Dark matter arising from extra spatial dimensions could be detected with existing or future experiments, according to the 18 November print issue of PRL. If an additional dimension were hidden in the right way, heavier replicas of the known particles might traverse space and account for the mysterious "dark" component of the Universe's mass. Detecting the unique particles would potentially confirm the existence of extra dimensions and solve at least part of the dark matter riddle.
Researchers think that 30% of the Universe's mass is made up of unknown particles that are invisible to telescopes but have gravitational effects on galaxies. Potential culprits called weakly-interacting massive particles (WIMPs) come from proposed extensions to the standard model of particle physics, such as supersymmetry and extra-dimensional theories. To verify the theories, searches for some of these particles look to space, because particle accelerators are too weak to produce them. But the big bang should have produced every particle. "Maybe the Universe made them for us and they're floating around out there and are dark matter," says Jonathan Feng of the University of California at Irvine. "Now the [difficulty] is you have to find them."
Kaluza-Klein particles are named for the two theorists who first proposed that extra dimensions could be "curled up" to a size too small for us to notice them. In the simplest case, these particles would be much like those of the standard model, but would move through four spatial dimensions instead of three. Their momentum along the fourth dimension would appear as additional mass in three dimensions, so we would observe heavy photons or heavy electrons, for example. The smaller the extra dimension, the greater the mass.
Now Feng and his colleagues have found that Kaluza-Klein particles would heat up or ionize a block of material such as germanium at rates comparable to other dark matter candidate particles. They could also annihilate each other in space, creating showers of ordinary particles. The researchers calculated that Kaluza-Klein dark matter would generate a unique, sharp positron signal, distinguishing it from the neutralinos of supersymmetry. Because dark matter feels the gravitational force, it would be drawn toward the sun's large mass and lead to an excess of neutrino and muon showers from the sun's direction. The AMANDA neutrino detector, buried within the Antarctic ice, or the Alpha-Magnetic Spectrometer (AMS), an antimatter detector scheduled to fly on the International Space Station in late 2005, could hunt out these signals.
"Kaluza-Klein dark matter is definitely worth looking for with AMS," says Kate Scholberg of the Massachusetts Institute of Technology in Cambridge, who works on the detector project. "The indirect signature would be quite dramatic for some of the possible parameters they discuss." She adds that detecting and distinguishing Kaluza-Klein and supersymmetric dark matter would depend strongly on how nature actually behaves, but coming up empty wouldn't rule them out, just constrain their possible characteristics.
Kaluza-Klein Dark Matter
Hsin-Chia Cheng, Jonathan L. Feng, and Konstantin T. Matchev
Phys. Rev. Lett. 89, 211301
(issue of 18 November 2002)
PDF file of the original letter is linked at the above page but is available only to subscribers or for a fee.