible for some of the most imaginative models for what’s going on in the dark sector, and is excited about the upcoming experimental prospects. If you want to know what particle physicists are thinking about dark matter these days, you’ve come to the right place.
For anyone in the New York area, Neal is giving a public lecture on dark matter at AMNH on Friday the 4th (tomorrow). If you have a chance to go, I’d recommend not missing it.
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The Era of Dark Matter Direct Detection
Commonly, when I speak to my friends who don’t spend their time obsessing about the prospects for dark matter discovery, I am confronted by indifference, or worse, pessimism, when I mention the next few years of dark matter experiment. The history of dark matter direct detection has largely been a string of experiments, increasingly able to better find nothing, interrupted by occasional unverified claims, they point out. Why should this era be any different?
In contrast, I remain incredibly optimistic about the coming era. I feel this level of sensitivity is special, and that if we are to discover WIMP scattering, it should be in the next few years.
Why am I so optimistic?
1)This level of sensitivity is special
When we talk about discovering dark matter through direct detection, we are typically referring to discovering WIMPs, or Weakly Interacting Massive Particles (although a variety of searches for axions are ongoing). These are particles with masses ranging from roughly the proton mass, to 1000 x the proton mass. The hope is that by putting large (~100 kg or larger) experiments underground, where cosmic rays are shielded, experiments can detect the rare scattering of one of these WIMPs as they pass through the detector. (Estimates of the local density suggest that for WIMPs 300 x the proton mass, there should be about 1000 of them in a cubic meter of space near Earth.)
For dark matter to scatter off of the nucleus, it must interact with it. In the standard model, there are only a limited number of possibilities, and for “renormalizable” interactions, there are only two. It can scatter by exchanging a Z-boson, or by exchanging a Higgs boson.
If the interaction is through a Z-boson, the strength is completely calculable. While a “weak” interaction, the Z-boson provides a relatively strong interaction as far as weak interactions go. Indeed, a WIMP exchanging a Z-boson to elastically scatter off a nucleus would have been seen already about a decade ago, and is excluded by about four orders of magnitude by present experiments (i.e., current experiments would have seen roughly 10^4 events, instead of few or none).
However there is a second possibility – that the WIMP interacts through a Higgs boson. The coupling of the Higgs to ordinary matter is orders of magnitude weaker, with a strength 10 – 100 times weaker than the current generation of experiments, but within reach of the next decade’s experiments. This is not something just pointed out now – Burgess, Pospelov and ter Veldhuis pointed this out a decade ago.
While other force carriers appear in new physics models, such as supersymmetry, even there, the Higgs is often the dominant one. Thus, if you had asked me twenty years ago* what the most interesting levels of sensitivity to think about were, I’d have told you to look for the Z and the Higgs exchange. We know it’s not the Z, and we’re about to know about the Higgs.
*OK, twenty years ago I’d actually have said “huh?”, but that misses the point.
2) If anomalies mean anything, we should find out soon
A great deal of thinking and excitement on the theoretical side has come from considering dark matter anomalies. The DAMA collaboration has reported an annual modulation in the flashes of light in a NaI(Tl) experiment for a decade. This modulation signature was pointed out by Drukier, Freese and Spergel in 1986. When the Earth orbits the sun, sometimes we move with the galactic rotation and sometimes we move against it, consequently the flux of WIMPs should change seasonally, and events in the detector should as well. This is precisely what the DAMA collaboration has observed.
Competing experiments, such as XENON, CDMS, Edelweiss, ZEPLIN and others have seen no such evidence, however, excluding the most conventional scenarios. This has prompted a variety of new ideas: light dark matter, inelastic dark matter, resonant dark matter, luminous dark matter… All of these allow a signal at DAMA consistent with other searches. When compelled by a novel result, theorists begin to see a wider range of possibilities. But even these possibilities make predictions.
