Probing Dark Matter Physics: What Properties Can We Derive?

[Floyd_13]

TL;DR Summary

What properties of dark matter can we derive from each of the available methods for probing the physics of dark matter?

This is probably a long shot but it's worth trying. My question is the following:

What properties of dark matter can we derive from each of the available methods for probing the physics of dark matter?

To elaborate a bit, my understanding is that the evidence for dark matter comes from its gravitational effects on large scale structures and from precision measurements on cosmological observables (e.g. CMB temperature anisotropies). The former method provides information about the structure formation of DM (e.g. DM halos) as well as that it interacts gravitationally with baryonic matter. The latter provides us with the relic density of DM which is around 0.26. What other properties can we derive from this kind of observations? (How do we know that DM is cold for instance?)

In addition to these methods, there are also direct, indirect and collider searches that aim in detecting signals from DM particles -- based on different principles and physical phenomena. Now, given that we eventually have a significant signal from these experiments, what can we learn from each type of experiment? Are all these methods able to give the mass of a DM particle for instance?

I 've read several papers and reviews on these methods and managed to get some answers, but any further insight (or recommendation) would be extremely helpful! Ideally what I am looking for is a table showing what kind of information can be derived by each method. Thanks!

 

[mfb]

How do we know that DM is cold for instance?

If it would be hot it could not clump together on the scale of galaxies.

Galaxy collisions give us upper bounds on the self-interaction of dark matter. Searches for annihilation signals contribute to these upper limits in some cases.
Earth-based detectors and accelerator searches give us upper limits on the interactions between dark matter and baryonic matter. They can give us the mass of a particle if we find a particle in the future.

 

[Floyd_13]

Thanks for the response.

Galaxy collisions give us upper bounds on the self-interaction of dark matter.

Searches for annihilation signals contribute to these upper limits in some cases.

Can you please provide any references with specific examples or even better a review paper for these?

 

[mfb]

See e.g. arXiv:astro-ph/0309303 and 1611.03184, and the various references in both directions.

 

[ohwilleke]

Summary:: What properties of dark matter can we derive from each of the available methods for probing the physics of dark matter?

This is probably a long shot but it's worth trying. My question is the following:

What properties of dark matter can we derive from each of the available methods for probing the physics of dark matter?

To elaborate a bit, my understanding is that the evidence for dark matter comes from its gravitational effects on large scale structures and from precision measurements on cosmological observables (e.g. CMB temperature anisotropies). The former method provides information about the structure formation of DM (e.g. DM halos) as well as that it interacts gravitationally with baryonic matter. The latter provides us with the relic density of DM which is around 0.26. What other properties can we derive from this kind of observations? (How do we know that DM is cold for instance?)

In addition to these methods, there are also direct, indirect and collider searches that aim in detecting signals from DM particles -- based on different principles and physical phenomena. Now, given that we eventually have a significant signal from these experiments, what can we learn from each type of experiment? Are all these methods able to give the mass of a DM particle for instance?

I've read several papers and reviews on these methods and managed to get some answers, but any further insight (or recommendation) would be extremely helpful! Ideally what I am looking for is a table showing what kind of information can be derived by each method. Thanks!

There are literally hundreds of papers, if not thousands, that bound the DM parameter space. Most of the limitations, however, are model specific and cover fairly small, often disjoint areas of that parameter space. I probably see five new ones every week. Integrating them into one coherent picture is tricky, and it pretty much has to be done in categories.

Simple Purely Collisionless Dark Matter Models

The simplest model is purely collisionless dark matter particle (maybe a fermion, maybe a boson) with no non-gravitational interactions of any kind, whose only real property is mean velocity, which can be connected to a characteristic mass in a model dependent manner in a thermal freeze out scenario by virtue of a virial theorem, but whose observable properties have more to do with mean velocity than with mass per se (which must be in a range known as "cold" or "warm" but not "hot" and cannot be completely static).

The only kind of direct detection of this kind of dark matter that is possible is via something like the "billion little pendulums" proposed experiment that measures gravitational responses to otherwise invisible objects. You can't, for example, by definition, use dark matter particle annihilations to detect it. You've got basically one observable free parameter that can be experimentally fit (particle mass) although you might throw in spin and mean velocity as independent variables.

The main way to constrain this very simple model is with galaxy and cluster dynamics. Dark matter that is really that simple behaves in a way that is possible to describe analytically (rather than just with N-body simulation models) and it is pretty strongly disfavored by observations because inferred dark matter halos are the wrong shape and because dark matter distributions are too tightly correlated with distributions of ordinary baryonic matter.

Arguably, though, with a boson DM candidate and/or a low enough mass that takes advantage of wave-particle dualities, you can beat these flaws of plain vanilla collisionless cold dark matter models.

As far as I'm concerned, however, papers that say that all observations are consistent with LambdaCDM have blinders on. They are ignoring not one, but a whole herd of elephants in the room and maybe a rhino and hippo to boot.

Simple Self-Interacting Dark Matter Models

One of the simplest variations of that model consists of a single kind of dark matter particle (usually a fermion, sometimes spin-1/2 and sometimes spin-3/2 as a "gravitino singlet") that has no non-gravitational interactions except that it has a self-interaction which takes the form of a Yukawa force mediated by a carrier boson with a non-zero rest mass (self-interacting dark matter). You need to add the carrier boson mass and the coupling constant of the force it mediates to the dark matter particle mass.

The virtue of SIDM is that is has the potential to produce inferred dark matter halos that more closely resemble what we observe, and there are lots of paper constraining SIDM parameter space, although its is tricky as the models fit are not always identical. The problem is getting those models to also reproduce the very tight correspondence between ordinary matter and dark matter distributions seen in such a wide variety of settings.

