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Catcher in the sky: The search for mystery matter
See more: Tour the mystery matter detector destined for space
Does the universe really hold hidden seams of primordial antimatter, dark matter and even “strange matter”? A massive orbital detector is set to find out
WHEN the space shuttle Endeavour blasts off on its final flight to theInternational Space Station this month, it will be carrying a very important payload. Secure in the cargo bay will be the Alpha Magnetic Spectrometer (AMS), a 6.9-tonne leviathan designed to sift through the perpetual sleet of particles from deep space.
Shortly after Endeavour’s arrival, astronauts will command the shuttle’s robotic arm to lift AMS from the cargo bay and hand it over, space-relay style, to the ISS’s robotic arm. Once AMS is lowered onto its final resting place on the station’s exterior, the astronauts will undertake a lengthy spacewalk to plug in and power up the space station’s greatest scientific experiment.
If AMS lives up to its billing, this $1.5 billion particle detector will change the way we think about the universe. It could tell us whether whole stars or even galaxies of antimatter exist somewhere out in space: it could tell us about the nature of the dark matter thought to pervade the universe: and it could reveal where the most powerful particle accelerators in nature are hiding.
Breakthroughs in any one of these areas would make the mission a roaring success. But AMS has some unfinished business too. Back in 1998, a prototype found hints of what seems to be an entirely new form of nuclear matter known as “strange matter”. If the latest mission confirms those findings, it will change our understanding of the stuff of the universe and even of our ultimate fate in the cosmos.
The detector’s arrival in space will mark the end of a long and bumpy journey that almost didn’t happen. It began in 1994 when the AMS team carried out the first feasibility study for NASA. Project leader and Nobel laureate Sam Ting, a particle physicist at the Massachusetts Institute of Technology, entranced NASA’s then-administrator Dan Goldin with one of AMS’s main goals – to hunt for primordial antimatter.
One of the biggest unanswered mysteries is why the universe is predominantly made of matter. Laboratory experiments show that every time a particle of matter is created out of energy, so is its antimatter counterpart. If this were a perfect process, the fireball of the big bang should have produced a universe with as much antimatter as matter. So where is it hiding?
The most likely possibility is that this process was not perfect, and ended up producing more matter than antimatter. In this scenario, the antimatter no longer exists because every bit of it went up in a puff of energy as it encountered matter during the universe’s early days. But our understanding of this process is far from complete, and that leaves open another possibility.
AMS could see a nucleus of anti-helium. That would be significant because most of the helium in the cosmos today was formed during the first 3 minutes after the big bang rather than inside stars, and we would expect the same to be true of any anti-helium out there. Any anti-helium AMS sees would therefore most likely have survived the great primordial annihiliation with matter, suggesting there must be other surviving fragments. “If AMS sees even a single piece of anti-helium, we will know that there is a concentration of antimatter somewhere in the galaxy,” says Martin Pohl, who heads the AMS group at the University of Geneva in Switzerland. “If it sees a particle of anti-carbon, we’ll know there’s an anti-star out there, because you need anti-stars to cook it up from the primordial antimatter.”
While some believe the search will be in vain (see “Hunt for the first antimatter”), Goldin was impressed with the simple elegance of such a quest, and his enthusiasm helped to bankroll the AMS project.
To find antiparticles, the AMS design called for a detector much like the ones used in today’s experiments at the Large Hadron Collider at CERN. Like them, it is built from a large magnet and several component detectors each designed to measure a different aspect of the 10,000 particles passing through every minute, including their energy, direction of travel, electrical charge and mass (see diagram). At any given moment, AMS will generate 300,000 pieces of information, which 650 microprocessors will process and record for transmission to Earth, where researchers from 16 countries will pore over it.
Progress in the late 1990s was fast. To prove the experiment was feasible, the collaboration built a prototype detector, which flew on the space shuttle Discovery in June 1998. The prototype operated for 100 hours – not long enough to find any anti-helium, though the results did allow the AMS team to set some punishingly small upper limits on how much primordial antimatter there could be in the galaxy. They showed that for every million regular helium nuclei, there had to be less than one anti-helium nucleus. “We’d like to push that to one in a billion,” says Pohl of the new version of the detector. “Then we’ll know for sure that there is no primordial antimatter in the galaxy.”