AMS tiptoes toward answer to dark-matter question
||The first result from the Alpha Magnetic Spectrometer experiment improves on previous measurements, promises precise future results. Photo: NASA
The space-based Alpha Magnetic Spectrometer experiment could be building toward evidence of dark matter, judging by its first result.
The AMS detector does its work more than 200 miles above Earth, latched to the side of the International Space Station. It detects charged cosmic rays, high-energy particles that for the most part originate outside our solar system.
The experiment's first result, released today, showed an excess of antimatter particles—over the number expected to come from cosmic-ray collisions—in a certain energy range.
There are two competing explanations for this excess. Extra antimatter particles called positrons could be forming in collisions between unseen dark-matter particles and their antiparticles in space. Or an astronomical object such as a pulsar could be firing them into our solar system.
Luckily, there are a couple of ways to find out which explanation is correct.
If dark-matter particles are the culprits, the excess of positrons should sink suddenly above a certain energy. But if a pulsar is responsible, at higher energies the excess will only gradually disappear.
"The way they drop off tells you everything," said AMS Spokesperson and Nobel laureate Sam Ting in [Wednesday's] presentation at CERN, the European center for particle physics.
The AMS result, to be published in Physical Review Letters on April 5, includes data from the energy range between 0.5 and 350 GeV. A graph of the flux of positrons over the flux of electrons and positrons takes the shape of a valley, dipping in the energy range between 0.5 to 10 GeV and then increasing steadily between 10 and 250 GeV. After that point, it begins to dip again—but the graph cuts off just before one can tell whether this is the great drop-off expected in dark-matter models or the gradual fade-out expected in pulsar models. This confirms previous results from the PAMELA experiment, with greater precision.
Ting smiled slightly while presenting this cliffhanger, pointing to the empty edge of the graph. "In here, what happens is of great interest," he said.
"We, of course, have a feeling what is happening," he said. "But probably it is too early to discuss that."