More recently, the CoGeNT experiment has seen event rates in their detector above what is expected from background. While no claim has been made of discovery, it is in a range where light dark matter should be expected to be found. XENON and CDMS (and in particular a recent low-energy analysis of the CDMS data, who use the same target) do not see what would have been expected, but a clear background explanation is lacking.
These may be signs of dark matter, and they may not be. If they are, we may already have guessed the correct model, or we may not have, but enough upcoming experiments have sensitivity that almost any scenario should be tested.
What should we be looking for this year?
- CoGeNT will update its data: with more exposure time, their radioactive backgrounds should decay, allowing the signal to be extracted more clearly. Does it modulate as expected? If so, theorists will have to go back to the drawing board.
- KIMS should report soon: the KIMS experiment (Korea Invisible Mass Search) is a CsI(Tl) experiment, with a 100kg target. DAMA began as a 100kg, and grew to 250 kg target of NaI(Tl). KIMS will not test WIMP-sodium scattering explanations of DAMA, but will test WIMP-iodine explanations, and even scenarios where the tiny amount of thallium is what the dark matter interacts with.
- COUPP: the Chicagoland Observatory for Underground Particle Physics is now operating a 4kg target of CF3I at SNOLAB in Canada. With both fluorine (which is light) and iodine (which is heavy and present in DAMA), it should have the ability to test most interpretations of DAMA as well as CoGeNT.
- XENON100: the gorilla in the room is the XENON100 experiment. With already a large exposure on a 30kg target of XENON recorded, the community is eagerly awaiting their results. They could come early in 2011 and may shake up the field.
Going forward, improvements to established detector technologies (such as CDMS) and the maturation of the liquid nobles (such as XENON, but also LUX, DEAP/CLEAN, WARP, DarkSide and more) promise an era of rapid progress, with sensitivity improving by orders of magnitude over the next decade. If WIMPs are there, this coming era is our best opportunity to see them. When coupled with the LHC and new data from astrophysics experiments (Fermi, and PLANCK among others), our attitudes of what dark matter is – or at least what it is not – will soon be entirely different.
Why does current experiment show there can be no Z boson coupling? Can’t you just adjust the Z-DM coupling constant until it’s so small that the cross section goes below noise? Or adjust the mass of the particle up until the number flux of the DM particles is so small that the probability of one hitting the detector goes vanishingly small?
Who (except maybe a rejected research grant applicant) could possibly argue with such a glowing recommendation for the self-expanding field of dark matter WIMP research?
(From Wikipedia:) “It is possible to devise models that reconcile a positive DAMA result with the other negative results, but as the sensitivity of other experiments improves, this becomes more difficult.”
Is it not possible that an exceptionally clever and imaginative inventor-genius could invent a model of a real physical system that invokes entities or actions that do not exist in physical reality? Who could then question the model? Would such a model would be a real model of physical reality? Who would know? Are there models now devised that reconcile conflicting experimental results? Is it not possible that such models, with suitably obscure weaknesses (say, one of the pink fairies has a twisted ankle), could be tweaked endlessly (or until the empire collapsed)? Is it not possible that a model could appear to be confirmed by some “confirming result” and yet be an incorrect model? And yet maybe no one would ever know, if the empire collapsed before the mistake was found.
@Valatan – The Z coupling to the WIMP is fully determined by its gauge multiplet, that is, its weak charge. The low energy interaction with nucleons has the 4-Fermi structure with strength determined by the nucleon and WIMP weak charges, the gauge coupling strength and the Z mass. All but the WIMP weak charge are already known, and the WIMP weak charge, if non-zero, can’t be any less than if it’s an SU(2)_W doublet. Hence a lower bound.
What if dark matter is like dark energy but attractive rather than repulsive? Virtual bosons anti-gravitate and virtual fermion-antifermion pairs gravitate. If something like this is true then the search for real on mass-shell dark matter particles will fail. Looking for WIMPS et-al would be like looking for the motion of Earth through the aether with a Michelson interferometer.
