Thread by @WKCosmo

@NewLeibniz OK, a wee bit more detail. :-)

@NewLeibniz A working Dark Matter theory is more than just speculating the existence of a hypothetical particle. You also have to answer the question: How is it produced?

@NewLeibniz This production mechanism needs to be specific, and to give the correct abundance of the particle in the universe, matching the indirect evidence from e.g. galactic rotation curves, galaxy clusters, etc.

@NewLeibniz This particle production typically happens in the very early universe. Nobody knows exactly when.

@NewLeibniz In order to produce a particle, it has to interact with the other stuff in the universe somehow.

@NewLeibniz A reasonable assumption is that, at some very early time, the DM particle was in thermal equilibrium with everything else, the Standard Model (SM) particles, like quarks and electrons and photons, etc.

@NewLeibniz *If* the DM particle was in thermal equilibrium in the early universe, you can calculate its present abundance using simple statistical physics.

@NewLeibniz It turns out that the present abundance of the DM particle doesn't depend much at all on the particle's mass, but only on how strongly it interacts with the SM particles.

@NewLeibniz If you calculate the interaction strength necessary to give the observed abundance of DM in the present universe, the answer that comes out is almost exactly the interaction strength of the weak nuclear force, mediated by exchange of W and Z bosons.

@NewLeibniz There is no a priori reason these numbers should be the same. This is a remarkable coincidence, and is referred to as the "WIMP miracle". It is the single largest reason that WIMP (Weakly Interacting Massive Particles) are the favored candidate for DM.

@NewLeibniz We already know of particles that interact only via the Weak Force -- neutrinos. WIMPs would be like a neutrino, but a lot heavier.

@NewLeibniz When Pauli first proposed the neutrino, he famously wrote: “I have done something very bad today by proposing a particle that cannot be detected; it is something no theorist should ever do,”

@NewLeibniz He was wrong, and we now detect neutrinos all the time: although the interaction of neutrinos with other particles is very weak, it is not zero. The same should be true of WIMPs: a sensitive enough detector should see them.

@NewLeibniz For example, the @Xenon1T detector looks for WIMPs bouncing off the nuclei of cryogenically cooled Xenon atoms.

@NewLeibniz @Xenon1T It hasn't seen anything: The vertical axis on this plot is the interaction strength ("cross section" in jargon), the horizontal axis is the particle mass. Everything above the colored band is excluded.

@NewLeibniz Here's the problem. The bound on the cross section is now so low that the "WIMP miracle" no longer works. The allowed interactions are far too weak.

@NewLeibniz You can still cook up models (e.g. Supersymmetry) that give the correct abundance with lower interaction strength, but the beautiful coincidence of the Weak Force appearing in two unrelated places no longer holds. Bummer.

@NewLeibniz We just aren't seeing WIMPs where we expected them to be. This is leading some people to wonder whether or not WIMPs exist at all. Certainly the motivation for their existence is much weaker than it once was.

@NewLeibniz Maybe Dark Matter wasn't produced in thermal equilibrium. That's an assumption. It does not have to be true.

@NewLeibniz But the expectation of interactions via the Weak Force comes from the assumption of thermal equilibrium! If you don't assume that, then the interaction could be something very different.

@NewLeibniz This would explain why nuclear recoil detectors like Xenon aren't seeing anything. They're looking in the wrong place.

@NewLeibniz Axions, for example, are not produced in thermal equilibrium, but by a non-thermal process called "vacuum misalignment".

@NewLeibniz There are many kinds of axions, but the one people usually mean when they talk about axions as a DM candidate does not interact via the Weak Force, but via the Electromagnetic Force instead.

@NewLeibniz There is much less money being spent on axion searches than on WIMP searches, but there are a few, and they too have so far seen nothing.

@NewLeibniz But all of these DM theories have one thing in common: producing the particle in the correct abundance requires that the particle interact somehow with the SM particles we know about.

@NewLeibniz If the DM particle interacts with the SM, then it can be detected, at least in principle.

@NewLeibniz It was once believed that there is no way to produce Dark Matter without *some* kind of interaction with the Standard Model taking place, and therefore that DM should be detectable.

@NewLeibniz This was shown to be incorrect by Dan Chung, Patrick Crotty, Rocky Kolb, and Toni Riotto, in this paper in 2001:

arxiv.org/abs/hep-ph/010…

@NewLeibniz They showed, remarkably, that it is possible to produce the right abundance of a Dark Matter particle *only* through gravitational effects. These particles can be much heavier than WIMPs, and they called them "WIMPzillas".

@NewLeibniz These particles would not interact with the Standard Model at all. Such particles are referred to as "sterile".

@NewLeibniz They would never show up in any detector. The only trace of them would be via their gravitation.

@NewLeibniz This is the nightmare scenario: a particle or particles which has exactly zero interaction with normal matter, but nonetheless is visible via gravity. We would never directly detect it, never produce it in particle accelerators, never see it decay into photons.

@NewLeibniz It could be true. Everybody is mostly just hoping it isn't.

@NewLeibniz Correction: the first paper proposing WIMPzillas was this one, in 1998, by Chung, Kolb, and Riotto. Apologies for the error.

arxiv.org/abs/hep-ph/980…