Decades-old scientific paper may hold clues to dark matter

By Adrian Cho

Here’s one reason libraries hang on to old science journals: A paper from an experiment conducted 32 years ago may shed light on the nature of dark matter, the mysterious stuff whose gravity appears to keep the galaxies from flying apart. The old data put a crimp in the newfangled concept of a "dark photon" and suggest that a simple bargain-basement experiment could put the idea to the test.

No one really knows what dark matter is. Since the 1980s, theorists' best hunch has been that it consists of so-called weakly interacting massive particles, or WIMPs. If they exist, WIMPs would have a mass between one and 1000 times that of a proton. They would interact only through the feeble weak nuclear force—one of two forces of nature that ordinarily flex their muscle only within the atomic nucleus—and could disappear only by colliding and annihilating one another. So if the infant universe cooked up lots of WIMPs, enough of them would naturally survive to produce the right amount of dark matter today. But physicists have yet to spot WIMPs, which every now and then should ping off atomic nuclei in sensitive detectors and send them flying.

More recently, theorists have explored other ideas, such as self-interacting dark matter. This would consist of a particle, known as a ? (pronounced chi), with a mass between 1/1000 and one times that of the proton. Those particles would interact with one another through a force like the electromagnetic force, which produces light. That force would be conveyed by a massive particle called a dark photon—a dark matter version of a particle of light—that might "mix" slightly with the ordinary ones. So with some small probability, a dark photon might interact with ordinary charged particles such as electrons and atomic nuclei—just as ordinary photons do.

Self-interacting dark matter has attractive properties. In particular, a dark photon could also explain a particle physics puzzle. A particle called the muon appears to be very slightly more magnetic than theory predicts, and that discrepancy could be resolved if the muon interacts with dark photons lurking in the vacuum. However, ?s and dark photons would be hard to detect with WIMP detectors; with their low masses, they couldn't whack a nucleus hard enough to create a signal.

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http://news.sciencemag.org/physics/2014/10/decades-old-scientific-paper-may-hold-clues-dark-matter