@Flash:
We know that dark matter doesn’t anti-gravitate, because the only reason we believe in it is because we can observe its gravitational field–it pulls on luminous matter and light.
Can you describe in some details each dark matter candidate you enumerated in the post? I remember some time ago there was a flurry of speculation about something called mirror matter being the source of dark matter. I don’t know how that compare to the serious models being considered by physicists.
Leaving aside the details of detection for a moment (which makes this slightly off-topic), I’ve always been confused by something: If you’ve got all of this stuff that is so weakly interacting, why hasn’t it all collapsed into black holes at the various nucleation points in the early universe? If I understand correctly, there’s considerable evidence that dark matter remains in (very large) halo form surrounding most galaxies. This seems weird to me: If the DM hasn’t collapsed, then it must at least be interacting with itself enough to produce some pressure against gravitational collapse. But if it does interact with itself, shouldn’t it flatten into an accretion disk over time?
I’m sure I’m missing something obvious here. Can somebody steer me in the right direction?
@TheRadicalModerate:
The reason the dark matter can stay in a diffuse halo without collapsing is because the interaction is so small.
If you consider a satellite in low earth orbit, if its energy was conserved the particle would orbit the Earth indefinitely. Instead interactions with the atmosphere mean the particle slowly loses energy and spirals inward, requiring us to continually add energy via boosters to make up for the loss. For interacting particles in a star of atmosphere, the energy lost via interactions is made up for via the pressure. For non- or weakly-interacting dark matter, these “frictional” or energy losing interactions don’t exist or are very small, leading to very long virualization times.
Observations (and the resultant supporting theories) suggest that only a small percentage of the universe is composed of ‘ordinary’ matter. And that the far more prevalent Dark Matter and Dark Energy have enormous cosomological influence. Except around here it seems. Why don’t we see any evidence of its gravitational (or other) influence in our local cosmic neighbourhood? Why aren’t the orbits of our nearby planets and moons in any way affected by it? Why aren’t laser beams bent and GPS satellites off-kilter? It would be a bit of a coincidence if the entire universe is chock full of the stuff except in our neck-of-the-woods? I prefer to guess that the observations are wrong, or that lightspeed and/or gravity vary over cosmological distances.
Excuse me if I’m being obtuse but aren’t there some rather large problems with the WIMP model. Of course the number one problem is that they have been searching for them for decades deep underground where earth shields the more mundane interactions. So far nothing. It seems quite optimistic to assume that still greater sensitivity in instrumentation will reveal them. An even more troubling problem is that the normal gravity from baryons fully accounts for the actions of galaxies as long as one does not move too far out from the center of galaxies. It is only as you move to the periphery of galaxies that baryonic gravity cannot account for confined orbital motion of matter. This is a very big deal. It means that for the WIMP model to be true requires not only the detection of wimps but a theoretical structure that explains the scarcity of WIMP action close in to the center of mass of galaxies. Why should there be more WIMPs farther out than closer in?
On the other hand there is already a well known discrete force we already know about that displays
this type of propensity: the strong force. The asymptotic properties of quarks in a nucleus display exactly this property of weak interaction close in and strong confinement at the nominal cross sectional length of the nucleon. Is this just a coincidence? I don’t think so.
Of course the resistance to this idea comes from the obvious difference in scale between the nucleus and the galaxy. This isn ‘t quite as big a problem as it might seem though. We just have to finally unload the baggage of the existence of a high energy density vacuum. The low energy density we observe in the cosmic microwave background is the stretched out version of what has been modeled in physics as the the zero point field. The high density ZPF that correlates to the Planck no longer exists, though it did at the dawn of the universe. Along with that stretching out of the universe with a correspondingly lower temperature we now see the asymptotic freedom also occurring on larger scales. We being humans have to give a name to this new variation of an old form so we call it axions. But we could also just call it low temperature gluons or low temperature asymptotic freedom.
