The first results from a huge—and hugely controversial—cosmic ray detector aboard the International Space Station confirm a previously reported excess of antiparticles from space. Readings from the $2 billion Alpha Magnetic Spectrometer (AMS) could be signs of particles of mysterious dark matter annihilating one another in the inky void. Or they could be merely subatomic exhaust from a pulsar or some other run-of-the-mill astronomical object.
The results were reported today during a seminar at the European particle physics laboratory, CERN, near Geneva, Switzerland, by Samuel C. C. Ting, a 77-year-old Nobel Prize-winning particle physicist and the force behind AMS. They settle the question of whether the tantalizing excess exists. "This is what has convinced me that this is real," says Stéphane Coutu, a cosmic ray physicist at Pennsylvania State University, University Park, who does not work on AMS. However, Coutu cautions, "what it means is not going to be clear for some time."
Bolted to the exterior of the space station on 19 May 2011, AMS has detected 30 billion cosmic rays and measured the ratio of antielectrons, or positrons, to the total number of electrons and positrons. According to standard astrophysics, that "positron fraction" should be small and should fall as energy increases. That's because sources such as exploding stars can pump out plenty of high-energy electrons, whereas high-energy positrons typically arise less frequently, through the collisions of cosmic rays. Instead, AMS scientists find that the positron fraction increases from roughly 5% at an energy of 10 giga-electron volts (GeV) to more than 15% at an energy 35 times as high.
Such an excess had been seen before. In April 2009, researchers with an Italian satellite experiment called Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) reported similar results. Their counterparts with NASA's orbiting Fermi Gammaray Space Telescope reported much the same thing in January 2012. But neither measurement clinched the case. PAMELA lacked systems to nail down the type of particle, so some researchers worried that it was mistaking protons for positrons. Fermi was designed to measure uncharged particles, so to distinguish between positrons and electrons, researchers had to rely on Earth's magnetic field to bend the particle's paths, a technique with a checkered history.
AMS measured the positron fraction to higher energies and with a precision unmatched by any previous experiment. "I'm happy that AMS confirms there's a positron excess," says Gregory Tarlé, a cosmic ray physicist at the University of Michigan, Ann Arbor, who worked on the High Energy Antimatter Telescope, a balloon-borne experiment that spotted an excess of positrons in the lower end of the energy range in 2001.
But what produced the excess? The most exciting possibility is that the positrons arise from dark matter, the mysterious stuff whose gravity binds the galaxies. According to popular theories, dark matter could consist of weakly interacting massive particles, or WIMPs. When two WIMPs collide, they could annihilate each other to produce an electron-positron pair, with the energy of those particles limited by the mass of the WIMP. In that case, the positron fraction should increase and then fall again beyond a certain "cutoff" energy. AMS researchers see tantalizing signs that the positron fraction levels off at 250 GeV, suggesting a cutoff lurks over the energy horizon.
However, excess positrons can emerge from other, more mundane astrophysical mechanisms, Coutu says. For example, a nearby radiation-spewing neutron star called a pulsar could crank out energetic positrons. So even though the positron excess appears to be real, it is not a smoking gun for dark matter, he says.
Ting and his 600 AMS colleagues acknowledge the point. In fact, in a carefully worded paper submitted to Physical Review Letters, they do not mention dark matter, referring instead to "new physical phenomena." But they also argue in a press release that with more data, AMS will be able to measure the exact shape of the spectrum to higher energies and determine whether the excess comes from dark-matter collisions or an astrophysical source. Cosmic ray physicists doubt that's possible, even if AMS sees a clear cutoff. "It's very easy to put a cutoff into an astrophysical model," Tarlé says. "All you have to do is limit the size of the particle-accelerating region."
Despite the uncertainty, the results mark a triumph for Ting, who all but willed AMS into orbit. Proposed in 1994, the detector made a test flight on NASA's space shuttle in 1998. But it appeared permanently grounded after the shuttle Columbia disintegrated on reentry in 2003, and NASA rethought the shuttle program. Ting worked tirelessly and in 2008 secured a congressional mandate that NASA launch AMS to the space station. (The AMS paper acknowledges nine current and former senators and representatives for their help.) By launch time, AMS's cost had ballooned from tens of millions of dollars to billions—although some observers say that Ting makes AMS sound as expensive as possible to accentuate how much countries such as China, South Korea, and Taiwan have contributed to it.
Those monumental costs still have some cosmic ray researchers shaking their heads in dismay. Tarlé says he's happy that AMS has confirmed the earlier results. But, he says, "I'm not happy it cost the world $2 billion, not to mention the cost of the extra shuttle flight." The controversy around AMS seems sure to continue.