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ILC Physics Studies: Dark Matter at the SUSY Focus Point

Dark matter is the name given to the unknown matter that comprises about 23% of the universe but remains invisible and almost completely inert. Despite its low key personality, this weakly interacting material plays a vital role in the evolution of the universe, by providing gravitating mass that leads to the formation of stars, galaxies, clusters of galaxies, and clusters of clusters... Without dark matter, we wouldn't be here.

Although we don't yet know what Dark Matter is, we can see its gravitational effects in galaxies, and can outline what general characteristics it must have:

  • It must have zero electric charge (so it isn't visible)
  • It must be weakly interacting (so it decouples from the Big Bang properly)
  • It must be massive (so it exerts and responds to gravity)
  • It must be stable (so it is still here after 13.7 billion years)

None of the known particles in the Standard Model of particle physics satisfies all these requirements: evidently we are looking for something genuinely new!

2L2o3jandfit_lr_slac_allsign.png

The invariant mass of two leptons emerging from a process that included production of a dark matter particle. The two leptons did not come from the dark matter particle, which after all escaped, but they reveal its presence nonetheless. The sharp edges in this figure indicate that the dark matter particle has "used up" some of the available energy, leaving no more for lepton production. We can deduce the mass of the dark matter candidate from these edges.

Essentially every theory of physics beyond the Standard Model now includes a candidate for Dark Matter -- and in general any such theory would not be considered viable if it did not.

In Supersymmetry, the dark matter candidate depends on what one thinks the parameters of Supersymmetry are. In one choice of parameters, known for technical reasons as the "Focus Point", the dark matter candidate is a so-called "neutralino", a particle that is a supersymmetric partner of some mixture of the photon, the Z0, and the two species of neutral Higgs particles that occur in minimal supersymmetry.

We should produce such neutralinos at the International Linear collider, and we can therefore expect to measure the properties of the dark matter in ILC experiments. Of course since the dark matter is invisible and weakly interacting it escapes our detectors without a trace, so we have to measure its properties by a kind of "process of elimination". That is, we measure everything else in the event, and by combining information from many, many events, create a statistical picture of the dark matter candidate. By this means we can measure its mass and some of its more subtle properties.

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