@kiwidamien:
Thanks for the info. So, in effect, you’re saying that gravitational scattering alone (as opposed to the gravitational and electromagnetic scattering for ordinary matter) is simply too weak to have caused the collapse over the available 13.8 billion year time scale.
And yet we’ve had a partial collapse (presumably only through gravitational scattering), or we wouldn’t detect DM in halo configurations at all. Is this yet another one of those cases where we happen to be observing in a goldilocks epoch where DM has aggregated into haloes but the halo boundaries are still outside their Schwarzchild radii? Do we have any observational evidence that DM haloes get bigger in galaxies that are further away?
Eric @10:
Remember that a mass inside a spherical DM halo will only feel the net gravitational force of the DM inside of its radius–the force from everything outside of the radius cancels out. So you can have a uniformly distributed sphere of WIMPs and get about the right behavior. Close to the center of a galaxy, the force on an object is proportional to r*(near-constant WIMP density + high average baryonic density), while near the periphery it’s r*(near-constant WIMP density + much lower average baryonic density).
TheRadicalModerate,
It seems to me you are making quite a few assumptions and it subsequently seems very jury rigged. I think the axion model is much better constrained and fits in with the low density vacuum we actually observe rather than the high density one that is hypothesized. Also the axion model does not really create a new kind of matter as the wimp model does. Simplicity is good as long as the simplification can explain all observations.
I think the real reason for resistance to the simpler axion model over the wimp model is that it eliminates the high energy density vacuum framework. There really is no observational evidence for a hog energy density vacuum. In fact both the observation of the cmb and the very low accelerating expansion of the universe argue against it. But that hasn’t stopped physics from building a rather creaky superstructure based on the ZPF. So if you accept a much, much simpler axion representation you will most likely have to do away with supersymmetry and much of string theory.
It will be very, very difficult for so many physicists to be feasting on so much crow. They won ‘t like it.
Freudian slip there about “hog energy”. I suppose it came from the feeling that that high energy density vacuum framework hogs the attention of physicists, yet there is no empirical evidence for it’s existence in today’s expanded universe.
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Excellent update. Thank you.
ORLY?
–o–
I had no idea that DM had been shown to not interact through the Weak Interaction. I thought the Weakly in WIMP referred to that.
Thank you.
If 70% of the Earth’s surface is water, why aren’t you drowning?
@Eric Habegger:
A right-handed neutrino with a mass through the seesaw mechanism isn’t really all THAT radical an extension of the standard model as far as I can tell. I don’t see how it is any more “exotic” than axions.
@Valatan,
A right handed neutrino perhaps isn’t more exotic than an axion. However the apparent heavy distribution of those right handed neutrinos at the edge of galaxies would be.
You’d expect any weakly coupled particle to be heavily distributed at the edge of galaxies–stuff falls into the middle of galaxies due to friction, and if you turn the friction off (which is certainly true of sterile neutrinos–they wouldn’t see ordinary matter), they just stay where they are, unlike luminous matter, which interacts with the ISM.
First question: what do you mean by stuff falls into the middle of galaxies due to friction?
Second question: what are you implying as the force causing friction?
Third question: why would turning the friction off in the case of sterile neutrinos move them to the edge of the galaxy?
You are assuming the normal forces that might somewhat randomize the distribution of regular massive particles do not do that to sterile neutrinos (one could also substitute Wimps here). That implies that the distribution of massive particles in the galaxy would not affect the distribution of
neutrinos. But on the other hand you are also saying that the heavy distribution of those neutrinos ARE affecting the massive particles on the edge of galaxies and keeping them from drifting off. Those are contradictory attributes that you are requiring neutrinos to have.
Why is the Milky way flat? Why does it have remnant clusters with a more spherical distribution that appear much older than your average Milky Way star? Why do these clusters seem to have roughly the same distribution as the dark matter?
hint: it all has to do with the interaction between stars and dust.
OK. 🙂
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