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EVER since a 1998 space shuttle experiment saw what appeared to be an anomalously heavy variety of matter, the hunt has been on for more of the same. Now, a search of lunar soil for so-called "strange matter" has come up short, casting doubt on whether it exists at all.
The standard model of particle physics describes six types of quark, including the up and down quarks which make up protons and neutrons, found inside ordinary atoms. Physicists have long theorised about strange matter that would also contain strange quarks. Strange matter is heavier and denser than ordinary matter, as the strange quark has roughly 10 times the mass of the up or down quark.
Some neutron stars - the dense remnants of supernovae - might actually be made of strange matter. If two such stars collided, bits of strange matter called strangelets could be spewed out. "You could get strange matter floating around in space," says Jack Sandweiss, a physicist at Yale University.
The Alpha Magnetic Spectrometer (AMS-01), which flew on the space shuttle Discovery, appears to have detected a strangelet - a nucleus like that of oxygen but with three times its mass. Delays in flying the more advanced AMS-02 instrument after the Columbia shuttle disaster have meant that this result has never been confirmed. So Sandweiss turned to an altogether different kind of detector: the moon. It has no magnetic field to deflect charged particles, so any strangelets arriving would hit its surface and stay embedded there.
Any strangelets arriving at the moon would hit its surface and stay embedded there
Sandweiss's team took 15 grams of lunar soil from the Apollo missions and accelerated the grains past a powerful magnet. Any strangelets present would curve less in the magnetic field than normal matter - but none was observed (www.arxiv.org/abs/0903.5055). "If AMS-01 had been a real event, we would have found it," says Sandweiss.
The study was designed to find the oxygen-like strangelet seemingly seen by AMS-01. Other strangelets might have slipped by unseen. The issue could be settled when AMS-02 is flown up to the International Space Station next year. "Then we'll finally answer the question," Sandweiss says. "AMS is really the right way to do this."