Simple Models With WIMP Dark Matter That Interacts Feebly With Ordinary Matter

Another kind of dark matter is "weakly interaction dark matter" or WIMPs. In its most naive forms, such as lightest supersymmetric particle WIMPs, it should be possible to detect in direct detection experiments since it literally interacts with ordinary matter via the weak force with the same weak force charge as other fundamental particles like the neutrinos. The direct detection experiments have ruled out these dark matter candidates over a broad range of masses where no interactions are seen down to the "neutrino threshold." In theory, if you can screen the neutrino background well enough, you could explore even lower mass ranges for these candidates.

Less strictly, WIMPs refer more generally to dark matter that has non-gravitational interactions with ordinary matter that are very feeble, but not actually zero, via either an existing force or some new physics 5th force. Direct detection experiments define regions of mass and cross-section of interaction where that is possible. This also includes milli-charged and micro-charged dark matter candidates. Warm dark matter WIMPs or "sterile neutrinos" are particularly attractive because at a characteristic mass on the order of keVs, they have some quantum properties that are rather similar to self-interacting dark matter and solve some of the same problems.

Sterile neutrinos also come in "true" sterile neutrino types that oscillate with "active" Standard Model neutrinos (either as part of the mass hierarchy of neutrinos or in something like a seesaw mechanism that imparts mass to ordinary neutrinos), and dark matter particles that only get the name because like hypothetical right handed neutrinos they don't have Standard Model force interactions and have similar properties to them except that they don't oscillate with Standard Model neutrinos.

Simple Dark Matter Models Without Beyond The Standard Model Particles

There are two kinds of dark matter candidates that haven't been ruled out that don't require beyond the Standard Model fundamental particles.

Stable Sterile Exotic Hadrons

One consists of strong force bound, color charged matter such as glueballs and hexaquarks that somehow sit in an island of stability unlike all other hadrons except protons and neutrons which have mean lifetimes of microseconds or less. There hasn't been a lot of constraint work done of them, but they'd have to have masses in the single to double digit GeV range and somehow evade direct detection experiments which greatly limit cross sections of interaction in that mass range.

Primordial Black Holes

Another consists of primordial black holes, which are black holes smaller than stellar black holes formed by star collapse that would have come into existence in the crazy high energy dense environment of the early universe. These would decay via Hawking radiation, which means that if they were created near the time of the Big Bang, they have a minimum size. There are also quite a few studies constraining their maximum size. The sweet spot is roughly asteroid mass PBHs (simply by process of elimination given available detection strategies) but there are problems with those as well (e.g. dynamics in galaxies and clusters and reproducing the CMB all of which should only work with DM candidates that are nearly collisionless) that investigators have been so focused on direct detection possibilities that they haven't investigated very closely.

Simple Axion-Like Dark Matter Models

Another popular candidate with lots of parameter space exclusions published involves axion-like particles, which are extremely light but evade "hot dark matter" exclusions inferred from the amount of structure seen in galaxies, etc. by having low mean velocity because they are produced by a means other than as thermal relics. The type example here is a QCD-axion originally designed to explain why the strong force doesn't experience CP violation the way that the weak force does, but particles with similar masses and non-thermal creation mechanisms get lumped in the same boat.

Complex Dark Sector Models With Dominant Components

One thing you will note from all of these models is that usually have only one or two dark matter particles, and that seems plausible from the inferred distributions of dark matters that don't seem to involve all that many degrees of freedom. But some models assume that there is an entire dark matter sector as rich as the one that we observe (e.g. mirror matter) and that just as the baryonic ordinary matter of the universe can make scores of different kinds of fundamental particles and hadrons but are dominated by protons, neutrons, electrons and neutrinos, maybe the dark sector is rich but we can only see one or two dominant elements of the dark sector through our available observational methods.

Gravity Based Explanations Of Dark Phenomena

And, as if that wasn't complicated enough, there is also a whole further world of modified gravity theories formulated in a variety of ways that presume that dark matter and/or dark energy don't exist and that these are merely gravitational effects for which we have the IR behavior wrong as we conventionally model it in classical General Relativity applications,.

This can be either because GR truly isn't correct, or because we model GR incorrectly.

Dark Phenomena As Quantum Gravity Effects

For example. dark matter and dark energy could actually be low energy quantum gravity effects for a quantum gravity theory that approximates GR in the limit, but differs in the find details with second or third order IR effects.

These can further be broken down, at first cut, between theories like loop quantum gravity and spin-foams that quantize space-time itself in a background independent way, and theories that basically generalize the Standard Model to describe a graviton that is just one more carrier boson, or are formulated as classical rather than quantum gravity theories.

Conclusion

This list of categories above isn't comprehensive, although it hits most of the leading types of active publication and research in the phenomenology of dark matter phenomena. (I have not been comprehensive at all in discussing dark energy effect models and models in which dark matter and dark energy phenomena have a common origin.)

It would really take a full fledge wiki, and maybe someone already has one, to keep all of the constraints organized.

It is also important to realize that lots of papers are written about constraints on types of DM that are already ruled out by some other kind of observation, because researchers generally don't know even the entire DM phenomenology literature. The mere fact that a new papers constraining some explanation of dark matter phenomena should not be taken to imply that the theory constrained is actually still viable when the sum total of the literature constraining dark matter models is considered.

 

[valenumr]

Were heavy sterile nutrinos mentioned? They are neglected from the standard model, even though they sort of have a place at the table. But since they would be unobservable by definition... Well, except gravity.