Who is Dark Matter?

Last week the colour was dark. I don't mean my depression but a very interesting dark matter workshop that was hosted by CERN. Some of the highlights i already covered here and here. Several other talks deserve being mentioned. But, instead of doing my reporting duty, i'm going to offer you some reflections on the all important question: what is this elusive particle that accounts for one fifth of the energy density in the universe.

The particle that constitutes the dominant dark matter component should have several properties. It has to be stable. It cannot have an electric charge, otherwise it wouldn't be dark. It cannot have strong interactions either, otherwise it would bind to nuclei. Therefore, it cannot be any of the known particles (see here why it cannot be one of the neutrinos). We badly need a new particle. Here is where the cavalry of particle theorists comes to rescue.

In fact, not much skill is required in order to come up with a dark matter canditate. The simplest possibilty is the WIMP, which stands for a weakly interacting massive particle. Take a stable particle with the mass of order the weak scale, that is 100 GeV. Assume its cross section for annihilation into ordinary matter is of order 1 picobarn, a typical weak interactions cross section. In the early universe, such particle would be in a thermal equilibrium with the photon plasma down to temperatures of order 10 GeV. At later times the annihillation would die off, leaving the relic density consistent with what is observed today.

That was easy. Too easy. The flip side is that the literature suffers from proliferation of dark matter candidates. There exist thousands of models that yield a stable WIMP. Even worse, the WIMP is not the only option. Theorists know the tricks that allow to get the correct relic density for a particle that has never been in a thermal equilibrium. With this problem of plenty in mind, theorists aim at obtaining a dark matter candidate as a by-product in a model designed to solve some other problem. Such particle is sociologically upgraded to a motivated dark matter candidate.

What are the motivated candidates? The most tempting approach is to explore the coincidence between the WIMP mass scale and the electroweak breaking scale. There is a hope that the underlying model of electroweak symmetry breaking could also explain dark matter. Indeed, models that try to solve the hierarchy problem often accommodate a dark matter candidate. Technically speaking, the connection arises as follows. A solution to the hierarchy problem always comes with a host of new particles at the weak scale. These new particles affect various low energy observables, which leads to a tension with experiment. One way to reduce the tension is to introduce a conserved parity symmetry, such that the Standard Model particles have the +1 charge and the new particles have the -1 charge. One consequence is that only pairs of the new particles can couple to the Standard Model ones, which usually helps to agree with experiment. Another consequence is that the lightest of the -1 charge particles is stable. Voila the dark matter particle.

This strategy has been recently applied to the Little Higgs models. Little Higgs is a scenario in which the higgs boson is a composite state, with the compositeness scale of order 10 TeV. The separation between the weak scale and the compositeness scale is stabilized by designing an approximate global symmetry and making the higgs a pseudo-goldstone boson. The global symmetry requires new states at the weak scale, which affects the electroweak precision observables. The remedy is the T-parity: a discrete symmetry ensuring that the new states couple only in pairs to the electroweak gauge bosons. The lightest T-odd particle (usually, some new gauge boson) is stable and can play a role of dark matter. Maxim Perelstein gave a nice account of this scenario last week; i also recommend his recent review article.

A similar story happened with supersymmetry, which used to be a well motivated candidate for new physics. Supersymmetry would immediately lead to a disaster by breaking the baryon and the lepton number, which would result in excessive proton decay. This time the remedy is called R-symmetry, and it assigns minus R-charge to all superpartners.

This scenario, connecting dark matter to new physics at the weak scale, would be a perfect news for us particle physicists. In this case the dark matter particle could be produced in the LHC, where it could show up via missing energy signatures. However, one should not be too much attached to this possibility, as there is no satisfactory model of electroweak symmetry breaking on the market. My guess is that there is a surprise in store. The reason why i think so is that there's no compelling scenario that would explain why the amount of dark matter and baryonic matter are roughly of the same order. In the WIMP case (and in the alternative ones as well) it is easy to produce an arbitrary dark matter to baryonic matter ratio, yet it happens to be a small number of order 5. Accident? Well, we certainly owe our lives to that one...